從一個(gè)例子開(kāi)頭：

@Slf4j
public class Test {

  public static void main(String[] args) throws InterruptedException {
      Thread thread_new = new Thread(() -> {
          try {
              Thread.sleep(2000);
          } catch (InterruptedException ignored) {
          }
          log.info("Thread------>:{}", Thread.currentThread().getName());
      },"thread_new");
      thread_new.start();
      thread_new.join();
      log.info("Thread------>:{}", Thread.currentThread().getName());
      log.info("end");
  }
}

毫無(wú)疑問(wèn)丁稀，會(huì)打印：

Thread------>:Thread-0
Thread------>:main
end

在Thread類中：

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

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

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

在主線程中調(diào)用 thread_new.join()方法會(huì)使主線程阻塞盏檐，進(jìn)入wait()狀態(tài)蜗巧，直到thread_new線程執(zhí)行完畢睹簇，那么為什么會(huì)產(chǎn)生這樣的現(xiàn)象呢。

thread.png

先從synchronized 說(shuō)起寥闪，synchronized在java中如果作為非靜態(tài)方法的描述符太惠，則等同于在方法第一行加上
synchronized(this){}，這時(shí)調(diào)用 thread_new.join()疲憋，則this指向的是當(dāng)前的thread_new對(duì)象凿渊，所以當(dāng)前thread_new對(duì)象的對(duì)象頭，會(huì)指向monitor對(duì)象缚柳，monitor中指向的線程為主線程埃脏，while (isAlive()) {wait(0);},isAlive()為Thread中的方法，會(huì)先判斷thread_new線程是否存活秋忙。顯然彩掐，isAlive()為true，這時(shí)直接調(diào)用wait(0)會(huì)使當(dāng)前thread_new對(duì)象頭中指向的monit對(duì)象中灰追，記錄的線程(主線程)堵幽，進(jìn)入waitSet。使主線程進(jìn)入wait狀態(tài)弹澎。等到thread_new的線程終結(jié)朴下，在jvm中會(huì)觸發(fā)thread_new對(duì)象的thread_exit()方法，在該方法中會(huì)調(diào)用notiyAll()方法苦蒿，喚醒該對(duì)象頭中waitSet中的線程殴胧。

ObjectMonitor() {
  _header       = NULL;
  _count        = 0; //記錄個(gè)數(shù)
  _waiters      = 0,
  _recursions   = 0;
  _object       = NULL;
  _owner        = NULL; //_owner指向持有ObjectMonitor對(duì)象的線程
  _WaitSet      = NULL; //處于wait狀態(tài)的線程，會(huì)被加入到_WaitSet
  _WaitSetLock  = 0 ;
  _Responsible  = NULL ;
  _succ         = NULL ;
  _cxq          = NULL ;
  FreeNext      = NULL ;
  _EntryList    = NULL ; //處于等待鎖block狀態(tài)的線程刽肠，會(huì)被加入到該列表
  _SpinFreq     = 0 ;
  _SpinClock    = 0 ;
  OwnerIsThread = 0 ;
}

綠色代表_owner指向的線程.png