multithreading - What is a class level lock in java

Question

What is a class level lock. Can you please explain with an example.

Answer 1

I am assuming you are referring to a synchronization lock. If you are unfamiliar with synchronization it is a means to prevent the same code being run by two different threads at the same time. In java synchronization is done with Object locks:

Object lock = new Object();

public void doSomething(){
  ...
  synchronized(lock){
    //something dangerous
  }
  ...
}

In this code it is guaranteed only one thread can do something dangerous at a given time, and it will complete everything in the synchronized block before another thread may start running the same code. I am guessing what you are referring to as a "class level lock" is the implicit lock on synchronized method:

public synchronized void somethingDangerous(){...}

Here again only one thread can execute the method at any given time and will always finish before another thread may begin executing the code. This is equivalent to a synchronized block on "this":

public void somethingDangerous(){
  synchronized(this){
    //something dangerous
  }
}

Now this lock isn't actually on the class, but rather on a single instance (i.e. not all clocks, but only your clock). If you want a true "class level lock" (which would typically be done in a static method), then you need to synchronize on something independent of any instances. For example:

public class Clock{
  private static Object CLASS_LOCK = new Object();

  public static void doSomething(){
    ...
    synchronized(CLASS_LOCK){
      //something dangerous to all clocks, not just yours
    }
    ...
  }
}

again there is an implicit lock for static methods:

public class Clock{
  public static synchronized void somethingDangerous(){}
}

which is the equivalent of locking on the class object:

public class Clock{
  public static void somethingDangerous(){
    synchronized(Clock.class){
      //do something dangerous
    }
  }
}