规则引擎Drools入门笔记（1）

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开发环境：IntelliJ IDEA，Java 8，Gradle，SpringBoot，Drools。

新建 Gradle 工程后，配置 SpringBoot 和 Drools 的引入：

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compile 'org.springframework.boot:spring-boot:1.5.9.RELEASE'
compile 'org.springframework.boot:spring-boot-starter:1.5.9.RELEASE'
compile 'org.springframework.boot:spring-boot-starter-log4j2:1.5.9.RELEASE'

compile 'org.kie:kie-api:7.5.0.Final'
compile 'org.drools:drools-compiler:7.5.0.Final'

配置 Main 类：

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package demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Main {

    public static void main(String[] args) {
        SpringApplication.run(Main.class, args);
    }

}

配置一个 Bean 类：

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package demo;

public class Message {

    public final static int HELLO = 1;
    public final static int GOODBYE = 2;

    private String message;
    private int status;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public int getStatus() {
        return status;
    }

    public void setStatus(int status) {
        this.status = status;
    }

}

配置 META-INF/kmodule.xml：

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<?xml version="1.0" encoding="UTF-8" ?>
<kmodule xmlns="http://www.drools.org/xsd/kmodule">
    <kbase name="demo" packages="rules">
        <ksession name="demo_ksession"/>
    </kbase>
</kmodule>

配置对应 packages「rules」下的规则文件：

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package rules;

import demo.Message;

rule "rule1"
    salience 1
    when
        $message: Message(status == Message.HELLO)
    then
        System.out.println("Status is 'Hello', set 'Hello World!' to message");
        $message.setMessage("Hello World!");
        System.out.println("Message: " + $message.getMessage());
end

配置 Demo 运行类：

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package demo;

import org.kie.api.KieServices;
import org.kie.api.runtime.KieContainer;
import org.kie.api.runtime.KieSession;
import org.springframework.boot.CommandLineRunner;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

@Component
@Order(1)
public class DroolsDemo implements CommandLineRunner{

    @Override
    public void run(String... args) throws Exception {

        KieServices ks = KieServices.Factory.get();
        KieContainer kieContainer = ks.getKieClasspathContainer();
        KieSession kieSession = kieContainer.newKieSession("demo_ksession");

        Message message = new Message();
        message.setStatus(Message.HELLO);
        kieSession.insert(message);
        kieSession.fireAllRules();
        kieSession.dispose();

    }

}

启动运行，输出：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!

完成。

把规则文件原有的规则调整一下，再增加一个新规则，如下所示：

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package rules;

import demo.Message;

rule "rule1"
    salience 1
    when
        $message: Message(status == Message.HELLO)
    then
        System.out.println("Status is 'Hello', set 'Hello World!' to message");
        $message.setMessage("Hello World!");
        System.out.println("Message: " + $message.getMessage());
        System.out.println("Then set 'Goodbye' to status");
        $message.setStatus(Message.GOODBYE);
        update($message);
end

rule "rule2"
    salience 1
    when
        $message: Message(status == Message.GOODBYE)
    then
        System.out.println("Status is 'Goodbye', set 'Goodbye World!' to message");
        $message.setMessage("Goodbye World!");
        System.out.println("Message: " + $message.getMessage());
end

再次运行，输出：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!

即 rule1 中的update($message)会使$message 对象再次经过规则验证。

如果把 rule2 改成和 rule1 类似的状况，如下所示：

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package rules;

import demo.Message;

rule "rule1"
   salience 1
   when
       $message: Message(status == Message.HELLO)
   then
       System.out.println("Status is 'Hello', set 'Hello World!' to message");
       $message.setMessage("Hello World!");
       System.out.println("Message: " + $message.getMessage());
       System.out.println("Then set 'Goodbye' to status");
       $message.setStatus(Message.GOODBYE);
       update($message);
end

rule "rule2"
   salience 1
   when
       $message: Message(status == Message.GOODBYE)
   then
       System.out.println("Status is 'Goodbye', set 'Goodbye World!' to message");
       $message.setMessage("Goodbye World!");
       System.out.println("Message: " + $message.getMessage());
       System.out.println("Then set 'Hello' to status");
       $message.setStatus(Message.HELLO);
       update($message);
end<

输出：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!
Then set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!
Then set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
......

这样造成了验证一直在循环。如果需要不让它一直循环的话，需要在 rule 的开头配置no-loop或lock-on-active。这两个属性在没有配置的情况下，默认为false，即不会阻止循环。
但要注意，no-loop只会在 rule 调用自己的时候才会阻止循环，lock-on-active则是其它 rule 调用该 rule 的时候也会阻止循环，使该 rule 只执行一次。

no-loop 例子：

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package rules;

import demo.Message;

rule "rule1"
   salience 1
   no-loop true
when
       $message: Message(status == Message.HELLO)
   then
       System.out.println("Status is 'Hello', set 'Hello World!' to message");
       $message.setMessage("Hello World!");
       System.out.println("Message: " + $message.getMessage());

        System.out.println("Then create a new Message and set 'Hello' to status");
        Message message2 = new Message();
        message2.setStatus(Message.HELLO);
        insert(message2);
end

当把no-loop true一句注释掉时，输出结果为：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then create a new Message and set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then create a new Message and set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
......

造成了循环，将注释解开后，输出结果为：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then create a new Message and set 'Hello' to status

没有出现循环。

lock-on-active 例子：

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package rules;

import demo.Message;

rule "rule1"
   salience 1
   no-loop true
   lock-on-active true
   when
       $message: Message(status == Message.HELLO)
   then
       System.out.println("Status is 'Hello', set 'Hello World!' to message");
       $message.setMessage("Hello World!");
       System.out.println("Message: " + $message.getMessage());
       System.out.println("Then set 'Goodbye' to status");
       $message.setStatus(Message.GOODBYE);
       update($message);
end

rule "rule2"
   salience 1
   when
       $message: Message(status == Message.GOODBYE)
   then
       System.out.println("Status is 'Goodbye', set 'Goodbye World!' to message");
       $message.setMessage("Goodbye World!");
       System.out.println("Message: " + $message.getMessage());
       System.out.println("Then set 'Hello' to status");
       $message.setStatus(Message.HELLO);
       update($message);
end

当把lock-on-active true一句注释掉时，输出结果为：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!
Then set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!
Then set 'Hello' to status
Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
......

即 rule1 中的 no-loop true 并没有阻止其被 rule2 调用，仍然造成了循环。当把注释解开后，输出结果则为：

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Status is 'Hello', set 'Hello World!' to message
Message: Hello World!
Then set 'Goodbye' to status
Status is 'Goodbye', set 'Goodbye World!' to message
Message: Goodbye World!
Then set 'Hello' to status

rule1 执行后调用了 rule2，rule2 没有再使 rule1 出现循环，rule1 仅执行了一次。

本 Demo 地址：https://github.com/HerbertGao/DroolsDemo