public void start()
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.
Throws:
IllegalThreadStateException - if the thread was already started.

public void run()
If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns.
Subclasses of Thread should override this method.
Specified by:
run in interface Runnable

上面是Thread类中Start()方法与run()方法的定义。
现有自定义的类如下：

public class Test2 {
	public static void main(String[] args) {
		Thread t1 = new Thread(new Runnable() {			
			@Override
			public void run() {
				System.out.println("in runnable() run().");
			}			
		});
		Thread t2 =new MyThread();
		t1.start();
		t2.start();
	}	
	static class MyThread extends Thread {
		@Override
		public void run() {
			System.out.println("in MyThread() run().");
		}
	}
}

由于t2并没有指定target,即没有这样定义

Thread t2 = new Thread(new MyThread());

所以根据API中run()方法的定义，t2.start()方法的调用应该没有调用MyThread的run()方法。
Thread类的run() 源代码如下：

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

Thread类无参构造函数

public Thread() {
	init(null, null, "Thread-" + nextThreadNum(), 0);
    }

private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
	Thread parent = currentThread();
	SecurityManager security = System.getSecurityManager();
	if (g == null) {
	    /* Determine if it's an applet or not */	    
	    /* If there is a security manager, ask the security manager
	       what to do. */
	    if (security != null) {
		g = security.getThreadGroup();
	    }
	    /* If the security doesn't have a strong opinion of the matter
	       use the parent thread group. */
	    if (g == null) {
		g = parent.getThreadGroup();
	    }
	}
	/* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
	g.checkAccess();
	/*
	 * Do we have the required permissions?
	 */
	if (security != null) {
	    if (isCCLOverridden(getClass())) {
	        security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
	    }
	}
        g.addUnstarted();
	this.group = g;
	this.daemon = parent.isDaemon();
	this.priority = parent.getPriority();
	this.name = name.toCharArray();
	if (security == null || isCCLOverridden(parent.getClass()))
	    this.contextClassLoader = parent.getContextClassLoader();
	else
	    this.contextClassLoader = parent.contextClassLoader;
	this.inheritedAccessControlContext = AccessController.getContext();
	this.target = target;
	setPriority(priority);
        if (parent.inheritableThreadLocals != null)
	    this.inheritableThreadLocals =
		ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;
        /* Set thread ID */
        tid = nextThreadID();
    }

但是根据API中start()方法的说明，run()方法是由JVM调用的，这是不是矛盾了？
求解答，谢谢啦。

问题补充：首先，谢谢AngelAndAngel和chen_yongkai的回答。

Thread t3 = new Thread(new MyThread());

Thread t4 = new MyThread();

上面两种定义方式，t3.start()后，启动此线程，调用流程是MyThread类的run()方法。因为它们调用的构造器是

Thread(Runnable target)

，此时Thread类的target就等于new MyThread()这个对象，所以它会直接调用target.run()这个方法。

而t4.start()后，启动此线程是通过JVM调用的start0()方法，再调用的run()的一个流程。

这样理解正确吗？

问题补充：问题补充。

AngelAndAngel 写道

调用start的时候不是马上调用run方法，而是触动调用了run。
但是你要是自己调用也不是不行（不会有线程调用的效果而已，这样和普通方法就木有区别了），只是说线程得通过启动（start）来调用

问题补充：问题补充

chen_yongkai 写道

    public synchronized void start() {
        if (started)
            throw new IllegalThreadStateException();
        started = true;
        group.add(this);
        start0();
    }

    private native void start0();

在start0()方法中jvm调用run()方法，因为是native所以看不到源码。

问题补充：对头，我怎么木有想到呢~ 谢谢啦。

chen_yongkai 写道

    static class MyThread extends Thread {   
        @Override  
        public void run() {   
            System.out.println("in MyThread() run().");   
        }   
    }

你已经覆盖了Thread 的run方法，就没有什么target的说法了

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

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