Drools学习(一)

Drool是jboss的规则引擎, 老早就想看看了, 前断时间忙的一直没有时间.现在抽空学习一下

什么是规则引擎, rete算法等等就交给google了.

鄙人是实干家.
一. 环境配置.
最快的方法:
下载Drool的eclipse插件:Drools 4.0.7 Eclipse Workbench for 3.2
http://www.jboss.org/drools/downloads.html

此插件功能不错, 写rule很方便, 并且能创建 drool project, 所有依赖的jar包都有包含.

?二: hello world
晦涩的理论先放一边, 跑一个hello world再说:

package com.sample;import java.io.InputStreamReader;import java.io.Reader;import org.drools.RuleBase;import org.drools.RuleBaseFactory;import org.drools.WorkingMemory;import org.drools.compiler.PackageBuilder;import org.drools.rule.Package;/** * This is a sample file to launch a rule package from a rule source file. */public class DroolsTest {    public static final void main(String[] args) {        try {                       //load up the rulebase            RuleBase ruleBase = readRule();            WorkingMemory workingMemory = ruleBase.newStatefulSession();                       //go !            Message message = new Message();            message.setMessage(  "Hello World" );            message.setStatus( Message.HELLO );            workingMemory.insert( message );            workingMemory.fireAllRules();                                } catch (Throwable t) {            t.printStackTrace();        }    }    /**     * Please note that this is the "low level" rule assembly API.     */    private static RuleBase readRule() throws Exception {        //read in the source        Reader source = new InputStreamReader( DroolsTest.class.getResourceAsStream( "/Sample.drl" ) );               //optionally read in the DSL (if you are using it).        //Reader dsl = new InputStreamReader( DroolsTest.class.getResourceAsStream( "/mylang.dsl" ) );        //Use package builder to build up a rule package.        //An alternative lower level class called "DrlParser" can also be used...               PackageBuilder builder = new PackageBuilder();        //this wil parse and compile in one step        //NOTE: There are 2 methods here, the one argument one is for normal DRL.        builder.addPackageFromDrl( source );        //Use the following instead of above if you are using a DSL:        //builder.addPackageFromDrl( source, dsl );               //get the compiled package (which is serializable)        Package pkg = builder.getPackage();               //add the package to a rulebase (deploy the rule package).        RuleBase ruleBase = RuleBaseFactory.newRuleBase();        ruleBase.addPackage( pkg );        return ruleBase;    }       public static class Message {        public static final int HELLO = 0;        public static final int GOODBYE = 1;        public static final int GAME_OVER = 2;               private String message;               private int status;               public String getMessage() {            return this.message;        }               public void setMessage(String message) {            this.message = message;        }               public int getStatus() {            return this.status;        }               public void setStatus( int status ) {            this.status = status;        }    }   }

rule文件:
?? Sample.drl

package com.sample import com.sample.DroolsTest.Message; rule "Hello World"    when        m : Message( status == Message.HELLO, message : message )    then        System.out.println( message );        m.setMessage( "Goodbye cruel world" );        m.setStatus( Message.GOODBYE );        update( m );endrule "GoodBye"    no-loop true    when        m : Message( status == Message.GOODBYE, message : message )    then        System.out.println( message );        m.setStatus(Message.GAME_OVER);        m.setMessage("game over now!");           update( m );   endrule "game over"    when        m : Message( status == Message.GAME_OVER)    then        System.out.println( m.getMessage() );        end

java代码就不说了, 主要说明一下rule文件, 借此熟悉一下rule语法:
1:规则的主体结构:
rule "name"
??? ATTRIBUTES
??? when
??????? LHS
??? then
??????? RHS
end

其中:
LHS(left hand side)是规则的条件部分, RHS(right hand side)是条件满足后的自行块.

2: 条件的书写:
m : Message( status == Message.HELLO, message : message )
意思是:
在working Memory中, 如果有一个Message的instance满足如下条件:
它的status属性等于Message.HELLO.
满足条件的Message instance 用变量 m表示, 它的message属性用message变量表示, 以便在RHS中使用.
?
3: 条件满足后的自行块(RHS)

         System.out.println( message );        m.setMessage( "Goodbye cruel world" );        m.setStatus( Message.GOODBYE );        update( m );

用到了在LHS中 定义的变量: m, message
update( m )表示把此Message instance在 working memory中更新.这便会触发名称为"GoodBye"的rule, 进而触发名称为"game over"的rule.

今天就到这里吧.

下课!