通过学习rete算法之后，这篇博客来介绍一下一个rete算法实现的规则引擎的框架Drools。

Drools是Jboss公司旗下一款开源的规则引擎，有如下特点；

1. 完整的实现了Rete算法；
2. 提供了强大的Eclipse Plugin开发支持；
3. 通过使用其中的DSL(Domain Specific Language)，可以实现用自然语言方式来描述业务规则，使得业务分析人员也可以看懂业务规则代码；
4. 提供了基于WEB的BRMS——Guvnor，Guvnor提供了规则管理的知识库，通过它可以实现规则的版本控制，及规则的在线修改与编译，使得开发人员和系统管理人员可以在线管理业务规则。

Drools 是业务逻辑集成平台，被分为4个项目：

1. Drools Guvnor (BRMS/BPMS)：业务规则管理系统
2. Drools Expert (rule engine)：规则引擎，drools的核心部分
3. Drools Flow (process/workflow)：工作流引擎
4. Drools Fusion (cep/temporal reasoning)：事件处理

Drools规则引擎的原理(原理解析见rete算法篇)

Drools 实例：

实体类：

package com.test;

public class Order {

	private String name = "";
	private Integer sumprice =0;
	private Integer DiscountPercent=0;;
	
	public Integer getSumprice() {
		return sumprice;
	}
	public void setSumprice(Integer sumprice) {
		this.sumprice = sumprice;
	}
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Integer getDiscountPercent() {
		return DiscountPercent;
	}
	public void setDiscountPercent(Integer DiscountPercent) {
		this.DiscountPercent = DiscountPercent;
	}
}

规则文件：

#created on: 2009-11-11
package com.test

#list any import classes here.
import com.test.Order;


#declare any global variables here




rule "First Rule"
	
	when
		#conditions
		order:Order(sumprice>30,sumprice<=50);
	then 
		#actions
		order.setDiscountPercent(98);
		
end

rule "Second Rule"
	#include attributes such as "salience" here...
	when
		#conditions
		order:Order(sumprice>50,sumprice<=100);
	then 
		#actions
		order.setDiscountPercent(95);
		
end

rule "third Rule"
	#include attributes such as "salience" here...
	when
		#conditions
		order:Order(sumprice>100);		
	then 
		#actions		
		order.setDiscountPercent(90);
end

测试类：

package com.test;

import java.util.Arrays;

import org.drools.KnowledgeBase;
import org.drools.KnowledgeBaseFactory;
import org.drools.builder.KnowledgeBuilder;
import org.drools.builder.KnowledgeBuilderFactory;
import org.drools.builder.ResourceType;
import org.drools.io.ResourceFactory;
import org.drools.runtime.StatelessKnowledgeSession;

public class ruleTest {
	public static final void main(String[] args) throws Exception {
		ruleTest launcher = new ruleTest();
	        launcher.executeExample();
	    }

	    public int executeExample() throws Exception {

	        
	        KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

	        kbuilder.add( ResourceFactory.newClassPathResource( "discountrule.drl",
	                                                            getClass() ),
	                      ResourceType.DRL);

	        if ( kbuilder.hasErrors() ) {
	            System.err.print( kbuilder.getErrors() );
	            return -1;
	        }

	        KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
	        kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );

	        
	        StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

	      
	        Order order = new Order();
	        order.setSumprice(159);
	        ksession.execute( Arrays.asList( new Object[]{order} ) );

	       
	        System.out.println( "DISCOUNT IS: " + order.getDiscountPercent() );

	        return order.getDiscountPercent();

	    }

	}

测试结果：

通过理论和demo应该对drools规则引擎有一定的了解，后面还会对drools规则引擎再介绍一篇博客，针对它的用法详细介绍一下。