差異處

這裏顯示兩個版本的差異處。

--- java:guava:eventbusbasicusage [2020/10/25 22:50]
tony 建立
+++ java:guava:eventbusbasicusage [2023/06/25 09:48] (目前版本)
@@ 行 4: / 行 4: @@
 最近在看Domain Event的實作方式，剛好看到Teddy分享使用Guava的EventBus來處理事件的註冊與分派，於是花了一些時間實驗API的用法。以下內容分享給大家。Source code可以從[[https://github.com/frank007love/GuavaEventBusPractice|link]]下載。
 ===== How to? =====
+==== Event ====
+我的範例事件為MJGotGodGuyCardEvent:
+<code java>
+public class MJGotGodGuyCardEvent {
+    private final String girlName;
+    public MJGotGodGuyCardEvent(String girlName) {
+        this.girlName = girlName;
+    }
+    public String getGirlName() {
+        return girlName;
+    }
+}
+</code>
+==== Event Handler ====
+我的事件處理者有兩個，第一個為MJGoToPronhub。需注意的是:
+  * @Subscribe: 用以宣告處理函式，函式只允許一個參數，且要與Event物件相同。
+  * @AllowConcurrentEvents: 用以告知EventBus此函式可以接受Concurrent存取，預設會使用循序方式存取此函式。
+<code java>
+public class MJGoToPronhub {
+    private static Logger logger = LoggerFactory.getLogger(MJGoToPronhub.class);
+    private List<MJGotGodGuyCardEvent> receivedEvents = new CopyOnWriteArrayList<>();
+    @AllowConcurrentEvents
+    @Subscribe
+    public void handle(MJGotGodGuyCardEvent event) {
+        receivedEvents.add(event);
+        logger.info("MJGoToPronhub due to {}", event.getGirlName());
+    }
+    public List<MJGotGodGuyCardEvent> getReceivedEvents(){
+        return receivedEvents;
+    }
+}
+</code>
+另外一個實作內容類似，不做贅述，名稱為MJGoToWanhua。
+==== EventBus ====
+用法相當簡單，只要透過EventBus的register把處理物件進去後，在負責發送通知的client使用post即可"循序"的讓handler處理訊息:
+<code java>
+public class TestEventBus {
+    private EventBus eventBus = new EventBus();
+    private MJGoToPronhub MJGoToPronhubHandler = new MJGoToPronhub();
+    private MJGoToWanhua MJGoToWanhuaHandler = new MJGoToWanhua();
+    private void thenTheHandlerShouldReceiveTheEvent() {
+        assertEquals(1, MJGoToPronhubHandler.getReceivedEvents().size());
+        assertEquals(1, MJGoToWanhuaHandler.getReceivedEvents().size());
+    }
+    private void givenEventBusRegisterTwoGoodHandler(EventBus eventBus) {
+        eventBus.register(MJGoToPronhubHandler);
+        eventBus.register(MJGoToWanhuaHandler);
+    }
+    @Test
+    public void ShouldGetReceivedEventsWhenPostEventToHandlers() {
+        givenEventBusRegisterTwoGoodHandler(eventBus);
+        MJGotGodGuyCardEvent event = new MJGotGodGuyCardEvent("Nancy");
+        eventBus.post(event);
+        thenTheHandlerShouldReceiveTheEvent();
+    }
+}
+</code>
+==== AsyncEventBus ====
+假如覺得循序處理太慢，可以使用AsyncEventBus，使用方法與EventBus相同，但它post是non-blocking的:
+<code java>
+public class TestAsyncEventBus {
+    private AsyncEventBus asyncEventBus = new AsyncEventBus(Executors.newCachedThreadPool());
+    private MJGoToPronhub MJGoToPronhubHandler = new MJGoToPronhub();
+    private MJGoToWanhua MJGoToWanhuaHandler = new MJGoToWanhua();
+    private void thenTheHandlerShouldReceiveTheEvent() {
+        assertEquals(1, MJGoToPronhubHandler.getReceivedEvents().size());
+        assertEquals(1, MJGoToWanhuaHandler.getReceivedEvents().size());
+    }
+    private void givenEventBusRegisterTwoGoodHandler(EventBus eventBus) {
+        eventBus.register(MJGoToPronhubHandler);
+        eventBus.register(MJGoToWanhuaHandler);
+    }
+    private MJGotGodGuyCardEvent givenDelayedMJGotGodGuyCardEvent(CountDownLatch latch) {
+        return new MJGotGodGuyCardEvent("Nancy") {
+            @Override
+            public String getGirlName() {
+                try {
+                    TimeUnit.SECONDS.sleep(1);
+                    return super.getGirlName();
+                } catch (InterruptedException e) {
+                    throw new RuntimeException(e);
+                } finally {
+                    latch.countDown();
+                }
+            }
+        };
+    }
+    private void thenPostShouldBeNotBlocked(long startTime) {
+        long afterTime = System.currentTimeMillis();
+        assertTrue((afterTime-startTime)<1000);
+    }
+    private void thenPostShouldBeDoneWithParallel(long startTime, CountDownLatch latch) throws InterruptedException {
+        latch.await();
+        long afterTime = System.currentTimeMillis();
+        assertTrue((afterTime-startTime)>1000);
+    }
+    @Test
+    public void testAsyncEventBus() throws InterruptedException {
+        givenEventBusRegisterTwoGoodHandler(asyncEventBus);
+        CountDownLatch latch = new CountDownLatch(2);
+        MJGotGodGuyCardEvent event = givenDelayedMJGotGodGuyCardEvent(latch);
+        long startTime = System.currentTimeMillis();
+        asyncEventBus.post(event);
+        thenPostShouldBeNotBlocked(startTime);
+        thenPostShouldBeDoneWithParallel(startTime, latch);
+        thenTheHandlerShouldReceiveTheEvent();
+    }
+}
+</code>
+==== Notes ====
+  * 我使用的guava版本為30.0-jre。
+  * 預設情況下，Handler處理發生例外時，並不影響工作繼續進行。
+  * 可以透過實做SubscriberExceptionHandler去達到自己的例外處理需求，可由constructor去注入。
 ===== Reference =====