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

ConcurrentHashMap 性能最好