多线程同步模式-黑马深入学习Java并发编程笔记

同步模式

保护性暂停

单任务版

Guarded Suspension，用在一个线程等待另一个线程的执行结果

• 有一个结果需要从一个线程传递到另一个线程，让它们关联同一个 GuardedObject
• 如果有结果不断从一个线程到另一个线程那么可以使用消息队列（见生产者/消费者）
• JDK 中，join 的实现、Future 的实现，采用的就是此模式

public static void main(String[] args) {
    GuardedObject object = new GuardedObjectV2();
    new Thread(() -> {
        sleep(1);
        object.complete(Arrays.asList("a", "b", "c"));
    }).start();
    
    Object response = object.get(2500);
    if (response != null) {
        log.debug("get response: [{}] lines", ((List<String>) response).size());
    } else {
        log.debug("can't get response");
    }
}

class GuardedObject {
    private Object response;
    private final Object lock = new Object();

    //获取结果
    //timeout :最大等待时间
    public Object get(long millis) {
        synchronized (lock) {
            // 1) 记录最初时间
            long begin = System.currentTimeMillis();
            // 2) 已经经历的时间
            long timePassed = 0;
            while (response == null) {
                // 4) 假设 millis 是 1000，结果在 400 时唤醒了，那么还有 600 要等
                long waitTime = millis - timePassed;
                log.debug("waitTime: {}", waitTime);
                //经历时间超过最大等待时间退出循环
                if (waitTime <= 0) {
                    log.debug("break...");
                    break;
                }
                try {
                    lock.wait(waitTime);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                // 3) 如果提前被唤醒，这时已经经历的时间假设为 400
                timePassed = System.currentTimeMillis() - begin;
                log.debug("timePassed: {}, object is null {}",
                        timePassed, response == null);
            }
            return response;
        }
    }

    //产生结果
    public void complete(Object response) {
        synchronized (lock) {
            // 条件满足，通知等待线程
            this.response = response;
            log.debug("notify...");
            lock.notifyAll();
        }
    }
}

多任务版

多任务版保护性暂停：

public static void main(String[] args) throws InterruptedException {
    for (int i = 0; i < 3; i++) {
        new People().start();
    }
    Thread.sleep(1000);
    for (Integer id : Mailboxes.getIds()) {
        new Postman(id, id + "号快递到了").start();
    }
}

@Slf4j(topic = "c.People")
class People extends Thread{
    @Override
    public void run() {
        // 收信
        GuardedObject guardedObject = Mailboxes.createGuardedObject();
        log.debug("开始收信i d:{}", guardedObject.getId());
        Object mail = guardedObject.get(5000);
        log.debug("收到信id:{}，内容:{}", guardedObject.getId(),mail);
    }
}

class Postman extends Thread{
    private int id;
    private String mail;
    //构造方法
    @Override
    public void run() {
        GuardedObject guardedObject = Mailboxes.getGuardedObject(id);
        log.debug("开始送信i d:{}，内容:{}", guardedObject.getId(),mail);
        guardedObject.complete(mail);
    }
}

class  Mailboxes {
    private static Map<Integer, GuardedObject> boxes = new Hashtable<>();
    private static int id = 1;

    //产生唯一的id
    private static synchronized int generateId() {
        return id++;
    }

    public static GuardedObject getGuardedObject(int id) {
        return boxes.remove(id);
    }

    public static GuardedObject createGuardedObject() {
        GuardedObject go = new GuardedObject(generateId());
        boxes.put(go.getId(), go);
        return go;
    }

    public static Set<Integer> getIds() {
        return boxes.keySet();
    }
}
class GuardedObject {
    //标识，Guarded Object
    private int id;//添加get set方法
}

顺序输出

顺序输出 2 1

public static void main(String[] args) throws InterruptedException {
    Thread t1 = new Thread(() -> {
        while (true) {
            //try { Thread.sleep(1000); } catch (InterruptedException e) { }
            // 当没有许可时，当前线程暂停运行；有许可时，用掉这个许可，当前线程恢复运行
            LockSupport.park();
            System.out.println("1");
        }
    });
    Thread t2 = new Thread(() -> {
        while (true) {
            System.out.println("2");
            // 给线程 t1 发放『许可』（多次连续调用 unpark 只会发放一个『许可』）
            LockSupport.unpark(t1);
            try { Thread.sleep(500); } catch (InterruptedException e) { }
        }
    });
    t1.start();
    t2.start();
}

交替输出

连续输出 5 次 abc

public class day2_14 {
    public static void main(String[] args) throws InterruptedException {
        AwaitSignal awaitSignal = new AwaitSignal(5);
        Condition a = awaitSignal.newCondition();
        Condition b = awaitSignal.newCondition();
        Condition c = awaitSignal.newCondition();
        new Thread(() -> {
            awaitSignal.print("a", a, b);
        }).start();
        new Thread(() -> {
            awaitSignal.print("b", b, c);
        }).start();
        new Thread(() -> {
            awaitSignal.print("c", c, a);
        }).start();

        Thread.sleep(1000);
        awaitSignal.lock();
        try {
            a.signal();
        } finally {
            awaitSignal.unlock();
        }
    }
}

class AwaitSignal extends ReentrantLock {
    private int loopNumber;

    public AwaitSignal(int loopNumber) {
        this.loopNumber = loopNumber;
    }
    //参数1：打印内容  参数二：条件变量  参数二：唤醒下一个
    public void print(String str, Condition condition, Condition next) {
        for (int i = 0; i < loopNumber; i++) {
            lock();
            try {
                condition.await();
                System.out.print(str);
                next.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                unlock();
            }
        }
    }
}

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