1、定义
要点
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有一个结果需要从一个线程传递到另一个线程,让他们关联同一个 GuardedObject
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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
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JDK 中,join 的实现、Future 的实现,采用的就是此模式
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因为要等待另一方的结果,因此归类到同步模式
2、实现
class GuardedObject { // 结果 private Object response; private final Object lock = new Object(); // 获取结果 public Object get() { synchronized (lock) { // 条件不满足则等待 while (response == null) { try { lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } 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; lock.notifyAll(); } } }
3、应用
一个线程等待另一个线程的执行结果
public static void main(String[] args) { GuardedObject guardedObject = new GuardedObject(); new Thread(() -> { try { // 子线程执行下载 List<String> response = download(); // 模拟下载操作 log.debug("download complete..."); guardedObject.complete(response); } catch (IOException e) { e.printStackTrace(); } }).start(); log.debug("waiting..."); // 主线程阻塞等待 Object response = guardedObject.get(); log.debug("get response: [{}] lines", ((List<String>) response).size()); }
执行结果
08:42:18.568 [main] c.TestGuardedObject - waiting... 08:42:23.312 [Thread-0] c.TestGuardedObject - download complete... 08:42:23.312 [main] c.TestGuardedObject - get response: [3] lines
4、带超时版 GuardedObject
如果要控制超时时间呢
class GuardedObjectV2 { private Object response; private final Object lock = new Object(); 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); if (waitTime <= 0) { log.debug("break..."); break; } try { lock.wait(waitTime); // 注意这里并不是 mills,防止虚假唤醒 } 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; } } 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(); } } }
测试,没有超时
public static void main(String[] args) { GuardedObjectV2 v2 = new GuardedObjectV2(); new Thread(() -> { sleep(1); // 睡眠1秒 v2.complete(null); sleep(1); v2.complete(Arrays.asList("a", "b", "c")); }).start(); Object response = v2.get(2500); if (response != null) { log.debug("get response: [{}] lines", ((List<String>) response).size()); } else { log.debug("can't get response"); } }
输出
08:49:39.917 [main] c.GuardedObjectV2 - waitTime: 2500 08:49:40.917 [Thread-0] c.GuardedObjectV2 - notify... 08:49:40.917 [main] c.GuardedObjectV2 - timePassed: 1003, object is null true 08:49:40.917 [main] c.GuardedObjectV2 - waitTime: 1497 08:49:41.918 [Thread-0] c.GuardedObjectV2 - notify... 08:49:41.918 [main] c.GuardedObjectV2 - timePassed: 2004, object is null false 08:49:41.918 [main] c.TestGuardedObjectV2 - get response: [3] lines
测试,超时
// 等待时间不足 List<String> lines = v2.get(1500);
输出
08:47:54.963 [main] c.GuardedObjectV2 - waitTime: 1500 08:47:55.963 [Thread-0] c.GuardedObjectV2 - notify... 08:47:55.963 [main] c.GuardedObjectV2 - timePassed: 1002, object is null true 08:47:55.963 [main] c.GuardedObjectV2 - waitTime: 498 08:47:56.461 [main] c.GuardedObjectV2 - timePassed: 1500, object is null true 08:47:56.461 [main] c.GuardedObjectV2 - waitTime: 0 08:47:56.461 [main] c.GuardedObjectV2 - break... 08:47:56.461 [main] c.TestGuardedObjectV2 - can't get response 08:47:56.963 [Thread-0] c.GuardedObjectV2 - notify...
5、多任务版 GuardedObject
图中 Futures 就好比居民楼一层的信箱(每个信箱有房间编号),左侧的 t0,t2,t4 就好比等待邮件的居民,右侧的 t1,t3,t5 就好比邮递员
如果需要在多个类之间使用 GuardedObject 对象,作为参数传递不是很方便,因此设计一个用来解耦的中间类,这样不仅能够解耦【结果等待者】和【结果生产者】,还能够同时支持多个任务的管理
新增 id 用来标识 Guarded Object
class GuardedObject { // 标识 Guarded Object private int id; public GuardedObject(int id) { this.id = id; } public int getId() { return id; } // 结果 private Object response; // 获取结果 // timeout 表示要等待多久 2000 public Object get(long timeout) { synchronized (this) { // 开始时间 15:00:00 long begin = System.currentTimeMillis(); // 经历的时间 long passedTime = 0; while (response == null) { // 这一轮循环应该等待的时间 long waitTime = timeout - passedTime; // 经历的时间超过了最大等待时间时,退出循环 if (timeout - passedTime <= 0) { break; } try { this.wait(waitTime); // 虚假唤醒 15:00:01 } catch (InterruptedException e) { e.printStackTrace(); } // 求得经历时间 passedTime = System.currentTimeMillis() - begin; // 15:00:02 1s } return response; } } // 产生结果 public void complete(Object response) { synchronized (this) { // 给结果成员变量赋值 this.response = response; this.notifyAll(); } } }
中间解耦类
class Mailboxes { private static Map<Integer, GuardedObject> boxes = new Hashtable<>(); private static int id = 1; // 产生唯一 id private static synchronized int generateId() { return id++; } public static GuardedObject getGuardedObject(int id) { return boxes.remove(id); // 注意这里的remove,防止堆溢出 } public static GuardedObject createGuardedObject() { GuardedObject go = new GuardedObject(generateId()); boxes.put(go.getId(), go); return go; } public static Set<Integer> getIds() { return boxes.keySet(); } }
业务相关类
class People extends Thread{ @Override public void run() { // 收信 GuardedObject guardedObject = Mailboxes.createGuardedObject(); log.debug("开始收信 id:{}", 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; public Postman(int id, String mail) { this.id = id; this.mail = mail; } @Override public void run() { GuardedObject guardedObject = Mailboxes.getGuardedObject(id); log.debug("送信 id:{}, 内容:{}", id, mail); guardedObject.complete(mail); } }
测试
public static void main(String[] args) throws InterruptedException { for (int i = 0; i < 3; i++) { new People().start(); } Sleeper.sleep(1);// 睡眠1秒 for (Integer id : Mailboxes.getIds()) { new Postman(id, "内容" + id).start(); } }
某次运行结果
10:35:05.689 c.People [Thread-1] - 开始收信 id:3 10:35:05.689 c.People [Thread-2] - 开始收信 id:1 10:35:05.689 c.People [Thread-0] - 开始收信 id:2 10:35:06.688 c.Postman [Thread-4] - 送信 id:2, 内容:内容2 10:35:06.688 c.Postman [Thread-5] - 送信 id:1, 内容:内容1 10:35:06.688 c.People [Thread-0] - 收到信 id:2, 内容:内容2 10:35:06.688 c.People [Thread-2] - 收到信 id:1, 内容:内容1 10:35:06.688 c.Postman [Thread-3] - 送信 id:3, 内容:内容3 10:35:06.689 c.People [Thread-1] - 收到信 id:3, 内容:内容3