Groovy - Main Thread

Main Thread

Whenever we run a Groovy program, main thread is created automatically. This thread is responsible for execution of groovy program. Groovy 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 - Printing Groovy 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.

Example.groovy

class Example {
   void printName() {
	   println("Thread Name: " + Thread.currentThread().getName());
	   println("Thread Priority: " +Thread.currentThread().getPriority());
   }	
   static void main(String[] args) {
	   Example thread = new Example();
	   thread.printName();	   
   }
}

Output

The above program produces the following output −

Thread Name: main
Thread Priority: 5

Example - Showcasing thread states

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.

Example.groovy

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

Output

The above program produces the following 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 - Checking subthreads

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.

Example.groovy

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

Output

The above program produces the following 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.