JavaThread多线程开发中Object类详细讲解

方法概览

Thread

wait notify notifyAll方法详解

作用

阻塞阶段

使用了wait方法之后，线程就会进入阻塞阶段，只有发生以下四种情况中的其中一个，线程才会被唤醒

• 另一个线程调用了这个线程的notify方法，刚好唤醒的是本线程
• 另一个线程调用了这个对象的notifyAll方法
• 过了wait规定的超时时间
• 线程调用了interrupt

唤醒阶段

notify会唤醒单个处于阻塞状态的线程，唤醒的线程是随机的

notify和wait都需要写在synchronized代码块里，不然会抛出异常

notifyAll会唤醒所有等待的线程

遇到中断

执行wait方法之后，被中断，会抛出InterruptedException这个异常

代码展示

• 展示wait和notify的基本用法
• 该代码执行wait方法之后会释放锁，然后thread2执行notify方法
• notify方法执行完毕之后，并没有立即释放锁，而是接着执行之后的代码,也就是打印“Thread2调用notify”这句话
• thread2执行完毕之后，会进行释放锁，thread1才会继续执行
• 在此期间，thread1虽然被唤醒，但是一直在等待thread2同步代码块里面的代码执行完毕

public class Wait {
  public static void main(String[] args) throws InterruptedException {
      Thread1 thread1 = new Thread1();
      Thread2 thread2 = new Thread2();
      thread1.start();
      Thread.sleep(200);
      thread2.start();
  }
  public static Object object = new Object();
  static class Thread1 extends Thread {
      @Override
      public void run() {
          synchronized (object) {
              System.out.println("Thread1执行");
              try {
                  object.wait();
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              System.out.println("Thread1获取锁");
          }
      }
  }
  static class Thread2 extends Thread {
      @Override
      public void run() {
          synchronized (object) {
              object.notify();
              System.out.println("Thread2调用notify");
          }
      }
  }
}
/*
Thread1执行
Thread2调用notify
Thread1获取锁
* */

• notify和notifyAll的展示
• 第一个输出：threadc调用notifyAll
• 第二个输出：threadc调用notify
• 调用notify的时候，程序并没有结束，threadb陷入等待

public class notifyOrAll implements Runnable{
  private static final Object a = new Object();
  public static void main(String[] args) throws InterruptedException {
      Runnable r = new notifyOrAll();
      Thread threada = new Thread(r);
      Thread threadb = new Thread(r);
      Thread threadc = new Thread(new Runnable() {
          @Override
          public void run() {
              synchronized (a) {
//                    a.notifyAll();
                  a.notify();
                  System.out.println(Thread.currentThread().getName() + "notify");
              }
          }
      });
      threada.start();
      Thread.sleep(200);
      threadb.start();
      Thread.sleep(200);
      threadc.start();
  }
  @Override
  public void run() {
      synchronized (a) {
          System.out.println(Thread.currentThread().getName() + "得到锁");
          try {
              System.out.println(Thread.currentThread().getName() + "wait");
              a.wait();
              System.out.println(Thread.currentThread().getName() + "wait结束");
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
  }
}
/*
Thread-0得到锁
Thread-0wait
Thread-1得到锁
Thread-1wait
Thread-2notifyAll
Thread-1wait结束
Thread-0wait结束
* */
/*
Thread-0得到锁
Thread-0wait
Thread-1得到锁
Thread-1wait
Thread-2notify
Thread-0wait结束
* */

• 只释放当前monitor
• 证明wait只释放当前的那把锁

public class OwnMonitor {
  private static volatile Object a = new Object();
  private static volatile Object b = new Object();
  public static void main(String[] args) throws InterruptedException {
      Thread threadA = new Thread(new Runnable() {
          @Override
          public void run() {
              synchronized (a) {
                  System.out.println("threadA得到a");
                  synchronized (b) {
                      System.out.println("threadA得到锁b");

                      try {
                          System.out.println("threadA释放a");
                          a.wait();
                      } catch (InterruptedException e) {
                          e.printStackTrace();
                      }
                  }
              }
          }
      });
      Thread threadB = new Thread(new Runnable() {
          @Override
          public void run() {
              synchronized (a) {
                  System.out.println("threadB得到a");
                  System.out.println("threadB要获取b");
                  synchronized (b) {
                      System.out.println("threadB得到b");
                  }
              }
          }
      });
      threadA.start();
      Thread.sleep(1000);
      threadB.start();
  }
}
/*
threadA得到a
threadA得到锁b
threadA释放a
threadB得到a
threadB要获取b
* */

特点

• 执行这些方法必须先获取锁
• notify只能换取一个，而且是随机的
• 都属于Object。任何对象都可以调用
• 都是native final修饰的

当线程从wait状态刚被唤醒时，通常不能直接得到锁，那就会从waiting状态转换到blocked状态，抢到锁之后状态转变为runnable

如果发生异常，则直接跳到Terminated状态

通过wait notify方法实现生产者和消费者

• 将storge当作生产者和消费者进行工作的仓库
• 如果storge中没有数据，生产者就开始wait
• 如果storge中数据满了，消费者就开始wait
• 生产者和消费者每进行一次生产和消费，就执行notify

public class ProducerConsumer {
  public static void main(String[] args) {
      Storge storge = new Storge();
      Producer producer = new Producer(storge);
      Consumer consumer = new Consumer(storge);
      new Thread(producer).start();
      new Thread(consumer).start();
  }
}
class Producer implements Runnable {
  private Storge storge;
  public Producer(Storge storge) {
      this.storge = storge;
  }
  @Override
  public void run() {
      for (int i = 0; i < 100; i++) {
          storge.put();
      }
  }
}
class Consumer implements Runnable {
  private Storge storge;
  public Consumer(Storge storge) {
      this.storge = storge;
  }
  @Override
  public void run() {
      for (int i = 0; i < 100; i++) {
          storge.take();
      }
  }
}
class Storge {
  private int maxSize;
  private LinkedList<Date> storge;
  public Storge() {
      maxSize = 10;
      storge = new LinkedList<>();
  }
  public synchronized void put() {
      while (storge.size() == maxSize) {
          try {
              wait();
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      storge.add(new Date());
      System.out.println("已经有了" + storge.size());
      notify();
  }
  public synchronized void take() {
      while (storge.size() == 0) {
          try {
              wait();
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      System.out.println("拿到了" + storge.poll() + "还剩" + storge.size());
      notify();
  }
}

sleep方法详解

作用：让线程在预期的时间执行，其他时间不占用CPU资源

特点：和wait不一样，sleep不释放锁

sleep不会释放锁

证明sleep不会释放 synchronized锁

public class SleepSyn implements Runnable{
  public static void main(String[] args) {
      SleepSyn sleepSyn = new SleepSyn();
      new Thread(sleepSyn).start();
      new Thread(sleepSyn).start();
  }
  @Override
  public void run() {
      syn();
  }
  private synchronized void syn() {
      System.out.println(Thread.currentThread().getName() + "获取锁");
      try {
          Thread.sleep(2000);
      } catch (InterruptedException e) {
          e.printStackTrace();
      }
      System.out.println(Thread.currentThread().getName() + "释放锁");
  }
}
/*
* Thread-0获取锁
Thread-0释放锁
Thread-1获取锁
Thread-1释放锁
* */

证明sleep不释放Lock锁

public class sleepLock implements Runnable{
  private static final Lock LOCK = new ReentrantLock();
  @Override
  public void run() {
      LOCK.lock();
      System.out.println(Thread.currentThread().getName() + "获取锁");
      try {
          Thread.sleep(2000);
      } catch (InterruptedException e) {
          e.printStackTrace();
      } finally {
          LOCK.unlock();
      }
      System.out.println(Thread.currentThread().getName() + "释放锁");
  }
  public static void main(String[] args) {
      sleepLock sleepLock = new sleepLock();
      new Thread(sleepLock).start();
      new Thread(sleepLock).start();
  }
}
/*
* Thread-0获取锁
Thread-0释放锁
Thread-1获取锁
Thread-1释放锁
* */

sleep响应中断

• 抛出InterruptedException
• 会清除中断状态
• 中断之后，抛出异常继续执行

public class sleepInterrupted implements Runnable{
  public static void main(String[] args) throws InterruptedException {
      Thread thread = new Thread(new sleepInterrupted());
      thread.start();
      Thread.sleep(2000);
      thread.interrupt();
  }
  @Override
  public void run() {
      for (int i = 0; i < 10; i++) {
          System.out.println(new Date());
          try {
              TimeUnit.SECONDS.sleep(1);
          } catch (InterruptedException e) {
              System.out.println("中断");
              e.printStackTrace();
          }
      }
  }
}
/*
* Fri Jan 27 21:11:57 CST 2023
Fri Jan 27 21:11:58 CST 2023
中断
Fri Jan 27 21:11:59 CST 2023
java.lang.InterruptedException: sleep interrupted
 at java.lang.Thread.sleep(Native Method)
 at java.lang.Thread.sleep(Thread.java:340)
 at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
 at com.jx.JavaTest.ThreadObjectMethod.sleepInterrupted.run(sleepInterrupted.java:21)
 at java.lang.Thread.run(Thread.java:748)
Fri Jan 27 21:12:00 CST 2023
Fri Jan 27 21:12:01 CST 2023
Fri Jan 27 21:12:02 CST 2023
Fri Jan 27 21:12:03 CST 2023
Fri Jan 27 21:12:04 CST 2023
Fri Jan 27 21:12:05 CST 2023
Fri Jan 27 21:12:06 CST 2023
Process finished with exit code 0
* */

总结

sleep方法可以让线程进入waiting状态，不占用CPU资源，但是不释放锁，规定时间之后再运行

休眠期间如果被打断，会抛出异常并清除中断状态

join方法详解

新线程加入，主线程等子线程执行完毕

代码展示

• 前一个结果是使用join
• 后一个结果是没使用join
• 可知使用join之后，主线程会等join的线程执行完毕再继续执行

public class join {
  public static void main(String[] args) throws InterruptedException {
      Thread thread1 = new Thread(new Runnable() {
          @Override
          public void run() {
              try {
                  Thread.sleep(1000);
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              System.out.println(Thread.currentThread().getName() + "执行完毕");
          }
      });
      Thread thread2 = new Thread(new Runnable() {
          @Override
          public void run() {
              try {
                  Thread.sleep(1000);
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              System.out.println(Thread.currentThread().getName() + "执行完毕");
          }
      });
      thread1.start();
      thread2.start();
      System.out.println("开始等待子线程运行");
//        thread1.join();
//        thread2.join();
      System.out.println("所有线程执行完毕");
  }
}
/*
* 开始等待子线程运行
Thread-0执行完毕
Thread-1执行完毕
所有线程执行完毕
* */
/*
* 开始等待子线程运行
所有线程执行完毕
Thread-1执行完毕
Thread-0执行完毕
* */

• 遇到中断
• 第一个的运行结果是主线程没中断的打印结果
• 第二个的运行结果是join期间进行中断的打印结果，可知在打印了“子线程运行完毕”之后，依然打印了“启动”两个字，可知会造成运行混乱
• 可以在捕获异常的代码块中，将join的线程也中断，可以解决上面的问题

public class joinInterrupt {
  public static void main(String[] args) {
      Thread main1 = Thread.currentThread();
      Thread thread1 = new Thread(new Runnable() {
          @Override
          public void run() {
              try {
                  main1.interrupt();
                  Thread.sleep(2000);
                  System.out.println("启动");
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
          }
      });
      thread1.start();
      System.out.println("join");
      try {
          thread1.join();
      } catch (InterruptedException e) {
          System.out.println(Thread.currentThread().getName() + "中断");
          // thread1.interrupt();
          e.printStackTrace();
      }
      System.out.println("子线程运行完毕");
  }
}
/*
* join
启动
子线程运行完毕
* */
/*
* join
main中断
子线程运行完毕
java.lang.InterruptedException
 at java.lang.Object.wait(Native Method)
 at java.lang.Thread.join(Thread.java:1252)
 at java.lang.Thread.join(Thread.java:1326)
 at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt.main(joinInterrupt.java:23)
启动
Process finished with exit code 0
* */
/*
* join
main中断
子线程运行完毕
java.lang.InterruptedException: sleep interrupted
 at java.lang.Thread.sleep(Native Method)
 at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt$1.run(joinInterrupt.java:13)
 at java.lang.Thread.run(Thread.java:748)
java.lang.InterruptedException
 at java.lang.Object.wait(Native Method)
 at java.lang.Thread.join(Thread.java:1252)
 at java.lang.Thread.join(Thread.java:1326)
 at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt.main(joinInterrupt.java:23)
Process finished with exit code 0
* */

join期间，线程处于WAITING状态

public class joinStates {
  public static void main(String[] args) throws InterruptedException {
      Thread main1 = Thread.currentThread();
      Thread thread = new Thread(new Runnable() {
          @Override
          public void run() {
              try {
                  Thread.sleep(3000);
                  System.out.println(main1.getState());
                  System.out.println("子线程运行结束");
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
          }
      });
      thread.start();
      System.out.println("join");
      thread.join();
      System.out.println("运行完毕");
  }
}
/*
* join
WAITING
子线程运行结束
运行完毕
* */

yield方法

用来释放CPU时间片，但是不一定能达到预期的效果，因为有时CPU资源不紧张，无需yield

和sleep的区别是：sleep期间不会被再次调度但是yield会立刻处于竞争状态，还会随时再次被调度

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