ConcurrentHashMap解析后续

HashMap详解：http://donald-draper.iteye.com/blog/2361702
ConcurrentMap介绍：http://donald-draper.iteye.com/blog/2361719
ConcurrentHashMap解析-Segment：http://donald-draper.iteye.com/blog/2363200
前言：
上一篇文章我们看了一下ConcurrentHashMap的Segment，今天来看ConcurrentHashMap的相关操作。先回顾一下上一篇的内容：
ConcurrentHashMap是线程安全的，可并发访问，不允许key或value的值为null，默认的容量为16，负载因子为0.75，并发访问量为16。ConcurrentHashMap中有一个Segment数组，默认数组大小为16，Segment中有一个HashEntry数组类似于HashMap中的table，Segment继承了可重入锁ReentrantLock，Segment的修改其hash table的操作都要使用lock。Segment的put操作，首先尝试获取锁，如果获取锁失败，则Key在片段hash table中的索引，遍历索引对应的Hash Entry链，如找不到key对应HashEntry,则创建一个HashEntry，这都是在尝试次数小于最大尝试次数MAX_SCAN_RETRIES情况下，MAX_SCAN_RETRIES默认为2。这样做的目的是为确保，进行put操作时，仍持有锁。然后定位key在片段table中的索引，并放在链头，如果实际size达到临界条件，则重新hash，创建2倍原始容量的hash table，重新建立hash table。Segment的remove操作，首先尝试获取锁失败，则继续尝试获取锁，在获取锁的过程中，定位key在片段table的HashEntry链表索引，遍历链表，如果找到对应HashEntry，则移除，如果尝试次数超过最大尝试次数，则lock，则遍历链表，找到对应的HashEntry，则移除。replace与remove的基本思路相同，唯一的区别是，当替换值时，修改计数要自增1。put，remove和replace操作是锁住片段table中，key对应的索引HashEntry链表，而Clear为锁住整个table。ConcurrentHashMap通过将所有HashEntry分散在不同的Segment，及锁机制实现了并发访问。
来看HashMap的put操作
 

/**
     * 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.
     *
     * @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;
        if (value == null)
            throw new NullPointerException();
	//获取key的hash值，定位Segment，
        int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
	    //确保片段索引对应的Segment存在，不存在则创建
            s = ensureSegment(j);
	//调用Segment的put操作，onlyIfAbsent为false，存在，则替换
        return s.put(key, hash, value, false);
    }

   /**
     * Returns the segment for the given index, creating it and
     * recording in segment table (via CAS) if not already present.
     *
     * @param k the index
     * @return the segment
     */
     //返回数组segment，指定索引的segment，如果不存在，则创建一个
    @SuppressWarnings("unchecked")
    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;
    }
/**
     * {@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
     */
     如果Map不包含对应的Key则，执行put的操作，否则返回旧值 
    @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);
        return s.put(key, hash, value, true);
    }

putIfAbsent与put操作的区别为onlyIfAbsent为true，存在返回旧值，否则放到对应
的Segment的hash table中。

 /**
     * 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
     */
     创建一个与参数Map相同HashEntry的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);
	 //委托给putAll
        putAll(m);
    }

    /**
     * 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.
     *
     * @param m mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        遍历HashEntry，放到新的Map中
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

Remove操作：

/**
     * 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) {
        int hash = hash(key);
	//定位片段
        Segment<K,V> s = segmentForHash(hash);
        //委托给Segment的remove操作
        return s == null ? null : s.remove(key, hash, null);
    }

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

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    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;
    }

再看repalce

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    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);
    }

    /**
     * {@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);
        if (value == null)
            throw new NullPointerException();
	//定位Segment，委托给Segment
        Segment<K,V> s = segmentForHash(hash);
        return s == null ? null : s.replace(key, hash, value);
    }

从上面分析的put，remove，replace操作来看，基本思路相同，先定位定位Segment，
再将操作委托给Segment相应的操作。
再看get操作

public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        HashEntry<K,V>[] tab;
        int h = hash(key);
	//定位Segment
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
	    //遍历key对应的HashEntry链表，找到，则返回值
            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;
    }

//再看判断是包含key

 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;
    }

思路与get操作类似；

 public boolean contains(Object value) {
        //委托给containsValue
        return containsValue(value);
    }
/**
     * 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>.
     *
     containsValue需要遍历内部的所有片段的Hash table，所以速度是非常慢的
     * @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
     */
    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)
		        //如果尝试次数为RETRIES_BEFORE_LOCK，则锁住所有片段
                        ensureSegment(j).lock(); // force creation
                }
                long hashSum = 0L;
                int sum = 0;
		//遍历所有片段的hash table的Hash Entry链表
                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;
    }

判断是否为空
 

  /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @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;
        final Segment<K,V>[] segments = this.segments;
	//遍历判断，如果片段的size，不为null，则返回false
        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;
            }
        }
	//利用Segment的modCount重新检查，以防遍历片段的时候，有修改操作
        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;
                }
            }
            if (sum != 0L)
                return false;
        }
        return true;
    }

获取size

 /**
     * 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>.
     *
     * @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.
        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 (;;) {
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                sum = 0L;
                size = 0;
                overflow = false;
		//遍历片段，获取片段size之和，如果超过Integer.MAX_VALUE，则返回最大值。
                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;
                    }
                }
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return overflow ? Integer.MAX_VALUE : size;
    }

clear操作

/**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        final Segment<K,V>[] segments = this.segments;
	//遍历片段，clear所有片段
        for (int j = 0; j < segments.length; ++j) {
            Segment<K,V> s = segmentAt(segments, j);
            if (s != null)
                s.clear();
        }
    }

ConCurrentHashMap实现ConCurrentMap和HashMap方法基本看完，我们再来看key，value，entrySet视图。
//EntrySet视图

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

//HashEntry Set集合类

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();
        }
    }

从EntrySet的所有操作依附于ConcurrentHashMap
再来看EntryIterator

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

    /**
     * Custom Entry class used by EntryIterator.next(), that relays
     * setValue changes to the underlying map.
     */
    包装EntryIterator的next元素为可修改值的WriteThroughEntry
    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;
        }
    }

再来看HashIterator

//HashIterator

abstract class HashIterator {
        int nextSegmentIndex;//next片段索引
        int nextTableIndex;//next片段table索引
        HashEntry<K,V>[] currentTable;当前片段
        HashEntry<K, V> nextEntry;//链表HashEntry
        HashEntry<K, V> lastReturned;

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

        /**
         * Set nextEntry to first node of next non-empty table
         * (in backwards order, to simplify checks).
         */
	 //循环遍历片段及片段table，返回不为null的hash table的hashEntry的第一个Node为nextEntry
        final void advance() {
            for (;;) {
                if (nextTableIndex >= 0) {
		    //如果hash table的索引大于0，则定位当前table，索引对应的HashEntry链表
                    if ((nextEntry = entryAt(currentTable,
                                             nextTableIndex--)) != null)
                        break;
                }
                else if (nextSegmentIndex >= 0) {
		    //定位片段
                    Segment<K,V> seg = segmentAt(segments, nextSegmentIndex--);
		    //获取当前片段的hash table
                    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
            if ((nextEntry = e.next) == null)
                advance();
            return e;
        }

        public final boolean hasNext() { return nextEntry != null; }
        public final boolean hasMoreElements() { return nextEntry != null; }
        //移除操作
        public final void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            ConcurrentHashMap.this.remove(lastReturned.key);
            lastReturned = null;
        }
    }

    //KeyIterator

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; }
    }

   //ValueIterator
  

 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; }
    }

//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();
        }
    }

//Key视图

 public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

//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();
        }
    }

//value视图

 public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

   /**
     * 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();
    }

总结：
put，remove，replace操作来看，基本思路相同，先定位定位Segment，
再将操作委托给Segment相应的操作。get操作是先定位定位Segment，再遍历key对应的HashEntry链表，找到，则返回值。视图基本思路为定位片段，在定位片段的hash table的Hash Entry链，遍历完当前片段，再遍历下一个片段的Hash table。