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 main(String[] 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() {
for(int i=0;i<5;i++){
try {
Thread.sleep(1000);
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。。。。
怪我咯2017-04-17 18:01:10
上邊已經回答你的問題，線程結束時會呼叫notifyAll。
JVM底層實際上是使用OS層提供的API來支援線程的，例如UNIX-Like的OS中一般使用pthread(Windows也有Pthread的實作），openjdk的hotspotVM使用的就是pthread，我們來看看openjdk8版本的hotspotVM底層實際代碼。
在Thread執行start的方法時，就會呼叫native方法的start0，start0底層實際經過很多層的封裝，最終會呼叫createJavaThread的方法，createJavaThread就會調pthread_create建立一個執行緒並執行。
過程大致是這樣的：Thread.start() -> start0() -> ... -> createJavaCreate() -> pthread_create() => threadStart() => attachThread() -> 執行Thread的run() -> detachThread() "這個方法最後會呼叫Object.notifyAll"。
openjdk-8-8u66-b17/jamvm-2.0.0/src/thread.c
void createJavaThread(Object *jThread, long long stack_size) {
Thread *self = threadSelf();
ExecEnv *ee = sysMalloc(sizeof(ExecEnv));
Thread *thread = sysMalloc(sizeof(Thread));
memset(ee, 0, sizeof(ExecEnv));
memset(thread, 0, sizeof(Thread));
thread->ee = ee;
ee->thread = jThread;
ee->stack_size = stack_size;
if(!classlibCreateJavaThread(thread, jThread)) {
sysFree(thread);
sysFree(ee);
return;
}
disableSuspend(self);
if(pthread_create(&thread->tid, &attributes, threadStart, thread)) {
classlibMarkThreadTerminated(jThread);
sysFree(ee);
enableSuspend(self);
signalException(java_lang_OutOfMemoryError, "can't create thread");
return;
}
pthread_mutex_lock(&lock);
/* Wait for thread to start */
while(classlibGetThreadState(thread) == CREATING)
pthread_cond_wait(&cv, &lock);
pthread_mutex_unlock(&lock);
enableSuspend(self);
}
....
void *threadStart(void *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 */
enableSuspend(thread);
/* Complete initialisation of the thread structure, create the thread
stack and add the thread to the thread list */
initThread(thread, INST_DATA(jThread, int, daemon_offset), &thread);
/* Add thread to thread ID map hash table. */
addThreadToHash(thread);
/* Set state to running and notify creating thread */
signalThreadRunning(thread);
/* Execute the thread's run method */
executeMethod(jThread, CLASS_CB(jThread->class)->method_table[run_mtbl_idx]);
/* Run has completed. Detach the thread from the VM and exit */
detachThread(thread);
TRACE("Thread %p id: %d exited\n", thread, thread->id);
return NULL;
}
void *detachThread(Thread *thread) {
Object *keep_alive;
ExecEnv *ee = thread->ee;
Object *java_thread = ee->thread;
Object *group = INST_DATA(java_thread, Object*, group_offset);
/* If there's an exception pending, it is uncaught */
if(exceptionOccurred0(ee))
uncaughtException();
/* Don't do anything if this is the main thread */
if(thread->prev == NULL)
return NULL;
/* remove thread from thread group */
executeMethod(group, (CLASS_CB(group->class))->
method_table[rmveThrd_mtbl_idx], java_thread);
/* Remove thread from the ID map hash table */
deleteThreadFromHash(thread);
objectLock(java_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 = classlibMarkThreadTerminated(java_thread);
/* Notify any threads waiting on the thread object -
these are joining this thread 这里调用objectNotifyAll */
objectNotifyAll(java_thread);
objectUnlock(java_thread);
/* Thread's about to die, so no need to enable suspend
afterwards. */
disableSuspend(thread);
/* 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 thread's 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. */
sysFree(ee->stack);
sysFree(ee);
/* 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 */
if(non_daemon_thrds == 0) {
/* No need to bother with disabling suspension
around lock, as we're 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 */
setThreadSelf(NULL);
TRACE("Thread %p id: %d detached from VM\n", thread, thread->id);
return keep_alive;
}
....
伊谢尔伦2017-04-17 18:01:10
你的推斷應該是對的,線程死的時候會調用自己的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