Question

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?

Answer 1

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.