Thread’s start method and run method

工作中用到了Thread，一开始用错了，仔细研究了一下，稍作整理。

前言，今天写代码居然这样写的

new Thread() {

	@Override
	public void run() {
		System.out.println("test");
	}

}.run();

天真得以为这样这样会新开启一个线程执行，可是打印了线程的信息之后，发现还是在主线程中。仔细一想，确实我这樣做和调用一个新的Object的toString方法有区别么，根本不会产生神马新的线程。


当然我们的主要实现是在run里面，这也是Thread唯一implements Runnable的一个方法。


于是我们调用start方法，可以在新线程中执行run中的处理逻辑，于是会一位start方法调用了run方法


然后却又不是这样


start的方法代码

/**
     * Starts the new Thread of execution. The <code>run()</code> method of
     * the receiver will be called by the receiver Thread itself (and not the
     * Thread calling <code>start()</code>).
     *
     * @throws IllegalThreadStateException if the Thread has been started before
     *
     * @see Thread#run
     */
    public synchronized void start() {
        if (hasBeenStarted) {
            throw new IllegalThreadStateException("Thread already started."); // TODO Externalize?
        }

        hasBeenStarted = true;

        VMThread.create(this, stackSize);
    }

请注意上面的javadoc The code run() method of the receiver will be called by the receiver Thread itself (and not the Thread calling start() )
而究竟是谁这么隐蔽地调用run方法呢
其实是start间接地调用，start调用VMThread的create方法，而系统（OS创建需要的线程），然后JVM调用run方法。直接调用的是jvm。
下面是一篇很geek的文章讲解：

start method

public void start() – this method is responsible for causing the thread (it’s called upon) to begin execution. The name may seem to be a misnomer here as this method only causes and not actually starts the execution. It just schedules the thread and when the CPU Scheduler picks this thread for excution then the JVM calls the run() method to actually start the execution of the thread.

This method will obviously result into two concurrent threads – one, from which this method is called and two, the new thread which executes the run() method of the new Thread instance.

A thread will throw IllegalStateException in case you try to call the start() method on an already started thread instance. That means, a thread can not be started more than once. As per the Java specifications a thread may not be restarted after completing its execution. You’ll be required to create and start the execution of a new thread in that case.

run method

public void run() – this is the only method of the Runnable interface and the classes which intend to execute their code in a separate thread of execution first implement the Runnable interface and subsequently define this method and put all the code expected to be executed in the separate thread inside the scope of this run method.

The other way, such classes can follow for the same is by extending the Thread class and in that case also they should oevrride the run method of the Thread class and put all the code supposed to be executed in the new thread inside the scope of the overriden run method.

Difference between start() and run() methods

start() methods only schedules the thread for execution and not actually begins the execution of the thread. The execution of the thread is started when the JVM calls the run() method of the thread once the CPU Scheduler picks this scheduled thread for execution.

文章来源：http://geekexplains.blogspot.com/2008/05/start-method-vs-run-method-in-java.html