讀ConcurrentHashMap源碼

//先看構造函數
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;
        }
	//默認情況下爲28
        this.segmentShift = 32 - sshift;
	//默認情況下爲15
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;

	 //計算每個segment存放多少個元素
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
	 //默認情況下爲2
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
	//初始化一個segment
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);
        //初始化segment數組
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
	//將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();
	//獲取hash值
        int hash = hash(key);
	//定位segments數組中的segment
        int j = (hash >>> segmentShift) & segmentMask;
	//獲取segment對象
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            //沒有獲取到新建一個segment
	    s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }

//利用CAS機制在Segments數組的u上新建一個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) {
           //嘗試獲取鎖獲取成功返回null失敗執行scanAndLockForPut(該方法也會獲取到鎖)
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);
	   //獲取到了鎖
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
		//獲取entry
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    if (e != null) {
                        K k;
			//找到了數組中原本有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;
                    }
		    //鏈表中沒有對應的key
                    else {
		       //在等待獲取鎖期間已經確定first位置沒有元素,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)
			//擴容
                            rehash(node);
                        else
			   //設置值
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

//在獲取鎖期間一直掃描看當前的key是否存在
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
            //通過hash獲取Segment位置上的第一個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
	    //無限循環直到獲取到鎖
            while (!tryLock()) {
                HashEntry<K,V> f; // to recheck first below
                if (retries < 0) {
		    //如果first位置沒有元素
                    if (e == null) {
                        if (node == null) // speculatively create node
			//創建元素
                            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;
        }

//獲取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);
    }

//獲取在table數組上的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);
    }

//擴容
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;
			//獲取HashEntry中計算出來和以前不相等的hash值,這些元素以及這些元素的next由於hash值一直所以不需要在計算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;
                            }
                        }
			//將lastIdx位置上設置爲lastRun。
                        newTable[lastIdx] = lastRun;
                        // 移動除了lastRun之外的所有元素,lastRun之後的元素已經在上面的代碼移動過了。
                        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);
                        }
                    }
                }
            }
	    //設置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方法的總結：
1、首先計算hash值定位到一個segment。如果沒有segment,初始化一個segment。（簡單理解一個segment相當於一個帶鎖的HashMap）
2、在segment中,首先嚐試獲取鎖。如果沒有獲取到鎖則無限循環去獲取鎖以及如果當前table[]上是空的創建firstNode。
3、獲得了鎖，根據Hash值獲取table[]上的node。循環遍歷如果找到了key值和put的key值相等則直接將值替換。
4、沒有找到。則如果第二部創建了firstNode則直接用firstNode設置next即可,第二部沒有成功創建firstNode則新建一個node。
5、如果大小超過當前的臨界值了則擴容並且設置值。沒有超過直接設置值。
*/


//如果有key值了則返回值沒有則插入。和put的實現差不多。
 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());
    }


//這個方法沒有上鎖那麼怎麼保證可見性？因爲HashEntry和table[]是volatile的。所以可以保證happen-before關係。
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;
    }

//根據key刪除元素
 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) {
           //嘗試獲得鎖
            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)
			        //設置爲first
                                setEntryAt(tab, index, next);
                            else
			        //刪掉
                                pred.setNext(next);
                            ++modCount;
                            --count;
                            oldValue = v;
                        }
                        break;
                    }
                    pred = e;
                    e = next;
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

//這段代碼的作用不太明白,網上說是爲了在cpu緩存,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;
	    //沒有獲取到鎖一直循環
            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;
                }
            }
        }

//給定的鍵值都匹配才刪除。實現和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;
    }


//根據key替換值(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;
        }

//只有當鍵值都相等時才替換
 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;
        }

//是否爲空
  public boolean isEmpty() {

        long sum = 0L;
        final Segment<K,V>[] segments = this.segments;
	//循環遍歷一次
        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;
            }
        }
	//再次檢查防止正好有元素加入進來了
        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;
                }
            }
	    //兩次modCount不相等說明肯定有元素加入
            if (sum != 0L)
                return false;
        }
        return true;
    }

//返回元素個數(這個算法很精妙,先不加鎖循環3次如果相鄰兩次獲得的modCount修改次數一致則直接返回,否則加鎖計算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 (;;) {
	        //當循環了3次之後還是沒有獲得一致性則上鎖計算
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
		        //爲了獲得更加準確的值,創建所有的segment並鎖定,lock是可重入鎖。下一次會重入。
                        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;
                    }
                }
		//當2次循環獲得的sum一致則表示這個瞬間size可以獲得
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
	   //解鎖
            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() {
            //獲取鎖
            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 {
	       //釋放鎖
                unlock();
            }
        }

//是否包含某個鍵(和get的實現一樣)
 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;
    }


//是否包含某個值（這個沒法計算hash值所以要掃描整個segments效率比containsKey慢）。算法和size()的實現差不多。都是先不加鎖循環，找不到再加鎖循環。
 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視圖
 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();
        }

        /**
        從尾巴開始尋找第一個entry
         */
        final void advance() {
            for (;;) {
                if (nextTableIndex >= 0) {
                    if ((nextEntry = entryAt(currentTable,
                                             nextTableIndex--)) != null)
                        break;
                }
                else if (nextSegmentIndex >= 0) {
		    //從最後面開始遍歷
                    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 再次循環返回下一個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視圖
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視圖
 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;
        }
    }

//對Entry的實現
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;
        }

    }


   /**
   總結：ConcurrentHashMap相當於數組+數組+鏈表的實現。最外層是一個Segmetns數組然後,然後每個segment數組裏面放着entry,
   每個entry都是鏈表結構。而在get的時候不需要上鎖，因爲是volatile的。而put的時候也是先計算hash值得到segment,只鎖住了一個segment。
   所以更加高效。
   */