o7planning

Java Multithreading Programming Tutorial with Examples

  1. The principle of operation of the thread
  2. Example start with Thread
  3. Runnable Interface
  4. Deamon Thread
  5. Using join() & join(long)
  6. Handling exception in thread
  7. Using yield()
  8. Compare sleep() and wait()

1. The principle of operation of the thread

2. Example start with Thread

We need 2 class participated in this example.
  • HelloMain is a class with the main method, it is a main thread.
  • HelloThread is a class extends the Thread class. It was created and is enabled to run within the main stream and will run parallel to the main thread.
package org.o7planning.tutorial.thread.hellothread;

public class HelloMain {

   public static void main(String[] args) throws InterruptedException {

       int idx = 1;

       for (int i = 0; i < 2; i++) {

           System.out.println("Main thread running " + idx++);
           // Sleep 2101 miliseconds.
           Thread.sleep(2101);
       }

       HelloThread helloThread = new HelloThread();

       // Run thread
       helloThread.start();

       for (int i = 0; i < 3; i++) {
           System.out.println("Main thread running " + idx++);

           // Sleep 2101 miliseconds.
           Thread.sleep(2101);
       }

       System.out.println("==> Main thread stopped");
   }
}
HelloThread.java
package org.o7planning.tutorial.thread.hellothread;

public class HelloThread extends Thread {


   // Code of method run() will be executed when
   // thread call start()
   @Override
   public void run() {
       int index = 1;

       for (int i = 0; i < 10; i++) {
           System.out.println("  - HelloThread running " + index++);

           try {
               // Sleep 1030 miliseconds.
               Thread.sleep(1030);
           } catch (InterruptedException e) {
           }

       }
       System.out.println("  - ==> HelloThread stopped");
   }
}
Results of running class HelloMain

3. Runnable Interface

You can also create a thread from one class to implements Runnable interface. See examples:
RunnableDemo.java
package org.o7planning.tutorial.thread.runnable;

public class RunnableDemo implements Runnable {

   @Override
   public void run() {
       int idx = 1;
       for (int i = 0; i < 5; i++) {
           System.out.println("Hello from RunnableDemo " + idx++);

           // Sleep 2 second.
           try {
               Thread.sleep(2000);
           } catch (InterruptedException e) {
           }
       }
   }

}
RunnableTest.java
package org.o7planning.tutorial.thread.runnable;

public class RunnableTest {

   public static void main(String[] args) throws InterruptedException {

       System.out.println("Main thread running..");

       // Create a thread from Runnable.
       Thread thread = new Thread(new RunnableDemo());

       thread.start();

       // Sleep 5 seconds.
       Thread.sleep(5000);
       System.out.println("Main thread stopped");
   }
}
Run class RunnableTest:
Main thread running..
Hello from RunnableDemo 1
Hello from RunnableDemo 2
Hello from RunnableDemo 3
Main thread stopped
Hello from RunnableDemo 4
Hello from RunnableDemo 5

4. Deamon Thread

Java divides Thread into 2 types: normal thread and Deamon Thread. The difference is the way of ending. In a program, normal threads and Deamon thread run parallel each other. When all normal thread finish, all Deamon threads will be ending.
Note:

Use setDeamon(boolean) to set up a thread that can be Deamon or not. Noticeably, you only can call setDeamon(boolean) when the thread has not ran already. It means that when thread has ran you cannot change the thread from non-deamon to deamon and vice versa.

When a new thread is created, it inherits the deamon feature from its father thread. Thus, when you create a thread in main method of a class, that thread naturally is non-deamon, so thread created by default is also non-deamon. Therefore, if you create a new thread in a Deamon thread, by default it also
Thread thread = new MyThread();

// marks this thread as a daemon thread
// This method is only called when the thread is not a start.
// In the case of start, it will be throws an exception.
thread.setDeamon(true);


// marks this thread as a none-daemon thread
// This method is only called when the thread is not a start.
// In the case of start, it will be throws an exception.
thread.setDeamon(false);
For ease of understanding we consider the following example. We have three classes involved in the example:
NoneDeamonThread.java
package org.o7planning.tutorial.thread.deamon;

public class NoneDeamonThread extends Thread {

   @Override
   public void run() {
       int i = 0;

       // Loop 10 times. This thread will end.
       while (i < 10) {
           System.out.println("  - Hello from None Deamon Thread " + i++);
           try {

               // Sleep 1 second
               Thread.sleep(1000);
           } catch (InterruptedException e) {
           }
       }

       // None deamon thread ending.
       System.out.println("\n==> None Deamon Thread ending\n");
   }
}
DeamonThread.java
package org.o7planning.tutorial.thread.deamon;

class DeamonThread extends Thread {

   @Override
   public void run() {
       int count = 0;
     
       // Infinite loop
       while (true) {
           System.out.println("+ Hello from Deamon Thread " + count++);
           try {
               // Sleep 2 second
               sleep(2000);
           } catch (InterruptedException e) {
           }
       }
   }
}
DaemonTest.java
package org.o7planning.tutorial.thread.deamon;

public class DaemonTest {

   public static void main(String[] args) {
       System.out.println("==> Main Thread running..\n");

       // Create thread
       Thread deamonThread = new DeamonThread();

       // Set deamon true
       deamonThread.setDaemon(true);
       deamonThread.start();

       // Create other thread
       new NoneDeamonThread().start();

       try {
           // Sleep 5 second
           Thread.sleep(5000);
       } catch (InterruptedException e) {
       }
     
       // Main Thread ending
       System.out.println("\n==> Main Thread ending\n");
   }

}
Results of running DeamonTest:
The above illustration shows that Deamon thread has been stopped when all of the normal threads stop although its code is infinite.
What is Deamon thread used for?
One of the important Deamon threads of Java is Garbage Collection Thread. It means to collect disused resources to liberate the memory. When all user's threads stop operating, Garbage Collection Thread also stop.

5. Using join() & join(long)

Thread.join() is a method notifying that please wait for this thread to be completed before the parent thread continues to run.
// Parent thread must wait until the end of this thread, before being continued.
// (This is equivalent to calling join(0))
public final void join() throws InterruptedException;

// Parent thread must wait 'millis' milliseconds to continue running.
// After call join(long).
// If the parameter millis = 0 means to wait until the end of this thread.
public final synchronized void join(long millis) throws InterruptedException;


// Parent thread must wait 'millis' milliseconds and 'nanos' nanoseconds to continue running.
// After call join(long,int).
// 1 second = 1000000 nanoseconds.
public final synchronized void join(long millis, int nanos) throws InterruptedException;
Consider an illustration:
JoinThread.java
package org.o7planning.tutorial.thread.join;

public class JoinThread extends Thread {
  private String threadName;
  private int count;

  public JoinThread(String threadName, int count) {
      this.threadName = threadName;
      this.count = count;
  }

  @Override
  public void run() {

      for (int i = 1; i < count + 1; i++) {
          System.out.println("Hello from " + this.threadName + " " + i);
          try {
              Thread.sleep(2000);
          } catch (InterruptedException e) {
          }
      }
      System.out.println("\n==> Thread " + threadName + " end!\n");
  }
}
JoinTest.java
package org.o7planning.tutorial.thread.join;

public class JoinTest {

   public static void main(String[] args) throws InterruptedException {

       System.out.println("\n==> Main thread starting..\n");

       Thread joinThreadA = new JoinThread("A*", 2);
       Thread joinThreadB = new JoinThread("B*", 3);


       // None join Thread.
       Thread noJoinThreadC = new JoinThread("C", 5);

       joinThreadA.start();
       joinThreadB.start();
       noJoinThreadC.start();

       // Using join()
       joinThreadA.join();
       joinThreadB.join();


       // The following code will have to wait until 2
       // JoinThread A, B completed.
       System.out.println("Hello from main thread...");

       System.out.println("Thread A isLive? " + joinThreadA.isAlive());
       System.out.println("Thread B isLive? " + joinThreadB.isAlive());
       System.out.println("Thread C isLive? " + noJoinThreadC.isAlive());

       System.out.println("\n==> Main Thread end!\n");
   }
}
The result of running JoinTest:
==> Main thread starting..

Hello from A* 1
Hello from C 1
Hello from B* 1
Hello from A* 2
Hello from C 2
Hello from B* 2

==> Thread A* end!

Hello from B* 3
Hello from C 3

==> Thread B* end!

Hello from C 4
Hello from main thread...
Thread A isLive? false
Thread B isLive? false
Thread C isLive? true

==> Main Thread end!

Hello from C 5

==> Thread C end!
Example of using join(long millis):
JoinTest2.java
package org.o7planning.tutorial.thread.join;

public class JoinTest2 {

   public static void main(String[] args) throws InterruptedException {

       System.out.println("\n==> Main thread starting..\n");

       Thread joinThreadA = new JoinThread("A*", 5);

       joinThreadA.start();
       
       // Main thread must wait to 5000 miliseconds,
       // and then continue running. (Not necessarily joinThreadA finish)
       joinThreadA.join(5000);

       System.out.println("Main thread after 5000 milli second");
       System.out.println("Hello from main thread...");

       System.out.println("Thread A isLive? " + joinThreadA.isAlive());

       System.out.println("\n==> Main Thread end!\n");
   }

}
Results of running the example:
==> Main thread starting..

Hello from A* 1
Hello from A* 2
Hello from A* 3
Main thread after 5000 milli second
Hello from main thread...
Thread A isLive? true

==> Main Thread end!

Hello from A* 4
Hello from A* 5

==> Thread A* end!

6. Handling exception in thread

The Thread.setDefaultUncaughtExceptionHandler() method set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.
ThreadExceptionDemo.java
package org.o7planning.tutorial.thread.exception;

import java.util.Random;

public class ThreadExceptionDemo {

   public static class RunnableTest implements Runnable {

       @Override
       public void run() {
           System.out.println("Thread running ..");

           while (true) {
               Random r = new Random();

               // A random number from 0-99
               int i = r.nextInt(100);
               System.out.println("Next value " + i);

               try {
                   Thread.sleep(2000);
               } catch (InterruptedException e) {
               }

               if (i > 70) {
                    // Simulate an exception was not handled in the thread.
                   throw new RuntimeException("Have a problem...");
               }
           }
       }

   }

   public static void main(String[] args) {
       System.out.println("==> Main thread running...");

       Thread thread = new Thread(new RunnableTest());
       Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {

           @Override
           public void uncaughtException(Thread t, Throwable e) {
               System.out.println("#Thread: " + t);
               System.out.println("#Thread exception message: " + e.getMessage());
           }
       });

       thread.start();
       System.out.println("==> Main thread end...");
   }

}
Results of running the example:
==> Main thread running...
==> Main thread end...
Thread running ..
Next value 21
Next value 42
Next value 67
Next value 18
Next value 34
Next value 40
Next value 11
Next value 1
Next value 92
#Thread: Thread[Thread-0,5,main]
#Thread exception message: Have a problem...

7. Using yield()

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();
so, yield() method is used when you see that thread is free, it's not doing anything important, it suggests operating system give priority temporarily to the other thread.
The example below, there are two threads, each thread print out a text 100K times (the numbers are large enough to see the difference). One thread is the highest priority, and other thread is lowest priority. See completion time of 2 threads.
YieldThreadExample.java
package org.o7planning.tutorial.thread.yield;

import java.util.Date;

public class YieldThreadExample {

	private static Date importantEndTime;
	private static Date unImportantEndTime;

	public static void main(String[] args) {
		importantEndTime = new Date();
		unImportantEndTime = new Date();

		System.out.println("Create thread 1");

		Thread importantThread = new ImportantThread();

		// Set the highest priority for this thread.
		importantThread.setPriority(Thread.MAX_PRIORITY);

		System.out.println("Create thread 2");

		Thread unImportantThread = new UnImportantThread();

		// Set the lowest priority for this thread.
		unImportantThread.setPriority(Thread.MIN_PRIORITY);

		// Start threads.
		unImportantThread.start();
		importantThread.start();

	}

	// A important job which requires high priority.
	static class ImportantThread extends Thread {

		@Override
		public void run() {
			for (int i = 0; i < 100000; i++) {
				System.out.println("\n Important work " + i);

				// Notifying the operating system,
				// this thread gives priority to other threads.
				Thread.yield();
			}

			// The end time of this thread.
			importantEndTime = new Date();
			printTime();
		}

	}

	static class UnImportantThread extends Thread {
		@Override
		public void run() {
			for (int i = 0; i < 100000; i++) {
				System.out.println("\n  -- UnImportant work " + i);
			}
			// The end time of this thread.
			unImportantEndTime = new Date();
			printTime();
		}
	}

	private static void printTime() {
		// Interval (Milliseconds)
		long interval = unImportantEndTime.getTime() - importantEndTime.getTime();

		System.out.println("UnImportant Thread - Important Thread = " //
				+ interval + " milliseconds");
	}

}
The result: the lower priority thread has completed the task 51 milliseconds faster than the thread with higher priority.
...

 Important work 99995

 Important work 99996

 Important work 99997

 Important work 99998

 Important work 99999
UnImportant Thread - Important Thread = -269 milliseconds

8. Compare sleep() and wait()

TODO.

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