java - join() 方法和《完整 Java 引用》第 9 版

Question

这个问题已经有答案了:

Getting different thread order even time I run my program (3 个回答)

已关闭 7 年前。

// Using join() to wait for threads to finish.
class NewThread implements Runnable {
    String name; // name of thread
    Thread t;
    NewThread(String threadname) {
        name = threadname;
        t = new Thread(this, name);
        System.out.println("New thread: " + t);
        t.start(); // Start the thread
    }
    // This is the entry point for thread.
    public void run() {
        try {
            for (int i = 5; i > 0; i--) {
                System.out.println(name + ": " + i);
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            System.out.println(name + " interrupted.");
        }
        System.out.println(name + " exiting.");
    }
}
class DemoJoin {
    public static void main(String args[]) {
        NewThread ob1 = new NewThread("One");
        NewThread ob2 = new NewThread("Two");
        NewThread ob3 = new NewThread("Three");

        System.out.println("Thread One is alive: " + ob1.t.isAlive());
        System.out.println("Thread Two is alive: " + ob2.t.isAlive());
        System.out.println("Thread Three is alive: " + ob3.t.isAlive());
        // wait for threads to finish
        try {
            System.out.println("Waiting for threads to finish.");
            ob1.t.join();
            ob2.t.join();
            ob3.t.join();
        } catch (InterruptedException e) {
            System.out.println("Main thread Interrupted");
        }
        System.out.println("Thread One is alive: " + ob1.t.isAlive());
        System.out.println("Thread Two is alive: " + ob2.t.isAlive());
        System.out.println("Thread Three is alive: " + ob3.t.isAlive());
        System.out.println("Main thread exiting.");
    }
}

嗨，那里。我对《完整的 Java 引用 - 第 9 版》中的这段和平代码有疑问。每次运行此代码时我都会得到不同的输出。示例如下:

New thread: Thread[One,5,main]
New thread: Thread[Two,5,main]
New thread: Thread[Three,5,main]
One: 5
Thread One is alive: true
Thread Two is alive: true
Thread Three is alive: true
Waiting for threads to finish.
Two: 5
Three: 5
One: 4
Three: 4
Two: 4
One: 3
Two: 3
Three: 3
One: 2
Two: 2
Three: 2
One: 1
Three: 1
Two: 1
One exiting.
Two exiting.
Three exiting.
Thread One is alive: false
Thread Two is alive: false
Thread Three is alive: false
Main thread exiting.

书中的输出是:

 New thread: Thread[One,5,main]
 New thread: Thread[Two,5,main]
 New thread: Thread[Three,5,main]
 Thread One is alive: true
 Thread Two is alive: true
 Thread Three is alive: true
 Waiting for threads to finish.
 One: 5
 Two: 5
 Three: 5
 One: 4
 Two: 4
 Three: 4
 One: 3
 Two: 3
 Three: 3
 One: 2
 Two: 2
 Three: 2
 One: 1
 Two: 1
 Three: 1
 Two exiting.
 Three exiting.
 One exiting.
 Thread One is alive: false
 Thread Two is alive: false
 Thread Three is alive: false
 Main thread exiting.

现在，我了解线程是如何工作的，但我不太明白为什么我的新线程与我的主线程同时启动。任何人都可以帮助我吗？泰。

编辑:主线程不应该运行直到 join()、sleep() 或其他熟悉的方法，然后，当该方法“发生”时，Thread::running 应该从就绪队列中取出下一个线程吗？

编辑:此代码:

// Create a second thread.
class NewThread implements Runnable {
 Thread t;
 NewThread() {
 // Create a new, second thread
 t = new Thread(this, "Demo Thread");
 System.out.println("Child thread: " + t);
 t.start(); // Start the thread
 }
 // This is the entry point for the second thread.
 public void run() {
 try {
 for(int i = 5; i > 0; i--) {
 System.out.println("Child Thread: " + i);
 Thread.sleep(500);
 }
 } catch (InterruptedException e) {
 System.out.println("Child interrupted.");
 }
 System.out.println("Exiting child thread.");
 }
}

class ThreadDemo {
 public static void main(String args[ ] ) {
 new NewThread(); // create a new thread
 try {
 for(int i = 5; i > 0; i--) {
 System.out.println("Main Thread: " + i);
 Thread.sleep(1000);
 }
 } catch (InterruptedException e) {
 System.out.println("Main thread interrupted.");
 }
 System.out.println("Main thread exiting.");
 }
}

始终保持相同的工作方式。怎么会？这是相同的东西，第二个线程在构造函数中启动()，但此代码每次都会给出相同的结果:

Child thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

Answer 1

因为一旦调用 start()，新线程就开始运行。

主线程创建第一个线程并启动它。现在，这是主线程启动其他两个线程并针对第一个线程开始倒计时打印等待消息之间的竞争。

谁赢得了比赛？任意且取决于硬件。