When to call the Thread.run() instead of Thread.start() in Java?

When we call the start() method on a thread it causes the thread to begin execution and run() method of a thread is called by the Java Virtual Machine(JVM). If we call directly the run() method, it will be treated as a normal overridden method of a thread class (or runnable interface) and it will be executed within the context of the current thread, not in a new thread.

Example

public class CallRunMethodTest extends Thread {
   @Override
   public void run() {
      System.out.println("In the run() method: " + Thread.currentThread().getName());
      for(int i = 0; i < 5 ; i++) {
         System.out.println("i: " + i);
         try {
            Thread.sleep(300);
         } catch (InterruptedException ie) {
            ie.printStackTrace();
         }
      }
  }
   public static void main(String[] args) {
      CallRunMethodTest t1 = new CallRunMethodTest();
      CallRunMethodTest t2 = new CallRunMethodTest();
      t1.run(); // calling run() method directly instead of start() method
      t2.run(); // calling run() method directly instead of start() method
   }
}

In the above example, two threads are created and the run() method is called directly on the threads rather than calling a start() method.

Output

In the run() method: main
i: 0
i: 1
i: 2
i: 3
i: 4
In the run() method: main
i: 0
i: 1
i: 2
i: 3
i: 4