Google Guava EventBus簡化生產/消費者模式使用

在Google Guava 10版本引入了EventBus, 它主要用來簡化我們處理生產/消費者編程模型.

[b]基本用法[/b]
使用Guava之後, 如果要訂閱消息, 就不用再繼承指定的接口, 只需要在指定的方法上加上@Subscribe註解即可:

public class EventListener {

    public int lastMessage = 0;

    @Subscribe
    public void listen(OurTestEvent event) {
        lastMessage = event.getMessage();
    }

    public int getLastMessage() {
        return lastMessage;
    }
}

上面的lastMessage用來接收消息.

下面定義的類用來對消息進行封裝:

public class OurTestEvent {

    private final int message;

    public OurTestEvent(int message) {
        this.message = message;
    }

    public int getMessage() {
        return message;
    }
}

通過寫一個測試來了解EventBus如何工作:

@Test
public void shouldReceiveEvent() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    EventListener listener = new EventListener();

    eventBus.register(listener);

    // when
    eventBus.post(new OurTestEvent(200));

    // then
    assertThat(listener.getLastMessage()).isEqualTo(200);
}

上面的測試是不是很簡單?

[b]MultiListener的使用[/b]

只需要在要訂閱消息的方法上加上@Subscribe註解即可實現對多個消息的訂閱:

public class MultipleListener {

    public Integer lastInteger;
    public Long lastLong;

    @Subscribe
    public void listenInteger(Integer event) {
        lastInteger = event;
    }

    @Subscribe
    public void listenLong(Long event) {
        lastLong = event;
    }

    public Integer getLastInteger() {
        return lastInteger;
    }

    public Long getLastLong() {
        return lastLong;
    }
}

下面是對應的測試:

@Test
public void shouldReceiveMultipleEvents() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    MultipleListener multiListener = new MultipleListener();

    eventBus.register(multiListener);

    // when
    eventBus.post(new Integer(100));
    eventBus.post(new Long(800));

    // then
    assertThat(multiListener.getLastInteger()).isEqualTo(100);
    assertThat(multiListener.getLastLong()).isEqualTo(800L);
}

[b]高級用法[/b]

[b]1.Dead Event[/b]

如果EventBus發送的消息都不是訂閱者關心的稱之爲Dead Event. 看下面的例子:

/**
 * Listener waiting for the event that any message was posted but not delivered to anyone
 */
public class DeadEventListener {

    boolean notDelivered = false;

    @Subscribe
    public void listen(DeadEvent event) {
        notDelivered = true;
    }

    public boolean isNotDelivered() {
        return notDelivered;
    }
}

下面是測試類:

@Test
public void shouldDetectEventWithoutListeners() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");

    DeadEventListener deadEventListener = new DeadEventListener();
    eventBus.register(deadEventListener);

    // when
    eventBus.post(new OurTestEvent(200));

    assertThat(deadEventListener.isNotDelivered()).isTrue();
}

如果沒有消息訂閱者監聽消息, EventBus將發送DeadEvent消息, 這時我們可以通過log的方式來記錄這種狀態.

[b]2.Event的繼承[/b]

如果Listener A監聽Event A, 而Event A有一個子類Event B, 此時Listener A將同時接收Event A和B消息

看下面的例子:

public class NumberListener {

    private Number lastMessage;

    @Subscribe
    public void listen(Number integer) {
        lastMessage = integer;
    }

    public Number getLastMessage() {
        return lastMessage;
    }
}


public class IntegerListener {

    private Integer lastMessage;

    @Subscribe
    public void listen(Integer integer) {
        lastMessage = integer;
    }

    public Integer getLastMessage() {
        return lastMessage;
    }
}

對應的測試類:

@Test
public void shouldGetEventsFromSubclass() throws Exception {

    // given
    EventBus eventBus = new EventBus("test");
    IntegerListener integerListener = new IntegerListener();
    NumberListener numberListener = new NumberListener();
    eventBus.register(integerListener);
    eventBus.register(numberListener);

    // when
    eventBus.post(new Integer(100));

    // then
    assertThat(integerListener.getLastMessage()).isEqualTo(100);
    assertThat(numberListener.getLastMessage()).isEqualTo(100);

    //when
    eventBus.post(new Long(200L));

    // then
    // this one should has the old value as it listens only for Integers
    assertThat(integerListener.getLastMessage()).isEqualTo(100);
    assertThat(numberListener.getLastMessage()).isEqualTo(200L);
}

參考原文:[url]http://tomaszdziurko.pl/2012/01/google-guava-eventbus-easy-elegant-publisher-subscriber-cases/[/url]