What does "synchronized" mean in Java?
I have been trying to learn design patterns. This site uses the synchronized keyword, but I don't understand what it does.
I searched on the net and found that it is somewhat related to multi-threading and memory, but I am a mechanical engineer and don't understand what that means.
Can anybody please help me understand threads and the synchronized keyword?
Answer
There is no synchronized keyword in C++.
There is one in Java, though, where for methods it means the following two things:
- It is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
- When a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.
Similar rules apply to arbitrary blocks.
Also, I recommend learning from a peer-reviewed book, not some arbitrary non-authoritative website.