java concurrenthashmap和hashmap的区别

java concurrenthashmap和hashmap的区别

最大的区别就是ConcurrentHashMap是线程安全的，hashMap不是线程安全的。
为什么线程安全呢：
ConcurrentHashMap代码中可以看出，它引入了一个“分段锁”的概念，具体可以理解为把一个大的Map拆分成N个小的HashTable，根据key.hashCode()来决定把key放到哪个HashTable中。
在ConcurrentHashMap中，就是把Map分成了N个Segment，put和get的时候，都是现根据key.hashCode()算出放到哪个Segment中：

前者是 a hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. this class obeys the same functional specification as hashtable, and includes versions of methods corresponding to each method of hashtable. however, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. this class is fully interoperable with hashtable in programs that rely on its thread safety but not on its synchronization details. 后者是 hash table based implementation of the map interface. this implementation provides all of the optional map operations, and permits null values and the null key. (the hashmap class is roughly equivalent to hashtable, except that it is unsynchronized and permits nulls.) this class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.