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 |
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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. |
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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. |
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Low Priority | Daemon threads have the lowest priority among all threads in Java. |
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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. |
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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. Setting a user thread as a daemon can be done using the'tU.setDaemon(true)', while setting a daemon thread as a user thread can be done using the 'tD.setDaemon(false)'.
Syntax:
public final void setDaemon(boolean on)
Parameter: on: If true, marks this thread as a daemon thread.
Exceptions:
- IllegalThreadStateException: if only this thread is active.
- SecurityException: if the current thread cannot modify this thread.
Example:
Java // Using setDaemon() Method // Class extending Thread Class class DaemonThread extends Thread{ // Override the run Method @Override public void run(){ System.out.println("Running the Daemon Thread"); } } // Driver Class public class Geeks { public static void main (String[] args) { DaemonThread t1 = new DaemonThread(); t1.setDaemon(true); t1.start(); } } Note: Yes, it will not show the output as JVM shuts down before the run() method is executed. The solution of the Problem is mentioned below after the isDaemon() method.
2. isDaemon()
This method is used to check that the current thread is a daemon. It returns true if the thread is Daemon. Else, it returns false.
Syntax:
public final boolean isDaemon()
Returns: This method returns true if this thread is a daemon thread; false otherwise
Example:
Java // Usage of setDaemon() and isDaemon() method class DaemonThread extends Thread { // Changing the name of Threads public DaemonThread(String name) { super(name); } // Overriding the run method public void run() { // Checking whether the thread is Daemon or not if(Thread.currentThread().isDaemon()) { System.out.println(getName() + " is Daemon thread"); } else { System.out.println(getName() + " is User thread"); } } } // Driver Class public class Geeks { public static void main(String[] args) { DaemonThread t1 = new DaemonThread("t1"); DaemonThread t2 = new DaemonThread("t2"); DaemonThread t3 = new DaemonThread("t3"); // Setting user thread t1 to Daemon t1.setDaemon(true); // Starting first 2 threads t1.start(); t2.start(); // Setting user thread t3 to Daemon t3.setDaemon(true); t3.start(); } } Outputt1 is Daemon thread t2 is User thread
Common Issues with Daemon Threads
Case 1: Output not Printed
This case arises when main thread which is not a Daemon Thread is executed and after it's execution JVM stops working so the daemon thread is not executed.
Solution: To, avoid such conditions we need to make main thread sleep for few seconds so that daemon thread is executed.
Example:
Java // Class extending Thread Class class DaemonThread extends Thread { // Override the run Method @Override public void run(){ System.out.println("Running the Daemon Thread"); } } // Driver Class public class Geeks { public static void main (String[] args) { DaemonThread t1 = new DaemonThread(); t1.setDaemon(true); t1.start(); // Ensuring JVM doesn't terminate immediately try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } } OutputRunning the Daemon Thread
So, the Output is finally printing , now let's move to the next issue.
Case 2: State Changed After Starting a Thread
If you call the setDaemon() method after starting the thread, it would throw IllegalThreadStateException.
Runtime exception thrown in such Case:
Exception in thread "main" java.lang.IllegalThreadStateException at java.lang.Thread.setDaemon(Thread.java:1352) at DaemonThread.main(DaemonThread.java:19)
Solution:
Java // Usage of exception in Daemon() Thread // Class Extending Thread Class class DaemonThread extends Thread { public DaemonThread(String name) { super(name); } // Override the run method 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 Geeks { public static void main(String[] args) { DaemonThread t1 = new DaemonThread("t1"); DaemonThread t2 = new DaemonThread("t2"); try { t1.start(); // Exception as the thread is already started t1.setDaemon(true); t2.start(); } catch (IllegalThreadStateException e) { System.out.println("Illegal Thread State Exception Thrown"); } } } OutputIllegal Thread State Exception Thrown Thread name: t1 Check if its DaemonThread: false
This clearly shows that we cannot call the setDaemon() method after starting the thread.
Note: the above try catch can save you from throwing exception cases. But, the best solution for this is to avoid such conditions.
Daemon vs User Threads
- Priority: When only daemon threads remain in a process, the JVM exits. This makes sense because when only daemon threads are running, there is no need for a daemon thread to provide a service to another thread.
- Usage: daemon threads are primarily used to provide background support to user threads. They handle tasks that support the main execution without interfering with the user's operations.
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