ConcurrentHashMap 源码赏析

HashMap 源码中主要了解其核心源码及实现逻辑。ConcurrentHashMap 就不再重复那些数据结构相关的内容咯，这里重点看一下它的并发安全实现。源码如下。

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable {
    /* ----------- 常量及成员变量的设计 几乎与HashMap相差无几 -------- *//*** 最大容量*/private static final int MAXIMUM_CAPACITY = 1 << 30;/*** 默认初始容量*/private static final int DEFAULT_CAPACITY = 16;/*** 单个数组最大容量*/static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;/*** 默认并发等级，也就分成多少个单独上锁的区域*/private static final int DEFAULT_CONCURRENCY_LEVEL = 16;/*** 扩容因子*/private static final float LOAD_FACTOR = 0.75f;/*** */transient volatile Node<K,V>[] table;/*** */private transient volatile Node<K,V>[] nextTable;/* --------- 系列构造方法，依然推荐在初始化时根据实际情况设置好初始容量 -------- */public ConcurrentHashMap() {
    }public ConcurrentHashMap(int initialCapacity) {
    if (initialCapacity < 0)throw new IllegalArgumentException();int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?MAXIMUM_CAPACITY :tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));this.sizeCtl = cap;}public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;putAll(m);}public ConcurrentHashMap(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, 1);}public ConcurrentHashMap(int initialCapacity,float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)throw new IllegalArgumentException();if (initialCapacity < concurrencyLevel)   // Use at least as many binsinitialCapacity = concurrencyLevel;   // as estimated threadslong size = (long)(1.0 + (long)initialCapacity / loadFactor);int cap = (size >= (long)MAXIMUM_CAPACITY) ?MAXIMUM_CAPACITY : tableSizeFor((int)size);this.sizeCtl = cap;}/** * ConcurrentHashMap 的核心就在于其put元素时 利用synchronized局部锁 和 * CAS乐观锁机制 大大提升了本集合的并发能力，比JDK7的分段锁性能更强*/public V put(K key, V value) {
    return putVal(key, value, false);}/*** 当前指定数组位置无元素时，使用CAS操作 将 Node键值对 放入对应的数组下标。* 出现hash冲突，则用synchronized局部锁锁住，若当前hash对应的节点是链表的头节点，遍历链表，* 若找到对应的node节点，则修改node节点的val，否则在链表末尾添加node节点；倘若当前节点是* 红黑树的根节点，在树结构上遍历元素，更新或增加节点*/final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();int hash = spread(key.hashCode());int binCount = 0;for (Node<K,V>[] tab = table;;) {
    Node<K,V> f; int n, i, fh;if (tab == null || (n = tab.length) == 0)tab = initTable();else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
    // 注意！这是一个CAS的方法，将新节点放入指定位置，不用加锁阻塞线程// 也能保证并发安全if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null)))break;                   // no lock when adding to empty bin}// 当前Map在扩容，先协助扩容，在更新值else if ((fh = f.hash) == MOVED)tab = helpTransfer(tab, f);else {
     // hash冲突V oldVal = null;// 局部锁，有效减少锁竞争的发生synchronized (f) {
     // f 是 链表头节点/红黑树根节点if (tabAt(tab, i) == f) {
    if (fh >= 0) {
    binCount = 1;for (Node<K,V> e = f;; ++binCount) {
    K ek;// 若节点已经存在，修改该节点的值if (e.hash == hash && ((ek = e.key) == key ||(ek != null && key.equals(ek)))) {
    oldVal = e.val;if (!onlyIfAbsent)e.val = value;break;}Node<K,V> pred = e;// 节点不存在，添加到链表末尾if ((e = e.next) == null) {
    pred.next = new Node<K,V>(hash, key,value, null);break;}}}// 如果该节点是 红黑树节点else if (f instanceof TreeBin) {
    Node<K,V> p;binCount = 2;if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,value)) != null) {
    oldVal = p.val;if (!onlyIfAbsent)p.val = value;}}}}// 链表节点超过了8，链表转为红黑树if (binCount != 0) {
    if (binCount >= TREEIFY_THRESHOLD)treeifyBin(tab, i);if (oldVal != null)return oldVal;break;}}}// 统计节点个数，检查是否需要resizeaddCount(1L, binCount);return null;}
}

与JDK1.7在同步机制上的区别 总结如下。JDK1.7 使用的是分段锁机制，其内部类Segment 继承了 ReentrantLock，将 容器内的数组划分成多段区域，每个区域对应一把锁，相比于HashTable确实提升了不少并发能力，但在数据量庞大的情况下，性能依然不容乐观，只能通过不断的增加锁来维持并发性能。而JDK1.8则使用了 CAS乐观锁 + synchronized局部锁 处理并发问题，锁粒度更细，即使数据量很大也能保证良好的并发性。