In Hashtable methods are synchronized, not the table variable. Does that mean put and putIfAbsent(or remove) can run in parallel?

Question

In Java, Hashtable has individual methods synchronized and not the table instance variable. That means a put and a putIfAbsent(or remove) can still run in parallel, right ?

This means one should use other synchronization options for absolute thread safety or write safety using Collections.synchronizedMap(object-mutex level synchronization) or ConcurrentHashMap(segment level synchronization) !

If so we should not simply say that Hashtable is threadsafe. Isn't it ?

user16632363 · Answer 1 · 2021-08-20T12:08:38.953

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I think you misunderstand what it means to declare a method 'synchronized'. Such methods are synchronized on the object instance, so that only one can execute at a time on that particular object instance.

edited Aug 20 '21 at 12:08

answered Aug 20 '21 at 11:56

user16632363

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Oh Ok. That is my misunderstanding. It now makes sense to me. Something to unlearn and relearn. :) – Chintan Mangukiya Aug 20 '21 at 12:00
So basically, Java Thread acquires an object level lock when it enters into an instance synchronized java method and acquires a class level lock when it enters into static synchronized java method. [From reference] – Chintan Mangukiya Aug 20 '21 at 12:09
Yes, that's it. – user16632363 Aug 20 '21 at 17:01

In Hashtable methods are synchronized, not the table variable. Does that mean put and putIfAbsent(or remove) can run in parallel?

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