Causes and solutions of deadlock in threads

Catalog

The deadlock problem

The dining problem of philosophers

  Avoid deadlock

---

The deadlock problem

Deadlock ： The circular waiting problem of locking objects , Use jconsole To check for deadlocks

Deadlock is a serious BUG. The thread that causes a program " stuck ", Not working properly ！

/**
 *  Deadlock 
 */
public class DeadLockTest {
    public static void main(String[] args) {
        Object projectLock = new Object();
        Object songLock = new Object();
        Thread t1 = new Thread(() -> {
            synchronized (songLock) {
                System.out.println(" Sing first , Write the project again ");
                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (projectLock) {
                    System.out.println(" The song is over , You can write the project ");
                }
            }
        }, "A");
        Thread t2 = new Thread(() -> {
            synchronized (projectLock) {
                System.out.println(" Write the project first , Sing again ");
                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (songLock) {
                    System.out.println(" Finish the project , A song ");
                }
            }
        }, "B");
        t1.start();
        t2.start();
    }
}

Use jconsole Check for deadlocks

The dining problem of philosophers

There's a table , Surrounded by philosophers , There is a plate of spaghetti in the middle of the table .

Between two philosophers , There is a chopstick .

Every philosopher does only two things ： Think about life perhaps Eat noodles .

When I think about life, I will put down my chopsticks , When you eat noodles, you will pick up the chopsticks on the left and right sides ( First pick up the left , Then pick up the right ).
If philosophers find that chopsticks can't be picked up ( Occupied by someone else ), It will block and wait .

[ Key points ] Suppose at the same time , The five philosophers picked up the chopsticks on their left hand side at the same time , Then try holding right-handed chopsticks , You will find that the chopsticks on your right hand are occupied . Because philosophers don't give in to each other , This is when Deadlock .

  Avoid deadlock

Deadlock Relieving , Just break the condition of this cycle waiting , Multiple threads acquire resources without forming a ring .

Let's go one step at a time , Avoid deadlock .

/**
 *  Avoid deadlock 
 */
public class DeadLockTest {
    public static void main(String[] args) {
        Object projectLock = new Object();
        Object songLock = new Object();
        Thread t1 = new Thread(() -> {
            synchronized (projectLock) {
                System.out.println(" The song is over , You can write the project ");
                synchronized (songLock) {
                    System.out.println(" Sing first , Write the project again √");
                    try {
                        Thread.sleep(300);
                    } catch (InterruptedException e) {
                        throw new RuntimeException(e);
                    }

                }
            }
        }, "A");
        Thread t2 = new Thread(() -> {
            synchronized (projectLock) {
                System.out.println(" Write the project first , Sing again ");
                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (songLock) {
                    System.out.println(" Finish the project , A song ");
                }
            }
        }, "B");
        t1.start();
        t2.start();
    }
}