本文主要是介绍Java规则引擎 Easy Rules，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
Easy Rules简单易用，只需两步：
首先，定义规则，方式有很多种
方式一：注解
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@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!");
}
}
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方式二：链式编程
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：表达式
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配置文件
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”));
接下来，应用规则
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public class Test {
public static void main(String[] args) {
// define facts
Facts facts = new Facts();
facts.put(“rain”, true);
// define rulesRule weatherRule = ...Rules rules = new Rules();rules.register(weatherRule);// fire rules on known factsRulesEngine rulesEngine = new DefaultRulesEngine();rulesEngine.fire(rules, facts);
}
}
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入门案例：Hello Easy Rules
mvn archetype:generate
-DarchetypeGroupId=org.jeasy
-DarchetypeArtifactId=easy-rules-archetype
-DarchetypeVersion=4.0.0
默认给我们生成了一个HelloWorldRule规则，如下：
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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");
}
}
在Easy Rules中，Rule接口代表规则
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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.
}
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evaluate方法封装了必须计算结果为TRUE才能触发规则的条件。execute方法封装了在满足规则条件时应该执行的动作。条件和操作由Condition和Action接口表示。
定义规则有两种方式：
通过在POJO类上添加注解
通过RuleBuilder API编程
可以在一个POJO类上添加@Rule注解，例如：
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@Rule(name = “my rule”, description = “my rule description”, priority = 1)
public class MyRule {
@Condition
public boolean when(@Fact("fact") fact) {//my rule conditionsreturn true;
}@Action(order = 1)
public void then(Facts facts) throws Exception {//my actions
}@Action(order = 2)
public void finally() throws Exception {//my final actions
}
}
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@Condition注解指定规则条件
@Fact注解指定参数
@Action注解指定规则执行的动作
RuleBuilder支持链式风格定义规则，例如：
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Rule rule = new RuleBuilder()
.name(“myRule”)
.description(“myRuleDescription”)
.priority(3)
.when(condition)
.then(action1)
.then(action2)
.build();
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组合规则
CompositeRule由一组规则组成。这是一个典型地组合设计模式的实现。
组合规则是一个抽象概念，因为可以以不同方式触发组合规则。
Easy Rules自带三种CompositeRule实现：
UnitRuleGroup : 要么应用所有规则，要么不应用任何规则（AND逻辑）
ActivationRuleGroup : 它触发第一个适用规则，并忽略组中的其他规则（XOR逻辑）
ConditionalRuleGroup : 如果具有最高优先级的规则计算结果为true，则触发其余规则
复合规则可以从基本规则创建并注册为常规规则：
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//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);
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