固定运行顺序

wait-notify 实现

采用 wait-notify 机制可以实现对线程执行顺序的控制，我们只需要一个执行标志即可，如下代码所示：

public class WaitNotify {
    // 用来同步的对象
    static Object obj = new Object();
    // t2 运行标记， 代表 t2 是否执行过
    static boolean t2runed = false;
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            synchronized (obj) {
                // 如果 t2 没有执行过
                while (!t2runed) {
                    try {
                        // t1 先等一会
                        obj.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            System.out.println(1);
        });
        Thread t2 = new Thread(() -> {
            System.out.println(2);
            synchronized (obj) {
                // 修改运行标记
                t2runed = true;
                // 通知 obj 上等待的线程（可能有多个，因此需要用 notifyAll）
                obj.notifyAll();
            }
        });
        t1.start();
        t2.start();
    }
}

park-unpark 实现

public class ParkUnpark {
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            LockSupport.park();
            System.out.println("1");
        }, "t1");
        Thread t2 = new Thread(() -> {
            System.out.println("2");
            LockSupport.unpark(t1);
        }, "t2");
        t1.start();
        t2.start();
    }
}

交替运行顺序

要求依次“线程1输出a，线程2输出b，线程3输出c”，这样循环5次。即利用三个线程输出“abcabcabcabcabc”这样的结构。

wait-notify 实现

public class SyncWaitNotify {
    private int flag;
    private final int loopNums;
    public SyncWaitNotify(int flag, int loopNums) {
        this.flag = flag;
        this.loopNums = loopNums;
    }
    @SneakyThrows
    public void print(int curFlag, int nextFlag, String content) {
        for (int i = 0; i < loopNums; i++) {
            synchronized (this) {
                //如果curFlag 和当前线程执行 flag 不相同
                while (this.flag != curFlag) {
                    wait();
                }
                System.out.print(content);
                flag = nextFlag;
                notifyAll();
            }
        }
    }
    public static void main(String[] args) {
        SyncWaitNotify syncWaitNotify = new SyncWaitNotify(1, 5);
        //线程1打印 a -->flag=1
        new Thread(() -> {
            syncWaitNotify.print(1, 2, "a");
        }).start();
        //线程2打印 b -->flag=2
        new Thread(() -> {
            syncWaitNotify.print(2, 3, "b");
        }).start();
        //线程3打印 c -->flag=3
        new Thread(() -> {
            syncWaitNotify.print(3, 1, "c");
        }).start();
    }
}

await-signal 实现

public class AwaitSignal extends ReentrantLock {
    // 循环次数
    private final int loopNumber;
    public AwaitSignal(int loopNumber) {
        this.loopNumber = loopNumber;
    }
    public void start(Condition first) {
        this.lock();
        try {
            first.signal();
        } finally {
            this.unlock();
        }
    }
    public void print(String str, Condition current, Condition next) {
        for (int i = 0; i < loopNumber; i++) {
            this.lock();
            try {
                current.await();
                System.out.print(str);
                next.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                this.unlock();
            }
        }
    }
    public static void main(String[] args) {
        AwaitSignal as = new AwaitSignal(5);
        //线程1的等待室
        Condition aWaitSet = as.newCondition();
        //线程2的等待室
        Condition bWaitSet = as.newCondition();
        //线程3的等待室
        Condition cWaitSet = as.newCondition();
        //线程1打印 a
        new Thread(() -> {
            as.print("a", aWaitSet, bWaitSet);
        }).start();
        //线程2打印 b
        new Thread(() -> {
            as.print("b", bWaitSet, cWaitSet);
        }).start();
        //线程3打印 c
        new Thread(() -> {
            as.print("c", cWaitSet, aWaitSet);
        }).start();
        //先唤醒 aWaitSet 中的线程1
        as.start(aWaitSet);
    }
}

park-unpark 实现

public class SyncPark {
    private final int loopNumber;
    private Thread[] threads;
    public SyncPark(int loopNumber) {
        this.loopNumber = loopNumber;
    }
    public void setThreads(Thread... threads) {
        this.threads = threads;
    }
    public void print(String str) {
        for (int i = 0; i < loopNumber; i++) {
            LockSupport.park();
            System.out.print(str);
            //输出str后，让下一个线程运行
            LockSupport.unpark(nextThread());
        }
    }
    private Thread nextThread() {
        Thread current = Thread.currentThread();
        int index = 0;
        for (int i = 0; i < threads.length; i++) {
            if (threads[i] == current) {
                index = i;
                break;
            }
        }
        if (index < threads.length - 1) {
            return threads[index + 1];
        } else {
            return threads[0];
        }
    }
    public void start() {
        for (Thread thread : threads) {
            thread.start();
        }
        //先让 threads[0] 执行
        LockSupport.unpark(threads[0]);
    }
    public static void main(String[] args) {
        SyncPark syncPark = new SyncPark(5);
        Thread t1 = new Thread(() -> {
            syncPark.print("a");
        });
        Thread t2 = new Thread(() -> {
            syncPark.print("b");
        });
        Thread t3 = new Thread(() -> {
            syncPark.print("c");
        });
        syncPark.setThreads(t1, t2, t3);
        syncPark.start();
    }
}