Is java.util.Hashtable thread safe?

Question

It's been a while since I've used hashtable for anything significant, but I seem to recall the get() and put() methods being synchronized.

The JavaDocs don't reflect this. They simply say that the class Hashtable is synchronized. What can I assume? If several threads access the hashtable at the same time (assuming they are not modifying the same entry), the operations will succeed, right? I guess what I'm asking is "Is java.util.Hashtable thread safe?"

Please Guide me to get out of this issue...

Answer 1

It is threadsafe because the get, put, contains methods etc are synchronized. Furthermore, Several threads will not be able to access the hashtable at the same time, regardless of which entries they are modifying.

edit - amended to include the provisio that synchronization makes the hashtable internally threadsafe in that it is modified atomically; it doesn't guard against race conditions in outside code caused by concurrent access to the hashtable by multiple threads.

Answer 2

No. It is 'threadsafe' only to the extent that its methods are synchronized. However it is not threadsafe in general, and it can't be, because classes that export internal state such as Iterators or Enumerations require the use of the internal state to be synchronized as well. That's why the new Collections classes are not synchronized, as the Java designers recognized that thread-safety is up to the user of the class, not the class itself.