1 /** 2      * Causes this thread to begin execution; the Java Virtual Machine 3      * calls the run method of this thread. 4      * 
 5      * The result is that two threads are running concurrently: the 6      * current thread (which returns from the call to the 7      * start method) and the other thread (which executes its 8      * run method). 9      * 
10      * It is never legal to start a thread more than once.11      * In particular, a thread may not be restarted once it has completed12      * execution.13      *14      * @exception  IllegalThreadStateException  if the thread was already15      *               started.16      * @see        #run()17      * @see        #stop()18      */19     public synchronized void start() {20         /**21          * This method is not invoked for the main method thread or "system"22          * group threads created/set up by the VM. Any new functionality added23          * to this method in the future may have to also be added to the VM.24          *25          * A zero status value corresponds to state "NEW".26          */27         if (threadStatus != 0)28             throw new IllegalThreadStateException();29 30         /* Notify the group that this thread is about to be started31          * so that it can be added to the group's list of threads32          * and the group's unstarted count can be decremented. */33         group.add(this);34 35         boolean started = false;36         try {37             start0();38             started = true;39         } finally {40             try {41                 if (!started) {42                     group.threadStartFailed(this);43                 }44             } catch (Throwable ignore) {45                 /* do nothing. If start0 threw a Throwable then46                   it will be passed up the call stack */47             }48         }49     }50 51     private native void start0();

从jdk的官方文档可以看出一些关于start方法的一些信息：

　　1.当一个thread被new出来后，线程的内部属性threadStatus==0;

　　2.new thread之后，都会重写run方法实现业务，纵观start方法中，只包含了一个native方法，从文档中看出   

        /* Causes this thread to begin execution; the Java Virtual Machine calls the <code>run</code> method of this thread   */　猜测推断出，native方法statr0调用了重写的run方法。

　　3.当一个线程被start之后，再次start的时候，会抛出 IllegalThreadStateException 异常。

　　4.  /* 　In particular, a thread may not be restarted once it has complete execution. */   当一个线程结束他的生命周期之后，是不能再次调用start的。

　　5.当线程start之后，会被加入到一个threadGroup中，最后会根据started标识位，判断是否移除此线程。