setDaemon method

[Arnab Saha]

setDaemon(true);
What does this statement do in Multithreading scenario.

[Srinivasa Raghavan]

Please have a look at this thread.
[ November 09, 2005: Message edited by: Srinivasa Raghavan ]

[Arnab Saha]

Please have a look at the code:

public class A extends Thread {
A() {
setDaemon(true);
}

public void run() {
(new B()).start();
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("A done");
}

class B extends Thread {
public void run() {
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("B done");
}
}

public static void main(String[] args) {
(new A()).start();
}
}
*************
No o/p is printed

But if in the sleep calls 60000 is replaced by 10
We get:
A done
B done.
Can u explain it?

[Lakshmanan Arunachalam]

Daemon threads are a kind of threads which provide services to other user(non-daemon) threads. When there is no live user threads, then JVM will abruptly kill the daemon threads. As log as the pgm has some live threads, daemon wont be killed abruptly. In your pgm, main thread is the user thread, it will end as soon as it starts. Hence the JVM kills the other daemon threads immly after the ending of main thread.

Can you make one of your thread, preferably B as non -daemon by calling setDaemon(false) inside the run method.

[Lakshmanan Arunachalam]

 
public class ADaemon extends Thread {
ADaemon() {
setDaemon(true);
}
 
public void run() {
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("A done");
}
 

 
public static void main(String[] args) {
(new ADaemon()).start();
BNonDaemon b = new BNonDaemon();
b.start();
}
}
class BNonDaemon extends Thread {
BNonDaemon()
{
setDaemon(false);
}
public void run() {
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("B done");
}
}
 

Sample 2:

 
public class ADaemon extends Thread {
ADaemon() {
setDaemon(true);
}
 
public void run() {
(new BNonDaemon()).start();
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("A done");
}
 
class BNonDaemon extends Thread {
BNonDaemon()
{
setDaemon(false);
}
public void run() {
try {
Thread.sleep(60000);
} catch (InterruptedException x) {}
System.out.println("B done");
}
}
 

public static void main(String[] args) {
(new ADaemon()).start();
 
}
}