public class SynchronizeDemo2 { public static void main(String[] args) { Data2 data = new Data2(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "A").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "B").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "C").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "D").start(); } } //线程操作的资源类,判断等待业务、通知 class Data2 { private int number = 0; //+1 public synchronized void increment() throws InterruptedException { if (number != 0) {//0 //等待 this.wait(); } number++; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我+1完毕了 this.notifyAll(); } //-1 public synchronized void decrement() throws InterruptedException { if (number == 0) { this.wait(); } number--; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我-1完毕了 this.notifyAll(); } } //执行以上的代码结果输出如下: A=>1 C=>0 B=>1 A=>2 B=>3 C=>2 C=>1 C=>0 B=>1 A=>2 B=>3 C=>2 B=>3 A=>4 D=>3 D=>2 D=>1 D=>0 A=>1 D=>0
站在上述两个消费者线程的角度上讲, 无论哪一个线程抢到了资源, 另一个线程的唤醒就可以被认为是没有必要的, 也就是被虚假唤醒了。
虚假唤醒发生的场景以及解决方式?
上述代码使用if判断,唤醒后线程会从wait之后的代码开始运行,但是不会重新判断if条件,直接继续运行if代码块之后的代码,而如果使用while的话,也会从wait之后的代码运行,但是唤醒后会重新判断循环条件,如果不成立再执行while代码块之后的代码块,成立的话继续wait。
public class SynchronizeDemo3 { public static void main(String[] args) { Data3 data = new Data3(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "A").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.decrement(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "B").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "C").start(); new Thread(() -> { for (int i = 0; i < 5; i++) { try { data.increment(); } catch (InterruptedException e) { e.printStackTrace(); } } }, "D").start(); } } //线程操作的资源类,判断等待业务、通知 class Data3 { private int number = 0; //+1 public synchronized void increment() throws InterruptedException { while (number != 0) {//0 //等待 this.wait(); } number++; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我+1完毕了 this.notifyAll(); } //-1 public synchronized void decrement() throws InterruptedException { while (number == 0) { this.wait(); } number--; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我-1完毕了 this.notifyAll(); } } //线程操作的资源类,判断等待业务、通知 class Data3 { private int number = 0; //+1 public synchronized void increment() throws InterruptedException { while (number != 0) {//0 //等待 this.wait(); } number++; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我+1完毕了 this.notifyAll(); } //-1 public synchronized void decrement() throws InterruptedException { while (number == 0) { this.wait(); } number--; System.out.println(Thread.currentThread().getName() + "=>" + number); //通知其他线程,我-1完毕了 this.notifyAll(); } } //多次运行代码输出如下: C=>1 B=>0 C=>1 A=>0 D=>1 B=>0 C=>1 A=>0 D=>1 B=>0 C=>1 A=>0 D=>1 B=>0 C=>1 B=>0 D=>1 A=>0 D=>1 A=>0