java线程的join方法源码分析的不解问题

1.首先通过源码可以看到join方法的底层逻辑还是使用当前线程对象的wait方法,也知道子线程执行完业务代码后,主线程才能解除阻塞。我认为既然使用的是wait方法,必然需要notify或notifyAll来唤醒,但唤醒的机制是什么?难道使用的线程的隐式钩子方式,当线程执行完后再进行notify?

2.伪代码,按自己的理解实现join方法,不知道这样对不对?
public class JoinTest {

public static void mainString[] args throws InterruptedException { ThreadTest tt=new ThreadTest; tt.start; synchronized tt { tt.wait; } System.out.println"主线程继续。。。。";
}

}

class ThreadTest extends Thread {

public void run { forint i=0;i<5;i++{ try { Thread.sleep1000; System.out.println"i="+i; } catch InterruptedException e { e.printStackTrace; } }
}

}
//—输出结果输下—-
i=0
i=1
i=2
i=3
i=4
主线程继续。。。。

上边已经回答你的问题,线程结束时会调用notifyAll。

JVM底层实际是使用OS层提供的API来支持线程的,比如UNIX-Like的OS中一般使用pthreadWindows也有Pthread的实现),openjdk的hotspotVM使用的就是pthread,我们来看一下openjdk8版本的hotspotVM底层实际代码。

在Thread执行start的方法时,就会调用native方法的start0,start0底层实际经过很多层的封装,最终会调用createJavaThread的方法,createJavaThread就会调pthread_create创建一个线程并执行。

过程大致是这样的:Thread.start -> start0 -> … -> createJavaCreate -> pthread_create => threadStart => attachThread -> 执行Thread的run -> detachThread “这个方法最后会调用Object.notifyAll”。

openjdk-8-8u66-b17/jamvm-2.0.0/src/thread.c

void createJavaThreadObject *jThread, long long stack_size { Thread *self = threadSelf; ExecEnv *ee = sysMallocsizeofExecEnv; Thread *thread = sysMallocsizeofThread; memsetee, 0, sizeofExecEnv; memsetthread, 0, sizeofThread; thread->ee = ee; ee->thread = jThread; ee->stack_size = stack_size; if!classlibCreateJavaThreadthread, jThread { sysFreethread; sysFreeee; return; } disableSuspendself; ifpthread_create&thread->tid, &attributes, threadStart, thread { classlibMarkThreadTerminatedjThread; sysFreeee; enableSuspendself; signalExceptionjava_lang_OutOfMemoryError, "cant create thread"; return; } pthread_mutex_lock&lock; /* Wait for thread to start */ whileclasslibGetThreadStatethread == CREATING pthread_cond_wait&cv, &lock; pthread_mutex_unlock&lock; enableSuspendself;
} .... void *threadStartvoid *arg { Thread *thread = Thread *arg; Object *jThread = thread->ee->thread; /* Parent thread created thread with suspension disabled. This is inherited so we need to enable */ enableSuspendthread; /* Complete initialisation of the thread structure, create the thread stack and add the thread to the thread list */ initThreadthread, INST_DATAjThread, int, daemon_offset, &thread; /* Add thread to thread ID map hash table. */ addThreadToHashthread; /* Set state to running and notify creating thread */ signalThreadRunningthread; /* Execute the threads run method */ executeMethodjThread, CLASS_CBjThread->class->method_table[run_mtbl_idx]; /* Run has completed. Detach the thread from the VM and exit */ detachThreadthread; TRACE"Thread %p id: %d exited
", thread, thread->id; return NULL;
} void *detachThreadThread *thread { Object *keep_alive; ExecEnv *ee = thread->ee; Object *java_thread = ee->thread; Object *group = INST_DATAjava_thread, Object*, group_offset; /* If theres an exception pending, it is uncaught */ ifexceptionOccurred0ee uncaughtException; /* Dont do anything if this is the main thread */ ifthread->prev == NULL return NULL; /* remove thread from thread group */ executeMethodgroup, CLASS_CBgroup->class-> method_table[rmveThrd_mtbl_idx], java_thread; /* Remove thread from the ID map hash table */ deleteThreadFromHashthread; objectLockjava_thread; /* Mark the thread as terminated. This state is used in determining if the thread is alive and so must be done before notifying joining threads. The VM thread structure is tied to a Java-level object see comment below. The keep_alive is an object which must be kept alive to prevent the structure from being freed while we are still accessing it */ keep_alive = classlibMarkThreadTerminatedjava_thread; /* Notify any threads waiting on the thread object - these are joining this thread 这里调用objectNotifyAll */ objectNotifyAlljava_thread; objectUnlockjava_thread; /* Threads about to die, so no need to enable suspend afterwards. */ disableSuspendthread; /* Grab global lock, and update thread structures protected by it thread list, thread ID and number of daemon threads */ pthread_mutex_lock&lock; /* It is safe to free the threads ExecEnv and stack now as these are only used within the thread. It is _not_ safe to free the native thread structure as another thread may be concurrently accessing it. However, they must have a reference to the java level thread -- therefore, it is safe to free during GC when the thread is determined to be no longer reachable. */ sysFreeee->stack; sysFreeee; /* If no more daemon threads notify the main thread which may be waiting to exit VM. Note, this is not protected by lock, but main thread checks again */ ifnon_daemon_thrds == 0 { /* No need to bother with disabling suspension around lock, as were no longer on thread list */ pthread_mutex_lock&exit_lock; pthread_cond_signal&exit_cv; pthread_mutex_unlock&exit_lock; } /* Finally, clear the thread local data */ setThreadSelfNULL; TRACE"Thread %p id: %d detached from VM
", thread, thread->id; return keep_alive;
} .... 

你的推断应该是对的,线程死的时候会调用自己的notifyAll方法
As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.
java/lang/Thread.java#1258

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