Java Daemon Thread

In Java, daemon threads are low-priority threads that run in the background to perform tasks such as garbage collection or provide services to user threads. The life of a daemon thread depends on the mercy of user threads, meaning that when all user threads finish their execution, the Java Virtual Machine (JVM) automatically terminates the daemon thread.

To put it simply, daemon threads serve user threads by handling background tasks and have no role other than supporting the main execution.

Some examples of daemon threads in Java include garbage collection (gc) and finalizer. These threads work silently in the background, performing tasks that support the main execution without interfering with the user’s operations.

Properties of Java Daemon Thread

Aspect	Description
No Preventing JVM Exit	Daemon threads cannot prevent the JVM from exiting when all user threads finish their execution. If all user threads complete their tasks, the JVM terminates itself, regardless of whether any daemon threads are running.
Automatic Termination	If the JVM detects a running daemon thread, it terminates the thread and subsequently shuts it down. The JVM does not check if the daemon thread is actively running; it terminates it regardless.
Low Priority	Daemon threads have the lowest priority among all threads in Java.

Default Nature of Daemon Thread

By default, the main thread is always a non-daemon thread. However, for all other threads, their daemon nature is inherited from their parent thread. If the parent thread is a daemon, the child thread is also a daemon, and if the parent thread is a non-daemon, the child thread is also a non-daemon.

Note: Whenever the last non-daemon thread terminates, all the daemon threads will be terminated automatically.

Methods of Daemon Thread

1. `setDaemon(boolean status)`

This method marks the current thread as a daemon thread or user thread.

public final void setDaemon(boolean on)

Parameter:

on: If true, marks this thread as a daemon thread.

Exceptions:

IllegalThreadStateException: If the thread is already started.
SecurityException: If the current thread cannot modify this thread.

Example:

// Using setDaemon() Method
 
// Class extending Thread Class
class DaemonThread extends Thread {
    @Override
    public void run() {
        System.out.println("Running the Daemon Thread");
    }
}
 
// Driver Class
public class Coding75 {
    public static void main (String[] args) {
        DaemonThread t1 = new DaemonThread();
        t1.setDaemon(true);
        t1.start();
    }
}

Note: The JVM may shut down before the daemon thread executes. To prevent this, use Thread.sleep().

2. `isDaemon()`

This method checks whether the current thread is a daemon thread.

public final boolean isDaemon()

Returns: true if the thread is a daemon thread, false otherwise.

Example:

// Usage of setDaemon() and isDaemon() method
class DaemonThread extends Thread {
    public DaemonThread(String name) {
        super(name);
    }
 
    @Override
    public void run() {
        if (Thread.currentThread().isDaemon()) {
            System.out.println(getName() + " is Daemon thread");
        } else {
            System.out.println(getName() + " is User thread");
        }
    }
}
 
// Driver Class
public class Coding75 {
    public static void main(String[] args) {
        DaemonThread t1 = new DaemonThread("t1");
        DaemonThread t2 = new DaemonThread("t2");
        DaemonThread t3 = new DaemonThread("t3");
 
        t1.setDaemon(true);
        t1.start();
        t2.start();
 
        t3.setDaemon(true);
        t3.start();
    }
}

Output:

t1 is Daemon thread
t2 is User thread

Common Issues with Daemon Threads

Case 1: Output Not Printed

If the main thread finishes execution before the daemon thread starts, the daemon thread will not execute.

Solution:

Use Thread.sleep() to ensure JVM doesn't terminate immediately.

// Class extending Thread Class
class DaemonThread extends Thread {
    @Override
    public void run() {
        System.out.println("Running the Daemon Thread");
    }
}
 
// Driver Class
public class Coding75 {
    public static void main (String[] args) {
        DaemonThread t1 = new DaemonThread();
        t1.setDaemon(true);
        t1.start();
        
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Output:

Running the Daemon Thread

Case 2: State Changed After Starting a Thread

Calling setDaemon(true) after starting a thread will throw IllegalThreadStateException.

Solution:

Avoid calling setDaemon(true) after starting a thread. Alternatively, use a try-catch block to handle exceptions.

// Usage of exception in Daemon Thread
class DaemonThread extends Thread {
    public DaemonThread(String name) {
        super(name);
    }
 
    @Override
    public void run() {
        System.out.println("Thread name: " + Thread.currentThread().getName());
        System.out.println("Check if its DaemonThread: " + Thread.currentThread().isDaemon());
    }
}
 
// Main Class
public class Coding75 {
    public static void main(String[] args) {
        DaemonThread t1 = new DaemonThread("t1");
        DaemonThread t2 = new DaemonThread("t2");
 
        try {
            t1.start();
            t1.setDaemon(true); // IllegalThreadStateException
            t2.start();
        } catch (IllegalThreadStateException e) {
            System.out.println("Illegal Thread State Exception Thrown");
        }
    }
}

Output:

Illegal Thread State Exception Thrown
Thread name: t1
Check if its DaemonThread: false

Daemon vs User Threads

Feature	Daemon Threads	User Threads
Priority	Low	Normal or High
JVM Behavior	JVM exits when only daemon threads remain	JVM waits for user threads to complete
Usage	Used for background tasks like garbage collection	Used for main program logic

Daemon threads provide essential background services, but they should not be relied upon for critical tasks.

Java Sleep() Thread Java Files