What is the Daemon Thread in Java?
- The thread executing in the running program’s background is called the daemon thread in Java.
- The garbage collector is one example of Daemon Thread in java.
- They are used to provide support for non-daemon threads in java.
- Example: If the main thread runs with low memory then JVM runs the garbage collector to destroy the useless objects and free some memory.
- Usually, this threads run with low priority but based on our requirement daemon threads can also run with high priority.
- We can check if a thread is a daemon or not by using the isDaemon() method.
- We can set any thread as a daemon thread using the setDaemon(boolean b) method.
- Changing the daemon nature is possible before starting a thread else we will get Runtime Exception as IllegalThreadStateException.
Let’s see the example below for Daemon Thread in Java. (Output sequence for the following program can be different)
class Test {
static int i = 0;
public static void main(String args[]) throws InterruptedException {
DaemonThread d = new DaemonThread();
MyThread t = new MyThread();
d.setDaemon(true);
System.out.println(“–Is demonThread d: ” + d.isDaemon());
System.out.println(“–Is demonThread t: ” + t.isDaemon());
d.start();
t.start();
Thread.currentThread().sleep(500);
System.out.println(Test.i);
}
}
class DaemonThread extends Thread {
public void run() {
while(true) {
Test.i = Test.i + 1;
}
}
}
class MyThread extends Thread {
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(“Running MyThread ” + i);
}
}
}
static int i = 0;
public static void main(String args[]) throws InterruptedException {
DaemonThread d = new DaemonThread();
MyThread t = new MyThread();
d.setDaemon(true);
System.out.println(“–Is demonThread d: ” + d.isDaemon());
System.out.println(“–Is demonThread t: ” + t.isDaemon());
d.start();
t.start();
Thread.currentThread().sleep(500);
System.out.println(Test.i);
}
}
class DaemonThread extends Thread {
public void run() {
while(true) {
Test.i = Test.i + 1;
}
}
}
class MyThread extends Thread {
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(“Running MyThread ” + i);
}
}
}
Output:
–Is demonThread d: true–Is demonThread t: false
Running MyThread 0
Running MyThread 1
Running MyThread 2
Running MyThread 3
Running MyThread 4
1069550001
- By default, the main thread is always a non-daemon thread.
- For remaining all threads daemon nature will be inherited from parent to child. That is if a parent is a daemon thread then a child is also a daemon.
- It’s impossible to change the daemon nature of the main thread since it is already stated by JVM at the beginning. After starting the thread daemon nature can not be changed.
- Whenever the last non-daemon thread is terminated automatically all daemon threads will be terminated irrespective of their position.
How to Stop a Thread?
- By using the stop() method of the Thread class we can stop the execution of the Thread. This method is deprecated.
- Syntax: public final void stop()
- Thread immediately enters into a dead state after the stop method.
- Since this method is deprecated it’s not recommended to use.
How to suspend & resume a Thread?
- A thread can suspend another thread by using the suspend() & resume by using the resume() method of thread class.
- Syntax for suspend: public final suspend void suspend();
- Syntax for resume: public final void resume();
