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This article mainly explains "what is the function of Drools rule engine". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what is the role of the Drools rules engine?"

I. description and determination of rules

Rule constraints and decisions are done through DRL (Drools Rule Language) files, and a file can contain one or more rules. Typical description rules are in the following format:

Package: a package is a folder of related assets in Drools, such as data objects, DRL files, decision tables, and other asset types. It acts primarily as a unique namespace for each rule group, must be at the top of the file, independent of the path of the related classes written in Java, and is guaranteed to be unique.

Import: import packages or classes of classes that need to be used

Function: function definition, such as:

Function String hello (String applicantName) {return "Hello" + applicantName + "!";}

Query: you can query eligible result objects in the engine according to the name of the rule

Declare: you can declare a class that has the same functionality as a class defined using java. Such as:

Declare Cheese name: String price: int end

Global: defining global variables

Rule: a unique identity that defines this rule

When: conditional decision, such as Person (age = = 50). In a Person object, satisfying the age attribute equals 50.

Then: actions triggered when conditions are met, including add, delete and change actions

End: the identifier of the end of the rule execution.

II. Main components

The first contact with Drools is rather deceptive about various configurations and component relationships. Combined with personal understanding, a simple diagram is given, which may be biased. If there is anything wrong, please also point out:

Kmodule.xml: defined in / META-INF/kmodule.xml, configure one or more Kiebase, each Kiebase can be configured with multiple KieSession. KieBases and KieSessions can usually be generated dynamically at run time based on project requirements, relying on KieFileSystem to provide a variety of methods. Dynamically create the content of kmodule.xml and add it to KieFileSystem.

KieBase: a repository defined in the project's KIE module descriptor file (kmodule. XML) that contains everything in Drools, but does not contain any runtime data, and is available through KieContainer: KieBase kBase = kContainer.getKieBase ()

KieServices: interface that provides access to all creation and runtime objects, such as logs, resources, environment, containers, etc. Some of these methods fall into two categories: getX () and newX (), where get only returns a reference to the corresponding singleton object, and new recreates an object.

KieContainer: load engine-related components and obtain them through KieServices.Factory.get () to provide services to other Kie tools. KieContainer can get another KieContainer through the ReleaseId,Artifact,Version parameter:

KieSession: in Drools, KIE sessions store and execute run-time data. If a KIE session is defined in the project's KIE module descriptor file (kmodu. XML), you can create a KIE session from the KIE base or directly from the KIE container, breaking down stateful and stateless connections.

.drl file: can be added dynamically, KieResources provides a variety of factory ways to convert to InputStream, URL, File, String

After creating the .drl file, reload:

Third, the determination of nested rules

Nested rules can also be understood as multiple condition determination. This Drools can do well, mainly to understand the syntax rules of its drl file, and can well handle complex logical judgments, such as nested objects, collections and other common requirements. Here is a simple example:

1) define the User object as follows:

@ Data public class User {private String name; private Integer age; private Pet pet;}

2) define Pet as follows:

@ Datapublic class Pet {private String name; private String color;}

3) the definition rules are as follows:

Rule "nested multiple condition" when $user: User (age > 15 & & age

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