Daemon thread in Java

Daemon thread is a low priority thread that runs in background to perform tasks such as garbage collection.

Properties:

• They can not prevent the JVM from exiting when all the user threads finish their execution.
• JVM terminates itself when all user threads finish their execution
• If JVM finds running daemon thread, it terminates the thread and after that shutdown itself. JVM does not care whether Daemon thread is running... or not.
• It is an utmost low priority thread.

Methods:

• void setDaemon(boolean status): This method is used to mark the current thread as daemon thread or user thread. For example if I have a user thread tU then tU.setDaemon(true) would make it Daemon thread. On the other hand if I have a Daemon thread tD then by calling tD.setDaemon(false) would make it user thread.
  Syntax:
  public final void setDaemon(boolean on)
  parameters:
  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.
• boolean isDaemon():
  This method is used to check that current is 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

// Java program to demonstrate the usage of
// setDaemon() and isDaemon() method.
public class DaemonThread extends Thread
{
public DaemonThread(String name){
super(name);
}

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");
}
}

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();
}
}
Output:

t1 is Daemon thread
t3 is Daemon thread
t2 is User thread
Exceptions in Daemon thread

If you call the setDaemon() method after starting the thread, it would throw IllegalThreadStateException.

// Java program to demonstrate the usage of
// exception in Daemon() Thread
public class DaemonThread extends Thread
{
public void run()
{
System.out.println("Thread name: " + Thread.currentThread().getName());
System.out.println("Check if its DaemonThread: "
+ Thread.currentThread().isDaemon());
}

public static void main(String[] args)
{
DaemonThread t1 = new DaemonThread();
DaemonThread t2 = new DaemonThread();
t1.start();

// Exception as the thread is already started
t1.setDaemon(true);

t2.start();
}
}
Runtime exception:

Exception in thread "main" java.lang.IllegalThreadStateException
at java.lang.Thread.setDaemon(Thread.java:1352)
at DaemonThread.main(DaemonThread.java:19)
Output:

Thread name: Thread-0
Check if its DaemonThread: false
This clearly shows that we cannot call the setDaemon() method after starting the thread.

Daemon vs User Threads

• Priority: When the only remaining threads in a process are daemon threads, the interpreter exits. This makes sense because when only daemon threads remain, there is no other thread for which a daemon thread can provide a service.
• Usage: Daemon thread is to provide services to user thread for background supporting task.