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關(guān)於信息/信件這擋事，讓我很快地想起古代的飛鴿傳書，電視電影常常都會(huì)帶到關(guān)鍵時(shí)刻讓救火信鴿扮演傳神的奇兵，然而他要是被人抓了煮熟吃了該怎麼辦 ?
在訊息導(dǎo)向中間件(MOM)的設(shè)計(jì)當(dāng)中，多半會(huì)特別去考慮到當(dāng)信息有發(fā)出，而對(duì)方?jīng)]收到時(shí)該怎麼做? Akka 也對(duì)此有基本的處理方式因應(yīng)，對(duì)於無(wú)法正確投遞的信件，會(huì)把它視為Dead Letter，系統(tǒng)開(kāi)發(fā)者可以針對(duì)特定的信息類型設(shè)計(jì)他對(duì)應(yīng)的處理機(jī)制，使得信息丟失時(shí)可被掌握。
package solid.humank.actor;
import akka.actor.AbstractActor;
import akka.actor.AbstractLoggingActor;
import akka.actor.Actor;
import akka.actor.DeadLetter;
/**
* Created by kim on 2017/4/25.
*/
public class MyDeadLetterListener extends AbstractLoggingActor {
@Override
public Receive createReceive() {
return receiveBuilder()
.match(DeadLetter.class, deadLetter -> {
log().info("received something: {}", deadLetter.message().toString());
})
.build();
}
}
在上面這個(gè)自訂的 Dead Letter 當(dāng)中，我們可以透過(guò)這方式去監(jiān)控死掉(已發(fā)送但收件者並沒(méi)有收件)的訊息，並自由決定你該對(duì)他做後續(xù)哪些處理。
@Test
public void dealWithDeadLetterByListener() {
final ActorSystem system = ActorSystem.create("sample");
final Props props = Props.create(ExplainActorLifeCycle.class);
final Props deadProps = Props.create(MyDeadLetterListener.class);
final TestActorRef<ExplainActorLifeCycle> ref = TestActorRef.create(system, props, "test");
final ActorRef deadLetterListener = TestActorRef.create(system, deadProps, "deadLetter");
system.eventStream().subscribe(deadLetterListener, DeadLetter.class);
assertTrue(system.eventStream().hasSubscriptions(deadLetterListener));
ref.tell(new String("Hello"), ref);
ref.tell(new String("Stop"), ref);
ref.tell(new String("Hello"), ref);
}
完成letter操作設(shè)計(jì)之後，試圖來(lái)做些驗(yàn)證，主要做法是把要通知的對(duì)象停停機(jī)，並試圖去看他的執(zhí)行結(jié)果：
- 對(duì)系統(tǒng)註冊(cè)你的 DeadLetter Listener，並告知要處理的類型(若有客製化內(nèi)容需要，可自行extends DeadLetter class)。
- akka framework 提供了方法讓你檢查你當(dāng)前的系統(tǒng)是否有註冊(cè)特定的事件，拿來(lái)查驗(yàn)還算可用的方法。
.... (略)
2017-05-15 15:13:06,134:INFO sample-akka.actor.default-dispatcher-2 (Slf4jLogger.scala:83) - received dead letter that receiever is not online : Hello
2017-05-15 15:13:06,135:INFO sample-akka.actor.default-dispatcher-2 (Slf4jLogger.scala:83) - Starting coordinated shutdown from JVM shutdown hook
2017-05-15 15:13:06,159 Thread-1 WARN Unable to register Log4j shutdown hook because JVM is shutting down. Using SimpleLogger
2017-05-15 15:13:06,170:INFO sample-akka.actor.default-dispatcher-2 (Slf4jLogger.scala:83) - Cluster Node [akka.tcp://sample@172.20.10.4:4711] - Exiting completed
2017-05-15 15:13:06,172:INFO sample-akka.actor.default-dispatcher-4 (Slf4jLogger.scala:83) - Cluster Node [akka.tcp://sample@172.20.10.4:4711] - Shutting down...
2017-05-15 15:13:06,174:INFO sample-akka.actor.default-dispatcher-4 (Slf4jLogger.scala:83) - Cluster Node [akka.tcp://sample@172.20.10.4:4711] - Successfully shut down
2017-05-15 15:13:06,183:INFO sample-akka.actor.default-dispatcher-4 (Slf4jLogger.scala:83) - Shutting down remote daemon.
2017-05-15 15:13:06,184:INFO sample-akka.actor.default-dispatcher-4 (Slf4jLogger.scala:83) - Remote daemon shut down; proceeding with flushing remote transports.
2017-05-15 15:13:06,203:INFO sample-akka.actor.default-dispatcher-3 (Slf4jLogger.scala:83) - Remoting shut down
[DEBUG] [05/15/2017 15:13:06.204] [sample-akka.actor.default-dispatcher-4] [EventStream] shutting down: StandardOutLogger started
2017-05-15 15:13:06,204:INFO sample-akka.actor.default-dispatcher-3 (Slf4jLogger.scala:83) - Remoting shut down.
2017-05-15 15:13:06,205:DEBUG sample-akka.actor.default-dispatcher-3 (Slf4jLogger.scala:88) - shutting down: StandardOutLogger started
[DEBUG] [05/15/2017 15:13:06.206] [sample-akka.actor.default-dispatcher-4] [EventStream] all default loggers stopped
Process finished with exit code 0
received dead letter that receiever is not online : Hello
**延伸思考，對(duì)於這類的非預(yù)期的結(jié)果(這不是意味著等同於錯(cuò)誤或例外)，在Enterprise Integration Pattern當(dāng)中多有介紹，接下來(lái)開(kāi)始透過(guò)akka來(lái)實(shí)踐這幾種實(shí)作模式。 **
