[thread / multithread] execution sequence of threads

Preface

public class ThreadSort {
    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            System.out.println(" Threads 1");
        });
        Thread thread2 = new Thread(() -> {
            System.out.println(" Threads 2");
        });
        Thread thread3 = new Thread(() -> {
            System.out.println(" Threads 3");
        });
        thread1.start();
        thread2.start();
        thread3.start();
    }
}

The code is simple , Three threads are declared , Print the text separately , Students who are idle and have nothing to do can run for many times , The order of each print , Not at all 123, And out of order , So this means ：start() The sequence of method calls cannot determine the execution order of the thread

Threads execute sequentially

The scenario of executing threads in code order is not uncommon , Let's look at the code above , How to make him according to start() The call is executed in sequence ？

The answer is ： Use Thread Class join() Method to ensure the execution order

public class ThreadSort {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread(() -> {
            System.out.println(" Threads 1");
        });
        Thread thread2 = new Thread(() -> {
            System.out.println(" Threads 2");
        });
        Thread thread3 = new Thread(() -> {
            System.out.println(" Threads 3");
        });
        thread1.start();
        thread1.join();

        thread2.start();
        thread2.join();
        
        thread3.start();
        thread3.join();
    }
}

Then run it many times and you will find , The result is always a thread 123 The order of

join() Method explanation

To explain how he realized the sequence , Then you need to enter the source code to have a look , Attach source code

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

Look at the source code , In fact, you can see , This join Method used synchronized To modify , This means that this method can only be called by one instance or method at a time , But when we call above, there is no input parameter , Say it will come if (millis == 0) In this logic , This method is mainly used to determine whether the thread has been started and is active , If it is active , Call wait() Method , So let's continue to look at wait() Method

public final native void wait(long timeout) throws InterruptedException;

Call this wait() when , Makes the main thread wait , Wait for the execution of the child thread to complete before continuing the downward execution