1、定義

即 Guarded Suspension，用在一個(gè)線程等待另一個(gè)線程的執(zhí)行結(jié)果

要點(diǎn)

• 有一個(gè)結(jié)果需要從一個(gè)線程傳遞到另一個(gè)線程，讓他們關(guān)聯(lián)同一個(gè) GuardedObject

• 如果有結(jié)果不斷從一個(gè)線程到另一個(gè)線程那么可以使用消息隊(duì)列

• JDK 中，join 的實(shí)現(xiàn)、Future 的實(shí)現(xiàn)，采用的就是此模式

• 因?yàn)橐却硪环降慕Y(jié)果，因此歸類到同步模式

2、實(shí)現(xiàn)

class GuardedObject {
?// 結(jié)果private Object response;private final Object lock = new Object();
?// 獲取結(jié)果public Object get() {synchronized (lock) {// 條件不滿足則等待while (response == null) {try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}return response;}}
?// 產(chǎn)生結(jié)果public void complete(Object response) {synchronized (lock) {// 條件滿足，通知等待線程this.response = response;lock.notifyAll();}}
}

3、應(yīng)用

一個(gè)線程等待另一個(gè)線程的執(zhí)行結(jié)果

public static void main(String[] args) {GuardedObject guardedObject = new GuardedObject();new Thread(() -> {try {// 子線程執(zhí)行下載List<String> response = download(); // 模擬下載操作log.debug("download complete...");guardedObject.complete(response);} catch (IOException e) {e.printStackTrace();}}).start();
?log.debug("waiting...");// 主線程阻塞等待Object response = guardedObject.get();log.debug("get response: [{}] lines", ((List<String>) response).size());
?
}

執(zhí)行結(jié)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、帶超時(shí)版 GuardedObject

如果要控制超時(shí)時(shí)間呢

class GuardedObjectV2 {
?private Object response;private final Object lock = new Object();
?public Object get(long millis) {synchronized (lock) {// 1) 記錄最初時(shí)間long begin = System.currentTimeMillis();// 2) 已經(jīng)經(jīng)歷的時(shí)間long timePassed = 0;while (response == null) {// 4) 假設(shè) millis 是 1000，結(jié)果在 400 時(shí)喚醒了，那么還有 600 要等long waitTime = millis - timePassed;log.debug("waitTime: {}", waitTime);if (waitTime <= 0) {log.debug("break...");break;}try {lock.wait(waitTime); ?// 注意這里并不是 mills，防止虛假喚醒} catch (InterruptedException e) {e.printStackTrace();}// 3) 如果提前被喚醒，這時(shí)已經(jīng)經(jīng)歷的時(shí)間假設(shè)為 400timePassed = System.currentTimeMillis() - begin;log.debug("timePassed: {}, object is null {}", timePassed, response == null);}return response;}}
?public void complete(Object response) {synchronized (lock) {// 條件滿足，通知等待線程this.response = response;log.debug("notify...");lock.notifyAll();}}
}

測(cè)試，沒有超時(shí)

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");}
}

輸出

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

測(cè)試，超時(shí)

// 等待時(shí)間不足
List<String> lines = v2.get(1500);

輸出

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、多任務(wù)版 GuardedObject

圖中 Futures 就好比居民樓一層的信箱（每個(gè)信箱有房間編號(hào)），左側(cè)的 t0，t2，t4 就好比等待郵件的居民，右側(cè)的 t1，t3，t5 就好比郵遞員

如果需要在多個(gè)類之間使用 GuardedObject 對(duì)象，作為參數(shù)傳遞不是很方便，因此設(shè)計(jì)一個(gè)用來解耦的中間類，這樣不僅能夠解耦【結(jié)果等待者】和【結(jié)果生產(chǎn)者】，還能夠同時(shí)支持多個(gè)任務(wù)的管理

新增 id 用來標(biāo)識(shí) Guarded Object

class GuardedObject {
?// 標(biāo)識(shí) Guarded Objectprivate int id;
?public GuardedObject(int id) {this.id = id;}
?public int getId() {return id;}
?// 結(jié)果private Object response;
?// 獲取結(jié)果// timeout 表示要等待多久 2000public Object get(long timeout) {synchronized (this) {// 開始時(shí)間 15:00:00long begin = System.currentTimeMillis();// 經(jīng)歷的時(shí)間long passedTime = 0;while (response == null) {// 這一輪循環(huán)應(yīng)該等待的時(shí)間long waitTime = timeout - passedTime;// 經(jīng)歷的時(shí)間超過了最大等待時(shí)間時(shí)，退出循環(huán)if (timeout - passedTime <= 0) {break;}try {this.wait(waitTime); // 虛假喚醒 15:00:01} catch (InterruptedException e) {e.printStackTrace();}// 求得經(jīng)歷時(shí)間passedTime = System.currentTimeMillis() - begin; // 15:00:02  1s}return response;}}
?// 產(chǎn)生結(jié)果public void complete(Object response) {synchronized (this) {// 給結(jié)果成員變量賦值this.response = response;this.notifyAll();}}
}

中間解耦類

class Mailboxes {private static Map<Integer, GuardedObject> boxes = new Hashtable<>();
?private static int id = 1;// 產(chǎn)生唯一 idprivate static synchronized int generateId() {return id++;}
?public static GuardedObject getGuardedObject(int id) {return boxes.remove(id); ?// 注意這里的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();}
}

業(yè)務(wù)相關(guān)類

class People extends Thread{@Overridepublic void run() {// 收信GuardedObject guardedObject = Mailboxes.createGuardedObject();log.debug("開始收信 id:{}", guardedObject.getId());Object mail = guardedObject.get(5000);log.debug("收到信 id:{}, 內(nèi)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:{}, 內(nèi)容:{}", id, mail);guardedObject.complete(mail);}
}

測(cè)試

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, "內(nèi)容" + id).start();}
}

某次運(yùn)行結(jié)果

10:35:05.689 c.People [Thread-1] - 開始收信 id:3
10:35:05.689 c.People [Thread-2] - 開始收信 id:1
10:35:05.689 c.People [Thread-0] - 開始收信 id:2
10:35:06.688 c.Postman [Thread-4] - 送信 id:2, 內(nèi)容:內(nèi)容2
10:35:06.688 c.Postman [Thread-5] - 送信 id:1, 內(nèi)容:內(nèi)容1
10:35:06.688 c.People [Thread-0] - 收到信 id:2, 內(nèi)容:內(nèi)容2
10:35:06.688 c.People [Thread-2] - 收到信 id:1, 內(nèi)容:內(nèi)容1
10:35:06.688 c.Postman [Thread-3] - 送信 id:3, 內(nèi)容:內(nèi)容3
10:35:06.689 c.People [Thread-1] - 收到信 id:3, 內(nèi)容:內(nèi)容3