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
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.

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