ConcurrentHashMap源码详解

今天药忘吃喽~ 2022-05-18 08:59 331阅读 0赞
成员变量
  1. private static final int MAXIMUM_CAPACITY = 1 << 30;
  3. private static final int DEFAULT_CAPACITY = 16;
  5. static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
  7. private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
  9. private static final float LOAD_FACTOR = 0.75f;
  11. static final int TREEIFY_THRESHOLD = 8;
  13. static final int UNTREEIFY_THRESHOLD = 6;
  15. static final int MIN_TREEIFY_CAPACITY = 64;
  17. private static final int MIN_TRANSFER_STRIDE = 16;
  19. private static int RESIZE_STAMP_BITS = 16;
  21. private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
  23. private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
  25. static final int MOVED     = -1; // hash for forwarding nodes
  26. static final int TREEBIN   = -2; // hash for roots of trees
  27. static final int RESERVED  = -3; // hash for transient reservations
  28. static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
  29. //存储key-value的Node节点
  30. static class Node<K,V> implements Map.Entry<K,V> {
  31.     final int hash;
  32.     final K key;
  33.     volatile V val;
  34.     volatile Node<K,V> next;
  36.     Node(int hash, K key, V val, Node<K,V> next) {
  37.         this.hash = hash;
  38.         this.key = key;
  39.         this.val = val;
  40.         this.next = next;
  41.     }
  42. }
  43. // 初始化发生在第一次插入操作，默认大小为16的数组，用来存储Node节点数据，扩容时大小总是2的幂次方。
  44. transient volatile Node<K,V>[] table;
  45. // 默认为null，扩容时新生成的数组，其大小为原数组的两倍。
  46. private transient volatile Node<K,V>[] nextTable;
  48. private transient volatile long baseCount;
  50. // sizeCtl默认为0，用来控制table的初始化和扩容操作
  51. /*
  52. -1 代表table正在初始化
  53. -N 表示有N-1个线程正在进行扩容操作
  54. 另外：
  55. 1 如果table未初始化，表示table需要初始化的大小
  56. 2 如果table初始化完成，表示table的容量，默认是table大小的0.75倍
  57. */
  58. private transient volatile int sizeCtl;
  60. private transient volatile int transferIndex;
  62. private transient volatile int cellsBusy;
  64. private transient volatile CounterCell[] counterCells;
  66. private transient KeySetView<K,V> keySet;
  67. private transient ValuesView<K,V> values;
  68. private transient EntrySetView<K,V> entrySet;
  70. //下面三个方法为原子操作，分别对应volatile的取值，CAS操作，赋值
  71. static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
  72.     return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
  73. }
  75. static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
  76.                                     Node<K,V> c, Node<K,V> v) {
  77.     return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
  78. }
  80. static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
  81.     U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
  82. }
构造方法初始化
  1. //创建一个对象，什么都不做
  2. public ConcurrentHashMap() {
  3. }
  4. //传入容量值，需要将这个值变为比这个值大的2^n
  5. public ConcurrentHashMap(int initialCapacity) {
  6.     if (initialCapacity < 0)
  7.         throw new IllegalArgumentException();
  8.     int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
  9.                MAXIMUM_CAPACITY :
  10.                tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
  11.     this.sizeCtl = cap;
  12. }
  14. public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
  15.     this.sizeCtl = DEFAULT_CAPACITY;
  16.     putAll(m);
  17. }
  19. public ConcurrentHashMap(int initialCapacity, float loadFactor) {
  20.     this(initialCapacity, loadFactor, 1);
  21. }
  23. public ConcurrentHashMap(int initialCapacity,
  24.                          float loadFactor, int concurrencyLevel) {
  25.     if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
  26.         throw new IllegalArgumentException();
  27.     if (initialCapacity < concurrencyLevel)   // Use at least as many bins
  28.         initialCapacity = concurrencyLevel;   // as estimated threads
  29.     long size = (long)(1.0 + (long)initialCapacity / loadFactor);
  30.     int cap = (size >= (long)MAXIMUM_CAPACITY) ?
  31.         MAXIMUM_CAPACITY : tableSizeFor((int)size);
  32.     this.sizeCtl = cap;
  33. }
table初始化
  1. //table的初始化不在构造方法中发生，而是在第一次put时，调用了initTable方法
  2. private final Node<K,V>[] initTable() {
  3.     Node<K,V>[] tab;
  4.     int sc;
  5.     while ((tab = table) == null || tab.length == 0) {
  6.         //如果一个线程发现sizeCtl<0，意味着另外的线程执行CAS操作成功，当前线程需要让出cpu时间片，重新进入就绪状态
  7.         if ((sc = sizeCtl) < 0)
  8.             Thread.yield(); // lost initialization race; just spin
  9.         else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
  10.             //sizeCtl未被设置过-1的值，则需要在本线程中创建对象
  11.             try {
  12.                 //首先通过CAS将sizeCtl置为-1，这样别的线程就知道这个线程在创建对象了
  13.                 if ((tab = table) == null || tab.length == 0) {
  14.                     int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
  15.                     @SuppressWarnings("unchecked")
  16.                     Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
  17.                     table = tab = nt;
  18.                     sc = n - (n >>> 2);//n-(n/4) 16-16/4=12,是不是记起了什么？
  19.                 }
  20.             } finally {
  21.                 sizeCtl = sc;
  22.             }
  23.             break;
  24.         }
  25.     }
  26.     return tab;
  27. }
put方法
  1. public V put(K key, V value) {
  2.     return putVal(key, value, false);
  3. }
  5. /** Implementation for put and putIfAbsent */
  6. final V putVal(K key, V value, boolean onlyIfAbsent) {
  7.     if (key == null || value == null) throw new NullPointerException();
  8.     int hash = spread(key.hashCode());
  9.     int binCount = 0;
  10.     for (Node<K,V>[] tab = table;;) {
  11.         Node<K,V> f; int n, i, fh;
  12.         if (tab == null || (n = tab.length) == 0)
  13.             tab = initTable();//初始化table
  14.         else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
  15.    //f为拿到的最新的元素值
  16.             // 如果f为空的话，说明table中这个位置第一次插入元素，利用Unsafe.compareAndSwapObject方法插入Node节点
  17.             if (casTabAt(tab, i, null,
  18.                          new Node<K,V>(hash, key, value, null)))
  19.                 break;                   // no lock when adding to empty bin
  20.         }
  21.         else if ((fh = f.hash) == MOVED) // 如果hash值为-1，说明正在进行扩容操作
  22.             tab = helpTransfer(tab, f);//数据迁移代码不太容易，日后再说
  23.         else {
  24.    //f 是该位置的头结点，而且不为空
  25.             V oldVal = null;
  26.             // 其余情况把新的Node节点按链表或红黑树的方式插入到合适的位置，这个过程采用同步内置锁实现并发
  27.             synchronized (f) {
  28.                 //在节点f上进行同步，节点插入之前，再次利用tabAt(tab, i) == f判断，防止被其它线程修改
  29.                 if (tabAt(tab, i) == f) {
  30.                     if (fh >= 0) {
  31.    //链表
  32.                         binCount = 1;
  33.                         //遍历链表
  34.                         for (Node<K,V> e = f;; ++binCount) {
  35.                             K ek;
  36.                             if (e.hash == hash &&
  37.                                 ((ek = e.key) == key ||
  38.                                  (ek != null && key.equals(ek)))) {
  39.                                 oldVal = e.val;
  40.                                 if (!onlyIfAbsent)
  41.                                     e.val = value;
  42.                                 break;
  43.                             }
  44.                             // 到了链表的最末端，将这个新值放到链表的最后面
  45.                             Node<K,V> pred = e;
  46.                             if ((e = e.next) == null) {
  47.                                 pred.next = new Node<K,V>(hash, key,
  48.                                                           value, null);
  49.                                 break;
  50.                             }
  51.                         }
  52.                     }
  53.                     else if (f instanceof TreeBin) {
  54.    //红黑树
  55.                         Node<K,V> p;
  56.                         binCount = 2;
  57.                         // 调用红黑树的插值方法插入新节点
  58.                         if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
  59.                                                        value)) != null) {
  60.                             oldVal = p.val;
  61.                             if (!onlyIfAbsent)
  62.                                 p.val = value;
  63.                         }
  64.                     }
  65.                 }
  66.             }
  67.             if (binCount != 0) {
  68.                 if (binCount >= TREEIFY_THRESHOLD)
  69.                     //满足某些条件的时候，转化为红黑树
  70.                     treeifyBin(tab, i);
  71.                 if (oldVal != null)
  72.                     return oldVal;
  73.                 break;
  74.             }
  75.         }
  76.     }
  77.     addCount(1L, binCount);
  78.     return null;
  79. }
get方法
  1. public V get(Object key) {
  2.     Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
  3.     int h = spread(key.hashCode());//得到hash值
  4.     //取值操作不需要考虑并发，依据HashMap的实现就可以了
  5.     if ((tab = table) != null && (n = tab.length) > 0 &&
  6.         (e = tabAt(tab, (n - 1) & h)) != null) {
  7.         if ((eh = e.hash) == h) {
  8.             if ((ek = e.key) == key || (ek != null && key.equals(ek)))
  9.                 return e.val;
  10.         }
  11.         else if (eh < 0)
  12.             return (p = e.find(h, key)) != null ? p.val : null;
  13.         while ((e = e.next) != null) {
  14.             if (e.hash == h &&
  15.                 ((ek = e.key) == key || (ek != null && key.equals(ek))))
  16.                 return e.val;
  17.         }
  18.     }
  19.     return null;
  20. }

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