设计模式之保护性暂停

1. 定义

即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给另一个线程，让他们关联到同一个GuarderObject（这就是保护性暂停模式，是两个线程之间交换结果的模式）
• 如果有结果不断从一个线程到另一个线程可以使用消息队列（这个是生产者-消费者模式）
• JDK中，Join实现，Futrue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

2. 实现保护性暂停模式

实现这个模式的关键是GuardedObject，response属性是用来保存中间结果。所以我们使用wait-notify来实现保护性暂停模式。

实现保护对象

class  GuardedObject{private Object response;//获取结果public Object get() {synchronized (this){while(response==null){try {this.wait();} catch (InterruptedException e) {e.printStackTrace();}}return  response;}}public void complete(Object response){synchronized (this){this.response=response;this.notify();}}
}

案例场景，线程1等待线程二的下载结果

public class jvm {public static List<String> downLoad() throws IOException {HttpURLConnection connection= (HttpURLConnection) new URL("https://www.baidu.com/").openConnection();List<String> list=new ArrayList<>();try(BufferedReader reader=new BufferedReader(new InputStreamReader(connection.getInputStream(), StandardCharsets.UTF_8))){String line;while((line= reader.readLine())!=null){list.add(line);}}return list;}public static void main(String[] args) {GuardedObject guardedObject=new GuardedObject();new Thread(()->{log.debug("等待结果");List<String> list= (List<String>) guardedObject.get();log.debug("结果大小,[{}]",list.size());},"线程1").start();new Thread(()->{log.debug("执行下载");try {List<String> list=downLoad();guardedObject.complete(list);} catch (IOException e) {e.printStackTrace();}},"线程2").start();}
}

3. Join原理

Join底层原理就是基于这种保护性暂停的模式，首先我们来看看Join的底层源码

public final synchronized void join(long millis)throws InterruptedException {//获得系统当前的时间戳long base = System.currentTimeMillis();//定义当前时间戳为0long now = 0;if (millis < 0) {throw new IllegalArgumentException("timeout value is negative");}//如果传入的等待时间为0if (millis == 0) {//如果线程是存活的就一直等待，调用wait(0)while (isAlive()) {wait(0);}} else {while (isAlive()) {long delay = millis - now;if (delay <= 0) {break;}wait(delay);now = System.currentTimeMillis() - base;}}}public final synchronized void join(long millis, int nanos)throws InterruptedException {if (millis < 0) {throw new IllegalArgumentException("timeout value is negative");}if (nanos < 0 || nanos > 999999) {throw new IllegalArgumentException("nanosecond timeout value out of range");}if (nanos >= 500000 || (nanos != 0 && millis == 0)) {millis++;}join(millis);}public final void join() throws InterruptedException {join(0);}

从源码可以看出，join的底层就是使用wait机制实现的。

4. 保护性暂停模式的扩展

途中Futures就好比居民楼的一层信箱（每个信箱都有自己的编号），左侧的t0，t2，t4就好比等待邮件的居民（等待结果的线程），右侧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，这样不仅仅能够解藕结果等待者和结果生产者，还能支持多个任务的管理。

改造GuardedObject类

class  GuardedObject{private Object response;private int id;public GuardedObject(){}public GuardedObject(int id){this.id=id;}public int getId(){return id;}//获取结果public Object get() {synchronized (this){while(response==null){try {this.wait();} catch (InterruptedException e) {e.printStackTrace();}}return  response;}}public void complete(Object response){synchronized (this){this.response=response;this.notify();}}
}

构造解耦类

class Boxes{private static Map<Integer,GuardedObject> box=new ConcurrentHashMap<>();//产生一个唯一的idpublic static int id=1;private static synchronized int increment(){return id++;}public static  GuardedObject getGuardedObject(int id){return box.remove(id);}public static GuardedObject creatGuardedObject(){GuardedObject guardedObject=new GuardedObject(increment());box.put(guardedObject.getId(),guardedObject);return guardedObject;}public static Set<Integer> getIds(){return box.keySet();}}

创造等待线程和生产线程

@Slf4j
class  PostMan extends Thread{private int id;private String mail_contex;//邮递员创建信件public PostMan(int id,String mail_contex){this.id=id;this.mail_contex=mail_contex;}@Overridepublic void run(){GuardedObject guardedObject=Boxes.getGuardedObject(id);log.debug("送信-{},内容-{}",id,mail_contex);guardedObject.complete(mail_contex);}
}
class Boxes{private static Map<Integer,GuardedObject> box=new Hashtable<>();//产生一个唯一的idpublic static int id=1;private static synchronized int increment(){return id++;}public static  GuardedObject getGuardedObject(int id){return box.remove(id);}public static GuardedObject creatGuardedObject(){GuardedObject guardedObject=new GuardedObject(increment());box.put(guardedObject.getId(),guardedObject);return guardedObject;}public static Set<Integer> getIds(){return box.keySet();}}

测试

public class jvm {public static void main(String[] args) throws InterruptedException {for (int i = 0; i < 3; i++) {new Poeple().start();}Thread.sleep(1000);for (Integer id : Boxes.getIds()) {new PostMan(id, "内容" + id).start();}}
}