用ConcurrentHashmap替换所有出现的Hashtable是否安全？

Question

Our legacy multi-threaded application has a lots of usage of Hashtable. Is it safe to replace the Hashtable instances with ConcurrentHashmap instances for performance gain? Will there be any side effect?

Answer 1

Is it safe to replace the Hashtable instances with ConcurrentHashmap instances for performance gain?

In most cases it should be safe and yield better performance. The effort on changing depends on whether you used the Map interface or Hashtable directly.

Will there be any side effect?

There might be side effects if your application expects to immediately be able to access elements that were put into the map by another thread.

From the JavaDoc on ConcurrentHashMap:

Retrieval operations (including get) generally do not block, so may overlap 
with update operations (including put and remove). Retrievals reflect the 
results of the most recently completed update operations holding upon their onset.

Edit : to clarify on "immediately" consider thread 1 adds element A to the map and while that write is executed thread 2 tries to whether A exists in the map. With Hashtable thread 2 would be blocked until after the write so the check would return true but when using ConcurrentHashMap it would return false since thread 2 would not be blocked and the write operation is not yet completed (thus thread 2 would see an outdated version of the bucket).

Answer 2

Depending on the size of your Hashtable objects you might get some performance gains by switching to ConcurrentHashmap .

ConcurrentHashmap is broken into segments which allow for the table to be only partially locked. This means that you can get more accesses per second than a Hashtable , which requires that you lock the entire table.

The tables themselves are both thread safe and both implement the Map interface, so replacement should be relatively easy.