多线程的 waitset

忘是亡心i 2023-06-30 02:58 8阅读 0赞

## 多线程的waitset介绍 ##

waitset 在线程wait()方法执行后，会被放入被锁对象的一个waitset里面去，而当前wait()的位置也会被记录。

private final static Object object = new Object();
    
    
            IntStream.rangeClosed(1, 10).forEach(s -> { 
                new Thread(() -> { 
                    synchronized (object) { 
                        System.out.println(Thread.currentThread().getName() + " --- > will wait ....");
                        try { 
                            object.wait();
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
    
                        System.out.println(Thread.currentThread().getName() + " --- > will leave ....");
    
                    }
                }).start();
            });
    
    
            try { 
                Thread.sleep(4_000);
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
    
    
            IntStream.rangeClosed(1, 10).forEach(s -> { 
                new Thread(() -> { 
                    synchronized (object) { 
    
                        object.notify();
    
                        try { 
                            Thread.sleep(1000);
                        } catch (InterruptedException e) { 
                            e.printStackTrace();
                        }
    
                    }
    
                }).start();
            });
    
    
        }

验证线程从wait set中被唤醒的顺序不一定是FIFO

Thread-1 --- > will wait ....
    Thread-0 --- > will wait ....
    Thread-8 --- > will wait ....
    Thread-5 --- > will wait ....
    Thread-9 --- > will wait ....
    Thread-4 --- > will wait ....
    Thread-2 --- > will wait ....
    Thread-3 --- > will wait ....
    Thread-7 --- > will wait ....
    Thread-6 --- > will wait ....
    Thread-1 --- > will leave ....
    Thread-6 --- > will leave ....
    Thread-7 --- > will leave ....
    Thread-3 --- > will leave ....
    Thread-2 --- > will leave ....
    Thread-4 --- > will leave ....
    Thread-9 --- > will leave ....
    Thread-5 --- > will leave ....
    Thread-8 --- > will leave ....
    Thread-0 --- > will leave ....

线程被唤醒后，必须重新去获取锁,会记录之前wait的位置,在wait的位置继续往下执行

private final static Object object = new Object();
    
        void work() { 
    
            synchronized (object) { 
    
                System.out.println(" 开始执行 work ");
    
                try { 
                    object.wait();  //此处等待后，再次回唤醒后继续从当前位置执行。
                    System.out.println(" wait 。。。");
                } catch (InterruptedException e) { 
                    e.printStackTrace();
                }
    
                System.out.println(" 结束执行 work ");
            }
    
    
        }

结果

开始执行 wait 
     wait 。。。
     结束执行 wait

## 结论 ##

1、所有的对象都会有一个wait set，用来存放调用了该对象的wait方法之后进入block状态的线程

2、wait() 调用者是Lock对象，是释放锁的；sleep() 调用者是线程自己，但不释放锁。

3、线程被notify唤醒后，不一定立即执行。

唤醒顺序不是FIFO。

当线程重新获得锁后，也不会从头开始执行，而是从wait处之后的代码开始继续执行（执行地址恢复）。