虚假唤醒
例子
wait（）是object类自带的方法，在jdk有介绍，有可能出现中断、虚假唤醒
也就是在下面的例子中
if(number != 0){ this.wait(); }
当线程成功进入if语句块中，发生了中断，cpu跑去调度别的进程了，再次调度这个线程的时候，应该需要再经历一次if的判断，但是并没有这样做。
于是下面的程序运行结果：
/** * @author zkw * @Description TODO * */ public class ThreadWaitNotify { public static void main(String[] args) { Resource resource = new Resource(); //increment new Thread(()->{ for (int i = 0; i < 10; i++) { try { resource.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } },"A-1").start(); new Thread(()->{ for (int i = 0; i < 10; i++) { try { resource.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } },"A-2").start(); //decrement new Thread(()->{ for (int i = 0; i < 10; i++) { try { resource.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } } },"B-1").start(); new Thread(()->{ try { resource.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } },"B-2").start(); } } class Resource{ private int number = 0; //number++ public synchronized void increment() throws InterruptedException { if(number != 0){ this.wait(); } number++; System.out.println(Thread.currentThread().getName()+":"+number); this.notifyAll(); } //number-- public synchronized void decrement() throws InterruptedException { if(number == 0){ this.wait(); } number--; System.out.println(Thread.currentThread().getName()+":"+number); this.notifyAll(); } }
解决办法
按照jdk文档的指示，应该将if换成while
while(number == 0){ this.wait(); }
修改后，正确的运行结果如下：
精准唤醒
JUC那套 lock、await、signal和 Object那套 synchronized、wait、notify的区别：
condition这一套能实现精确唤醒某一个线程，而不是像notifyAll那样唤醒全部线程。
下面这个例子中，一共创建了三个Condition，可以根据业务对这三个Condition依次唤醒。来达到精准唤醒的功能
import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; /** * @author zkw * @Description 线程顺序访问 * 多线程之间的顺序调用 A-》B-》C * 三个线程启动，要求如下： * AA打印5次，BB打印10次，CC打印15次 * 接着 * AA打印5次，BB打印10次，CC打印15次 * ....来10轮 * * 1 高内聚低耦合的前提下，线程 操作 资源类 * 2 判断/干活/通知 * 3 多线程交互中，必须要防止多线程的虚假唤醒，也即（判断只用while不用if） * 4 标志位 * */ public class ThreadOrderAccess { public static void main(String[] args) { ShareResource shareResource = new ShareResource(); new Thread(()->{ try { for (int i = 0; i < 10; i++) { shareResource.printf5(); } } catch (InterruptedException e) { e.printStackTrace(); } },"Thread1").start(); new Thread(()->{ try { for (int i = 0; i < 10; i++) { shareResource.printf10(); } } catch (InterruptedException e) { e.printStackTrace(); } },"Thread2").start(); new Thread(()->{ try { for (int i = 0; i < 10; i++) { shareResource.printf15(); } } catch (InterruptedException e) { e.printStackTrace(); } },"Thread3").start(); } } class ShareResource{ private int number = 1; //1:A 2:B 3C private Lock lock = new ReentrantLock(); private Condition condition1 = lock.newCondition(); private Condition condition2 = lock.newCondition(); private Condition condition3 = lock.newCondition(); public void printf5() throws InterruptedException { lock.lock(); try { while (number!=1){ condition1.await(); } for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName()+"--AA"); } number = 2; condition2.signal(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void printf10() throws InterruptedException { lock.lock(); try { while (number!=2){ condition2.await(); } for (int i = 0; i < 10; i++) { System.out.println(Thread.currentThread().getName()+"--BB"); } number = 3; condition3.signal(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void printf15() throws InterruptedException { lock.lock(); try { while (number!=3){ condition3.await(); } for (int i = 0; i < 15; i++) { System.out.println(Thread.currentThread().getName()+"--CC"); } number = 1; condition1.signal(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } }
