Java規則引擎 Easy Rules-有解無憂

1. Easy Rules 概述

Easy Rules是一個Java規則引擎，靈感來自一篇名為《Should I use a Rules Engine?》的文章

規則引擎就是提供一種可選的計算模型，與通常的命令式模型（由帶有條件和回圈的命令依次組成）不同，規則引擎基于生產規則系統，這是一組生產規則，每條規則都有一個條件（condition）和一個動作（action）———— 簡單地說，可以將其看作是一組if-then陳述句，

精妙之處在于規則可以按任何順序撰寫，引擎會決定何時使用對順序有意義的任何方式來計算它們，考慮它的一個好方法是系統運行所有規則，選擇條件成立的規則，然后執行相應的操作，這樣做的好處是，很多問題都很自然地符合這個模型：
if car.owner.hasCellPhone then premium += 100;
if car.model.theftRating > 4 then premium += 200;
if car.owner.livesInDodgyArea && car.model.theftRating > 2 then premium += 300;
規則引擎是一種工具，它使得這種計算模型編程變得更容易，它可能是一個完整的開發環境，或者一個可以在傳統平臺上作業的框架，生產規則計算模型最適合僅解決一部分計算問題，因此規則引擎可以更好地嵌入到較大的系統中，

你可以自己構建一個簡單的規則引擎，你所需要做的就是創建一組帶有條件和動作的物件，將它們存盤在一個集合中，然后遍歷它們以評估條件并執行這些動作，

Easy Rules它提供Rule抽象以創建具有條件和動作的規則，并提供RuleEngine API，該API通過一組規則運行以評估條件并執行動作，

Easy Rules簡單易用，只需兩步：

首先，定義規則，方式有很多種

方式一：注解

@Rule(name = "weather rule", description = "if it rains then take an umbrella")
public class WeatherRule {

    @Condition
    public boolean itRains(@Fact("rain") boolean rain) {
        return rain;
    }
    
    @Action
    public void takeAnUmbrella() {
        System.out.println("It rains, take an umbrella!");
    }
}

方式二：鏈式編程

Rule weatherRule = new RuleBuilder()
        .name("weather rule")
        .description("if it rains then take an umbrella")
        .when(facts -> facts.get("rain").equals(true))
        .then(facts -> System.out.println("It rains, take an umbrella!"))
        .build();

方式三：運算式

Rule weatherRule = new MVELRule()
        .name("weather rule")
        .description("if it rains then take an umbrella")
        .when("rain == true")
        .then("System.out.println(\"It rains, take an umbrella!\");");

方式四：yml組態檔

例如：weather-rule.yml

name: "weather rule"
description: "if it rains then take an umbrella"
condition: "rain == true"
actions:
  - "System.out.println(\"It rains, take an umbrella!\");"

MVELRuleFactory ruleFactory = new MVELRuleFactory(new YamlRuleDefinitionReader());
Rule weatherRule = ruleFactory.createRule(new FileReader("weather-rule.yml"));

接下來，應用規則

public class Test {
    public static void main(String[] args) {
        // define facts
        Facts facts = new Facts();
        facts.put("rain", true);

        // define rules
        Rule weatherRule = ...
        Rules rules = new Rules();
        rules.register(weatherRule);

        // fire rules on known facts
        RulesEngine rulesEngine = new DefaultRulesEngine();
        rulesEngine.fire(rules, facts);
    }
}

入門案例：Hello Easy Rules

<dependency>
    <groupId>org.jeasy</groupId>
    <artifactId>easy-rules-core</artifactId>
    <version>4.0.0</version>
</dependency>

通過骨架創建maven專案：

mvn archetype:generate \
    -DarchetypeGroupId=org.jeasy \
    -DarchetypeArtifactId=easy-rules-archetype \
    -DarchetypeVersion=4.0.0

默認給我們生成了一個HelloWorldRule規則，如下：

package com.cjs.example.rules;

import org.jeasy.rules.annotation.Action;
import org.jeasy.rules.annotation.Condition;
import org.jeasy.rules.annotation.Rule;

@Rule(name = "Hello World rule", description = "Always say hello world")
public class HelloWorldRule {

    @Condition
    public boolean when() {
        return true;
    }

    @Action
    public void then() throws Exception {
        System.out.println("hello world");
    }

}

2. 規則定義

2.1. 定義規則

大多數業務規則可以用以下定義表示：

Name : 一個命名空間下的唯一的規則名稱
Description : 規則的簡要描述
Priority : 相對于其他規則的優先級
Facts : 事實，可立即為要處理的資料
Conditions : 為了應用規則而必須滿足的一組條件
Actions : 當條件滿足時執行的一組動作

Easy Rules為每個關鍵點提供了一個抽象來定義業務規則，

在Easy Rules中，Rule介面代表規則

public interface Rule {

    /**
    * This method encapsulates the rule's conditions.
    * @return true if the rule should be applied given the provided facts, false otherwise
    */
    boolean evaluate(Facts facts);

    /**
    * This method encapsulates the rule's actions.
    * @throws Exception if an error occurs during actions performing
    */
    void execute(Facts facts) throws Exception;

    //Getters and setters for rule name, description and priority omitted.

}

evaluate方法封裝了必須計算結果為TRUE才能觸發規則的條件，execute方法封裝了在滿足規則條件時應該執行的動作，條件和操作由Condition和Action介面表示，

定義規則有兩種方式：

通過在POJO類上添加注解
通過RuleBuilder API編程

可以在一個POJO類上添加@Rule注解，例如：

@Rule(name = "my rule", description = "my rule description", priority = 1)
public class MyRule {

    @Condition
    public boolean when(@Fact("fact") fact) {
        //my rule conditions
        return true;
    }

    @Action(order = 1)
    public void then(Facts facts) throws Exception {
        //my actions
    }

    @Action(order = 2)
    public void finally() throws Exception {
        //my final actions
    }

}

@Condition注解指定規則條件
@Fact注解指定引數
@Action注解指定規則執行的動作

RuleBuilder支持鏈式風格定義規則，例如：

Rule rule = new RuleBuilder()
                .name("myRule")
                .description("myRuleDescription")
                .priority(3)
                .when(condition)
                .then(action1)
                .then(action2)
                .build();

組合規則

CompositeRule由一組規則組成，這是一個典型地組合設計模式的實作，

組合規則是一個抽象概念，因為可以以不同方式觸發組合規則，

Easy Rules自帶三種CompositeRule實作：

UnitRuleGroup : 要么應用所有規則，要么不應用任何規則（AND邏輯）
ActivationRuleGroup : 它觸發第一個適用規則，并忽略組中的其他規則（XOR邏輯）
ConditionalRuleGroup : 如果具有最高優先級的規則計算結果為true，則觸發其余規則

復合規則可以從基本規則創建并注冊為常規規則：

//Create a composite rule from two primitive rules
UnitRuleGroup myUnitRuleGroup = new UnitRuleGroup("myUnitRuleGroup", "unit of myRule1 and myRule2");
myUnitRuleGroup.addRule(myRule1);
myUnitRuleGroup.addRule(myRule2);

//Register the composite rule as a regular rule
Rules rules = new Rules();
rules.register(myUnitRuleGroup);

RulesEngine rulesEngine = new DefaultRulesEngine();
rulesEngine.fire(rules, someFacts);

每個規則都有優先級，它代表觸發注冊規則的默認順序，默認情況下，較低的值表示較高的優先級，可以重寫compareTo方法以提供自定義優先級策略，

2.2. 定義事實

在Easy Rules中，Fact API代表事實

public class Fact<T> {
     private final String name;
     private final T value;
}

舉個栗子：

Fact<String> fact = new Fact("foo", "bar");
Facts facts = new Facts();
facts.add(fact);

或者，也可以用這樣簡寫形式

Facts facts = new Facts();
facts.put("foo", "bar");

用@Fact注解可以將Facts注入到condition和action方法中

@Rule
class WeatherRule {

    @Condition
    public boolean itRains(@Fact("rain") boolean rain) {
        return rain;
    }

    @Action
    public void takeAnUmbrella(Facts facts) {
        System.out.println("It rains, take an umbrella!");
        // can add/remove/modify facts
    }

}

2.3. 定義規則引擎

Easy Rules提供兩種RulesEngine介面實作：

DefaultRulesEngine : 根據規則的自然順序應用規則
InferenceRulesEngine : 持續對已知事實應用規則，直到不再適用任何規則為止

創建規則引擎：

RulesEngine rulesEngine = new DefaultRulesEngine();

// or

RulesEngine rulesEngine = new InferenceRulesEngine();

然后，注冊規則

rulesEngine.fire(rules, facts);

規則引擎有一些可配置的引數，如下圖所示：

舉個栗子：

RulesEngineParameters parameters = new RulesEngineParameters()
    .rulePriorityThreshold(10)
    .skipOnFirstAppliedRule(true)
    .skipOnFirstFailedRule(true)
    .skipOnFirstNonTriggeredRule(true);

RulesEngine rulesEngine = new DefaultRulesEngine(parameters);

2.4. 定義規則監聽器

通過實作RuleListener介面

public interface RuleListener {

    /**
     * Triggered before the evaluation of a rule.
     *
     * @param rule being evaluated
     * @param facts known before evaluating the rule
     * @return true if the rule should be evaluated, false otherwise
     */
    default boolean beforeEvaluate(Rule rule, Facts facts) {
        return true;
    }

    /**
     * Triggered after the evaluation of a rule.
     *
     * @param rule that has been evaluated
     * @param facts known after evaluating the rule
     * @param evaluationResult true if the rule evaluated to true, false otherwise
     */
    default void afterEvaluate(Rule rule, Facts facts, boolean evaluationResult) { }

    /**
     * Triggered on condition evaluation error due to any runtime exception.
     *
     * @param rule that has been evaluated
     * @param facts known while evaluating the rule
     * @param exception that happened while attempting to evaluate the condition.
     */
    default void onEvaluationError(Rule rule, Facts facts, Exception exception) { }

    /**
     * Triggered before the execution of a rule.
     *
     * @param rule the current rule
     * @param facts known facts before executing the rule
     */
    default void beforeExecute(Rule rule, Facts facts) { }

    /**
     * Triggered after a rule has been executed successfully.
     *
     * @param rule the current rule
     * @param facts known facts after executing the rule
     */
    default void onSuccess(Rule rule, Facts facts) { }

    /**
     * Triggered after a rule has failed.
     *
     * @param rule the current rule
     * @param facts known facts after executing the rule
     * @param exception the exception thrown when attempting to execute the rule
     */
    default void onFailure(Rule rule, Facts facts, Exception exception) { }

}

3. 示例

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.cjs.example</groupId>
    <artifactId>easy-rules-quickstart</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <dependencies>
        <dependency>
            <groupId>org.jeasy</groupId>
            <artifactId>easy-rules-core</artifactId>
            <version>4.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.jeasy</groupId>
            <artifactId>easy-rules-support</artifactId>
            <version>4.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.jeasy</groupId>
            <artifactId>easy-rules-mvel</artifactId>
            <version>4.0.0</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-simple</artifactId>
            <version>1.7.30</version>
        </dependency>
    </dependencies>
</project>

4. 擴展

規則本質上是一個函式，如y=f(x1,x2,..,xn)

規則引擎就是為了解決業務代碼和業務規則分離的引擎，是一種嵌入在應用程式中的組件，實作了將業務決策從應用程式代碼中分離，

還有一種常見的方式是Java+Groovy來實作，Java內嵌Groovy腳本引擎進行業務規則剝離，

https://github.com/j-easy/easy-rules/wiki

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標籤：Java

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