代碼：public class SysHashMao {
    private  static  Map<String,String> map= Collections.synchronizedMap(new HashMap<String,String>());
    public static void main(String[] args) {
        map.put("11","11");
        map.get("11");
    }
}

源碼解讀：可以看到，SynchronizedMap的每一個方法  都加上了Synchronized關鍵字，通過鎖定mutex，達到加鎖的限制，從而保障了線程的安全性
 

private static class SynchronizedMap<K,V>
    implements Map<K,V>, Serializable {
    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();}
    }

3.3.2線程安全的HashMap