Difference between Thread.start() and Thread.run() in Java

In Java’s multi-threading concept, start() and run() are the two most important methods. Below are some of the differences between the Thread.start() and Thread.run() methods:

  1. New Thread creation: When a program calls the start() method, a new thread is created and then the run() method is executed. But if we directly call the run() method then no new thread will be created and run() method will be executed as a normal method call on the current calling thread itself and no multi-threading will take place.
    Let us understand it with an example:

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    class MyThread extends Thread {
        public void run()
        {
            System.out.println("Current thread name: "
                               + Thread.currentThread().getName());
            System.out.println("run() method called");
        }
    }
      
    class GeeksforGeeks {
        public static void main(String[] args)
        {
            MyThread t = new MyThread();
            t.start();
        }
    }

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    Output:



    Current thread name: Thread-0
    run() method called
    

    As we can see in the above example, when we call the start() method of our thread class instance, a new thread is created with default name Thread-0 and then run() method is called and everything inside it is executed on the newly created thread.
    Now, let us try to call run() method directly instead of start() method:

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    class MyThread extends Thread {
        public void run()
        {
            System.out.println("Current thread name: "
                               + Thread.currentThread().getName());
      
            System.out.println("run() method called");
        }
    }
      
    class GeeksforGeeks {
        public static void main(String[] args)
        {
            MyThread t = new MyThread();
            t.run();
        }
    }

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    Output:

    Current thread name: main
    run() method called
    

    As we can see in the above example, when we called the run() method of our MyThread class, no new thread is created and the run() method is executed on the current thread i.e. main thread. Hence, no multi-threading took place. The run() method is called as a normal function call.

  2. Multiple invocation: In Java’s multi-threading concept, another most important difference between start() and run() method is that we can’t call the start() method twice otherwise it will throw an IllegalStateException whereas run() method can be called multiple times as it is just a normal method calling.
    Let us understand it with an example:

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    class MyThread extends Thread {
        public void run()
        {
            System.out.println("Current thread name: "
                               + Thread.currentThread().getName());
      
            System.out.println("run() method called");
        }
    }
      
    class GeeksforGeeks {
        public static void main(String[] args)
        {
            MyThread t = new MyThread();
            t.start();
            t.start();
        }
    }

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    Output:

    Current thread name: Thread-0
    run() method called
    Exception in thread "main" java.lang.IllegalThreadStateException
        at java.lang.Thread.start(Thread.java:708)
        at GeeksforGeeks.main(File.java:11)
    

    As we can see in the above example, calling start() method again raises java.lang.IllegalThreadStateException.
    Now, let us try to call run() method twice:

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    class MyThread extends Thread {
        public void run()
        {
            System.out.println("Current thread name: "
                               + Thread.currentThread().getName());
            System.out.println("run() method called");
        }
    }
      
    class GeeksforGeeks {
        public static void main(String[] args)
        {
            MyThread t = new MyThread();
            t.run();
            t.run();
        }
    }

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    Output:

    Current thread name: main
    run() method called
    Current thread name: main
    run() method called
    

    As we can see in the above example, calling run() method twice doesn’t raise any exception and it is executed twice as expected but on the main thread itself.

    Summary

    start() run()
    Creates a new thread and the run() method is executed on the newly created thread. No new thread is created and the run() method is executed on the calling thread itself.
    Can’t be invoked more than one time otherwise throws java.lang.IllegalStateException Multiple invocation is possible
    Defined in java.lang.Thread class. Defined in java.lang.Runnable interface and must be overriden in the implementing class.


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