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》

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