what's a monitor?

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.