expression language support

Easy Rules provides support for defining rules with MVEL and SpEL.

MVEL and SpEL support is provided through the easy-rules-mvel and easy-rules-spel modules respectively. These modules provide APIs to define conditions, actions and rules.

Define rules in a programmatic way

Conditions, actions and rules are represented respectively by the MVELCondition/SpELCondition, MVELAction/SpELAction and MVELRule/SpELRule classes. The following is an example of rule definition using MVEL (similar API for SpEL as well):

Rule ageRule = new MVELRule()
        .name("age rule")
        .description("Check if person's age is > 18 and marks the person as adult")
        .priority(1)
        .when("person.age > 18")
        .then("person.setAdult(true);");

Create rules from a rule descriptor

You can also define a rule in a descriptor file and use the MVELRuleFactory/SpELRuleFactory to create a rule from the descriptor. Here is an example of a rule defined in YAML format in alcohol-rule.yml:

name: "alcohol rule"
description: "children are not allowed to buy alcohol"
priority: 2
condition: "person.isAdult() == false"
actions:
  - "System.out.println(\"Shop: Sorry, you are not allowed to buy alcohol\");"

MVELRuleFactory ruleFactory = new MVELRuleFactory(new YamlRuleDefinitionReader());
MVELRule alcoholRule = ruleFactory.createRule(new FileReader("alcohol-rule.yml"));

You can also create multiple rules at the same time from a single file. For instance, here is a rules.yml file:

---
name: adult rule
description: when age is greater than 18, then mark as adult
priority: 1
condition: "person.age > 18"
actions:
  - "person.setAdult(true);"
---
name: weather rule
description: when it rains, then take an umbrella
priority: 2
condition: "rain == true"
actions:
  - "System.out.println(\"It rains, take an umbrella!\");"

To load these rules into a Rules object, you can use the following snippet:

MVELRuleFactory ruleFactory = new MVELRuleFactory(new YamlRuleDefinitionReader());
Rules rules = ruleFactory.createRules(new FileReader("rules.yml"));

You can find a complete example of how to use Easy Rules with MVEL in the shop tutorial.

Easy Rules provides support to load rules from a JSON descriptor as well. You can use the JsonRuleDefinitionReader with the MVELRuleFactory as follows:

MVELRuleFactory ruleFactory = new MVELRuleFactory(new JsonRuleDefinitionReader());

Please note that rules descriptors should be defined as an array of JSON object (each object defines a rule) even if a single rule is defined. Here is the previous alcohol-rule expressed in JSON:

[
  {
    "name": "alcohol rule",
    "description": "children are not allowed to buy alcohol",
    "priority": 2,
    "condition": "person.isAdult() == false",
    "actions": [
      "System.out.println(\"Shop: Sorry, you are not allowed to buy alcohol\");"
    ]
  }
]