Daemon Thread in Java

Properties of Daemon Thread in Java

Below are some of the properties of the daemon thread:

• It has the lowest priority possible.
   
• If the JVM discovers a daemon thread running, it kills it and shuts it.
   
• Once all user threads have finished their work, the daemon thread cannot prevent the JVM from terminating.
   
• JVM shuts down after all user threads have completed running.
   
• Whether the daemon thread is active or inactive is unimportant to the JVM.
   
• JVM terminates the daemon thread whenever he encounters no user threads.
   
• Daemon threads are created before user threads are created and die after all other user threads die.

Default Nature of Daemon Thread 

The main thread is always non-daemon by default, but all the other threads will inherit their daemon characteristics from their parents. In other words, if a parent is a daemon, the child will likewise be a daemon, and if a parent is not a daemon, the child will also be a non-daemon.

One point to note is that all the daemon threads will be terminated automatically whenever the last non-daemon thread is terminated.

Methods of Daemon Thread

1. void setDaemon(boolean status):

This method involves designating the current thread either as a daemon or user thread. You can designate a user thread as a daemon using 'tU.setDaemon(true)', and conversely, you can designate a daemon thread as a user thread using 'tD.setDaemon(false)'

Syntax: 

public void setDaemon(boolean status)

Parameters:

• status (boolean): This parameter specifies whether the thread should be set as a daemon or a user thread. If true, the thread is set as a daemon thread; if false, the thread is designated as a user thread.

Exceptions:

• This method must be called before the thread starts, that is, before invoking the start() method on the thread.
• If you attempt to set the daemon status of a thread after it has already started, an IllegalThreadStateException will be thrown.

2. boolean isDaemon(): 

Utilized for verifying the current thread's daemon status, this method     returns 'true' if the thread is a daemon and 'false' otherwise.

Syntax: 

public boolean isDaemon()

Returns:

If the current thread is a daemon thread, the method returns true; otherwise, it returns false.

Code:

• Java

Java

public class IsDaemonExample {
   public static void main(String[] args) {
       Thread currentThread = Thread.currentThread();
       System.out.println("Is the current thread a daemon thread? " + currentThread.isDaemon());
       Thread daemonThread = new Thread(() -> {
           System.out.println("Daemon thread isDaemon: " + Thread.currentThread().isDaemon());
       });
       daemonThread.setDaemon(true);
       Thread userThread = new Thread(() -> {
           System.out.println("User thread isDaemon: " + Thread.currentThread().isDaemon());
       });
       daemonThread.start();
       userThread.start();
       try {
           daemonThread.join();
           userThread.join();
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
       System.out.println("Main thread isDaemon: " + currentThread.isDaemon());
   }
}

You can also try this code with Online Java Compiler

Run Code

Output:

Is the current thread a daemon thread? false
Daemon thread isDaemon: true
User thread isDaemon: false
Main thread isDaemon: false

Example of Daemon Thread in Java 

Examples of daemon threads in Java consist of garbage collection (gc) and finalizer.

When a program invokes the start() function, it creates a new thread and then calls the run() method. These methods are helpful whenever we want to create a thread. 

Let's now look at an example to understand the two methods of the daemon thread in Java.

• Java

Java

class DaemonThreadExample extends Thread{
    String name;
    // Constructor
    public DaemonThreadExample(String name){
        this.name = name;
    }

    // Calling the run method 
    public void run(){
        // Checking whether the thread is Daemon or not
        if (!Thread.currentThread().isDaemon()){
            System.out.println(this.name + " is User thread");
        }
        else{
            System.out.println(this.name + " is Daemon thread");
        }
    }

    public static void main(String[] args){

        // Creating objects
        DaemonThreadExample T1 = new DaemonThreadExample("First");
        DaemonThreadExample T2 = new DaemonThreadExample("Second");

        // Setting user thread T1 to Daemon
        T1.setDaemon(true);

        // Starting first two threads
        T1.start();
        T2.start();
    }
}

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Run Code

Output 

Explanation

In the above code of the daemon thread, the objects T1 and T2 are created with a default type, i.e., User thread. Once we call the setDaemon() method on the T1 object, it will change its type from a user thread to a daemon thread.  

Exceptions in Daemon Thread

Below are some exceptions that can occur while using the daemon thread:

Example:

Let’s now see an example to understand one of the examples of demon thread exceptions.

• Java

Java

class DaemonThreadExample extends Thread{
    String name;
    public DaemonThreadExample(String name){
        this.name = name;
    }

    public void run(){
        // Checking whether the thread is Daemon or not
        if (!Thread.currentThread().isDaemon()){
            System.out.println(this.name + " is User thread");
        }
        else{
            System.out.println(this.name + " is Daemon thread");
        }
    }

public static void main(String[] args){

        DaemonThreadExample T1 = new DaemonThreadExample("First");
        DaemonThreadExample T2 = new DaemonThreadExample("Second");

        // Starting thread T1
        T1.start();
        /* 
        	Setting user thread T1 to Daemon
        	will get an error at this line as the thread is already running 
        */
        T1.setDaemon(true);
    }
}

You can also try this code with Online Java Compiler

Run Code

Output 

Explanation

In the above code of the daemon thread, we tried to set object T1 as a daemon thread. But as the object T1 was already put in a running state using the start() method. We encountered an error stating that the thread's state is not valid to perform that action.

Daemon Thread Vs User Thread

Now, let’s see the difference between a daemon thread and a user thread.

Frequently Asked Questions

Why is the daemon thread used?

Daemon threads are used for background tasks that don't prevent the program from exiting. They terminate when all non-daemon threads finish.

What is a daemon thread?

A daemon thread is a background thread that runs without blocking the program's termination.

What is difference between user thread and daemon thread?

User threads are foreground threads initiated by the user, while daemon threads are background threads that support user threads.

What is the difference between daemon and regular thread?

Daemon threads terminate when the program exits, while regular threads don't.

Can we stop daemon thread?

Yes, daemon threads can be stopped, but they automatically terminate when the program exits.

Conclusion

This article discusses the topic of daemon threads in Java. We also discussed various methods in the daemon thread and some exceptions which we may encounter while using them. We hope this blog has helped you enhance your knowledge of daemon threads. If you want to learn more, then check out our articles.

• Threads and their types
• Benefits of Multithreading
• Processes and threads
• Open Source Operating System
   

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