同步模式之保护性暂停

定义

即 Guarded Suspension，用在一个线程等待另一个线程的执行结果
要点

• 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
• 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
• 因为要等待另一方的结果，因此归类到同步模式
  

实现

class GuardedObject {private Object response;private final Object lock = new Object();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;}}public void complete(Object response) {synchronized (lock) {// 条件满足，通知等待线程this.response = response;lock.notifyAll();}}}

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();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

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);} 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 400timePassed = 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);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: 250008:49:40.917 [Thread-0] c.GuardedObjectV2 - notify...08:49:40.917 [main] c.GuardedObjectV2 - timePassed: 1003, object is null true08:49:40.917 [main] c.GuardedObjectV2 - waitTime: 149708:49:41.918 [Thread-0] c.GuardedObjectV2 - notify...08:49:41.918 [main] c.GuardedObjectV2 - timePassed: 2004, object is null false08:49:41.918 [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: 150008:47:55.963 [Thread-0] c.GuardedObjectV2 - notify...08:47:55.963 [main] c.GuardedObjectV2 - timePassed: 1002, object is null true08:47:55.963 [main] c.GuardedObjectV2 - waitTime: 49808:47:56.461 [main] c.GuardedObjectV2 - timePassed: 1500, object is null true08:47:56.461 [main] c.GuardedObjectV2 - waitTime: 008:47:56.461 [main] c.GuardedObjectV2 - break...08:47:56.461 [main] c.TestGuardedObjectV2 - can't get response08:47:56.963 [Thread-0] c.GuardedObjectV2 - notify...

扩展——原理之join

join的底层就是使用保护性暂停的设计模式

扩展——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。

中间用来解耦的类要维护一个集合，同时为了把多个guardedObject区分开，要加上id。

新增 id 用来标识 Guarded Object

 class GuardedObject {// 标识 Guarded Objectprivate 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 2000public Object get(long timeout) {synchronized (this) {// 开始时间 15:00:00long 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 idprivate 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();}

业务相关类

class People extends Thread{@Overridepublic 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;}}@Overridepublic 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);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:310:35:05.689 c.People [Thread-2] - 开始收信 id:110:35:05.689 c.People [Thread-0] - 开始收信 id:210:35:06.688 c.Postman [Thread-4] - 送信 id:2, 内容:内容210:35:06.688 c.Postman [Thread-5] - 送信 id:1, 内容:内容110:35:06.688 c.People [Thread-0] - 收到信 id:2, 内容:内容210:35:06.688 c.People [Thread-2] - 收到信 id:1, 内容:内容110:35:06.688 c.Postman [Thread-3] - 送信 id:3, 内容:内容310:35:06.689 c.People [Thread-1] - 收到信 id:3, 内容:内容3