Synchronizing two methods in Java

问题

I have a class like this one:

public class IClass{

   public void draw(){...};  //is called periodically by the rendering thread
   public void foo(){...};    //is called asynchronously from another Thread(it could be an onTouchEvent() method for example)
}

I want the foo() method to wait until the draw method is finished and vice versa. How can I do this in Java?

regards

回答1:

Make the methods synchronized.

public synchronized void draw() {   System.out.println("draw"); }

public synchronized void foo() {    System.out.println("foo"); }

Or synchronize on the same object.

private static final Object syncObj = new Object();

public void draw() {
    synchronized (syncObj) {
        System.out.println("draw");
    }
}

public void foo() {
    synchronized (syncObj) {
        System.out.println("foo");
    }
}

回答2:

Putting synchronized on a method means the thread has to acquire the lock on the object instance before entering that method, so if you have two different methods marked synchronized the threads entering them will be contending for the same lock, and once one thread gets the lock all other threads are shut out of all methods that synchronize on that same lock. So in order for the two methods to run concurrently they would have to use different locks, like this:

public class IClass {

      private final Object lockDraw = new Object();
      private final Object lockFoo = new Object();

      public void draw() {
          synchronized(lockDraw) {
             //method draw
          }
      }

      public void foo() {
          synchronized(lockFoo) {
             //method foo
          }
      }
}

Both methods lock the same monitor. Therefore, you can't simultaneously execute them on the same object from different threads (one of the two methods will block until the other is finished).

来源：https://stackoverflow.com/questions/24341447/synchronizing-two-methods-in-java