走近Guava(六): 事件總線EventBus

EventBus:

建立EventBus實例：

EventBus eventBus = new EventBus();
//或者
EventBus eventBus = new EventBus(TradeAccountEvent.class.getName());//帶標識符,用於日誌記錄

訂閱事件：

• 模擬一個交易過程。

• 事件類：

/**
 * 事件類
 */
public class TradeAccountEvent {
	private double amount;
	private Date tradeExecutionTime;
	private TradeType tradeType;
	private TradeAccount tradeAccount;

	public TradeAccountEvent(TradeAccount account, double amount,
			Date tradeExecutionTime, TradeType tradeType) {
		this.amount = amount;
		this.tradeExecutionTime = tradeExecutionTime;
		this.tradeAccount = account;
		this.tradeType = tradeType;
	}
}

/**
 * 購買事件
 */
public class BuyEvent extends TradeAccountEvent {
	public BuyEvent(TradeAccount tradeAccount, double amount,
			Date tradExecutionTime) {
		super(tradeAccount, amount, tradExecutionTime, TradeType.BUY);
	}
}

/**
 * 賣出事件
 */
public class SellEvent extends TradeAccountEvent {
	public SellEvent(TradeAccount tradeAccount, double amount,
			Date tradExecutionTime) {
		super(tradeAccount, amount, tradExecutionTime, TradeType.SELL);
	}
}

• 訂閱者

/**
 * 賣出和購買審計，即訂閱者
 */
public class AllTradesAuditor {
	private List<BuyEvent> buyEvents = Lists.newArrayList();
	private List<SellEvent> sellEvents = Lists.newArrayList();

	public AllTradesAuditor(EventBus eventBus) {
		eventBus.register(this);
	}

	/**
	 * 訂閱賣出事件
	 */
	@Subscribe
	public void auditSell(SellEvent sellEvent) {
		sellEvents.add(sellEvent);
		System.out.println("Received TradeSellEvent " + sellEvent);
	}

	/**
	 * 訂閱購買事件
	 */
	@Subscribe
	public void auditBuy(BuyEvent buyEvent) {
		buyEvents.add(buyEvent);
		System.out.println("Received TradeBuyEvent " + buyEvent);
	}
}

• 發佈者

/**
 * 執行交易, 即發佈者
 */
public class SimpleTradeExecutor {
	private EventBus eventBus;

	public SimpleTradeExecutor(EventBus eventBus) {
		this.eventBus = eventBus;
	}

	/**
	 * 執行交易
	 */
	public void executeTrade(TradeAccount tradeAccount, double amount,
			TradeType tradeType) {
		TradeAccountEvent tradeAccountEvent = processTrade(tradeAccount,
				amount, tradeType);
		eventBus.post(tradeAccountEvent); // 發佈事件
	}

	/**
	 * 處理交易
	 * 
	 * @return 交易事件
	 */
	private TradeAccountEvent processTrade(TradeAccount tradeAccount,
			double amount, TradeType tradeType) {
		Date executionTime = new Date();
		String message = String.format(
				"Processed trade for %s of amount %n type %s @ %s",
				tradeAccount, amount, tradeType, executionTime);
		TradeAccountEvent tradeAccountEvent;
		if (tradeType.equals(TradeType.BUY)) { //購買動做
			tradeAccountEvent = new BuyEvent(tradeAccount, amount,
					executionTime);
		} else { //賣出動做
			tradeAccountEvent = new SellEvent(tradeAccount, amount,
					executionTime);
		}
		System.out.println(message);
		return tradeAccountEvent;
	}
}

• 測試用例

EventBus eventBus = new EventBus();
AllTradesAuditor auditor = new AllTradesAuditor(eventBus);
SimpleTradeExecutor tradeExecutor = new SimpleTradeExecutor(eventBus);
tradeExecutor.executeTrade(new TradeAccount(), 1000, TradeType.SELL);
tradeExecutor.executeTrade(new TradeAccount(), 2000, TradeType.BUY);

取消訂閱：

• 訂閱者來取消註冊

public void unregister(){
      this.eventBus.unregister(this);
}

AsyncEventBus類

• 聞其名，就是異步事件總線，當處理耗時的處理時頗有用，咱們要依賴Executors來實現異步事件總線

AsyncEventBus asyncEventBus = new AsyncEventBus(executorService);

DeadEvents:

• 當總線接收到發佈者發佈的信息時，但這時沒有訂閱者，那麼該事件會被包裝爲DeadEvent事件

public class DeadEventSubscriber {
	private static final Logger logger = 
			Logger.getLogger(DeadEventSubscriber.class.getName());

	public DeadEventSubscriber(EventBus eventBus) {
		eventBus.register(this);
	}
	
	/**
	 * 沒有訂閱者時被觸發
	 */
	@Subscribe
	public void handleUnsubscribedEvent(DeadEvent event){
		logger.warning("No subscribers for "+event.getEvent());
	}
}

依賴注入

• 咱們能夠經過DI框架(Spring或Guice)來注入一樣的EventBus

@Component
public class SimpleTradeExecutor {
       private EventBus eventBus;
       @Autowired
       public SimpleTradeExecutor(EventBus eventBus) {
           this.eventBus = checkNotNull(eventBus, "EventBus can't be null");                                             }
}

@Component
public class SimpleTradeAuditor {
       private List<TradeAccountEvent> tradeEvents =
    Lists.newArrayList();
    @Autowired
    public SimpleTradeAuditor(EventBus eventBus){
           checkNotNull(eventBus,"EventBus can't be null");
           eventBus.register(this);
   }
}

以上就介紹了Guava的EventBus。

不吝指正。