16.Java源码分析系列笔记-JDK1.7的ConcurrentHashMap

目录

• 1. 构造方法
  
• 2. put方法
  
  
  
  • 2.1. hash
    
  • 2.2. ensureSegment
    
  • 2.3. segment的put方法
    
    
    
    • 2.3.1. scanAndLockForPut
      
    • 2.3.2. rehash
      
    
    
  
  
• 3. get
  
• 4. containsKey方法
  
• 5. remove
  
  
  
  • 5.1. segmentForHash
    
  • 5.2. segment的remove
    
  
  
• 总结
  
• 6. 参考
  

1. 构造方法

1. public ConcurrentHashMap() {
2.         //16,0.75f，16
3.     this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
4. }

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2. put方法

1. public V put(K key, V value) {
2.     Segment<K,V> s;
3.     //value不能为空
4.     if (value == null)
5.         throw new NullPointerException();
6.     //计算key的hash值
7.     int hash = hash(key);
8.     //在用hash值计算j
9.     int j = (hash >>> segmentShift) & segmentMask;
10.     if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
11.          //用j计算地址，并且cas获取segment对象
12.          (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
13.         //初始化
14.         s = ensureSegment(j);
15.     //调用segment的put方法
16.     return s.put(key, hash, value, false);
17. }

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2.2. ensureSegment

1. private int hash(Object k) {
2.     int h = hashSeed;
4.     if ((0 != h) && (k instanceof String)) {
5.         return sun.misc.Hashing.stringHash32((String) k);
6.     }
8.     h ^= k.hashCode();
10.     // Spread bits to regularize both segment and index locations,
11.     // using variant of single-word Wang/Jenkins hash.
12.     h += (h <<  15) ^ 0xffffcd7d;
13.     h ^= (h >>> 10);
14.     h += (h <<   3);
15.     h ^= (h >>>  6);
16.     h += (h <<   2) + (h << 14);
17.     return h ^ (h >>> 16);
18. }

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2.3.1. scanAndLockForPut

1. private Segment<K,V> ensureSegment(int k) {
2.     final Segment<K,V>[] ss = this.segments;
3.     long u = (k << SSHIFT) + SBASE; // raw offset
4.     Segment<K,V> seg;
5.     //segment为空，需要初始化。为什么使用cas而不是直接用下标？？？
6.     if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
7.             //原型模式，使用第一个segment作为原型
8.         Segment<K,V> proto = ss[0]; // use segment 0 as prototype
9.         //获取第一个segment的capacity，loadFactor，threshold，（后续用于创建entry table和segment）
10.         int cap = proto.table.length;
11.         float lf = proto.loadFactor;
12.         int threshold = (int)(cap * lf);
13.         
14.         //创建entry table（后续用于创建segment）
15.         HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
16.         //重新确认为空，不为空说明已经有其他线程初始化了，直接退出，避免进入死循环+cas浪费cpu
17.         if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
18.             == null) { // recheck
19.             //创建segment
20.             Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
21.             //死循环+cas设置segment
22.             while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
23.                    == null) {
24.                 if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
25.                     break;
26.             }
27.         }
28.     }
29.     return seg;
30. }

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2.3.2. rehash

[code]private void rehash(HashEntry 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.         */        HashEntry[] oldTable = table;        int oldCapacity = oldTable.length;        //新的容量为旧容量*2        int newCapacity = oldCapacity >> segmentShift) & segmentMask) > segmentShift) & segmentMask) > segmentShift) & segmentMask)