Java创建线程

文章目錄

1. 1. 实现Runnable接口
2. 2. 继承Thread类

在Java中创建线程有两中方法，一种是实现Runnable接口，一种是继承Thread类

实现Runnable接口

1. 将任务代码迁移到实现Runnable接口的类的run方法中

   class MyRunnable implements Runnable {
       public void run() {
           task code
       }
   }

2. 创建一个类对象:
   Runnable r = new MyRunnable()

3. 由Runnable创建一个Thread对象
   Thread t = new Thread(r)
4. 启动线程
   t.start()
   完整例子如下:

   public class MyRunnable implements Runnable {
       @Override
       public void run() {
           System.out.println("Child: " + Thread.currentThread().getId());
           
       }
       public static void main(String[] arg) {
           for (int i = 0; i < 5; i++) {
               Runnable r = new MyRunnable();
               Thread t = new Thread(r);
               t.start();
           }
           System.out.println("Parent: " + Thread.currentThread().getId());
       }
   }

输出结果如下：
Child: 8
Child: 9
Child: 10
Child: 11
Parent: 1
Child: 12
这里不能直接调用run方法，因为这样不会创建新的线程:

public class MyRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Child: " + Thread.currentThread().getId());
    }
    public static void main(String[] arg) {
        for (int i = 0; i < 5; i++) {
            Runnable r = new MyRunnable();
//          Thread t = new Thread(r);
//          t.start();
            r.run();
        }
        System.out.println("Parent: " + Thread.currentThread().getId());
    }
}

输出结果如下：
Child: 1
Child: 1
Child: 1
Child: 1
Child: 1
Parent: 1
可以看到id都是一样的,也就是这里没有创建新的线程.

继承Thread类

class MyThread extends Thread {
    public void run() {
        task code
    }
}

完整例子如下:

public class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Child: " + Thread.currentThread().getId()); 
    }
    public static void main(String[] arg) {
        for (int i = 0; i < 5; i++) {
            Thread t = new MyThread();
            t.start();
        }
        System.out.println("Parent: " + Thread.currentThread().getId());
    }
}

输出结果如下：
Child: 8
Child: 9
Child: 10
Child: 11
Parent: 1
Child: 12
这里不能直接调用t.run()，因为这样不会创建新的线程

public class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Child: " + Thread.currentThread().getId()); 
    }
    public static void main(String[] arg) {
        for (int i = 0; i < 5; i++) {
            Thread t = new MyThread();
//          t.start();
            t.run();
        }
        System.out.println("Parent: " + Thread.currentThread().getId());
    }
}

结果如下：
Child: 1
Child: 1
Child: 1
Child: 1
Child: 1
Parent: 1
可以看到id都是一样的。
查看Thread类的源码就会发现问题的所在.

public synchronized void start() {
    /**
     * This method is not invoked for the main method thread or "system"
     * group threads created/set up by the VM. Any new functionality added
     * to this method in the future may have to also be added to the VM.
     *
     * A zero status value corresponds to state "NEW".
     */
    if (threadStatus != 0)
        throw new IllegalThreadStateException();

    /* Notify the group that this thread is about to be started
     * so that it can be added to the group's list of threads
     * and the group's unstarted count can be decremented. */
    group.add(this);

    boolean started = false;
    try {
        start0();
        started = true;
    } finally {
        try {
            if (!started) {
                group.threadStartFailed(this);
            }
        } catch (Throwable ignore) {
            /* do nothing. If start0 threw a Throwable then
              it will be passed up the call stack */
        }
    }
}

private native void start0();

在start方法中调用native方法start0()，虽然看不到它的具体实现，但可以推测这里创建了新的线程，然后调用run方法。而run方法中，则没有创建线程相关的代码

public void run() {
    if (target != null) {
        target.run();
    }
}

关于两种方法的区别，可以看http://stackoverflow.com/questions/541487/implements-runnable-vs-extends-thread, 推荐使用实现Runnable接口的方法。