先看下Thread類中之前沒有太多關(guān)注的類定義：

public
class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */

    /**
     * The synchronization object responsible for this thread's join/sleep/park operations.
     */
    private final Object lock = new Object();

    private volatile long nativePeer;
/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

public final void join(long millis) throws InterruptedException {
        synchronized(lock) {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                lock.wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                lock.wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
        }
    }

join

從上面看到實(shí)際上檢測(cè)isAlive，因此線程未開始執(zhí)行時(shí)宙暇，是不生效的暴匠；
關(guān)鍵點(diǎn)調(diào)用了lock.wait函數(shù)精耐，那么在哪里notifyAll()呢希太？
在線程將要退出的時(shí)候克饶，要查看jvm中的實(shí)現(xiàn)
ensure_join方法中，調(diào)用 lock.notify_all(thread); 喚醒所有等待thread鎖的線程誊辉。

void JavaThread::run() {
  ...
  thread_main_inner();
}

void JavaThread::thread_main_inner() {
  ...
  this->exit(false);
  delete this;
}

void JavaThread::exit(bool destroy_vm, ExitType exit_type) {
  ...
  // Notify waiters on thread object. This has to be done after exit() is called
  // on the thread (if the thread is the last thread in a daemon ThreadGroup the
  // group should have the destroyed bit set before waiters are notified).
  ensure_join(this);
  ...
}

static void ensure_join(JavaThread* thread) {
  // We do not need to grap the Threads_lock, since we are operating on ourself.
  Handle threadObj(thread, thread->threadObj());
  assert(threadObj.not_null(), "java thread object must exist");
  ObjectLocker lock(threadObj, thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
  // Thread is exiting. So set thread_status field in  java.lang.Thread class to TERMINATED.
  java_lang_Thread::set_thread_status(threadObj(), java_lang_Thread::TERMINATED);
  // Clear the native thread instance - this makes isAlive return false and allows the join()
  // to complete once we've done the notify_all below
  java_lang_Thread::set_thread(threadObj(), NULL);
  lock.notify_all(thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
}

final線程安全

而final可以防止此類事情的發(fā)生：如果某個(gè)成員是final的矾湃，JVM規(guī)范做出如下明確的保證：一旦對(duì)象引用對(duì)其他線程可見，則其final成員也必須正確的賦值了堕澄。

volatile