Java - Main Thread

Main Thread in Java

Whenever we run a Java program, main thread is created automatically. This thread is responsible for execution of java program. Java runtime searches for main method to execute and create a main thread based on it. If we're creating multiple threads then all child threads will be spawned from main thread. This main thread is the first thread to be created and is generally the last thread and it is used to perform shut down tasks.

How to Control Main Thread?

The main thread is created by the JVM automatically when a program starts. But you can control a Main thread by using different Thread methods and techniques.

The following are some of the methods for controlling the Main thread.

• sleep() Method

• join() Method

• interrupt() Method

Example of Java Main Thread

In this example, we're showing a simple one thread program where we're not declaring any thread and checking the thread name in the program execution.

package com.tutorialspoint;
public class TestThread {
   public void printName() {
	   System.out.println("Thread Name: " + Thread.currentThread().getName());
	   System.out.println("Thread Priority: " +Thread.currentThread().getPriority());
   }	
   public static void main(String args[]) {
	   TestThread thread = new TestThread();
	   thread.printName();	   
   }
}

Output

Thread Name: main
Thread Priority: 5

More Example of Main Thread

Example

In this example, we've created a ThreadDemo class which extends Thread class. We're not passing any name to the Thread and it will print the default names assigned to the threads by the system. In main method, we've created two threads. In output, you can check, current thread name is printed as main while threads are created using constructor() method call.

package com.tutorialspoint;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: New");
   }
   public void run() {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Running");
      for(int i = 4; i > 0; i--) {
         System.out.println("Thread: " + Thread.currentThread().getName() + ", " + i); 
      }
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
   public void start () {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Start");
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  thread1.start();
	  thread2.start();
   }
}

Output

Thread: main, State: New
Thread: main, State: New
Thread: main, State: Start
Thread: main, State: Start
Thread: Thread-0, State: Running
Thread: Thread-0, 4
Thread: Thread-0, 3
Thread: Thread-1, State: Running
Thread: Thread-1, 4
Thread: Thread-0, 2
Thread: Thread-1, 3
Thread: Thread-0, 1
Thread: Thread-1, 2
Thread: Thread-0, State: Dead
Thread: Thread-1, 1
Thread: Thread-1, State: Dead

Example 2

In this example, we've created a ThreadDemo class which extends Thread class. We're not passing any name to the Thread and it will print the default names assigned to the threads by the system. In main method, we've created two threads. In output, you can check, current thread name is printed as main while threads are created using constructor() method call. In the end of main method, we're printing the state of main thread.

package com.tutorialspoint;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: New");
   }
   public void run() {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Running");
      for(int i = 4; i > 0; i--) {
         System.out.println("Thread: " + Thread.currentThread().getName() + ", " + i); 
      }
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
   public void start () {
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Start");
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  thread1.start();
	  thread2.start();
      System.out.println("Thread: " + Thread.currentThread().getName() + ", " + "State: Dead");
   }
}

Output

Thread: main, State: New
Thread: main, State: New
Thread: main, State: Start
Thread: main, State: Start
Thread: Thread-0, State: Running
Thread: main, State: Dead
Thread: Thread-1, State: Running
Thread: Thread-0, 4
Thread: Thread-1, 4
Thread: Thread-1, 3
Thread: Thread-1, 2
Thread: Thread-1, 1
Thread: Thread-1, State: Dead
Thread: Thread-0, 3
Thread: Thread-0, 2
Thread: Thread-0, 1
Thread: Thread-0, State: Dead

In this output, you can check that main thread was finished in earlier stages but threads were still running and finished their execution.