如何创建一个线程安全的Map

1，使用普通的旧的Hashtable

　　HashMap允许null作为key，而Hashtable不可以

2，使用Collections中同步化的包装方法synchronizedMap

3，使用concurrent包下的ConcurrentHashMap 

//Hashtable Example Code
Map<String, Integer> threadSafeMap = new Hashtable<String, Integer>);
//synchronizedMap Example Code.
threadSafeMap = Collections.synchronizedMapnew HashMap<String, Integer>));
//ConcurrentHashMap Example Code
threadSafeMap = new ConcurrentHashMap<String, Integer>);
threadSafeMap .put"Key1", 123)

ConcurrentHashMap 性能最好