原文：http://www.cnblogs.com/skywang12345/p/3479083.html
start() 和 run()的区别说明
start():它的作用是启动一个新线程，新线程会执行相应的run()方法。start()不能被重复调用。
run():run()就和普通的成员方法一样，可以被重复调用。单独调用run()的话，会在当前线程中执行run()，而并不会启动新线程！
下面以代码来进行说明。
class MyThread extends Thread{
public void run(){
...
}
}
MyThread mythread = new MyThread();
mythread.start()会启动一个新线程，并在新线程中运行run()方法。
而mythread.run()则会直接在当前线程中运行run()方法，并不会启动一个新线程来运行run()。
start() 和run()的区别示例
下面，通过一个简单示例演示它们之间的区别。源码如下：
class MyThread extends Thread{
public MyThread(String name) {
super(name);
}
public void run(){
System.out.println(Thread.currentThread().getName()+" is running");
}
}
public class Demo {
public static void main(String[] args) {
Thread myThread=new MyThread("myThread");
System.out.println(Thread.currentThread().getName()+" call myThread.run()");
myThread.run();
System.out.println(Thread.currentThread().getName()+" call myThread.start()");
myThread.start();
}
}
运行结果：
main call mythread.run() main is running main call mythread.start() mythread is running
结果说明：
(01) Thread.currentThread().getName()是用于获取“当前线程”的名字。当前线程是指正在cpu中调度执行的线程。
(02) mythread.run()是在“主线程main”中调用的，该run()方法直接运行在“主线程main”上。
(03) mythread.start()会启动“线程mythread”，“线程mythread”启动之后，会调用run()方法；此时的run()方法是运行在“线程mythread”上。
start() 和 run()相关源码
Thread.java中start()方法的源码如下：
/**
* Causes this thread to begin execution; the Java Virtual Machine
* calls the run method of this thread.
* It is never legal to start a thread more than once.
* In particular, a thread may not be restarted once it has completed
* execution.*/
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();
说明：start()实际上是通过本地方法start0()启动线程的。而start0()会新运行一个线程，新线程会调用run()方法。
private native void start0();
Thread.java中run()的代码如下：
public void run() {
if (target != null) {
target.run();
}
}
说明：target是一个Runnable对象。run()就是直接调用Thread线程的Runnable成员的run()方法，并不会新建一个线程。