Why the execution thread for inner class, which extends thread, is the main thread?

Question

I am having difficulty understanding the behavior of inner class threads.

I have this simple test program.

public class Test {
    private static Random rand = new Random(System.currentTimeMillis());

    public class TestThread extends Thread {
        @Override
        public void start() {
            System.out.println("in start " + Thread.currentThread().getName());
            try {
                Thread.sleep(rand.nextInt(5000));
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

    }

    public static void main(String[] args){
        System.out.println(Thread.currentThread().getName());
        for(int i = 0; i < 5; ++i){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(rand.nextInt(5000));
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName());
                }
            }).start();
        }

        System.out.println("new run");
        Test test = new Test();
        for(int i = 0; i < 5; ++i){
            (test. new TestThread()).start();
        }
    }
}

When the first for loop runs, the threads behaves as I expected. Total of 6 threads, main, thread-0, thread-1, thread-2, thread-3, and thread-4. Thread 0 - 4 are printed out of order as I expected.

The result for second for loop has me confused.

system.out.println("in start " + Thread.currentThread().getName());

It always prints out "main," and the threads are executed sequentially. Why is the execution of the inner class thread carried out by the main thread?

Thanks!

score 4 · Answer 1 · answered Oct 29 '15 at 13:36

4

Don't override Thread.start(). The start method is always called from the thread that is starting it. You need to override Thread.run().

(See examples in the Thread class' javadoc documentation)

answered Oct 29 '15 at 13:36

dsh

12,037
3
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Thank you. Now it makes complete sense. :) – user3520211 Oct 29 '15 at 13:40
2

Or even better [don't extend Thread at all...](http://stackoverflow.com/questions/541487/implements-runnable-vs-extends-thread) – assylias Oct 29 '15 at 13:43

score 0 · Answer 2 · answered Oct 29 '15 at 13:37

0

You shouldn't @Override the start() method. You should @Override run() instead. Then call start().

answered Oct 29 '15 at 13:37

lance-java

25,497
4
59
101

Better still if the OP would not override `run()` either, but provide the `Thread` with a `Runnable` _delegate_ instead. – Solomon Slow Oct 29 '15 at 14:19

Why the execution thread for inner class, which extends thread, is the main thread?

2 Answers2