Thread的run方法和start方法

thread调用start()方法，可以理解为，start()方法通知“线程规划器”此线程已经准备就绪，等待调用线程对象的run()方法。这个过程其实就是让系统安排一个时间来调用Thread中的run()方法，也就是使线程得到运行，启动线程，具有异步执行的效果。如果直接调用代码thread.run()方法，就不是异步执行了，而是同步，那么此线程对象并不交给“线程规划器”来进行处理，而是由main主线程来调用run()方法，也就是必须等run()方法中的代码执行完后才可以执行后面的代码。源代码如下：

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();

   /**
    * If this thread was constructed using a separate
    * <code>Runnable</code> run object, then that
    * <code>Runnable</code> object's <code>run</code> method is called;
    * otherwise, this method does nothing and returns.
    * <p>
    * Subclasses of <code>Thread</code> should override this method.
    *
    * @see     #start()
    * @see     #stop()
    * @see     #Thread(ThreadGroup, Runnable, String)
    */
   @Override
   public void run() {
       if (target != null) {
           target.run();
       }
   }

另外，还需要注意一下，执行start()方法的顺序不代表线程启动的顺序。