A Strange Java Multi-Thread Case

Today, we have a nice problem about Java multi-thread and other java related tips. I want to write it down to remind myself that keep curious about whatever strange thing you met because it may open a brand new window for you. Ok, first of all, here is the java code:


class Daemon extends Thread{
private static final int SIZE = 100;
private Thread[] t= new Thread[SIZE];
private static volatile int i = 0;

public Daemon(){
   start();
}
public void run(){
   for( i = 0; i < SIZE; i++){
t[i] = new DaemonSpawn(i);
//t[i].start();  [1]
   }
while(true)
yield();
}
}

class DaemonSpawn extends Thread{
private int n ;
public DaemonSpawn(int i){
this.n = i;
start();
}
public void run(){
System.out.println("DaemonSpawn" + n + "started");
while(true)
yield();
}
}

public class Daemons {
public static void main(String...args){
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Daemon());
//Thread d = new Daemon();  [2]
//System.out.println("d.isDaemon() = " + d.isDaemon());

}
}

Ok, Just as it is, we have a class named Daemons, it will new a ExecutorService and run its execute method on the Daemon class. The Daemon class constructs a serial of DaemonSpawn threads and run them. It should be an easy java code, and I guess the result should be a serial of unsorted print statements from 0 to 100?

You can not be more wrong.

The result is a serial of unsorted print statements, but there are replications.

Why?

1, The first lesson i should learn is "NEVER WRITE A START METHOD IN THE THREAD OR RUNNABLE CONSTRUCTOR". The reason is when you call start() in a thread's constructor, in fact, most of the constructor has not finished yet, and the thread starts already because of your start() method. So wired things would happen, and you never image what these things could be.

2, The second tips, The ExecutorService will make existed thread two times due to the 'exec' instance will initialize the Thread instance again by calling constructor twice. This means Daemon thread will start again, so you will see the replicated print statements.

If you use any [1] or [2] part instead of the existed codes, you would get the right results.


REFERENCE:
----------------------
[1] http://stackoverflow.com/questions/5623285/java-why-not-to-start-a-thread-in-the-constructor-how-to-terminate