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Java Thread多线程的start()和run()

时间:2022-06-29 02:55:28 编辑:袖梨 来源:一聚教程网

1.start()方法来启动线程,真正实现了多线程运行,这时无需等待run方法体代码执行完毕而直接继续执行下面的代码:
 
通过调用Thread类的start()方法来启动一个线程,
这时此线程是处于就绪状态,
并没有运行。
然后通过此Thread类调用方法run()来完成其运行操作的,
这里方法run()称为线程体,
它包含了要执行的这个线程的内容,
Run方法运行结束,
此线程终止,
而CPU再运行其它线程,

2.run()方法当作普通方法的方式调用,程序还是要顺序执行,还是要等待run方法体执行完毕后才可继续执行下面的代码:
而如果直接用Run方法,
这只是调用一个方法而已,
程序中依然只有主线程--这一个线程,
其程序执行路径还是只有一条,
这样就没有达到写线程的目的。
通过implements Runnable接口来实现多线程

示例代码:

 代码如下 复制代码

package com;

public class TestRunnable implements Runnable{
 private int num = 0;
 public static void main(String[] args) {
  for(int i=0; i<5; i++){
   new Thread(new TestRunnable(i)).start();
  }
 }

 public TestRunnable(int num) {
  this.num = num;
 }

 //实现方法
 public void run(){
  System.out.println("线程:"+this.num);
 }
}

上例打印出来的结果是无序的:
1 4 3 2 0

也可能是别的顺序,通过Thread类调用start,真正实现了多线程,不用等待run方法执行完毕再起新线程。

如果将

 代码如下 复制代码

new Thread(new TestRunnable(i)).start()

改成

new Thread(new TestRunnable(i)).run(),

则打印出来的结果是由顺序的,也就是:

0 1 2 3 4


通过Thread类直接调用run方法,实际上就是调用TestRunnable类的run方法,查看Thread类的run方法:

 代码如下 复制代码

 


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

发现Thread类run方法就是调用了TestRunnable的run(),所以通过

 代码如下 复制代码

new Thread(new TestRunnable(i)).run()

并不是多线程,其执行线程还是主线程。

 

下面再将Thread类的start方法贴出来,可与上面Thread类的run方法做个对比:

 代码如下 复制代码

 /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the run method of this thread.
     *


     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * start method) and the other thread (which executes its
     * run method).
     *


     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    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 */
            }
        }
    }

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