What is a guardian thread

Daemon threads are different from user threads , The user thread is the thread we created manually , The daemon thread is a thread that provides a general service in the background when the program is running .

I use my own vernacular to summarize ：
Why is it called a daemon ？ What is it guarding ？
It guards the thread that created this thread , So the creator is the protected object .

So how does the created guard thread guard ？
The daemons have the feature of automatically ending their life cycle , Non Guardian threads don't have

eg：
The guardian thread ：

    public static void main(String[] args) throws InterruptedException {
    
        Thread t1 = new Thread(()-> {
    
                while (true) {
    
                    try {
    
                        Thread.sleep(1000);
                        System.out.println(" I'm a child thread ( User threads .I am running");
                    } catch (Exception e) {
    
                    }
                }
        });
        // Marked as a daemon thread 
        t1.setDaemon(true);
        // Start thread 
        t1.start();

        Thread.sleep(3000);
        System.out.println(" Main thread execution completed ...");
    }

You can find that after marking as a guard thread , Main thread destroy stop , The daemons are destroyed together （ Self destruction ）.

eg：
Non-daemon thread ：
Get rid of t1.setDaemon(true) Guardian mark effect

therefore , When the main thread exits ,JVM And then it goes out of operation , The daemons are also recycled , Even if it's a dead circle . If it's a user thread , It's going to stop in an endless loop . This is the difference between a daemon thread and a non daemon thread .

Application scenarios

Through the example code above , I believe you have understood the difference between daemons and ordinary threads , that , Let's talk about why we need daemons , And when to use , What is its application scenario ？

above , We already know that , If JVM There is no running non - daemonic thread in , This is the time ,JVM Will quit . let me put it another way , The daemons have the feature of automatically ending their life cycle , The non - daemons do not have this feature .

JVM The garbage collection thread in is a typical Guardian thread , If we don't have this feature , What will happen ？

When JVM To exit , Because the garbage collection thread is still running , Causes the program to be unable to exit , This is awkward ！！！ thus it can be seen , The importance of daemons .

Generally speaking , Daemons are often used to perform background tasks , But what? , You want the program to exit , Or say JVM Exit time , Threads can automatically shut down , here , Daemons are your first choice .