Java读写锁

Java读写锁，ReadWriteLock.java接口， RentrantReadWriteLock.java实现。通过读写锁，可以实现读-读线程并发，读-写，写-读线程互斥进行。以前面试遇到一个问题，ConcurrentHashMap的实现原理，如何封装Map效率更高。今天看了《Java并发编程实战》，封装的ReadWriteMap类，效率就比ConcurrentHashMap效率更高，在读多写少的场景。

ReadWriteMap.java

public class ReadWriteMap<K, V> {
    private final Map<K, V> map;
    private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    private final Lock readLock = readWriteLock.readLock();
    private final Lock writeLock = readWriteLock.writeLock();

    public ReadWriteMap(Map<K, V> map){
        this.map = map;
    }

    public V put(K key, V value){
        writeLock.lock();
        try{
            return map.put(key, value);
        } finally {
            //释放锁，一定要写在finally里面
            writeLock.unlock();
        }
    }

    public V get(K key){
        readLock.lock();
        try{
            return map.get(key);
        } finally {
            readLock.unlock();
        }
    }

}

ConcurrentHashMap里面的putVal()方法

 /** Implementation for put and putIfAbsent */
    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;
    }

使用的是synchronized进行同步，synchronized是独占锁，在读多写少的情况下效率不高，极端情况就是所有都是读线程，串行执行。