读ConcurrentHashMap源码_JAVA_编程开发_程序员俱乐部

class="java" name="code">
//先看构造函数
public ConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

 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);
    }

public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
	//默认情况下为28
        this.segmentShift = 32 - sshift;
	//默认情况下为15
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
	  
	 //计算每个segment存放多少个元素
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
	 //默认情况下为2
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
	//初始化一个segment
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);
        //初始化segment数组
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
	//将Segment[0] 指向s0
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

 public V put(K key, V value) {
        Segment<K,V> s;
        if (value == null)
            throw new NullPointerException();
	//获取hash值
        int hash = hash(key);
	//定位segments数组中的segment
        int j = (hash >>> segmentShift) & segmentMask;
	//获取segment对象
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            //没有获取到新建一个segment
	    s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }

//利用CAS机制在Segments数组的u上新建一个segment
private Segment<K,V> ensureSegment(int k) {
        final Segment<K,V>[] ss = this.segments;
        long u = (k << SSHIFT) + SBASE; // raw offset
        Segment<K,V> seg;
        if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
            Segment<K,V> proto = ss[0]; // use segment 0 as prototype
            int cap = proto.table.length;
            float lf = proto.loadFactor;
            int threshold = (int)(cap * lf);
            HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
            if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                == null) { // recheck
                Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
                while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                       == null) {
                    if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                        break;
                }
            }
        }
        return seg;
    }

 final V put(K key, int hash, V value, boolean onlyIfAbsent) {
           //尝试获取锁获取成功返回null失败执行scanAndLockForPut(该方法也会获取到锁)
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);
	   //获取到了锁
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
		//获取entry
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    if (e != null) {
                        K k;
			//找到了数组中原本有key直接修改value值
                        if ((k = e.key) == key ||
                            (e.hash == hash && key.equals(k))) {
                            oldValue = e.value;
                            if (!onlyIfAbsent) {
                                e.value = value;
                                ++modCount;
                            }
                            break;
                        }
                        e = e.next;
                    }
		    //链表中没有对应的key
                    else {
		       //在等待获取锁期间已经确定first位置没有元素,初始化了node。
                        if (node != null)
                            node.setNext(first);
                        else
                            node = new HashEntry<K,V>(hash, key, value, first);
                        int c = count + 1;
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)
			//扩容
                            rehash(node);
                        else
			   //设置值
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

//在获取锁期间一直扫描看当前的key是否存在
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
            //通过hash获取Segment位置上的第一个HashEntry
            HashEntry<K,V> first = entryForHash(this, hash);
            HashEntry<K,V> e = first;
            HashEntry<K,V> node = null;
            int retries = -1; // negative while locating node
	    //无限循环直到获取到锁
            while (!tryLock()) {
                HashEntry<K,V> f; // to recheck first below
                if (retries < 0) {
		    //如果first位置没有元素
                    if (e == null) {
                        if (node == null) // speculatively create node
			//创建元素
                            node = new HashEntry<K,V>(hash, key, value, null);
                        retries = 0;
                    }
                    else if (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; // re-traverse if entry changed
                    retries = -1;
                }
            }
            return node;
        }

//获取segment中的entry
static final <K,V> HashEntry<K,V> entryForHash(Segment<K,V> seg, int h) {
        HashEntry<K,V>[] tab;
        return (seg == null || (tab = seg.table) == null) ? null :
            (HashEntry<K,V>) UNSAFE.getObjectVolatile
            (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
    }

//获取在table数组上的entry
 @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);
    }

//扩容
private void rehash(HashEntry<K,V> node) {
           
            HashEntry<K,V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            int newCapacity = oldCapacity << 1;
            threshold = (int)(newCapacity * loadFactor);
            HashEntry<K,V>[] newTable =
                (HashEntry<K,V>[]) new HashEntry[newCapacity];
            int sizeMask = newCapacity - 1;
            for (int i = 0; i < oldCapacity ; i++) {
                HashEntry<K,V> e = oldTable[i];
                if (e != null) {
                    HashEntry<K,V> next = e.next;
                    int idx = e.hash & sizeMask;
                    if (next == null)   //  Single node on list
                        newTable[idx] = e;
                    else { // Reuse consecutive sequence at same slot
                        HashEntry<K,V> lastRun = e;
                        int lastIdx = idx;
			//获取HashEntry中计算出来和以前不相等的hash值,这些元素以及这些元素的next由于hash值一直所以不需要在计算hash了。
                        for (HashEntry<K,V> last = next;
                             last != null;
                             last = last.next) {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
			//将lastIdx位置上设置为lastRun。
                        newTable[lastIdx] = lastRun;
                        // 移动除了lastRun之外的所有元素,lastRun之后的元素已经在上面的代码移动过了。
                        for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
                            V v = p.value;
                            int h = p.hash;
                            int k = h & sizeMask;
                            HashEntry<K,V> n = newTable[k];
                            newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
                        }
                    }
                }
            }
	    //设置node的值
            int nodeIndex = node.hash & sizeMask; // add the new node
            node.setNext(newTable[nodeIndex]);
            newTable[nodeIndex] = node;
            table = newTable;
        }

static final <K,V> void setEntryAt(HashEntry<K,V>[] tab, int i,
                                       HashEntry<K,V> e) {
        UNSAFE.putOrderedObject(tab, ((long)i << TSHIFT) + TBASE, e);
    }

/**
put方法的总结：
1、首先计算hash值定位到一个segment。如果没有segment,初始化一个segment。（简单理解一个segment相当于一个带锁的HashMap）
2、在segment中,首先尝试获取锁。如果没有获取到锁则无限循环去获取锁以及如果当前table[]上是空的创建firstNode。
3、获得了锁，根据Hash值获取table[]上的node。循环遍历如果找到了key值和put的key值相等则直接将值替换。
4、没有找到。则如果第二部创建了firstNode则直接用firstNode设置next即可,第二部没有成功创建firstNode则新建一个node。
5、如果大小超过当前的临界值了则扩容并且设置值。没有超过直接设置值。
*/


//如果有key值了则返回值没有则插入。和put的实现差不多。
 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);
        return s.put(key, hash, value, true);
    }


public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }



//这个方法没有上锁那么怎么保证可见性？因为HashEntry和table[]是volatile的。所以可以保证happen-before关系。
public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        HashEntry<K,V>[] tab;
        int h = hash(key);
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            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;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return e.value;
            }
        }
        return null;
    }

//根据key删除元素
 public V remove(Object key) {
        int hash = hash(key);
	//定位Segment
        Segment<K,V> s = segmentForHash(hash);
        return s == null ? null : s.remove(key, hash, null);
    }

final V remove(Object key, int hash, Object value) {
           //尝试获得锁
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
                HashEntry<K,V> e = entryAt(tab, index);
                HashEntry<K,V> pred = null;
                while (e != null) {
                    K k;
                    HashEntry<K,V> next = e.next;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        V v = e.value;
                        if (value == null || value == v || value.equals(v)) {
                            if (pred == null)
			        //设置为first
                                setEntryAt(tab, index, next);
                            else
			        //删掉
                                pred.setNext(next);
                            ++modCount;
                            --count;
                            oldValue = v;
                        }
                        break;
                    }
                    pred = e;
                    e = next;
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

//这段代码的作用不太明白,网上说是为了在cpu缓存,提升效率。
private void scanAndLock(Object key, int hash) {
            // similar to but simpler than scanAndLockForPut
	    //找到table[]上的元素
            HashEntry<K,V> first = entryForHash(this, hash);
            HashEntry<K,V> e = first;
            int retries = -1;
	    //没有获取到锁一直循环
            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;
                }
            }
        }

//给定的键值都匹配才删除。实现和remove差不多。
public boolean remove(Object key, Object value) {
        int hash = hash(key);
        Segment<K,V> s;
        return value != null && (s = segmentForHash(hash)) != null &&
            s.remove(key, hash, value) != null;
    }


//根据key替换值(比用put效率高)
 public V replace(K key, V value) {
        int hash = hash(key);
        if (value == null)
            throw new NullPointerException();
        Segment<K,V> s = segmentForHash(hash);
        return s == null ? null : s.replace(key, hash, value);
    }

final V replace(K key, int hash, V value) {
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K,V> e;
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        oldValue = e.value;
                        e.value = value;
                        ++modCount;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

//只有当键值都相等时才替换
 public boolean replace(K key, V oldValue, V newValue) {
        int hash = hash(key);
        if (oldValue == null || newValue == null)
            throw new NullPointerException();
        Segment<K,V> s = segmentForHash(hash);
        return s != null && s.replace(key, hash, oldValue, newValue);
    }

final boolean replace(K key, int hash, V oldValue, V newValue) {
            if (!tryLock())
                scanAndLock(key, hash);
            boolean replaced = false;
            try {
                HashEntry<K,V> e;
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        if (oldValue.equals(e.value)) {
                            e.value = newValue;
                            ++modCount;
                            replaced = true;
                        }
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return replaced;
        }

//是否为空
  public boolean isEmpty() {
        
        long sum = 0L;
        final Segment<K,V>[] segments = this.segments;
	//循环遍历一次
        for (int j = 0; j < segments.length; ++j) {
            Segment<K,V> seg = segmentAt(segments, j);
            if (seg != null) {
                if (seg.count != 0)
                    return false;
                //count是0了
                sum += seg.modCount;
            }
        }
	//再次检查防止正好有元素加入进来了
        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;
                    sum -= seg.modCount;
                }
            }
	    //两次modCount不相等说明肯定有元素加入
            if (sum != 0L)
                return false;
        }
        return true;
    }

//返回元素个数(这个算法很精妙,先不加锁循环3次如果相邻两次获得的modCount修改次数一致则直接返回,否则加锁计算size值。)
public int size() {
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        final Segment<K,V>[] segments = this.segments;
        int size;
        boolean overflow; // true if size overflows 32 bits
        long sum;         // sum of modCounts
        long last = 0L;   // previous sum
        int retries = -1; // first iteration isn't retry
        try {
            for (;;) {
	        //当循环了3次之后还是没有获得一致性则上锁计算
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
		        //为了获得更加准确的值,创建所有的segment并锁定,lock是可重入锁。下一次会重入。
                        ensureSegment(j).lock(); // force creation
                }
                sum = 0L;
                size = 0;
                overflow = false;
                for (int j = 0; j < segments.length; ++j) {
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null) {
                        sum += seg.modCount;
                        int c = seg.count;
                        if (c < 0 || (size += c) < 0)
                            overflow = true;
                    }
                }
		//当2次循环获得的sum一致则表示这个瞬间size可以获得
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
	   //解锁
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return overflow ? Integer.MAX_VALUE : size;
    }

//清空元素
public void clear() {
        final Segment<K,V>[] segments = this.segments;
        for (int j = 0; j < segments.length; ++j) {
            Segment<K,V> s = segmentAt(segments, j);
            if (s != null)
                s.clear();
        }
    }

final void clear() {
            //获取锁
            lock();
            try {
                HashEntry<K,V>[] tab = table;
                for (int i = 0; i < tab.length ; i++)
		    清空
                    setEntryAt(tab, i, null);
                ++modCount;
                count = 0;
            } finally {
	       //释放锁
                unlock();
            }
        }

//是否包含某个键(和get的实现一样)
 public boolean containsKey(Object key) {
        Segment<K,V> s; // same as get() except no need for volatile value read
        HashEntry<K,V>[] tab;
        int h = hash(key);
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            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;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return true;
            }
        }
        return false;
    }


//是否包含某个值（这个没法计算hash值所以要扫描整个segments效率比containsKey慢）。算法和size()的实现差不多。都是先不加锁循环，找不到再加锁循环。
 public boolean containsValue(Object value) {
        // Same idea as size()
        if (value == null)
            throw new NullPointerException();
        final Segment<K,V>[] segments = this.segments;
        boolean found = false;
        long last = 0;
        int retries = -1;
        try {
            outer: for (;;) {
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                long hashSum = 0L;
                int sum = 0;
                for (int j = 0; j < segments.length; ++j) {
                    HashEntry<K,V>[] tab;
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null && (tab = seg.table) != null) {
                        for (int i = 0 ; i < tab.length; i++) {
                            HashEntry<K,V> e;
                            for (e = entryAt(tab, i); e != null; e = e.next) {
                                V v = e.value;
                                if (v != null && value.equals(v)) {
                                    found = true;
                                    break outer;
                                }
                            }
                        }
                        sum += seg.modCount;
                    }
                }
                if (retries > 0 && sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return found;
    }

 public boolean contains(Object value) {
        return containsValue(value);
    }

 //返回key的set视图
 public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

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();
        }
    }

 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; }
    }

 abstract class HashIterator {
        int nextSegmentIndex;
        int nextTableIndex;
        HashEntry<K,V>[] currentTable;
        HashEntry<K, V> nextEntry;
        HashEntry<K, V> lastReturned;

        HashIterator() {
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        /**
        从尾巴开始寻找第一个entry
         */
        final void advance() {
            for (;;) {
                if (nextTableIndex >= 0) {
                    if ((nextEntry = entryAt(currentTable,
                                             nextTableIndex--)) != null)
                        break;
                }
                else if (nextSegmentIndex >= 0) {
		    //从最后面开始遍历
                    Segment<K,V> seg = segmentAt(segments, nextSegmentIndex--);
                    if (seg != null && (currentTable = seg.table) != null)
                        nextTableIndex = currentTable.length - 1;
                }
                else
                    break;
            }
        }

        final HashEntry<K,V> nextEntry() {
            HashEntry<K,V> e = nextEntry;
            if (e == null)
                throw new NoSuchElementException();
            lastReturned = e; // cannot assign until after null check
	    //如果nextEntry.next==null 再次循环返回下一个nextEntry
            if ((nextEntry = e.next) == null)
                advance();
            return e;
        }

        public final boolean hasNext() { return nextEntry != null; }
        public final boolean hasMoreElements() { return nextEntry != null; }

        public final void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            ConcurrentHashMap.this.remove(lastReturned.key);
            lastReturned = null;
        }
    }

//返回value的collection视图
public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

 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 ValueIterator
        extends HashIterator
        implements Iterator<V>, Enumeration<V>
    {
        public final V next()        { return super.nextEntry().value; }
        public final V nextElement() { return super.nextEntry().value; }
    }

//返回Map.entry的set视图
 public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

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();
        }
    }

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);
        }
    }

final class WriteThroughEntry
        extends AbstractMap.SimpleEntry<K,V>
    {
        WriteThroughEntry(K k, V v) {
            super(k,v);
        }

        public V setValue(V value) {
            if (value == null) throw new NullPointerException();
            V v = super.setValue(value);
            ConcurrentHashMap.this.put(getKey(), value);
            return v;
        }
    }

//对Entry的实现
public static class SimpleEntry<K,V>
        implements Entry<K,V>, java.io.Serializable
    {
        private static final long serialVersionUID = -8499721149061103585L;

        private final K key;
        private V value;

     
        public SimpleEntry(K key, V value) {
            this.key   = key;
            this.value = value;
        }

      
        public SimpleEntry(Entry<? extends K, ? extends V> entry) {
            this.key   = entry.getKey();
            this.value = entry.getValue();
        }

        public K getKey() {
            return key;
        }

       
        public V getValue() {
            return value;
        }

       
        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

       
        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry e = (Map.Entry)o;
            return eq(key, e.getKey()) && eq(value, e.getValue());
        }

       
        public int hashCode() {
            return (key   == null ? 0 :   key.hashCode()) ^
                   (value == null ? 0 : value.hashCode());
        }

        public String toString() {
            return key + "=" + value;
        }

    }

   
   /**
   总结：ConcurrentHashMap相当于数组+数组+链表的实现。最外层是一个Segmetns数组然后,然后每个segment数组里面放着entry,
   每个entry都是链表结构。而在get的时候不需要上锁，因为是volatile的。而put的时候也是先计算hash值得到segment,只锁住了一个segment。
   所以更加高效。
   */