对同步和线程安全感到困惑吗？ 爪哇

Question

Actually, I am a bit confused in regards of several explanation from website or blog about synchronization and thread-safe. I've done some research on different class of Core Java Api or Java Framework (Collections). And i've often noticed that some class are synchronize and thread-safe which means, at a time, only one thread can access the code.

But i need some precision :

• A class is synchronize so its thread-safe ?
• Or synchronize and thread-safe have two different meaning ?

Best regards

Answer 1

A class is synchronize so its thread-safe ?

A class is not synchronized. Rather a method, or a block of code is synchronized.

Synchronization (using synchronized ) is one way to make code thread-safe. There are other ways.

Or synchronize and thread-safe have two different meaning ?

Yes. They have different meanings.

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And i've often noticed that some class are synchronize and thread-safe which means, at a time, only one thread can access the code.

Actually, if you "noticed" that, you were not paying attention!

With a synchronized method , only one thread can access the code while holding a given lock; ie you get mutual exclusion. If two threads use different locks, then you won't get mutual exclusion.

The other thing to note is that merely using synchronized does not guarantee thread-safety. You need to use it in the right way:

• threads need to synchronize on the appropriate objects / locks
• threads need to synchronize in all appropriate code
• if the code entails acquiring multiple locks, the locks need to be acquired in an order that avoids deadlocks.