What is a daemon thread in Java?

任何人都可以告诉我Java中的守护线程是什么？

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A daemon thread is a thread that does not prevent the JVM from exiting when the program finishes but the thread is still running. An example for a daemon thread is the garbage collection.

You can use the setDaemon(boolean) method to change the Thread daemon properties before the thread starts.

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A few more points (Reference: Java Concurrency in Practice)

When a new thread is created it inherits the daemon status of its parent.

Normal thread and daemon threads differ in what happens when they exit. When the JVM halts any remaining daemon threads are abandoned:

finally blocks are not executed,

stacks are not unwound - the JVM just exits.

Due to this reason daemon threads should be used sparingly and it is dangerous to use them for tasks that might perform any sort of I/O.

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All the above answers are good. Here's a simple little code snippet, to illustrate the difference. Try it with each of the values of true and false in setDaemon .

public class DaemonTest {

    public static void main(String[] args) {
        new WorkerThread().start();

        try {
            Thread.sleep(7500);
        } catch (InterruptedException e) {
            // handle here exception
        }

        System.out.println("Main Thread ending") ;
    }

}

class WorkerThread extends Thread {

    public WorkerThread() {
        // When false, (i.e. when it's a user thread),
        // the Worker thread continues to run.
        // When true, (i.e. when it's a daemon thread),
        // the Worker thread terminates when the main 
        // thread terminates.
        setDaemon(true); 
    }

    public void run() {
        int count = 0;

        while (true) {
            System.out.println("Hello from Worker "+count++);

            try {
                sleep(5000);
            } catch (InterruptedException e) {
                // handle exception here
            }
        }
    }
}

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