// 创建一个带有默认初始容量（16）、加载因子（0.75）和concurrencyLevel（16）的新的空映射
ConcurrentHashMap()
// 创建一个带有指定初始容量、默认加载因子（0.75）和concurrencyLevel（16）的新的空映射
ConcurrentHashMap(int initialCapacity)
// 创建一个带有指定初始容量、加载因子和默认concurrencyLevel（16）的新的空映射
ConcurrentHashMap(int initialCapacity, float loadFactor)
// 构造一个与给定映射具有相同映射关系的新映射
ConcurrentHashMap(Map<? extends K, ? extends V> m)
// 创建一个带有指定初始容量、加载因子和并发级别的新的空映射
ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)

// 从该映射中移除所有映射关系
void clear()
// 一种遗留方法，测试此表中是否有一些与指定值存在映射关系的键
boolean contains(Object value)
// 测试指定对象是否为此表中的键
boolean containsKey(Object key)
// 如果此映射将一个或多个键映射到指定值，则返回true
boolean containsValue(Object value)
// 返回此表中值的枚举
Enumeration<V> elements()
// 返回此映射所包含的映射关系的Set视图
Set<Map.Entry<K, V>> entrySet()
// 返回指定键所映射到的值，如果此映射不包含该键的映射关系，则返回null
V get(Object key)
// 如果此映射不包含键-值映射关系，则返回true
boolean isEmpty()
// 返回此表中键的枚举
Enumeration<K> keys()
// 返回此映射中包含的键的Set视图
Set<K> keySet()
// 将指定键映射到此表中的指定值
V put(K key, V value)
// 将指定映射中所有映射关系复制到此映射中
void putAll(Map<? extends K, ? extends V> m)
// 如果指定键已经不再与某个值相关联，则将它与给定值关联
V putIfAbsent(K key, V value)
// 从此映射中移除键（及其相应的值）
V remove(Object key)
// 只有目前将键的条目映射到给定值时，才移除该键的条目
boolean remove(Object key, Object value)
// 只有目前将键的条目映射到某一值时，才替换该键的条目
V replace(K key, V value)
// 只有目前将键的条目映射到给定值时，才替换该键的条目
boolean replace(K key, V oldValue, V newValue)
// 返回此映射中的键-值映射关系数
int size()
// 返回此映射中包含的值的Collection视图
Collection<V> values()

package com.coderap.collection;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapTest {
    // 循环次数
    private final static int loopCount = 5;
    // 操作的HashMap和ConcurrentHashMap
    private final static Map<String, String> hashMap = new HashMap<>();
    private final static ConcurrentHashMap<String, String> concurrentHashMap = new ConcurrentHashMap();

    public static void main(String[] args) {
        // 开启两个线程同时操作
        new Thread(new ConcurrentHashMapTest.OperateThread(concurrentHashMap, loopCount), "T-1").start();
        new Thread(new ConcurrentHashMapTest.OperateThread(concurrentHashMap, loopCount), "T-2").start();

        //new Thread(new ConcurrentHashMapTest.OperateThread(hashMap, loopCount), "T-3").start();
        //new Thread(new ConcurrentHashMapTest.OperateThread(hashMap, loopCount), "T-4").start();
    }

    // 遍历方法
    private static void iterate(Map<String, String> map) {
        StringBuilder stringBuilder = new StringBuilder();
        for (Map.Entry<String, String> entry : map.entrySet()) {
            stringBuilder.append(entry.getKey() + " => " + entry.getValue());
            stringBuilder.append(", ");
        }
        // 删除最后多余的字符
        stringBuilder.delete(stringBuilder.length() - 2, stringBuilder.length() - 1);
        // 打印
        System.out.println(Thread.currentThread().getName() + " iterate: " + stringBuilder.toString());
    }

    private static class OperateThread implements Runnable {

        private Map map;
        private int loopCount;

        public OperateThread(Map map, int loopCount) {
            this.map = map;
            this.loopCount = loopCount;
        }

        @Override
        public void run() {
            // 循环添加并遍历打印
            while (loopCount >= 0) {
                map.put(Thread.currentThread().getName() + " - " + loopCount, "" + loopCount);
                iterate(map);
                loopCount--;
            }
        }
    }
}

T-1 iterate: T-1 - 5 => 5
T-1 iterate: T-1 - 5 => 5, T-1 - 4 => 4
T-1 iterate: T-1 - 5 => 5, T-1 - 4 => 4, T-1 - 3 => 3
T-1 iterate: T-1 - 2 => 2, T-1 - 5 => 5, T-1 - 4 => 4, T-1 - 3 => 3
T-1 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 5 => 5, T-1 - 4 => 4, T-1 - 3 => 3
T-1 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-1 - 5 => 5, T-1 - 4 => 4, T-1 - 3 => 3
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-1 - 5 => 5, T-1 - 4 => 4, T-2 - 5 => 5, T-1 - 3 => 3
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-1 - 5 => 5, T-1 - 4 => 4, T-2 - 5 => 5, T-1 - 3 => 3, T-2 - 4 => 4
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-2 - 3 => 3, T-1 - 5 => 5, T-1 - 4 => 4, T-2 - 5 => 5, T-1 - 3 => 3, T-2 - 4 => 4
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-2 - 3 => 3, T-1 - 5 => 5, T-2 - 2 => 2, T-1 - 4 => 4, T-2 - 5 => 5, T-1 - 3 => 3, T-2 - 4 => 4
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-2 - 1 => 1, T-2 - 3 => 3, T-1 - 5 => 5, T-2 - 2 => 2, T-1 - 4 => 4, T-2 - 5 => 5, T-1 - 3 => 3, T-2 - 4 => 4
T-2 iterate: T-1 - 2 => 2, T-1 - 1 => 1, T-1 - 0 => 0, T-1 - 5 => 5, T-1 - 4 => 4, T-1 - 3 => 3, T-2 - 1 => 1, T-2 - 0 => 0, T-2 - 3 => 3, T-2 - 2 => 2, T-2 - 5 => 5, T-2 - 4 => 4

static final class HashEntry<K, V> {
    // hash值
    final int hash;
    // 键
    final K key;
    // 值，volatile修饰，实现线程可见性
    volatile V value;
    // 下一个HashEntry，构成链表
    volatile HashEntry<K, V> next;

    // 构造方法
    HashEntry(int hash, K key, V value, HashEntry<K, V> next) {
        this.hash = hash;
        this.key = key;
        this.value = value;
        this.next = next;
    }

    // Unsafe方式设置后继HashEntry
    final void setNext(HashEntry<K, V> n) {
        UNSAFE.putOrderedObject(this, nextOffset, n);
    }

    // Unsafe相关
    static final sun.misc.Unsafe UNSAFE;
    static final long nextOffset;
    static {
        try {
            // 初始化UNSAFE常量
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class k = HashEntry.class;
            // 初始化nextOffset，用于Unsafe操作
            nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

/**
 * The per-segment table. Elements are accessed via
 * entryAt/setEntryAt providing volatile semantics.
 * 每个Segment Table装载HashEntry的数组，volatile修饰
 * 元素主要通过具有volatile语义的entryAt/setEntryAt方法进行获取和设置
 */
transient volatile HashEntry<K, V>[] table;

/**
 * Gets the ith element of given table (if nonnull) with volatile
 * read semantics. Note: This is manually integrated into a few
 * performance-sensitive methods to reduce call overhead.
 * 获取tab数组上i位置的元素，看不懂不要紧，后面有解释
 */
@SuppressWarnings("unchecked")
static final <K, V> HashEntry<K, V> entryAt(HashEntry<K, V>[] tab, int i) {
    return (tab == null) ? null : (HashEntry<K, V>) UNSAFE.getObjectVolatile(tab, ((long) i << TSHIFT) + TBASE);
}

/**
 * Sets the ith element of given table, with volatile write
 * semantics. (See above about use of putOrderedObject.)
 * 设置tab数组上i位置的元素为e，看不懂不要紧，后面有解释
 */
static final <K, V> void setEntryAt(HashEntry<K, V>[] tab, int i, HashEntry<K, V> e) {
    UNSAFE.putOrderedObject(tab, ((long) i << TSHIFT) + TBASE, e);
}

static final class Segment<K, V> extends ReentrantLock implements Serializable {

    private static final long serialVersionUID = 2249069246763182397L;

    /**
     * The maximum number of times to tryLock in a prescan before
     * possibly blocking on acquire in preparation for a locked
     * segment operation. On multiprocessors, using a bounded
     * number of retries maintains cache acquired while locating
     * nodes.
     * 尝试获取锁的最大扫描次数，当在多核计算机上为64次，单核计算机上为1次
     */
    static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;

    /**
     * The per-segment table. Elements are accessed via
     * entryAt/setEntryAt providing volatile semantics.
     * 每个Segment Table装载HashEntry的数组，volatile修饰
     * 元素主要通过具有volatile语义的entryAt/setEntryAt方法进行获取和设置
     */
    transient volatile HashEntry<K, V>[] table;

    /**
     * The number of elements. Accessed only either within locks
     * or among other volatile reads that maintain visibility.
     * 元素的个数
     */
    transient int count;

    /**
     * The total number of mutative operations in this segment.
     * Even though this may overflows 32 bits, it provides
     * sufficient accuracy for stability checks in CHM isEmpty()
     * and size() methods.  Accessed only either within locks or
     * among other volatile reads that maintain visibility.
     * 当前Segment的修改次数
     */
    transient int modCount;

    /**
     * The table is rehashed when its size exceeds this threshold.
     * (The value of this field is always <tt>(int)(capacity *
     * loadFactor)</tt>.)
     * 当HashEntry数组大小超过这个极值，就会触发rehash操作
     * threshold = (int)(capacity * loadFactor)
     * 即 threshold = 容量 * 加载因子
     */
    transient int threshold;

    /**
     * The load factor for the hash table.  Even though this value
     * is same for all segments, it is replicated to avoid needing
     * links to outer object.
     * 加载因子，所有Segment都一样
     * @serial
     */
    final float loadFactor;

    // 从一个HashEntry数组构造Segment
    Segment(float lf, int threshold, HashEntry<K, V>[] tab) {
        this.loadFactor = lf;
        this.threshold = threshold;
        this.table = tab;
    }

    // 添加元素
    final V put(K key, int hash, V value, boolean onlyIfAbsent) { ... }

    /**
     * Doubles size of table and repacks entries, also adding the
     * given node to new table
     */
    @SuppressWarnings("unchecked")
    private void rehash(HashEntry<K, V> node) { ... }

    /**
     * Scans for a node containing given key while trying to
     * acquire lock, creating and returning one if not found. Upon
     * return, guarantees that lock is held. UNlike in most
     * methods, calls to method equals are not screened: Since
     * traversal speed doesn't matter, we might as well help warm
     * up the associated code and accesses as well.
     *
     * @return a new node if key not found, else null
     */
    private HashEntry<K, V> scanAndLockForPut(K key, int hash, V value) { ... }

    /**
     * Scans for a node containing the given key while trying to
     * acquire lock for a remove or replace operation. Upon
     * return, guarantees that lock is held.  Note that we must
     * lock even if the key is not found, to ensure sequential
     * consistency of updates.
     * 该方法与 {@link #scanAndLockForPut} 类似，但只有扫描上锁操作
     */
    private void scanAndLock(Object key, int hash) { ... }

    /**
     * Remove; match on key only if value null, else match both.
     * 移除操作，返回移除的元素
     */
    final V remove(Object key, int hash, Object value) { ... }

    // 替换节点，返回结果标识是否进行了更新
    final boolean replace(K key, int hash, V oldValue, V newValue) { ... }

    /**
     * 替换节点的值为value，并返回旧值
     * 与上面的replace相比这个方法不会比较旧值是否一致，同时返回值不一样，其他流程一样
     */
    final V replace(K key, int hash, V value) { ... }

    // 清理操作，会将table中已有的节点全部置为null
    final void clear() { ... }
}

// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
// arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址
// Segment数组中第一个Segment的地址偏移量
private static final long SBASE;
// arrayIndexScale方法是一个本地方法，可以计算数组中元素的增量地址
// SSHIFT是Segment数组中元素的增量地址
private static final int SSHIFT;
// HashEntry数组中第一个HashEntry的地址偏移量
private static final long TBASE;
// SSHIFT是HashEntry数组中元素的增量地址
private static final int TSHIFT;
// hashSeed的偏移量
private static final long HASHSEED_OFFSET;

static {
    int ss, ts;
    try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class tc = HashEntry[].class;
        Class sc = Segment[].class;
        TBASE = UNSAFE.arrayBaseOffset(tc);
        SBASE = UNSAFE.arrayBaseOffset(sc);
        ts = UNSAFE.arrayIndexScale(tc);
        ss = UNSAFE.arrayIndexScale(sc);
        HASHSEED_OFFSET = UNSAFE.objectFieldOffset(ConcurrentHashMap.class.getDeclaredField("hashSeed"));
    } catch (Exception e) {
        throw new Error(e);
    }
    // ss和ts必然是2的次方
    if ((ss & (ss - 1)) != 0 || (ts & (ts - 1)) != 0)
        throw new Error("data type scale not a power of two");
    // Segemnt数组中元素的增量地址值，按位数据转换后除掉高位补0后非零的位数
    SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
    // HashEntry数组中元素的增量地址值，按位数据转换后除掉高位补0后非零的位数
    TSHIFT = 31 - Integer.numberOfLeadingZeros(ts);
}

if ((ss & (ss - 1)) != 0 || (ts & (ts - 1)) != 0)
        throw new Error("data type scale not a power of two");

// 获取tab数组上i位置的元素
@SuppressWarnings("unchecked")
static final <K, V> HashEntry<K, V> entryAt(HashEntry<K, V>[] tab, int i) {
    return (tab == null) ? null : (HashEntry<K, V>) UNSAFE.getObjectVolatile(tab, ((long) i << TSHIFT) + TBASE);
}

// 设置tab数组上i位置的元素为e
static final <K, V> void setEntryAt(HashEntry<K, V>[] tab, int i, HashEntry<K, V> e) {
    UNSAFE.putOrderedObject(tab, ((long) i << TSHIFT) + TBASE, e);
}

@SuppressWarnings("unchecked")
static final <K, V> Segment<K, V> segmentAt(Segment<K, V>[] ss, int j) {
    long u = (j << SSHIFT) + SBASE;
    return ss == null ? null : (Segment<K, V>) UNSAFE.getObjectVolatile(ss, u);
}

// 默认初始容量
static final int DEFAULT_INITIAL_CAPACITY = 16;
// 加载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 默认并发级别
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
// 最大容量，1,073,741,824
static final int MAXIMUM_CAPACITY = 1 << 30;
// 分段表最小容量
static final int MIN_SEGMENT_TABLE_CAPACITY = 2;
// 分段表最大数量，65,535
static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

/**
 * Number of unsynchronized retries in size and containsValue
 * methods before resorting to locking. This is used to avoid
 * unbounded retries if tables undergo continuous modification
 * which would make it impossible to obtain an accurate result.
 * 在size()方法和containsValue()方法中上锁钱的重试次数，
 * 可避免因哈希表被其他线程修改导致的无限重试
 */
static final int RETRIES_BEFORE_LOCK = 2;

/**
 * Mask value for indexing into segments. The upper bits of a
 * key's hash code are used to choose the segment.
 * 用于定位Segment的Mask码
 */
final int segmentMask;

/**
 * Shift value for indexing within segments.
 */
final int segmentShift;

/**
 * The segments, each of which is a specialized hash table.
 * 存放所有的Segment
 */
final Segment<K, V>[] segments;

// 遍历相关
transient Set<K> keySet;
transient Set<Map.Entry<K, V>> entrySet;
transient Collection<V> values;

/**
 * A randomizing value associated with this instance that is applied to
 * hash code of keys to make hash collisions harder to find.
 * 随机Hash种子
 */
private transient final int hashSeed = randomHashSeed(this);
// 产生随机哈希种子的随机方法
private static int randomHashSeed(ConcurrentHashMap instance) {
    if (sun.misc.VM.isBooted() && Holder.ALTERNATIVE_HASHING) {
        return sun.misc.Hashing.randomHashSeed(instance);
    }
    return 0;
}

package java.util.concurrent;

import java.util.concurrent.locks.*;
import java.util.*;
import java.io.Serializable;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
        implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments,
     * each of which itself is a concurrently readable hash table.  To
     * reduce footprint, all but one segments are constructed only
     * when first needed (see ensureSegment). To maintain visibility
     * in the presence of lazy construction, accesses to segments as
     * well as elements of segment's table must use volatile access,
     * which is done via Unsafe within methods segmentAt etc
     * below. These provide the functionality of AtomicReferenceArrays
     * but reduce the levels of indirection. Additionally,
     * volatile-writes of table elements and entry "next" fields
     * within locked operations use the cheaper "lazySet" forms of
     * writes (via putOrderedObject) because these writes are always
     * followed by lock releases that maintain sequential consistency
     * of table updates.
     *
     * Historical note: The previous version of this class relied
     * heavily on "final" fields, which avoided some volatile reads at
     * the expense of a large initial footprint.  Some remnants of
     * that design (including forced construction of segment 0) exist
     * to ensure serialization compatibility.
     */

    /* ---------------- Constants -------------- */

    // 默认初始容量
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    // 加载因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    // 默认并发级别
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    // 最大容量，1,073,741,824
    static final int MAXIMUM_CAPACITY = 1 << 30;

    // 分段表最小容量
    static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

    // 分段表最大数量，也是最大并发级别，65,535
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue
     * methods before resorting to locking. This is used to avoid
     * unbounded retries if tables undergo continuous modification
     * which would make it impossible to obtain an accurate result.
     * 在size()方法和containsValue()方法中上锁钱的重试次数，
     * 可避免因哈希表被其他线程修改导致的无限重试
     */
    static final int RETRIES_BEFORE_LOCK = 2;

    /* ---------------- Fields -------------- */

    /**
     * holds values which can't be initialized until after VM is booted.
     * 存放一些在虚拟机启动后才能初始化的值
     */
    private static class Holder {

        /**
         * Enable alternative hashing of String keys?
         *
         * <p>Unlike the other hash map implementations we do not implement a
         * threshold for regulating whether alternative hashing is used for
         * String keys. Alternative hashing is either enabled for all instances
         * or disabled for all instances.
         */
        static final boolean ALTERNATIVE_HASHING;

        static {
            // Use the "threshold" system property even though our threshold behaviour is "ON" or "OFF".
            // 获得JVM的参数jdk.map.althashing.threshold的值，这个值将作为阈值
            String altThreshold = java.security.AccessController.doPrivileged(new sun.security.action.GetPropertyAction("jdk.map.althashing.threshold"));

            int threshold;
            try {
                // 强转altThreshold，如果altThreshold为null则取2,147,483,647
                threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : Integer.MAX_VALUE;

                // disable alternative hashing if -1
                // 当threshold为-1时，关掉alternative hashing
                if (threshold == -1) {
                    threshold = Integer.MAX_VALUE;
                }

                if (threshold < 0) {
                    throw new IllegalArgumentException("value must be positive integer.");
                }
            } catch (IllegalArgumentException failed) {
                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
            }
            /**
             * 当threshold <= 1,073,741,824时ALTERNATIVE_HASHING为true
             * 其实这种情况表明，当-Djdk.map.althashing.threshold参数传入-1时，ALTERNATIVE_HASHING必然为false，也即关掉alternative hashing
             */
            ALTERNATIVE_HASHING = threshold <= MAXIMUM_CAPACITY;
        }
    }

    /**
     * A randomizing value associated with this instance that is applied to
     * hash code of keys to make hash collisions harder to find.
     * 随机Hash种子，这个值取决于 {@link Holder#ALTERNATIVE_HASHING}
     */
    private transient final int hashSeed = randomHashSeed(this);

    // 根据Holder.ALTERNATIVE_HASHING计算得到hashSeed
    private static int randomHashSeed(ConcurrentHashMap instance) {
        if (sun.misc.VM.isBooted() && Holder.ALTERNATIVE_HASHING) {
            return sun.misc.Hashing.randomHashSeed(instance);
        }

        return 0;
    }

    /**
     * Mask value for indexing into segments. The upper bits of a
     * key's hash code are used to choose the segment.
     * 用于定位Segment的Mask码
     */
    final int segmentMask;

    /**
     * Shift value for indexing within segments.
     */
    final int segmentShift;

    /**
     * The segments, each of which is a specialized hash table.
     * 存放所有的Segment
     */
    final Segment<K, V>[] segments;

    // 遍历相关
    transient Set<K> keySet;
    transient Set<Map.Entry<K, V>> entrySet;
    transient Collection<V> values;

    /**
     * ConcurrentHashMap list entry. Note that this is never exported
     * out as a user-visible Map.Entry.
     */
    static final class HashEntry<K, V> {
        // hash值
        final int hash;
        // 键
        final K key;
        // 值，volatile修饰，实现线程可见性
        volatile V value;
        // 下一个HashEntry，构成链表
        volatile HashEntry<K, V> next;

        // 构造方法
        HashEntry(int hash, K key, V value, HashEntry<K, V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        // Unsafe方式设置后继HashEntry
        final void setNext(HashEntry<K, V> n) {
            UNSAFE.putOrderedObject(this, nextOffset, n);
        }

        // Unsafe相关
        static final sun.misc.Unsafe UNSAFE;
        static final long nextOffset;

        static {
            try {
                // 初始化UNSAFE常量
                UNSAFE = sun.misc.Unsafe.getUnsafe();
                Class k = HashEntry.class;
                // 初始化nextOffset，用于Unsafe操作
                nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next"));
            } catch (Exception e) {
                throw new Error(e);
            }
        }
    }

    /**
     * Gets the ith element of given table (if nonnull) with volatile
     * read semantics. Note: This is manually integrated into a few
     * performance-sensitive methods to reduce call overhead.
     * 获取tab数组上i位置的元素
     */
    @SuppressWarnings("unchecked")
    static final <K, V> HashEntry<K, V> entryAt(HashEntry<K, V>[] tab, int i) {
        return (tab == null) ? null : (HashEntry<K, V>) UNSAFE.getObjectVolatile(tab, ((long) i << TSHIFT) + TBASE);
    }

    /**
     * Sets the ith element of given table, with volatile write
     * semantics. (See above about use of putOrderedObject.)
     * 设置tab数组上i位置的元素为e
     */
    static final <K, V> void setEntryAt(HashEntry<K, V>[] tab, int i, HashEntry<K, V> e) {
        UNSAFE.putOrderedObject(tab, ((long) i << TSHIFT) + TBASE, e);
    }

    /**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because ConcurrentHashMap uses power-of-two length hash tables,
     * that otherwise encounter collisions for hashCodes that do not
     * differ in lower or upper bits.
     * hash方法，根据k计算hash值
     */
    private int hash(Object k) {
        int h = hashSeed;

        if ((0 != h) && (k instanceof String)) {
            return sun.misc.Hashing.stringHash32((String) k);
        }

        h ^= k.hashCode();

        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    /**
     * Segments are specialized versions of hash tables.  This
     * subclasses from ReentrantLock opportunistically, just to
     * simplify some locking and avoid separate construction.
     * 继承自ReentrantLock，即自身是一个重入锁
     */
    static final class Segment<K, V> extends ReentrantLock implements Serializable {

        private static final long serialVersionUID = 2249069246763182397L;

        /**
         * The maximum number of times to tryLock in a prescan before
         * possibly blocking on acquire in preparation for a locked
         * segment operation. On multiprocessors, using a bounded
         * number of retries maintains cache acquired while locating
         * nodes.
         * 尝试获取锁的最大扫描次数，当在多核计算机上为64次，单核计算机上为1次
         */
        static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;

        /**
         * The per-segment table. Elements are accessed via
         * entryAt/setEntryAt providing volatile semantics.
         * 每个Segment Table装载HashEntry的数组，volatile修饰
         * 元素主要通过具有volatile语义的entryAt/setEntryAt方法进行获取和设置
         */
        transient volatile HashEntry<K, V>[] table;

        /**
         * The number of elements. Accessed only either within locks
         * or among other volatile reads that maintain visibility.
         * 元素的个数
         */
        transient int count;

        /**
         * The total number of mutative operations in this segment.
         * Even though this may overflows 32 bits, it provides
         * sufficient accuracy for stability checks in CHM isEmpty()
         * and size() methods.  Accessed only either within locks or
         * among other volatile reads that maintain visibility.
         * 当前Segment的修改次数
         */
        transient int modCount;

        /**
         * The table is rehashed when its size exceeds this threshold.
         * (The value of this field is always <tt>(int)(capacity *
         * loadFactor)</tt>.)
         * 当HashEntry数组大小超过这个极值，就会触发rehash操作
         * threshold = (int)(capacity * loadFactor)
         * 即 threshold = 容量 * 加载因子
         */
        transient int threshold;

        /**
         * The load factor for the hash table.  Even though this value
         * is same for all segments, it is replicated to avoid needing
         * links to outer object.
         * 加载因子，所有Segment都一样
         *
         * @serial
         */
        final float loadFactor;

        // 从一个HashEntry数组构造Segment
        Segment(float lf, int threshold, HashEntry<K, V>[] tab) {
            this.loadFactor = lf;
            this.threshold = threshold;
            this.table = tab;
        }

        // 添加元素
        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            /**
             * 先尝试获取锁，tryLock()方法是继承自ReentrantLock的方法
             * 如果获取到锁了，node为null，否则执行scanAndLockForPut(key, hash, value)获取node
             * scanAndLockForPut()内部会不断自旋尝试获取锁，一旦其返回了即说明获取到锁了
             * 如果返回值不为null，表示是相应HashEntry链表的头节点
             *
             * 当获取到锁之后，会根据hash找到对应的HashEntry链表（此处具体实现是找到链表的头元素）
             * 然后在链表中根据key进行遍历查找
             * 如果查找到了key相同的HashEntry，就判断onlyIfAbsent是否为true，如果是就更新该HashEntry的值为value，否则直接跳出
             * 如果没有查找到，可能有两种情况，链表为空或者确实没有
             * 则根据传入的key、hash、value创建一个HashEntry，设置到index位置上
             */
            HashEntry<K, V> node = tryLock() ? null : scanAndLockForPut(key, hash, value);
            V oldValue;
            try {
                HashEntry<K, V>[] tab = table;
                // 计算index，即 (table的长度 - 1) & hash值
                int index = (tab.length - 1) & hash;
                // 获取table上索引为index上的元素
                HashEntry<K, V> first = entryAt(tab, index);
                for (HashEntry<K, V> e = first; ; ) {
                    if (e != null) {
                        // 获取到的元素不为null，表示index上已经有元素了
                        K k;
                        if ((k = e.key) == key || (e.hash == hash && key.equals(k))) {
                            // 走到这里，表示put进的键与已经存在于index位置上HashEntry的键相同
                            // 先记录旧值
                            oldValue = e.value;
                            if (!onlyIfAbsent) {
                                /**
                                 * 如果没有指定onlyIfAbsent为true，即没有要求必须是不存在的情况下添加
                                 * 就更新index位置上已存在的元素的值为value
                                 */
                                e.value = value;
                                // modCount自增
                                ++modCount;
                            }
                            break;
                        }
                        // 走到这里表示key与e.key不相同，则遍历链表下一个元素进行判断
                        e = e.next;
                    } else {
                        // 获取到的元素为null，表示index上没有元素
                        if (node != null)
                            // 如果node不为null，将first设置为node的后继元素
                            node.setNext(first);
                        else
                            // 如果node为null，则创建一个新的HashEntry赋值给node，并将first作为node的后继节点
                            node = new HashEntry<K, V>(hash, key, value, first);
                        /**
                         * 计算Segment中元素个数是否超过阈值，如果超过了就进行rehash操作
                         * 否则将node放在table的index位置上
                         */
                        int c = count + 1;
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                            rehash(node);
                        else
                            setEntryAt(tab, index, node);
                        // 更新modCount
                        ++modCount;
                        // 更新Segment元素个数计数器
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                // 解锁
                unlock();
            }
            return oldValue;
        }

        /**
         * Doubles size of table and repacks entries, also adding the
         * given node to new table
         *
         * 重哈希操作，会对Segment进行扩容，然后将传入的节点添加到Segment的table数组中
         */
        @SuppressWarnings("unchecked")
        private void rehash(HashEntry<K, V> node) {
            /*
             * Reclassify nodes in each list to new table.  Because we
             * are using power-of-two expansion, the elements from
             * each bin must either stay at same index, or move with a
             * power of two offset. We eliminate unnecessary node
             * creation by catching cases where old nodes can be
             * reused because their next fields won't change.
             * Statistically, at the default threshold, only about
             * one-sixth of them need cloning when a table
             * doubles. The nodes they replace will be garbage
             * collectable as soon as they are no longer referenced by
             * any reader thread that may be in the midst of
             * concurrently traversing table. Entry accesses use plain
             * array indexing because they are followed by volatile
             * table write.
             */
            // 记录旧table
            HashEntry<K, V>[] oldTable = table;
            // 记录旧的容量
            int oldCapacity = oldTable.length;
            // 新的容量是旧的容量的两倍
            int newCapacity = oldCapacity << 1;
            // 计算新的阈值
            threshold = (int) (newCapacity * loadFactor);
            // 先创建一个新的HashEntry数组newtable
            HashEntry<K, V>[] newTable = (HashEntry<K, V>[]) new HashEntry[newCapacity];
            int sizeMask = newCapacity - 1;
            for (int i = 0; i < oldCapacity; i++) {
                // 遍历取出旧元素e
                HashEntry<K, V> e = oldTable[i];
                if (e != null) {
                    // e的后继
                    HashEntry<K, V> next = e.next;
                    // 计算新的index为idx
                    int idx = e.hash & sizeMask;
                    if (next == null)   //  Single node on list
                        // 如果e的后继为空，表示e是一个单节点，则直接将其存入newTable的idx位置
                        newTable[idx] = e;
                    else {
                        // Reuse consecutive sequence at same slot
                        // 走到这里说明e是一个HashEntry链的头节点
                        HashEntry<K, V> lastRun = e;
                        int lastIdx = idx;
                        /**
                         * 这是一种优化做法，可以将一部分新索引一致的元素一次性迁移
                         * 从next开始向后遍历，该过程主要是找到第一个新的index与旧的index不同的节点
                         * 通过这个遍历过程，可以找到由于HashEntry链容量较之原来扩大了一倍，计算出的新的index与原来不同的第一个元素，赋值给lastRun
                         * 而这个lastRun元素之后的所有元素的新的index都应该与之相同，否则后续的遍历还会更新lastRun的指向
                         */
                        for (HashEntry<K, V> last = next; last != null; last = last.next) {
                            // 计算当前遍历到的节点新的index为k
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                // 如果当前节点新的index与前驱节点的新的index不同，则记录当前节点新的index为lastIdx
                                lastIdx = k;
                                // 记录当前节点为lastRun
                                lastRun = last;
                            }
                        }
                        /**
                         * 将上面计算出来的lastRun放在newTable的lastIdx位置的槽中，这个lastIdx也是上面新计算出来的
                         * 由于HashEntry链是一个链表，因此lastRun后面的元素也会一并移动到newTable的lastIdx位置的槽中
                         */
                        newTable[lastIdx] = lastRun;
                        // 复制e元素对应的HashEntry链上剩余的元素到新的newTable中
                        for (HashEntry<K, V> p = e; p != lastRun; p = p.next) {
                            V v = p.value;
                            int h = p.hash;
                            // 计算新的index为k
                            int k = h & sizeMask;
                            // 取出原来k位置上的元素n
                            HashEntry<K, V> n = newTable[k];
                            // 构建新节点，将n作为新节点的后继节点，然后将新节点放在newTable的k位置上，相当于把新节点插在链表头部
                            newTable[k] = new HashEntry<K, V>(h, p.key, v, n);
                        }
                    }
                }
            }
            // 将传入的node添加到对应的HashEntry链表头
            int nodeIndex = node.hash & sizeMask; // add the new node
            node.setNext(newTable[nodeIndex]);
            newTable[nodeIndex] = node;
            // 将newTable赋值给table
            table = newTable;
        }

        /**
         * Scans for a node containing given key while trying to
         * acquire lock, creating and returning one if not found. Upon
         * return, guarantees that lock is held. UNlike in most
         * methods, calls to method equals are not screened: Since
         * traversal speed doesn't matter, we might as well help warm
         * up the associated code and accesses as well.
         *
         * 扫描元素并尝试加锁，扫描元素主要用于寻找与给定key相同的HashEntry节点，
         * 如果没找到就创建一个新的节点，然后将这个节点返回。
         * 该方法可以保证锁已获取到
         *
         * @return a new node if key not found, else null
         */
        private HashEntry<K, V> scanAndLockForPut(K key, int hash, V value) {
            // 尝试获取HashEntry链的第一个元素
            HashEntry<K, V> first = entryForHash(this, hash);
            HashEntry<K, V> e = first;
            HashEntry<K, V> node = null;
            int retries = -1; // negative while locating node
            while (!tryLock()) {
                // 走到这里说明获取锁失败
                HashEntry<K, V> f; // to recheck first below
                if (retries < 0) {
                    // 如果重试次数retries小于0，表示可能是第一次尝试
                    /**
                     * 从entryForHash()方法可知，e为null有两种情况：
                     * 1. Segment为空
                     * 2. Segment的table为空则返回空
                     */
                    if (e == null) {
                        // 创建一个HashEntry节点
                        if (node == null) // speculatively create node
                            node = new HashEntry<K, V>(hash, key, value, null);
                        // retries次数置为0，下次就不会走if (retries < 0)的分支了
                        retries = 0;
                    } else if (key.equals(e.key))
                        // 当e不为null，且key与e.key相同时，将retries次数置为0
                        retries = 0;
                    else
                        // 走到这里，说明e不为null，且key与e.key不同，则继续往后查找
                        e = e.next;
                } else if (++retries > MAX_SCAN_RETRIES) {
                    // 当重试次数超过限制，强制上锁（可能会进入阻塞状态）
                    lock();
                    break;
                } else if ( // 是否是偶数次尝试
                        (retries & 1) == 0 &&
                                // 重新尝试获取HashEntry链的第一个元素，然后判断本次获取的第一个元素是否与之前的first相同
                                (f = entryForHash(this, hash)) != first) {
                    /**
                     * 走到这里说明是偶数次重试，并且HashEntry链的第一个元素发生了改变
                     * 如果是的话，就应该重新遍历，
                     * 将e和first置为新获取的HashEntry链第一个元素
                     * 重置retries为-1，然后进入下一次循环
                     */
                    e = first = f; // re-traverse if entry changed
                    retries = -1;
                }
            }
            /**
             * 该方法最后返回node，即新创建的HashEntry表头节点
             * 只有在tryLock()方法或lock()获取到锁的时候才会返回，否则会一直自旋
             */
            return node;
        }

        /**
         * Scans for a node containing the given key while trying to
         * acquire lock for a remove or replace operation. Upon
         * return, guarantees that lock is held.  Note that we must
         * lock even if the key is not found, to ensure sequential
         * consistency of updates.
         * 该方法与 {@link #scanAndLockForPut} 类似，但只有扫描上锁操作
         */
        private void scanAndLock(Object key, int hash) {
            // similar to but simpler than scanAndLockForPut
            HashEntry<K, V> first = entryForHash(this, hash);
            HashEntry<K, V> e = first;
            int retries = -1;
            while (!tryLock()) {
                HashEntry<K, V> f;
                if (retries < 0) {
                    if (e == null || key.equals(e.key))
                        retries = 0;
                    else
                        e = e.next;
                } else if (++retries > MAX_SCAN_RETRIES) {
                    lock();
                    break;
                } else if ((retries & 1) == 0 &&
                        (f = entryForHash(this, hash)) != first) {
                    e = first = f;
                    retries = -1;
                }
            }
        }

        /**
         * Remove; match on key only if value null, else match both.
         * 移除操作，返回移除的元素
         */
        final V remove(Object key, int hash, Object value) {
            // 先尝试上锁
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K, V>[] tab = table;
                int index = (tab.length - 1) & hash;
                // 获取对应的元素e
                HashEntry<K, V> e = entryAt(tab, index);
                HashEntry<K, V> pred = null;
                while (e != null) {
                    K k;
                    // e的后继
                    HashEntry<K, V> next = e.next;
                    // key相同，hash值也相同
                    if ((k = e.key) == key || (e.hash == hash && key.equals(k))) {
                        // 获取元素的value
                        V v = e.value;
                        if (value == null || value == v || value.equals(v)) {
                            if (pred == null)
                                // 如果pred为null，将e的后继节点放在e现在的位置上
                                setEntryAt(tab, index, next);
                            else
                                // 否则将e的后继节点作为pred的后继，即跳过了原来的e节点
                                pred.setNext(next);
                            // 更新modCount计数器
                            ++modCount;
                            // 更新map大小
                            --count;
                            // 赋值给oldValue
                            oldValue = v;
                        }
                        break;
                    }
                    // 如果key不同，hash值也不同，则往后遍历
                    pred = e;
                    e = next;
                }
            } finally {
                // 解锁
                unlock();
            }
            return oldValue;
        }

        // 替换节点，返回结果标识是否进行了更新
        final boolean replace(K key, int hash, V oldValue, V newValue) {
            // 上锁
            if (!tryLock())
                scanAndLock(key, hash);
            boolean replaced = false;
            try {
                HashEntry<K, V> e;
                // 遍历
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key || (e.hash == hash && key.equals(k))) {
                        // 走到这里说明找到了key和hash都相同的节点
                        if (oldValue.equals(e.value)) {
                            // 如果找到的节点当前的value与oldValue一致则进行修改
                            e.value = newValue;
                            // 更新计数器
                            ++modCount;
                            replaced = true;
                        }
                        break;
                    }
                }
            } finally {
                // 解锁
                unlock();
            }
            // 返回的结果标识是否进行了更新
            return replaced;
        }

        /**
         * 替换节点的值为value，并返回旧值
         * 与上面的replace相比这个方法不会比较旧值是否一致，同时返回值不一样，其他流程一样
         */
        final V replace(K key, int hash, V value) {
            // 加锁
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K, V> e;
                // 遍历寻找节点
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key || (e.hash == hash && key.equals(k))) {
                        // key相同，hash值也相同，表示找到了节点
                        // 记录节点旧值
                        oldValue = e.value;
                        // 修改节点的value值
                        e.value = value;
                        ++modCount;
                        break;
                    }
                }
            } finally {
                // 解锁
                unlock();
            }
            // 返回旧值
            return oldValue;
        }

        // 清理操作，会将table中已有的节点全部置为null
        final void clear() {
            lock();
            try {
                // 遍历table数组，将每个元素都置为null
                HashEntry<K, V>[] tab = table;
                for (int i = 0; i < tab.length; i++)
                    setEntryAt(tab, i, null);
                ++modCount;
                count = 0;
            } finally {
                unlock();
            }
        }
    }

    // Accessing segments

    /**
     * Gets the jth element of given segment array (if nonnull) with
     * volatile element access semantics via Unsafe. (The null check
     * can trigger harmlessly only during deserialization.) Note:
     * because each element of segments array is set only once (using
     * fully ordered writes), some performance-sensitive methods rely
     * on this method only as a recheck upon null reads.
     * 获取Segment数组ss中索引为j的Segment元素
     */
    @SuppressWarnings("unchecked")
    static final <K, V> Segment<K, V> segmentAt(Segment<K, V>[] ss, int j) {
        long u = (j << SSHIFT) + SBASE;
        return ss == null ? null : (Segment<K, V>) UNSAFE.getObjectVolatile(ss, u);
    }

    /**
     * Returns the segment for the given index, creating it and
     * recording in segment table (via CAS) if not already present.
     * 返回指定索引位置的Segment，当不存在时将创建它
     *
     * @param k the index
     * @return the segment
     */
    @SuppressWarnings("unchecked")
    private Segment<K, V> ensureSegment(int k) {
        // 获取Segment数组
        final Segment<K, V>[] ss = this.segments;
        // 计算索引为k的Segment在Segment数组中的原始偏移量
        long u = (k << SSHIFT) + SBASE; // raw offset
        Segment<K, V> seg;
        // 尝试获取
        if ((seg = (Segment<K, V>) UNSAFE.getObjectVolatile(ss, u)) == null) {
            // 走到这里表示获取到的Segment为null
            // 使用索引为0位置上的Segment作为原型，主要将其table长度、加载因子等值用于新Segment的创建
            Segment<K, V> proto = ss[0]; // use segment 0 as prototype
            // table长度
            int cap = proto.table.length;
            // 加载因子
            float lf = proto.loadFactor;
            // 阈值
            int threshold = (int) (cap * lf);
            // 创建HashEntry链数组
            HashEntry<K, V>[] tab = (HashEntry<K, V>[]) new HashEntry[cap];
            // 重新检查索引为k的Segment是否为null
            if ((seg = (Segment<K, V>) UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck
                // 创建新的Segment
                Segment<K, V> s = new Segment<K, V>(lf, threshold, tab);
                while ((seg = (Segment<K, V>) UNSAFE.getObjectVolatile(ss, u)) == null) {
                    // CAS方式赋值
                    if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                        // 赋值成功即跳出while循环
                        break;
                }
            }
        }
        return seg;
    }

    // Hash-based segment and entry accesses

    /**
     * Get the segment for the given hash
     */
    @SuppressWarnings("unchecked")
    private Segment<K, V> segmentForHash(int h) {
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        return (Segment<K, V>) UNSAFE.getObjectVolatile(segments, u);
    }

    /**
     * Gets the table entry for the given segment and hash
     * 获取某个Segment中相应hash值的HashEntry
     */
    @SuppressWarnings("unchecked")
    static final <K, V> HashEntry<K, V> entryForHash(Segment<K, V> seg, int h) {
        HashEntry<K, V>[] tab;
        /**
         * 如果Segment为空或者Segment的table为空则返回空
         * 否则通过Unsafe方法获取相应的HashEntry
         */
        return (seg == null || (tab = seg.table) == null) ? null :
                (HashEntry<K, V>) UNSAFE.getObjectVolatile
                        (tab, ((long) (((tab.length - 1) & h)) << TSHIFT) + TBASE);
    }

    /* ---------------- Public operations -------------- */

    /**
     * Creates a new, empty map with the specified initial
     * capacity, load factor and concurrency level.
     * 根据初始容量、平衡因子和并发级别创建一个ConcurrentHashMap
     *
     * @param initialCapacity  初始容量
     * @param loadFactor       平衡因子，用于计算阈值
     * @param concurrencyLevel 并发级别
     * @throws IllegalArgumentException 当参数不规范时会抛出异常
     */
    @SuppressWarnings("unchecked")
    public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        // 检查并发级别，从这里可以得知，最大并发级别为65,535
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        /**
         * 根据并发级别，来计算Segment的最大个数，最大为65535
         * 其中sshift记录了左移次数，ssize则计算了大小
         */
        int sshift = 0;
        int ssize = 1;
        // 2的次方，不小于concurrencyLevel的那个值
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        // 这两个值后面会讲解
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        /**
         * 检查哈希表的初始容量
         * 哈希表的实际容量 = Segments的容量 * Segment中数组的长度
         */
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        // 计算每个Segment Table，即HashEntry链，应该装载的数据量
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        // cap即是Segments中HashEntry链的长度，2的次方，不小于c的那个值
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
        // 创建Segment数组的第0个Segment对象s0
        Segment<K, V> s0 = new Segment<K, V>(loadFactor, (int) (cap * loadFactor), (HashEntry<K, V>[]) new HashEntry[cap]);
        // 创建Segment数组
        Segment<K, V>[] ss = (Segment<K, V>[]) new Segment[ssize];
        // 将s0装入到Segment数组ss索引为0的位置
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

    /**
     * Creates a new, empty map with the specified initial capacity
     * and load factor and with the default concurrencyLevel (16).
     *
     * @param initialCapacity The implementation performs internal
     *                        sizing to accommodate this many elements.
     * @param loadFactor      the load factor threshold, used to control resizing.
     *                        Resizing may be performed when the average number of elements per
     *                        bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative or the load factor is nonpositive
     * @since 1.6
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity,
     * and with default load factor (0.75) and concurrencyLevel (16).
     *
     * @param initialCapacity the initial capacity. The implementation
     *                        performs internal sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative.
     */
    public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with a default initial capacity (16),
     * load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new map with the same mappings as the given map.
     * The map is created with a capacity of 1.5 times the number
     * of mappings in the given map or 16 (whichever is greater),
     * and a default load factor (0.75) and concurrencyLevel (16).
     *
     * @param m the map
     */
    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                DEFAULT_INITIAL_CAPACITY),
                DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     * 判断Map是否为空
     *
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        /*
         * Sum per-segment modCounts to avoid mis-reporting when
         * elements are concurrently added and removed in one segment
         * while checking another, in which case the table was never
         * actually empty at any point. (The sum ensures accuracy up
         * through at least 1<<31 per-segment modifications before
         * recheck.)  Methods size() and containsValue() use similar
         * constructions for stability checks.
         */
        long sum = 0L;
        // 获取Segment数组
        final Segment<K, V>[] segments = this.segments;
        // 循环遍历Segment数组
        for (int j = 0; j < segments.length; ++j) {
            Segment<K, V> seg = segmentAt(segments, j);
            if (seg != null) {
                // 一旦某个Segment中的元素不为0，就返回false
                if (seg.count != 0)
                    return false;
                // 更新modCount总和，这次是加操作
                sum += seg.modCount;
            }
        }
        // 重新检查
        if (sum != 0L) { // recheck unless no modifications
            for (int j = 0; j < segments.length; ++j) {
                Segment<K, V> seg = segmentAt(segments, j);
                if (seg != null) {
                    if (seg.count != 0)
                        return false;
                    // 更新modCount总和，这次是减操作
                    sum -= seg.modCount;
                }
            }
            // 二次检查后，sum应该为0，如果不为0表示有其他线程修改了Map导致元素数量发生了变化，则返回false
            if (sum != 0L)
                return false;
        }
        return true;
    }

    /**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     * 返回Map中键值对的个数，当超过Integer.MAX_VALUE时会返回Integer.MAX_VALUE
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        /**
         * Try a few times to get accurate count. On failure due to continuous async changes in table, resort to locking.
         * 多次尝试以获取准确的数量。当持续进行table的异步更新、重排和锁定时可能会失败
         */
        // 获取Segment数组
        final Segment<K, V>[] segments = this.segments;
        int size;
        // 标识是否溢出
        boolean overflow; // true if size overflows 32 bits
        // 本次计算后得到的modCount
        long sum;         // sum of modCounts
        // 上一次计算后得到的modCount
        long last = 0L;   // previous sum
        int retries = -1; // first iteration isn't retry
        try {
            // 循环不断尝试
            for (; ; ) {
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    // 遍历操作，对所有的Segment上锁
                    for (int j = 0; j < segments.length; ++j)
                        /**
                         * 获取Segment数组中索引为j位置上的Segment，并上锁
                         * 这个操作可以保证如果索引为j位置上没有Segment，就新创建一个
                         */
                        ensureSegment(j).lock(); // force creation
                }
                sum = 0L;
                size = 0;
                overflow = false;
                // 遍历所有的Segment，叠加计算其存储的元素的数量
                for (int j = 0; j < segments.length; ++j) {
                    // 获取Segment
                    Segment<K, V> seg = segmentAt(segments, j);
                    if (seg != null) {
                        // 更新modCount计数器
                        sum += seg.modCount;
                        int c = seg.count;
                        // 叠加并判断相关数量是否溢出
                        if (c < 0 || (size += c) < 0)
                            overflow = true;
                    }
                }
                /**
                 * 将当前这一次计算后的得到modCount与上一次计算后的得到modCount对比，
                 * 如果相同就跳出for循环，结束计数
                 * 这种方式可以保证计数的精确性，如果两次计算得到modCount不一致表示可能有其他的线程修改了Map导致元素数量发生了变化
                 */
                if (sum == last)
                    break;
                // 记录这一次计算后得到modCount
                last = sum;
            }
        } finally {
            // 循环对所有的Segment解锁
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        // 返回值
        return overflow ? Integer.MAX_VALUE : size;
    }

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * 获取指定键对应的值，如果在Map中没有找到该键，将返回null
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        Segment<K, V> s; // manually integrate access methods to reduce overhead
        HashEntry<K, V>[] tab;
        // 哈希值
        int h = hash(key);
        // 获取Segment对应的索引
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        // 获取Segment
        if ((s = (Segment<K, V>) UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) {
            // 获取并遍历HashEntry链
            for (HashEntry<K, V> e = (HashEntry<K, V>) UNSAFE.getObjectVolatile
                    (tab, ((long) (((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                // 键相同，哈希值也相同
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    // 返回值
                    return e.value;
            }
        }
        // 否则返回null
        return null;
    }

    /**
     * Tests if the specified object is a key in this table.
     * 判断某个键是否存在于Map中
     *
     * @param key possible key
     * @return <tt>true</tt> if and only if the specified object
     * is a key in this table, as determined by the
     * <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException if the specified key is null 键不可为空
     */
    @SuppressWarnings("unchecked")
    public boolean containsKey(Object key) {
        Segment<K, V> s; // same as get() except no need for volatile value read
        HashEntry<K, V>[] tab;
        // 获取key的哈希值
        int h = hash(key);
        // 根据哈希值计算相应的Segment在Segment数组中的偏移量
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        // 根据偏移量获取Segment
        if ((s = (Segment<K, V>) UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) {
            // 获取到的Segment不为null，且该Segment的table不为null，就遍历该Segment的table中的HashEntry对象
            for (HashEntry<K, V> e = (HashEntry<K, V>) UNSAFE.getObjectVolatile
                    (tab, ((long) (((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                // key相同，key的hash也相同，表示找到了，就返回true；否则继续查找下一个HashEntry
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return true;
            }
        }
        return false;
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value. Note: This method requires a full internal
     * traversal of the hash table, and so is much slower than
     * method <tt>containsKey</tt>.
     *
     * 判断某个值是否存在于Map中，可能需要遍历整个Map，因此比containsKey要慢
     *
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the
     * specified value
     * @throws NullPointerException if the specified value is null 值不能为null
     */
    public boolean containsValue(Object value) {
        // Same idea as size() 实现方式与size()方法类似
        // 检查value是否为空
        if (value == null)
            throw new NullPointerException();
        // 获取Segment数组
        final Segment<K, V>[] segments = this.segments;
        // 标识是否找到
        boolean found = false;
        long last = 0;
        int retries = -1;
        try {
            outer:
            for (; ; ) {
                // 先对所有Segment上锁
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                long hashSum = 0L;
                int sum = 0;
                // 遍历Segment
                for (int j = 0; j < segments.length; ++j) {
                    HashEntry<K, V>[] tab;
                    // 获取指定索引的Segment
                    Segment<K, V> seg = segmentAt(segments, j);
                    if (seg != null && (tab = seg.table) != null) {
                        // 遍历Segment中的HashEntry链
                        for (int i = 0; i < tab.length; i++) {
                            HashEntry<K, V> e;
                            // 遍历HashEntry链
                            for (e = entryAt(tab, i); e != null; e = e.next) {
                                V v = e.value;
                                // 判断值是否相同
                                if (v != null && value.equals(v)) {
                                    // 如果有相同的表示是包含这个值的
                                    found = true;
                                    // 跳出到外层循环
                                    break outer;
                                }
                            }
                        }
                        sum += seg.modCount;
                    }
                }
                // 判断得到的modCount是否一致，保证精确性
                if (retries > 0 && sum == last)
                    break;
                last = sum;
            }
        } finally {
            // 遍历Segment并解锁
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return found;
    }

    /**
     * Legacy method testing if some key maps into the specified value
     * in this table.  This method is identical in functionality to
     * {@link #containsValue}, and exists solely to ensure
     * full compatibility with class {@link java.util.Hashtable},
     * which supported this method prior to introduction of the
     * Java Collections framework.
     *
     * @param value a value to search for
     * @return <tt>true</tt> if and only if some key maps to the
     * <tt>value</tt> argument in this table as
     * determined by the <tt>equals</tt> method;
     * <tt>false</tt> otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean contains(Object value) {
        return containsValue(value);
    }

    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p> The value can be retrieved by calling the <tt>get</tt> method
     * with a key that is equal to the original key.
     *
     * 添加键值对到Map中，键和值都不可以为null
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     * <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V put(K key, V value) {
        Segment<K, V> s;
        // 检查value是否为null
        if (value == null)
            throw new NullPointerException();
        // 在hash()方法中如果key为null会抛出NullPointerException异常
        int hash = hash(key);
        // 根据hash计算对应Segment的偏移量
        int j = (hash >>> segmentShift) & segmentMask;
        // nonvolatile; recheck
        // 获取对应的Segment
        if ((s = (Segment<K, V>) UNSAFE.getObject(segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            // 如果对应的Segment为空就使用ensureSegment()方法来创建
            s = ensureSegment(j);
        // 使用Segment的put方法添加元素，onlyIfAbsent传入的是false，即如存在就覆盖旧值
        return s.put(key, hash, value, false);
    }

    /**
     * {@inheritDoc}
     *
     * 该方法与{@link #put} 方法的实现基本一样
     *
     * @return the previous value associated with the specified key,
     * or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V putIfAbsent(K key, V value) {
        Segment<K, V> s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K, V>) UNSAFE.getObject(segments, (j << SSHIFT) + SBASE)) == null)
            s = ensureSegment(j);
        // 与put方法的地方在于，此处onlyIfAbsent传入的是true，即如存在就不添加
        return s.put(key, hash, value, true);
    }

    /**
     * Copies all of the mappings from the specified map to this one.
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified map.
     *
     * 将m中的键值对拷贝到ConcurrentHashMap中，会覆盖ConcurrentHashMap中原有的键值对
     *
     * @param m mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        // 循环遍历m中所有的键值对，使用put方法依次添加到ConcurrentHashMap中
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * Removes the key (and its corresponding value) from this map.
     * This method does nothing if the key is not in the map.
     *
     * @param key the key that needs to be removed
     * @return the previous value associated with <tt>key</tt>, or
     * <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key is null
     */
    public V remove(Object key) {
        // 根据哈希值获取Segment
        int hash = hash(key);
        Segment<K, V> s = segmentForHash(hash);
        // 调用Segment的删除方法
        return s == null ? null : s.remove(key, hash, null);
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        int hash = hash(key);
        Segment<K, V> s;
        // 调用Segment的删除方法
        return value != null && (s = segmentForHash(hash)) != null &&
                s.remove(key, hash, value) != null;
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        int hash = hash(key);
        // 检查传入参数是否合法
        if (oldValue == null || newValue == null)
            throw new NullPointerException();
        // 根据哈希值获取对应的Segment
        Segment<K, V> s = segmentForHash(hash);
        // 调用Segment的replace方法
        return s != null && s.replace(key, hash, oldValue, newValue);
    }

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V replace(K key, V value) {
        int hash = hash(key);
        // 检查传入参数是否合法
        if (value == null)
            throw new NullPointerException();
        // 根据哈希值获取对应的Segment
        Segment<K, V> s = segmentForHash(hash);
        // 调用Segment的replace方法
        return s == null ? null : s.replace(key, hash, value);
    }

    /**
     * Removes all of the mappings from this map.
     * 清空Map中所有的键值对
     */
    public void clear() {
        final Segment<K, V>[] segments = this.segments;
        // 遍历Segment进行清空处理
        for (int j = 0; j < segments.length; ++j) {
            Segment<K, V> s = segmentAt(segments, j);
            if (s != null)
                s.clear();
        }
    }

    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from this map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  The collection
     * supports element removal, which removes the corresponding
     * mapping from this map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from the map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    /**
     * Returns an enumeration of the keys in this table.
     *
     * @return an enumeration of the keys in this table
     * @see #keySet()
     */
    public Enumeration<K> keys() {
        return new KeyIterator();
    }

    /**
     * Returns an enumeration of the values in this table.
     *
     * @return an enumeration of the values in this table
     * @see #values()
     */
    public Enumeration<V> elements() {
        return new ValueIterator();
    }

    /* ---------------- Iterator Support -------------- */

    abstract class HashIterator {
        // ConcurrentHashMap里下一个Segment的索引
        int nextSegmentIndex;
        // 下一个HashEntry链的索引
        int nextTableIndex;
        // 当前的HashEntry链
        HashEntry<K, V>[] currentTable;
        // 下一个HashEntry
        HashEntry<K, V> nextEntry;
        // 最后返回
        HashEntry<K, V> lastReturned;

        HashIterator() {
            // 初始化nextSegmentIndex为Segment数组最大索引
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        /**
         * Set nextEntry to first node of next non-empty table
         * (in backwards order, to simplify checks).
         * 设置nextEntry为下一个不为空的HashEntry链的头节点
         */
        final void advance() {
            for (; ; ) {
                if (nextTableIndex >= 0) {
                    // 获取table中的HashEntry，，即在table数组中从后向前的顺序依次获取
                    if ((nextEntry = entryAt(currentTable, nextTableIndex--)) != null)
                        break;
                } else if (nextSegmentIndex >= 0) {
                    // 获取Segment，即在Segment数组中从后向前的顺序依次获取
                    Segment<K, V> seg = segmentAt(segments, nextSegmentIndex--);
                    // 获取Segment相应的table数组
                    if (seg != null && (currentTable = seg.table) != null)
                        // 将nextTableIndex置为table数组最大索引
                        nextTableIndex = currentTable.length - 1;
                } else
                    break;
            }
        }

        final HashEntry<K, V> nextEntry() {
            // 获取nextEntry为返回的HashEntry
            HashEntry<K, V> e = nextEntry;
            if (e == null)
                throw new NoSuchElementException();
            lastReturned = e; // cannot assign until after null check
            if ((nextEntry = e.next) == null)
                advance();
            return e;
        }

        // 判断是否有下一个元素
        public final boolean hasNext() {
            return nextEntry != null;
        }

        // 判断是否还有更多的元素
        public final boolean hasMoreElements() {
            return nextEntry != null;
        }

        public final void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            // 从ConcurrentHashMap中移除当前遍历到的元素
            ConcurrentHashMap.this.remove(lastReturned.key);
            lastReturned = null;
        }
    }

    // 键迭代器
    final class KeyIterator
            extends HashIterator
            implements Iterator<K>, Enumeration<K> {
        public final K next() {
            return super.nextEntry().key;
        }

        public final K nextElement() {
            return super.nextEntry().key;
        }
    }

    // 值迭代器
    final class ValueIterator
            extends HashIterator
            implements Iterator<V>, Enumeration<V> {
        public final V next() {
            return super.nextEntry().value;
        }

        public final V nextElement() {
            return super.nextEntry().value;
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays
     * setValue changes to the underlying map.
     */
    final class WriteThroughEntry
            extends AbstractMap.SimpleEntry<K, V> {
        WriteThroughEntry(K k, V v) {
            super(k, v);
        }

        /**
         * Set our entry's value and write through to the map. The
         * value to return is somewhat arbitrary here. Since a
         * WriteThroughEntry does not necessarily track asynchronous
         * changes, the most recent "previous" value could be
         * different from what we return (or could even have been
         * removed in which case the put will re-establish). We do not
         * and cannot guarantee more.
         */
        public V setValue(V value) {
            if (value == null) throw new NullPointerException();
            V v = super.setValue(value);
            ConcurrentHashMap.this.put(getKey(), value);
            return v;
        }
    }

    final class EntryIterator
            extends HashIterator
            implements Iterator<Entry<K, V>> {
        public Map.Entry<K, V> next() {
            HashEntry<K, V> e = super.nextEntry();
            return new WriteThroughEntry(e.key, e.value);
        }
    }

    // 键集类
    final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }

        public int size() {
            return ConcurrentHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }

        public boolean contains(Object o) {
            return ConcurrentHashMap.this.containsKey(o);
        }

        public boolean remove(Object o) {
            return ConcurrentHashMap.this.remove(o) != null;
        }

        public void clear() {
            ConcurrentHashMap.this.clear();
        }
    }

    // 值集类
    final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        public int size() {
            return ConcurrentHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }

        public boolean contains(Object o) {
            return ConcurrentHashMap.this.containsValue(o);
        }

        public void clear() {
            ConcurrentHashMap.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K, V>> {
        public Iterator<Map.Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            V v = ConcurrentHashMap.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }

        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return ConcurrentHashMap.this.remove(e.getKey(), e.getValue());
        }

        public int size() {
            return ConcurrentHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }

        public void clear() {
            ConcurrentHashMap.this.clear();
        }
    }

    /* ---------------- Serialization Support -------------- */

    /**
     * Save the state of the <tt>ConcurrentHashMap</tt> instance to a
     * stream (i.e., serialize it).
     *
     * 序列化写方法
     *
     * @param s the stream
     * @serialData the key (Object) and value (Object)
     * for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s) throws IOException {
        // force all segments for serialization compatibility
        for (int k = 0; k < segments.length; ++k)
            ensureSegment(k);
        // 调用缺省序列化写
        s.defaultWriteObject();

        // 遍历Segment数组
        final Segment<K, V>[] segments = this.segments;
        for (int k = 0; k < segments.length; ++k) {
            Segment<K, V> seg = segmentAt(segments, k);
            // 上锁
            seg.lock();
            try {
                // 遍历Segment的table
                HashEntry<K, V>[] tab = seg.table;
                for (int i = 0; i < tab.length; ++i) {
                    HashEntry<K, V> e;
                    // 分别写出HashEntry的键和值
                    for (e = entryAt(tab, i); e != null; e = e.next) {
                        s.writeObject(e.key);
                        s.writeObject(e.value);
                    }
                }
            } finally {
                // 解锁
                seg.unlock();
            }
        }
        // 写出两个null
        s.writeObject(null);
        s.writeObject(null);
    }

    /**
     * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a
     * stream (i.e., deserialize it).
     *
     * 序列化读方法
     *
     * @param s the stream
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
            throws IOException, ClassNotFoundException {
        s.defaultReadObject();

        // set hashMask
        UNSAFE.putIntVolatile(this, HASHSEED_OFFSET, randomHashSeed(this));

        // Re-initialize segments to be minimally sized, and let grow.
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        final Segment<K, V>[] segments = this.segments;
        for (int k = 0; k < segments.length; ++k) {
            Segment<K, V> seg = segments[k];
            if (seg != null) {
                seg.threshold = (int) (cap * seg.loadFactor);
                seg.table = (HashEntry<K, V>[]) new HashEntry[cap];
            }
        }

        // Read the keys and values, and put the mappings in the table
        for (; ; ) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            if (key == null)
                break;
            put(key, value);
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    // arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址
    // 第一个Segment的地址偏移量
    private static final long SBASE;
    // arrayIndexScale方法是一个本地方法，可以计算数组中元素的增量地址
    // SSHIFT是Segment数组中元素的增量地址
    private static final int SSHIFT;
    // 第一个HashEntry的地址偏移量
    private static final long TBASE;
    // SSHIFT是HashEntry数组中元素的增量地址
    private static final int TSHIFT;
    // hashSeed的偏移量
    private static final long HASHSEED_OFFSET;

    static {
        int ss, ts;
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class tc = HashEntry[].class;
            Class sc = Segment[].class;
            TBASE = UNSAFE.arrayBaseOffset(tc);
            SBASE = UNSAFE.arrayBaseOffset(sc);
            ts = UNSAFE.arrayIndexScale(tc);
            ss = UNSAFE.arrayIndexScale(sc);
            HASHSEED_OFFSET = UNSAFE.objectFieldOffset(ConcurrentHashMap.class.getDeclaredField("hashSeed"));
        } catch (Exception e) {
            throw new Error(e);
        }
        // ss和ts必然是2的次方
        if ((ss & (ss - 1)) != 0 || (ts & (ts - 1)) != 0)
            throw new Error("data type scale not a power of two");
        // Segemnt数组中元素的增量地址值，按位数据转换后除掉高位补0后非零的位数
        SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
        // HashEntry数组中元素的增量地址值，按位数据转换后除掉高位补0后非零的位数
        TSHIFT = 31 - Integer.numberOfLeadingZeros(ts);
    }

}