Java Concurrency - Thread Priority

What is Thread Priority?

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The priority in the order of execution that the operating system assigns to each thread when scheduling threads. Threads with a higher priority are executed before threads with a lower priority.

The default thread priority in Java is 5, and you can specify the priority of thread execution in the range 1 to 10.

Generally, threads with higher priorities will have a higher chance of being executed than threads with lower priorities. We can use the setPriority() method of the Thread class to set the priority of a thread.

 

 

The reason for not recommending to use the thread priority 

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It is not advised to set the thread priority because the operating system's thread scheduling algorithm determines the actual order of execution. Therefore, a result of multiple threads set priorities can be different from what you expected.

 

// This priority setting is really not reliable since the result is not same as expected(t3 -> t2 -> t1)
public class TestExecuteOrder {

    static class MyRunnable implements Runnable {

        @Override
        public void run() {
            System.out.printf("The currently executing thread is :%s, priority:%s",
                    Thread.currentThread().getName(), Thread.currentThread().getPriority() + "\n");
        }
    }
    public static void main(String[] args) {
        Thread t1 = new Thread(new MyRunnable());
        t1.setPriority(1);

        Thread t2 = new Thread(new MyRunnable());
        t2.setPriority(5);

        Thread t3 = new Thread(new MyRunnable());
        t3.setPriority(10);

        t3.start();
        t2.start();
        t1.start();
    }
}

 

Result

The currently executing thread is :Thread-2, priority:10
The currently executing thread is :Thread-0, priority:1
The currently executing thread is :Thread-1, priority:5

 

If the priority of a thread is different from that of its thread group?

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The thread's priority will be invalidated and replaced with the maximum priority of the thread group.

public class ThreadGroupOrder {
    public static void main(String[] args) {
        ThreadGroup myThreadGroup = new ThreadGroup("myThreadGroup");
        myThreadGroup.setMaxPriority(6);
        Thread myThread = new Thread(myThreadGroup, "myThread");
        myThread.setPriority(8);
        System.out.println("myThreadGroup = " + myThreadGroup.getMaxPriority());
        System.out.println("myThread = " + myThread.getPriority());
    }
}

 

Result

myThreadGroup = 6
myThread = 6

 

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https://medium.com/@junfeng0828/java-concurrency-6-what-is-thread-priority-why-is-it-not-rcommended-to-use-it-69fd9c52112b