java 线程中run 和start方法

Java线程中run和start方法

1.两者的区别说明

start()方法：start的作用是开启一个新线程执行新开启线程的run()方法，start方法不能被重复调用

run()方法：run方法的就相当于一个普通的成员方法，直接调用在当前线程中处理run方法的主体，可以重复调用

2.run方法和start方法示例

	public static void main(String args[]) {
		Thread mythread=new ThreadDemo("mythread");
		mythread.run();
		mythread.run();
		mythread.start();}

public class ThreadDemo extends Thread{
	 public ThreadDemo(String name) {
		  super(name);
         }
         public void run(){
	      System.out.println(Thread.currentThread().getName()+" is running");
	 } 
}

运行结果：

main is running
main is running
mythread is running

可以看到，当直接调用run方法时时在当前线程执行run方法，当前线程的名称是main，此时可以进行多次调用

当调用start方法时，将新开启一个线程执行run方法的主体，此时如果重复调用则会出现以下异常信息

Exception in thread "main" mythread is running
java.lang.IllegalThreadStateException
	at java.lang.Thread.start(Thread.java:705)
	at com.test.demo.thread.ThreadTest.main(ThreadTest.java:13)

我们查看Thread类的源码

      /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * 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 */
            }
        }
    }

从以上红色代码部分不难发现，当threadStatus的状态不为0时都抛出 IllegalThreadStateException()异常，而从注释部分我们可以看出只有当线程的状态为新建 （NEW）时，其threadStatus的值才为0  因此当连续调用线程的start方法则会抛出上面的异常。关于线程的几种状态我们将在下一张详细讲述。