ConcurrentHashMap

转载于 2018-09-07 08:05:50 发布 · 55 阅读

CC 4.0 BY-SA版权

原文链接：https://juejin.im/post/5b922b90e51d450e5c472390

本文详细解析了ConcurrentHashMap的工作原理，包括其线程安全特性、不允许null的key和value，以及与Hashtable和Collections.synchronizedMap的区别。介绍了ConcurrentHashMap的基本属性、构造器和主要方法，如put方法的实现细节。

ConcurrentHashMap

线程安全
key 和 value 不允许null
Collections.synchronizedMap(map) 和 Hashtable 用synchronized 进行同步，效率不高

基本属性和构造器

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentMap<K,V>, Serializable {
    
    // 最大容量
    private static final int MAXIMUM_CAPACITY = 1 << 30;
    // 默认容量
    private static final int DEFAULT_CAPACITY = 16;
    // 最大数组大小
    static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    
    private static final float LOAD_FACTOR = 0.75f;
    static final int TREEIFY_THRESHOLD = 8;
    static final int UNTREEIFY_THRESHOLD = 6;
    static final int MIN_TREEIFY_CAPACITY = 64;
    private static final int MIN_TRANSFER_STRIDE = 16;
    private static int RESIZE_STAMP_BITS = 16;
    private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
    
    static final int MOVED     = -1; // hash for forwarding nodes
    static final int TREEBIN   = -2; // hash for roots of trees
    static final int RESERVED  = -3; // hash for transient reservations
    static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
}
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主要方法

Put 方法

public V put(@NotNull K key,@NotNull V value) {
    return putVal(key, value, false);
}

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) {
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            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;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    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;
                        }
                    }
                }
            }
            if (binCount != 0) {
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}

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