javamultithreadingconcurrencymonitor

What's a monitor in Java?

What's a monitor referred to in concurrent programming in Java?

When I read that "every object has associated a monitor" what does it meaning?

Is it a special object?

Solution

  • A monitor is mechanism to control concurrent access to an object.

    This allows you to do:

    Thread 1:

    public void a()
{
    synchronized(someObject) {
        // do something (1)
    }
}

    Thread 2:

    public void b()
{
    synchronized(someObject) {
        // do something else (2)
    }
}

    This prevents Threads 1 and 2 accessing the monitored (synchronized) section at the same time. One will start, and monitor will prevent the other from accessing the region before the first one finishes.

    It's not a special object. It's synchronization mechanism placed at class hierarchy root: java.lang.Object.

    There are also wait and notify methods that will also use object's monitor to communication among different threads.