Java Concurrency – Difference between yield() and join()

yield() method

Theoretically, to ‘yield’ means to let go, to give up, to surrender. A yielding thread tells the virtual machine that it’s willing to let other threads be scheduled in its place. This indicates that it’s not doing something too critical. Note that it’s only a hint, though, and not guaranteed to have any effect at all.

yield() is defined as following in Thread.java.

public static native void yield();


Let’s list down important points from above definition:

• Yield is a Static method and Native too.
• Yield tells the currently executing thread to give a chance to the threads that have equal priority in the Thread Pool.
• There is no guarantee that Yield will make the currently executing thread to runnable state immediately.
• It can only make a thread from Running State to Runnable State, not in wait or blocked state.

yield() method example usage

In below example program, I have created two threads named producer and consumer for no specific reason. Producer is set to minimum priority and consumer is set to maximum priority. I will run below code with/without commenting the line Thread.yield(). Without yield(), though the output changes sometimes, but usually first all consumer lines are printed and then all producer lines.

With using yield() method, both prints one line at a time and pass the chance to another thread, almost all the time.

public class YieldExample
{
   public static void main(String[] args)
   {
      Thread producer = new Producer();
      Thread consumer = new Consumer();
       
      producer.setPriority(Thread.MIN_PRIORITY); //Min Priority
      consumer.setPriority(Thread.MAX_PRIORITY); //Max Priority
       
      producer.start();
      consumer.start();
   }
}
 
class Producer extends Thread
{
   public void run()
   {
      for (int i = 0; i < 5; i++)
      {
         System.out.println("I am Producer : Produced Item " + i);
         Thread.yield();
      }
   }
}
 
class Consumer extends Thread
{
   public void run()
   {
      for (int i = 0; i < 5; i++)
      {
         System.out.println("I am Consumer : Consumed Item " + i);
         Thread.yield();
      }
   }
}

Output of above program “without” yield() method

I am Consumer : Consumed Item 0
I am Consumer : Consumed Item 1
I am Consumer : Consumed Item 2
I am Consumer : Consumed Item 3
I am Consumer : Consumed Item 4
I am Producer : Produced Item 0
I am Producer : Produced Item 1
I am Producer : Produced Item 2
I am Producer : Produced Item 3
I am Producer : Produced Item 4

Output of above program “with” yield() method added

I am Producer : Produced Item 0
I am Consumer : Consumed Item 0
I am Producer : Produced Item 1
I am Consumer : Consumed Item 1
I am Producer : Produced Item 2
I am Consumer : Consumed Item 2
I am Producer : Produced Item 3
I am Consumer : Consumed Item 3
I am Producer : Produced Item 4
I am Consumer : Consumed Item 4

join() method

The join() method of a Thread instance can be used to “join” the start of a thread’s execution to the end of another thread’s execution so that a thread will not start running until another thread has ended. If join() is called on a Thread instance, the currently running thread will block until the Thread instance has finished executing.

public final void join() throws InterruptedException


Giving a timeout within join(), will make the join() effect to be nullified after the specific timeout. When the timeout is reached, the main thread and taskThread are equally probable candidates to execute. However, as with sleep, join is dependent on the OS for timing, so you should not assume that join will wait exactly as long as you specify.

Like sleep, join responds to an interrupt by exiting with an InterruptedException.

join() method example usage

public class JoinExample
{
   public static void main(String[] args) throws InterruptedException
   {
      Thread t = new Thread(new Runnable()
         {
            public void run()
            {
               System.out.println("First task started");
               System.out.println("Sleeping for 2 seconds");
               try
               {
                  Thread.sleep(2000);
               } catch (InterruptedException e)
               {
                  e.printStackTrace();
               }
               System.out.println("First task completed");
            }
         });
      Thread t1 = new Thread(new Runnable()
         {
            public void run()
            {
               System.out.println("Second task completed");
            }
         });
      t.start(); // Line 15
      t.join(); // Line 16
      t1.start();
   }
}

First task started
Sleeping for 2 seconds
First task completed
Second task completed