What does "synchronized" mean in Java?

 

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  • What does 'synchronized' mean? 15 answers

    • 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.

    In the (Java) example 

    public static synchronized Singleton getInstance()

    means that only one thread at a time should be able to access the getInstance() method this to avoid a racing condition.

    As the commenters already pointed out, synchronized is a Java keyword.

    It means that two threads cannot execute the method at the same time and the JVM takes care of enforcing that.

    In C++, you will have to use some synchronization construct, like a critical section or a mutex. You can consult this.

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