SynchronizedMap是另外一种保证线程安全的Map容器，实现方式是将线程不安全的Map容器包装成自己的属性m，对m的操作使用同步代码块包裹着，这种保证线程安全的效率低下。
代码一览：

private static class SynchronizedMap<K,V>
        implements Map<K,V>, Serializable {
        private static final long serialVersionUID = 1978198479659022715L;

        private final Map<K,V> m;     //被包装的Map对象
        final Object      mutex;        // 对象锁，默认为this对象

        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;