thread.start()到底做了什么?

这篇文章作为Thread类源码剖析的补充,从一个侧面来看Thread源码。也解答了面试高频问题:“多次start一个线程会怎么样?”

答案是:java.lang.IllegalThreadStateException 线程状态非法异常 继承关系是:—>extends IllegalArgumentException—>extends RuntimeException一个运行时异常,下面我们从源码来透彻分析一下start()时做了什么。

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1

/**
 2      * Causes this thread to begin execution; the Java Virtual Machine
 3      * calls the run method of this thread.
 4      * 

线程被执行,JVM调用run方法 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.多次调用start方法启动一个线程是非法的 11 * In particular, a thread may not be restarted once it has completed 12 * execution. 13 * 14 * @exception IllegalThreadStateException if the thread was already已经启动的线程再次start,异常 15 * 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 added 23 * 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)//状态校验 0:NEW 新建状态 28 throw new IllegalThreadStateException(); 29 30 /* Notify the group that this thread is about to be started 31 * so that it can be added to the group's list of threads 32 * and the group's unstarted count can be decremented. */ 33 group.add(this);//添加进线程组 34 35 boolean started = false; 36 try { 37 start0();//调用native方法执行线程run方法 38 started = true; 39 } finally { 40 try { 41 if (!started) { 42 group.threadStartFailed(this);//启动失败,从线程组中移除当前前程。 43 } 44 } catch (Throwable ignore) { 45 /* do nothing. If start0 threw a Throwable then 46 it will be passed up the call stack */ 47 } 48 } 49 } 50 51 private native void start0();

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greop.add(this),把当前线程添加进线程组,源码如下:

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1

 /**
 2      * Adds the specified thread to this thread group.
 3      *
 4      * 

Note: This method is called from both library code 5 * and the Virtual Machine. It is called from VM to add 6 * certain system threads to the system thread group. 7 * 8 * @param t 9 * the Thread to be added 10 * 11 * @throws IllegalThreadStateException 12 * if the Thread group has been destroyed 13 */ 14 void add(Thread t) { 15 synchronized (this) { 16 if (destroyed) {//线程组状态校验 17 throw new IllegalThreadStateException(); 18 } 19 if (threads == null) { 20 threads = new Thread[4];//初始化长度为4的Thread数组 21 } else if (nthreads == threads.length) {//数组满了就扩容2倍 22 threads = Arrays.copyOf(threads, nthreads * 2); 23 } 24 threads[nthreads] = t;//新线程t添加进数组 25 26 // This is done last so it doesn't matter in case the 27 // thread is killed 28 nthreads++;//线程数加1 29 30 // The thread is now a fully fledged member of the group, even 31 // though it may, or may not, have been started yet. It will prevent 32 // the group from being destroyed so the unstarted Threads count is 33 // decremented. 34 nUnstartedThreads--;//未启动线程数-1 35 } 36 }

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启动失败后调用group.threadStartFailed(this),都是加锁方法,从线程组中移除当前线程,源码如下

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1 void threadStartFailed(Thread t) {
 2         synchronized(this) {
 3             remove(t);//移除线程t
 4             nUnstartedThreads++;//未启动线程+1
 5         }
 6     }
 7 
 8 private void remove(Thread t) {
 9         synchronized (this) {
10             if (destroyed) {
11                 return;
12             }
13             for (int i = 0 ; i < nthreads ; i++) {
14                 if (threads[i] == t) {
15                     System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
16                     // Zap dangling reference to the dead thread so that
17                     // the garbage collector will collect it.
18                     threads[nthreads] = null;
19                     break;
20                 }
21             }
22         }
23     }

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