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2. 规则定义
2.1. 定义规则
大多数业务规则可以用以下定义表示：
Name : 一个命名空间下的唯一的规则名称
Description : 规则的简要描述
Priority : 相对于其他规则的优先级
Facts : 事实，可立即为要处理的数据
Conditions : 为了应用规则而必须满足的一组条件
Actions : 当条件满足时执行的一组动作
Easy Rules为每个关键点提供了一个抽象来定义业务规则。
在Easy Rules中，Rule接口代表规则
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：
@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：
Rule rule = new RuleBuilder()
.name("myRule")
.description("myRuleDescription")
.priority(3)
.when(condition)
.then(action1)
.then(action2)
.build();
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：
//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);
每个规则都有优先级。它代表触发注册规则的默认顺序。默认情况下，较低的值表示较高的优先级。可以重写compareTo方法以提供自定义优先级策略。
2.2. 定义事实
在Easy Rules中，Fact API代表事实
public class Fact<T> {
private final String name;
private final T value;
}
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举个栗子：
Fact<String> fact = new Fact("foo", "bar");
Facts facts = new Facts();
facts.add(fact);
或者，也可以用这样简写形式
Facts facts = new Facts();
facts.put("foo", "bar");
用@Fact注解可以将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. 定义规则引擎
Easy Rules提供两种RulesEngine接口实现：
DefaultRulesEngine : 根据规则的自然顺序应用规则
InferenceRulesEngine : 持续对已知事实应用规则，直到不再适用任何规则为止
创建规则引擎：
RulesEngine rulesEngine = new DefaultRulesEngine();
// or
RulesEngine rulesEngine = new InferenceRulesEngine();
然后，注册规则
rulesEngine.fire(rules, facts);
规则引擎有一些可配置的参数，如下图所示：
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举个栗子：
RulesEngineParameters parameters = new RulesEngineParameters()
.rulePriorityThreshold(10)
.skipOnFirstAppliedRule(true)
.skipOnFirstFailedRule(true)
.skipOnFirstNonTriggeredRule(true);
RulesEngine rulesEngine = new DefaultRulesEngine(parameters);
2.4. 定义规则监听器
通过实现RuleListener接口
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>
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4. 扩展
规则本质上是一个函数，如y=f(x1,x2,..,xn)
规则引擎就是为了解决业务代码和业务规则分离的引擎，是一种嵌入在应用程序中的组件，实现了将业务决策从应用程序代码中分离。
还有一种常见的方式是Java+Groovy来实现，Java内嵌Groovy脚本引擎进行业务规则剥离。
