1、定義
要點
-
有一個結果需要從一個線程傳遞到另一個線程，讓他們關聯同一個 GuardedObject
-
如果有結果不斷從一個線程到另一個線程那麼可以使用消息隊列
-
JDK 中，join 的實現、Future 的實現，採用的就是此模式
-
因爲要等待另一方的結果，因此歸類到同步模式
2、實現
class GuardedObject {
// 結果
private Object response;
private final Object lock = new Object();
// 獲取結果
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;
}
}
// 產生結果
public void complete(Object response) {
synchronized (lock) {
// 條件滿足，通知等待線程
this.response = response;
lock.notifyAll();
}
}
}3、應用
一個線程等待另一個線程的執行結果
public static void main(String[] args) {
GuardedObject guardedObject = new GuardedObject();
new Thread(() -> {
try {
// 子線程執行下載
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());
}執行結果
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] lines4、帶超時版 GuardedObject
如果要控制超時時間呢
class GuardedObjectV2 {
private Object response;
private final Object lock = new Object();
public Object get(long millis) {
synchronized (lock) {
// 1) 記錄最初時間
long begin = System.currentTimeMillis();
// 2) 已經經歷的時間
long timePassed = 0;
while (response == null) {
// 4) 假設 millis 是 1000，結果在 400 時喚醒了，那麼還有 600 要等
long waitTime = millis - timePassed;
log.debug("waitTime: {}", waitTime);
if (waitTime <= 0) {
log.debug("break...");
break;
}
try {
lock.wait(waitTime); // 注意這裏並不是 mills，防止虛假喚醒
} catch (InterruptedException e) {
e.printStackTrace();
}
// 3) 如果提前被喚醒，這時已經經歷的時間假設爲 400
timePassed = 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();
}
}
}測試，沒有超時
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測試，超時
// 等待時間不足
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、多任務版 GuardedObject
圖中 Futures 就好比居民樓一層的信箱（每個信箱有房間編號），左側的 t0，t2，t4 就好比等待郵件的居民，右側的 t1，t3，t5 就好比郵遞員
如果需要在多個類之間使用 GuardedObject 對象，作爲參數傳遞不是很方便，因此設計一個用來解耦的中間類，這樣不僅能夠解耦【結果等待者】和【結果生產者】，還能夠同時支持多個任務的管理
新增 id 用來標識 Guarded Object
class GuardedObject {
// 標識 Guarded Object
private int id;
public GuardedObject(int id) {
this.id = id;
}
public int getId() {
return id;
}
// 結果
private Object response;
// 獲取結果
// timeout 表示要等待多久 2000
public Object get(long timeout) {
synchronized (this) {
// 開始時間 15:00:00
long begin = System.currentTimeMillis();
// 經歷的時間
long passedTime = 0;
while (response == null) {
// 這一輪循環應該等待的時間
long waitTime = timeout - passedTime;
// 經歷的時間超過了最大等待時間時，退出循環
if (timeout - passedTime <= 0) {
break;
}
try {
this.wait(waitTime); // 虛假喚醒 15:00:01
} catch (InterruptedException e) {
e.printStackTrace();
}
// 求得經歷時間
passedTime = System.currentTimeMillis() - begin; // 15:00:02 1s
}
return response;
}
}
// 產生結果
public void complete(Object response) {
synchronized (this) {
// 給結果成員變量賦值
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;
// 產生唯一 id
private static synchronized int generateId() {
return id++;
}
public static GuardedObject getGuardedObject(int id) {
return boxes.remove(id); // 注意這裏的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();
}
}業務相關類
class People extends Thread{
@Override
public 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:{}, 內容:{}", id, mail);
guardedObject.complete(mail);
}
}測試
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, "內容" + id).start();
}
}某次運行結果
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, 內容:內容2
10:35:06.688 c.Postman [Thread-5] - 送信 id:1, 內容:內容1
10:35:06.688 c.People [Thread-0] - 收到信 id:2, 內容:內容2
10:35:06.688 c.People [Thread-2] - 收到信 id:1, 內容:內容1
10:35:06.688 c.Postman [Thread-3] - 送信 id:3, 內容:內容3
10:35:06.689 c.People [Thread-1] - 收到信 id:3, 內容:內容3