Guava ---- EventBus事件驱动模型

在软件开发过程中， 难免有信息的共享或者对象间的协作。 怎样让对象间信息共享高效， 而且耦合性低。 这是一个难题。 而耦合性高将带来编码改动牵一发而动全身的连锁效应。 Spring的风靡正是由于攻克了高耦合问题。 本篇介绍的EventBus中也用到了Spring中的依赖注入。 来进行对象和对象间的解耦（如@Subscribe）。

Guava解决高耦合採用的是事件驱动模型的思路。 对象能够订阅（subscribe）特定的事件或者公布（publish）特定的事件去被消费。 从以下的代码能够看出， EventBus对生产者和消费者是透明的， 它无需知道他们的类型。 从而实现了解耦。

TradeAccountEvent： 基本对象兼測试类

package com.wenniuwuren.eventbus;
import com.google.common.eventbus.EventBus;

import java.util.Date;

/**
 *  无论什么时候买卖交易运行， 都会产生一个TradeAccountEvent实例
 */
public class TradeAccountEvent {
    private double amount;
    private Date tradeExecutionTime;
    private String tradeType;
    private String tradeAccount;

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

    public static void main(String[] args) {
        // 消费者和生产者依据EventBus对象来一一相应
        EventBus eventBus1 = new EventBus();
        SimpleTradeAuditor simpleTradeAuditor = new SimpleTradeAuditor(eventBus1);
        SimpleTradeExecutor simpleTradeExecutor = new SimpleTradeExecutor(eventBus1);
        simpleTradeExecutor.executeTrade("zhangsan", 10, "Money");

        System.out.println("----This is devil dividing line------");

        EventBus eventBus2 = new EventBus();
        BuySellTradeExecutor buySellTradeExecutor = new BuySellTradeExecutor(eventBus2);
        AllTradesAuditor allTradesAuditor = new AllTradesAuditor(eventBus2);
        buySellTradeExecutor.executeTrade("lisi", 100, "SELL");
        System.out.println("---------------------");
        buySellTradeExecutor.executeTrade("wangwu", 1000, "BUY");
    }
}

AllTradesAuditor：依据不同生产者订阅不同内容

package com.wenniuwuren.eventbus;

import java.util.List;

import com.google.common.collect.Lists;
import com.google.common.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

public class AllTradesAuditor {
	private List<BuyEvent> buyEvents = Lists.newArrayList();
	private List<SellEvent> sellEvents = Lists.newArrayList();

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

	@Subscribe
	public void auditSell(SellEvent sellEvent) {
		sellEvents.add(sellEvent);
		System.out.println("Received TradeSellEvent " + sellEvent);
	}

	@Subscribe
	public void auditBuy(BuyEvent buyEvent) {
		buyEvents.add(buyEvent);
		System.out.println("Received TradeBuyEvent " + buyEvent);
	}
}

BuyEvent：

package com.wenniuwuren.eventbus;

import java.util.Date;

/**
 * 购买事件
 * @author wenniuwuren
 *
 */
public class BuyEvent extends TradeAccountEvent {
	public BuyEvent(String tradeAccount, double amount,
			Date tradExecutionTime) {
		super(tradeAccount, amount, tradExecutionTime, "BUY");
	}
}

SellEvent：

package com.wenniuwuren.eventbus;

import java.util.Date;

/**
 * 销售事件
 * @author wenniuwuren
 *
 */
public class SellEvent extends TradeAccountEvent {
	public SellEvent(String tradeAccount, double amount, Date tradExecutionTime) {
		super(tradeAccount, amount, tradExecutionTime, "SELL");
	}
}

BuySellTradeExecutor： 分类型（BUY、SELL）公布事件

package com.wenniuwuren.eventbus;

import java.util.Date;

import com.google.common.eventbus.EventBus;

/**
 * 分类型（SELL BUY）运行器
 * @author wenniuwuren
 *
 */

public class BuySellTradeExecutor {

	private EventBus eventBus;

    public BuySellTradeExecutor(EventBus eventBus) {

        this.eventBus = eventBus;
    }

	private TradeAccountEvent processTrade(String tradeAccount, double amount,
			String tradeType) {
		Date executionTime = new Date();
		String message = String.format("Processed trade for" + tradeAccount
				+ "of amount" + amount + "type" + tradeType + "@"
				+ executionTime);
		TradeAccountEvent tradeAccountEvent;
		if (tradeType.equals("BUY")) {
			tradeAccountEvent = new BuyEvent(tradeAccount, amount,
					executionTime);
		} else {
			tradeAccountEvent = new SellEvent(tradeAccount, amount,
					executionTime);
		}
		System.out.println(message);
		return tradeAccountEvent;
	}

	public void executeTrade(String tradeAccount, double amount, String tradeType) {
        TradeAccountEvent tradeAccountEvent = processTrade(tradeAccount, amount, tradeType);
        // 公布, 通知订阅者
        eventBus.post(tradeAccountEvent);
    }
}

SimpleTradeAuditor： 最简单的事件消费者（或者说订阅者）

package com.wenniuwuren.eventbus;
import com.google.common.collect.Lists;
import com.google.common.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

import java.util.List;

/**
 * 审核交易
 */
public class SimpleTradeAuditor {
    private List<TradeAccountEvent> tradeEvents = Lists.newArrayList();

    public SimpleTradeAuditor(EventBus eventBus) {
        // 注冊， 以便获取TradeAccountEvent的通知
        eventBus.register(this);
    }

    /**
     * 事件处理（用@Subscribe注解表示）
     * @param tradeAccountEvent
     */
    @Subscribe
    public void auditTrade(TradeAccountEvent tradeAccountEvent) {
        tradeEvents.add(tradeAccountEvent);
        System.out.println("Received trade " + tradeAccountEvent);
    }
}

TradeBuyAuditor：分类型事件消费

package com.wenniuwuren.eventbus;

import java.util.List;

import com.google.common.collect.Lists;
import com.google.common.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

/**
 * 购买审查
 * @author wenniuwuren
 *
 */
public class TradeBuyAuditor {
	private List<BuyEvent> buyEvents = Lists.newArrayList();

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

	@Subscribe
	public void auditBuy(BuyEvent buyEvent) {
		buyEvents.add(buyEvent);
		System.out.println("Received TradeBuyEvent " + buyEvent);
	}

	public List<BuyEvent> getBuyEvents() {
		return buyEvents;
	}
}

TradeSellAuditor：

package com.wenniuwuren.eventbus;

import java.util.List;

import com.google.common.collect.Lists;
import com.google.common.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

/**
 * 销售审查
 * @author wenniuwuren
 *
 */
public class TradeSellAuditor {
	private List<SellEvent> sellEvents = Lists.newArrayList();

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

	@Subscribe
	public void auditSell(SellEvent sellEvent) {
		sellEvents.add(sellEvent);
		System.out.println("Received SellEvent " + sellEvent);
	}

	public List<SellEvent> getSellEvents() {
		return sellEvents;
	}
}

输出结果：

Processed trade forzhangsanof amount10.0typeMoney@Fri Jun 12 02:29:03 CST 2015
Received trade com.wenniuwuren.eventbus.TradeAccountEvent@7c53a9eb
----This is devil dividing line------
Processed trade forlisiof amount100.0typeSELL@Fri Jun 12 02:29:03 CST 2015
Received TradeSellEvent com.wenniuwuren.eventbus.SellEvent@14899482
---------------------
Processed trade forwangwuof amount1000.0typeBUY@Fri Jun 12 02:29:03 CST 2015
Received TradeBuyEvent com.wenniuwuren.eventbus.BuyEvent@21588809

參考资料：

《Getting Started with Google Guava》