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众所周知，HashMap 本身非线程安全的，但是当使用 Collections.synchronizedMap(new HashMap()) 进行包装后就返回一个线程安全的Map。

怎么实现的呢？今天比较有兴趣就看了 Collections.synchronizedMap 的实现，发现其实还是比较简单的。

public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
return new SynchronizedMap<K,V>(m);
}

 

当调用synchronizedMap后返回了一个SynchronizedMap 

    private static class SynchronizedMap<K,V>
    implements Map<K,V>, Serializable {
    // use serialVersionUID from JDK 1.2.2 for interoperability
    private static final long serialVersionUID = 1978198479659022715L;

    private final Map<K,V> m;     // Backing Map
        final Object      mutex;    // Object on which to synchronize

    SynchronizedMap(Map<K,V> m) {
            if (m==null)
                throw new NullPointerException();
            this.m = m;
            mutex = this;
        }

    SynchronizedMap(Map<K,V> m, Object mutex) {
            this.m = m;
            this.mutex = mutex;
        }

    public int size() {
        synchronized(mutex) {return m.size();}
        }
    public boolean isEmpty(){
        synchronized(mutex) {return m.isEmpty();}
        }
    public boolean containsKey(Object key) {
        synchronized(mutex) {return m.containsKey(key);}
        }
    public boolean containsValue(Object value){
        synchronized(mutex) {return m.containsValue(value);}
        }
    public V get(Object key) {
        synchronized(mutex) {return m.get(key);}
        }

    public V put(K key, V value) {
        synchronized(mutex) {return m.put(key, value);}
        }
    public V remove(Object key) {
        synchronized(mutex) {return m.remove(key);}
        }
    public void putAll(Map<? extends K, ? extends V> map) {
        synchronized(mutex) {m.putAll(map);}
        }
    public void clear() {
        synchronized(mutex) {m.clear();}
    }

    private transient Set<K> keySet = null;
    private transient Set<Map.Entry<K,V>> entrySet = null;
    private transient Collection<V> values = null;

    public Set<K> keySet() {
            synchronized(mutex) {
                if (keySet==null)
                    keySet = new SynchronizedSet<K>(m.keySet(), mutex);
                return keySet;
            }
    }

    public Set<Map.Entry<K,V>> entrySet() {
            synchronized(mutex) {
                if (entrySet==null)
                    entrySet = new SynchronizedSet<Map.Entry<K,V>>(m.entrySet(), mutex);
                return entrySet;
            }
    }

    public Collection<V> values() {
            synchronized(mutex) {
                if (values==null)
                    values = new SynchronizedCollection<V>(m.values(), mutex);
                return values;
            }
        }

    public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized(mutex) {return m.equals(o);}
        }
    public int hashCode() {
            synchronized(mutex) {return m.hashCode();}
        }
    public String toString() {
        synchronized(mutex) {return m.toString();}
        }
        private void writeObject(ObjectOutputStream s) throws IOException {
        synchronized(mutex) {s.defaultWriteObject();}
        }
    }

可以看到SynchronizedMap 是一个实现了Map接口的代理类，该类中对Map接口中的方法使用synchronized 同步关键字来保证对Map的操作是线程安全的。