From the Thread.Start () method to see the thread source code, multiple start a thread will do

This article as the thread class source analysis of the supplement, from a side view thread source code. Also answer the interview high-frequency question: "How many start a thread?" ”

The answer is: java.lang.IllegalThreadStateException thread state Illegal exception inheritance relationship is:--->extends illegalargumentexception--->extends RuntimeException a run-time exception, let's take a thorough look at start () from the source.

1 /**2 * Causes this thread to begin execution; the Java Virtual machine3 * Calls the <code>run</code> method of this thread.4 * <p> thread is executed, JVM calls the Run method5 * The result is this and the threads is running concurrently:the6 * Current thread (which returns the7 * <code>start</code> method) and the other thread (which executes its8 * <code>run</code> method).9 * <p>Ten * It's never legal to start a thread more than once. Multiple calls to start method starting a thread is illegal One * In particular, a thread is not being restarted once it has completed A * execution. -      * -      * @exceptionIllegalthreadstateexception If the thread was already start again, the exception the * started. -      * @see#run () -      * @see#stop () -      */ +      Public synchronized voidstart () { -         /** + * This method isn't invoked for the main method thread or "system" A * Group Threads Created/set up by the VM. Any new functionality added at * To the The also is added to the VM. -          * - * A Zero status value corresponds to state "NEW". -          */ -         if(Threadstatus! = 0)//Status check 0:new new state  -             Throw Newillegalthreadstateexception (); in  -         /*Notify The group , the This thread is an about-to-be started to * So, it can be added to the group ' s list of threads + * and the group ' s unstarted count can be decremented.*/ -Group.add ( This);// add into thread group  the  *         Booleanstarted =false; $         Try {Panax Notoginseng start0 ();//Call the native method to execute the thread run method -started =true; the}finally { +             Try { A                 if(!started) { theGroup.threadstartfailed ( This);// failed to start, removing the current prospect from the thread group.  +                 } -}Catch(Throwable ignore) { $                 /*Do nothing . If Start0 threw a throwable then $ It 'll be passed up the call stack*/ -             } -         } the     } - Wuyi     Private native voidStart0 ();

Greop.add (this), add the current thread into the thread group, the source code is as follows:

1 /**2 * Adds The specified thread to this thread group.3      *4 * <p> Note:this method is called from both library code5 * and the Virtual machine. It is called from VMs to add6 * Certain system threads to the system thread group.7      *8      * @paramT9 * The Thread to be addedTen      * One      * @throwsillegalthreadstateexception A * If the Thread group has been destroyed -      */ -     voidAdd (Thread t) { the         synchronized( This) { -             if(destroyed) {//thread group State check -                 Throw Newillegalthreadstateexception (); -             } +             if(Threads = =NULL) { -Threads =NewThread[4];//Initializes a thread array of length 4 +}Else if(Nthreads = =threads.length) {// array expands twice times when full  AThreads = arrays.copyof (Threads, Nthreads * 2); at             } -Threads[nthreads] =t;// new Thread T added into array  -  -             //This was done by the last so it doesn ' t matter in case the -             //thread is killed -nthreads++;// number of threads plus 1  in  -             //The thread is now a fully fledged member of the group, even to             //though it may, or could not, has been started yet. It'll prevent +             //The group from being destroyed so the unstarted Threads count is -             //decremented. thenunstartedthreads--;// number of threads not started-1  *         } $}

Call Group.threadstartfailed (this) after a failed start,is the lock method to remove the current thread from the thread group, the source code is as follows

1 voidthreadstartfailed (Thread t) {2         synchronized( This) {3 Remove (t);//Remove thread T4nunstartedthreads++;//thread +1 not started5         }6     }7 8 Private voidRemove (Thread t) {9         synchronized( This) {Ten             if(destroyed) { One                 return; A             } -              for(inti = 0; i < nthreads; i++) { -                 if(Threads[i] = =t) { theSystem.arraycopy (threads, i + 1, threads, I,--nthreads-i); -                     //Zap dangling reference to the dead thread so -                     //The garbage collector would collect it. -Threads[nthreads] =NULL; +                      Break; -                 } +             } A         } at}

From the Thread.Start () method to see the thread source code, multiple start a thread will do