feat: add rule engine design pattern by MohamedMBG · Pull Request #3548 · iluwatar/java-design-patterns

MohamedMBG · 2026-07-23T01:20:46Z

Adds a new rule-engine module implementing the Rule Engine design pattern.

Rule<T> abstraction separating evaluate (decision) from execute (action)
Three concrete rules: minimum age, minimum income, credit score
RuleEngine<T> evaluates all rules in insertion order (no short-circuit) against a LoanApplication context
Immutable RuleEngineResult reporting approved flag plus passed/failed rule names
Defensive copies / immutability throughout; null contexts and null rules rejected
Unit tests (19) covering pass/fail per rule, engine composition, ordering, empty engine, null inputs, and defensive copying
Module README documenting intent, behavioral decisions, trade-offs, and a PlantUML class diagram

Registered in the root pom.xml modules list.

Add an educational rule-engine module demonstrating the Rule Engine pattern. Each business rule is an independent object exposing evaluate (decision) and execute (action); a RuleEngine evaluates a collection of rules against an immutable loan-application context in insertion order, runs the passing rules, and reports every passed and failed rule via an immutable RuleEngineResult. Includes three concrete rules, an App demo, unit tests, README, and class diagram.

github-actions · 2026-07-23T01:20:56Z

PR Summary

Closes #1219. Added a new rule-engine module implementing the Rule Engine design pattern.

Rule<T> abstraction separating evaluate (decision) from execute (action)
Three concrete rules: MinimumAgeRule, MinimumIncomeRule, CreditScoreRule
RuleEngine<T> evaluates all rules in insertion order (no short-circuit) against a LoanApplication context
Immutable RuleEngineResult reporting approved flag plus passed/failed rule names
Defensive copies / immutability throughout; null contexts and null rules rejected
Unit tests (19) covering pass/fail per rule, engine composition, ordering, empty engine, null inputs, and defensive copying
Module README documenting intent, behavioral decisions, trade-offs, and a PlantUML class diagram
Registered in the root pom.xml modules list.

Changes

File	Summary
`pom.xml`	Updated root Maven `pom.xml` to include the new `rule-engine` module, registering it in the multi-module build.
`rule-engine/README.md`	Documentation for the Rule Engine pattern, describing intent, usage, semantics and examples.
`rule-engine/etc/rule-engine.urm.puml`	PlantUML class diagram for the Rule Engine pattern showing interfaces and relationships.
`rule-engine/pom.xml`	Module POM for `rule-engine` with dependencies (SLF4J, Logback, JUnit) and build configuration.
`rule-engine/src/main/java/com/iluwatar/ruleengine/App.java`	Demo app that builds a `RuleEngine<LoanApplication>` with three rules and runs two sample applications, printing results.
`rule-engine/src/main/java/com/iluwatar/ruleengine/CreditScoreRule.java`	Concrete `Rule<LoanApplication>` implementing minimum credit score check; provides `name`, `evaluate`, and `execute` methods.
`rule-engine/src/main/java/com/iluwatar/ruleengine/LoanApplication.java`	Immutable `LoanApplication` context (record) used by all rules.
`rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumAgeRule.java`	Concrete `Rule<LoanApplication>` enforcing minimum age; implements `evaluate` and `execute` with logging.
`rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumIncomeRule.java`	Concrete `Rule<LoanApplication>` enforcing minimum income; implements `evaluate` and `execute`.
`rule-engine/src/main/java/com/iluwatar/ruleengine/Rule.java`	Abstract `Rule<T>` interface defining `name`, `evaluate`, and `execute` to separate decision from action.
`rule-engine/src/main/java/com/iluwatar/ruleengine/RuleEngine.java`	Engine that coordinates rules, evaluates all in order, and returns `RuleEngineResult`; supports defensive copies and exposes rules.
`rule-engine/src/main/java/com/iluwatar/ruleengine/RuleEngineResult.java`	Immutable `RuleEngineResult` capturing approval status and lists of passed/failed rules with defensive copies.
`rule-engine/src/test/java/com/iluwatar/ruleengine/AppTest.java`	Test ensuring `App.main` runs without throwing exceptions.
`rule-engine/src/test/java/com/iluwatar/ruleengine/LoanRulesTest.java`	Unit tests for individual rules across thresholds (pass/fail) and that `execute` runs when applicable.
`rule-engine/src/test/java/com/iluwatar/ruleengine/RuleEngineResultTest.java`	Tests for defensive copying and immutability of `RuleEngineResult` lists.
`rule-engine/src/test/java/com/iluwatar/ruleengine/RuleEngineTest.java`	Integration tests covering approvals, rejections, ordering, empty engine, and null input handling.

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github-actions

🚨 Pull request needs attention.

Review Summary

Commits Considered (1)

1e959a1: feat: add rule engine design pattern

Add an educational rule-engine module demonstrating the Rule Engine
pattern. Each business rule is an independent object exposing evaluate
(decision) and execute (action); a RuleEngine evaluates a collection of
rules against an immutable loan-application context in insertion order,
runs the passing rules, and reports every passed and failed rule via an
immutable RuleEngineResult. Includes three concrete rules, an App demo,
unit tests, README, and class diagram.

Files Processed (16)

pom.xml (1 hunk)
rule-engine/README.md (1 hunk)
rule-engine/etc/rule-engine.urm.puml (1 hunk)
rule-engine/pom.xml (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/App.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/CreditScoreRule.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/LoanApplication.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumAgeRule.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumIncomeRule.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/Rule.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/RuleEngine.java (1 hunk)
rule-engine/src/main/java/com/iluwatar/ruleengine/RuleEngineResult.java (1 hunk)
rule-engine/src/test/java/com/iluwatar/ruleengine/AppTest.java (1 hunk)
rule-engine/src/test/java/com/iluwatar/ruleengine/LoanRulesTest.java (1 hunk)
rule-engine/src/test/java/com/iluwatar/ruleengine/RuleEngineResultTest.java (1 hunk)
rule-engine/src/test/java/com/iluwatar/ruleengine/RuleEngineTest.java (1 hunk)

Actionable Comments (4)

rule-engine/src/main/java/com/iluwatar/ruleengine/App.java [64-64]

best_practice: "Logger name mismatch with Lombok"
rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumAgeRule.java [56-56]

best_practice: "Logger name mismatch in MinimumAgeRule.execute"
rule-engine/src/main/java/com/iluwatar/ruleengine/MinimumIncomeRule.java [56-60]

best_practice: "Logger name mismatch in MinimumIncomeRule.execute"
rule-engine/src/main/java/com/iluwatar/ruleengine/CreditScoreRule.java [56-60]

best_practice: "Logger name mismatch in CreditScoreRule.execute"

Skipped Comments (0)

github-actions · 2026-07-23T01:22:05Z

+
+  private static void report(RuleEngineResult result) {
+    if (result.approved()) {
+      LOGGER.info("Loan approved. Passed rules: {}", result.passedRules());


Logger name mismatch with Lombok's @slf4j annotation. Lombok typically exposes a 'log' field when using @slf4j; update to use log.info(...) (or switch to a logger annotation that matches LOGGER).

github-actions · 2026-07-23T01:22:05Z

+
+  @Override
+  public void execute(LoanApplication context) {
+    LOGGER.info("Applicant age {} meets the minimum age of {}.", context.age(), minimumAge);


Logger name mismatch in execute() for MinimumAgeRule. Replace LOGGER with log to align with @slf4j usage.

github-actions · 2026-07-23T01:22:05Z

+    LOGGER.info(
+        "Applicant income {} meets the minimum income of {}.",
+        context.monthlyIncome(),
+        minimumIncome);
+  }


Logger name mismatch across the multi-line execute() in MinimumIncomeRule. Replace LOGGER with log and maintain formatting.

github-actions · 2026-07-23T01:22:05Z

+    LOGGER.info(
+        "Applicant credit score {} meets the minimum score of {}.",
+        context.creditScore(),
+        minimumScore);
+  }


Logger name mismatch across the multi-line execute() in CreditScoreRule. Replace LOGGER with log to align with Lombok @slf4j usage.

codecov · 2026-07-23T01:37:08Z

Codecov Report

❌ Patch coverage is 98.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.27%. Comparing base (74d2dbe) to head (1e959a1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ine/src/main/java/com/iluwatar/ruleengine/App.java	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3548      +/-   ##
============================================
+ Coverage     83.24%   83.27%   +0.02%     
- Complexity     4025     4056      +31     
============================================
  Files          1060     1067       +7     
  Lines         14246    14306      +60     
  Branches        686      688       +2     
============================================
+ Hits          11859    11913      +54     
- Misses         2100     2108       +8     
+ Partials        287      285       -2

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HattoriHenzo

As a globale review, I would suggest you to organize your project into different package or module with the RuleEngine (Abstraction) and the rule implementation (Implementations).

github-actions Bot reviewed Jul 23, 2026

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MohamedMBG mentioned this pull request Jul 23, 2026

Fix busy-waiting loops #2977

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HattoriHenzo reviewed Jul 23, 2026

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feat: add rule engine design pattern#3548

feat: add rule engine design pattern#3548
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MohamedMBG:issue-1219-rule-engine

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