介绍一款基于MVEL简单规则引擎

参考原文A Really Simple But Powerful Rule Engine

MVEL 介绍

最近正好有做规则引擎相关的需求，预研了一下，并找到一些可以满足我需求的技术。引入一个东西——MVEL，怎么定义MVEL表达式，参看MVEL Guide。

来一段代码看一下MVEL的功能，首先引入MVEL解析库：

    compile 'org.mvel:mvel2:2.3.2.Final'

下图显示的是本文所用到的Java Bean类：

MVEL example:

 @Test    public void testMvel() {        String rule1 = "(!(input.person.age < 26) && !(input.person.age > 59)) && input.account.ageInMonths > 24";        // compile表达式 异常表达式会报错         Serializable serializable = MVEL.compileExpression(rule1);        TarifRequest request = new TarifRequest();        request.setPerson(new Person());        request.setAccount(new Account());        request.getPerson().setAge(30);        request.getPerson().setName("jack");        request.getAccount().setAgeInMonths(25);        Map<String, TarifRequest> vars = new HashMap<>();        vars.put("input", request);        boolean res = (boolean) MVEL.executeExpression(serializable, vars);        System.out.println(res);   // true        String rule2 = "input.person.name == 'tom'";        boolean res1 = (boolean) MVEL.executeExpression(MVEL.compileExpression(rule2), vars);        System.out.println(res1);  // false        String rule3 = "['jack', 'tom', 'hanson'].contains(input.person.name)";        boolean res2 = (boolean) MVEL.executeExpression(MVEL.compileExpression(rule3), vars);        System.out.println(res2);    //true        // MVEL.eval() 直接计算出表达式的值         System.out.println(MVEL.eval(rule2, vars));  //false    }

基于MVEL的 简单规则引擎

github源码地址
引入依赖：

    compile 'ch.maxant:rules:2.2.1'

1. 执行规则

这个规则引擎比较简单，我居然都能看懂里面的源码，看测试代码：

 @Test    public void testRules() throws Exception {        Rule r1 = new Rule("one", "input.person.age < 26", "YT2011", 3, "ch.maxant.someapp.tarifs", null);        Rule r2 = new Rule("two", "input.person.age > 59", "ST2011", 3, "ch.maxant.someapp.tarifs", null);        Rule r3 = new Rule("three", "!#one && !#two", "DT2011", 3, "ch.maxant.someapp.tarifs", null);        Rule r4 = new Rule("four", "#three && input.account.ageInMonths > 24", "LT2011", 4, "ch.maxant.someapp.tarifs", null);        // 默认规则        Rule defaultRule = new Rule("five", "true", "default", -1, "ch.maxant.someapp.tarifs", null);        List<Rule> rules = Arrays.asList(r1, r2, r3, r4, defaultRule);        // 初始化并用MVEL解析器编译规则        Engine engine = new Engine(rules, true);        TarifRequest request = new TarifRequest();        request.setPerson(new Person());        request.setAccount(new Account());        request.setDefaultRuleFlag(true);        request.getPerson().setAge(30);        request.getAccount().setAgeInMonths(5);        // 获取优先级最高的匹配规则        String tarif = engine.getBestOutcome(request);        System.out.println(tarif);    }

执行大概流程图如下：

当没有规则匹配的时候，就返回默认规则的结果。

2. 使用SubRule编写更复杂的规则

   @Test    public void testSubRules() throws Exception {        Rule rule1 = new SubRule("longdistance", "input.distance > 100", "ch.maxant.produkte", null);        Rule rule2 = new SubRule("firstclass", "input.map[\"travelClass\"] == 1", "ch.maxant.produkte", null);        Rule rule3 = new Rule("productA", "#longdistance && #firstclass", "productA", 3, "ch.maxant.produkte", null);        List<Rule> rules = Arrays.asList(rule1, rule2, rule3);        Engine e = new Engine(rules, true);        TravelRequest request = new TravelRequest(150);        request.put("travelClass", 1);        List<Rule> rs = e.getMatchingRules(request);        System.out.println(rs);    }

规则引擎在解析的时候，SubRule的规则都将被丢弃，只执行rule3的规则，rule3被替换成了：

Rule rule3 = new Rule("productA", "(input.distance > 100) && (input.map["travelClass"] == 1)", "productA", 3, "ch.maxant.produkte", null);

3. 根据规则执行具体的操作

@Test    public void testRulesAction() throws Exception {        Rule r1 = new Rule("SendEmailToUser", "input.config.sendUserEmail == true", "SendEmailToUser", 1, "ch.maxant.someapp.config", null);        Rule r2 = new Rule("SendEmailToModerator", "input.config.sendAdministratorEmail == true and input.user.numberOfPostings < 5", "SendEmailToModerator", 2, "ch.maxant.someapp.config", null);        List<Rule> rules = Arrays.asList(r1, r2);        final List<String> log = new ArrayList<>();        AbstractAction<ForumSetup, Void> a1 = new AbstractAction<ForumSetup, Void>("SendEmailToUser") {            @Override            public Void execute(ForumSetup input) {                log.add("Sending email to user!");                return null;            }        };        AbstractAction<ForumSetup, Void> a2 = new AbstractAction<ForumSetup, Void>("SendEmailToModerator") {            @Override            public Void execute(ForumSetup input) {                log.add("Sending email to moderator!");                return null;            }        };        Engine engine = new Engine(rules, true);        ForumSetup setup = new ForumSetup();        Config config = new Config();        setup.setConfig(config);        User user = new User();        setup.setUser(user);        setup.getConfig().setSendUserEmail(true);        setup.getConfig().setSendAdministratorEmail(true);        setup.getUser().setNumberOfPostings(2);        // executeBestAction() 执行优先级较高规则的action        engine.executeAllActions(setup, Arrays.asList(a1, a2));

执行操作也挺简单的，rule的outcome和action的name是一一对应的关系，从匹配到的规则里获取outcome，然后通过outcome获取action，最后action执行具体的操作，这个逻辑没毛病，但感觉rule和action有辣么一点耦合关系，有值得改进的地方，等大伙看源码去深挖吧。

其他的规则引擎：
easy-rules
Rulette