What does 'synchronized' mean?

Question

I have some questions regarding the usage and significance of the synchronized keyword.

• What is the significance of the synchronized

Answer 1

The synchronized keyword causes a thread to obtain a lock when entering the method, so that only one thread can execute the method at the same time (for the given object instance, unless it is a static method).

This is frequently called making the class thread-safe, but I would say this is a euphemism. While it is true that synchronization protects the internal state of the Vector from getting corrupted, this does not usually help the user of Vector much.

Consider this:

 if (vector.isEmpty()){
     vector.add(data);
 }

Even though the methods involved are synchronized, because they are being locked and unlocked individually, two unfortunately timed threads can create a vector with two elements.

So in effect, you have to synchronize in your application code as well.

Because method-level synchronization is a) expensive when you don't need it and b) insufficient when you need synchronization, there are now un-synchronized replacements (ArrayList in the case of Vector).

More recently, the concurrency package has been released, with a number of clever utilities that take care of multi-threading issues.