BAEL-9162 - Simple Rule Engine

rule-engines-modules/pom.xml

@@ -18,6 +18,7 @@
         <module>evrete</module>
         <module>openl-tablets</module>
         <module>rulebook</module>
+        <module>simple-rule-engine</module>
         <!-- <module>jess</module> --> <!-- requires dependencies which are not publicly available -->
     </modules>
 

rule-engines-modules/simple-rule-engine/pom.xml

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<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/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>com.baeldung.simpleruleengine</groupId>
+    <artifactId>simple-rule-engine</artifactId>
+    <version>1.0</version>
+    <name>simple-rule-engine</name>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>9</source>
+                    <target>9</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>rule-engines-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>${lombok.version}</version>
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-expression</artifactId>
+            <version>7.0.0-M7</version>
+        </dependency>
+    </dependencies>
+
+</project>

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/FirstOrderHighValueSpecialDiscountRule.java

@@ -0,0 +1,17 @@
+package com.baeldung.ruleengine;
+
+import com.baeldung.ruleengine.model.Order;
+
+public class FirstOrderHighValueSpecialDiscountRule implements IRule {
+
+    @Override
+    public boolean evaluate(Order order) {
+        return order.getCustomer()
+            .isFirstOrder() && order.getAmount() > 500;
+    }
+
+    @Override
+    public String description() {
+        return "First Order Special Discount Rule: First Time customer with high value order";
+    }
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/IRule.java

@@ -0,0 +1,8 @@
+package com.baeldung.ruleengine;
+
+import com.baeldung.ruleengine.model.Order;
+
+public interface IRule {
+    boolean evaluate(Order order);
+    String description();
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/LoyaltyDiscountRule.java

@@ -0,0 +1,16 @@
+package com.baeldung.ruleengine;
+
+import com.baeldung.ruleengine.model.Order;
+
+public class LoyaltyDiscountRule implements IRule{
+
+    @Override
+    public boolean evaluate(Order order) {
+        return order.getCustomer().getLoyaltyPoints() > 500;
+    }
+
+    @Override
+    public String description() {
+        return "Loyalty Discount Rule: Customer has more than 500 points";
+    }
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/RuleEngine.java

@@ -0,0 +1,21 @@
+package com.baeldung.ruleengine;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+import com.baeldung.ruleengine.model.Order;
+
+public class RuleEngine {
+    private final List<IRule> rules;
+
+    public RuleEngine(List<IRule> rules) {
+        this.rules = rules;
+    }
+
+    public List<String> evaluate(Order order) {
+        return rules.stream()
+            .filter(rule -> rule.evaluate(order))
+            .map(IRule::description)
+            .collect(Collectors.toList());
+    }
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/SpelRule.java

@@ -0,0 +1,31 @@
+package com.baeldung.ruleengine;
+
+import org.springframework.expression.ExpressionParser;
+import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.expression.spel.support.StandardEvaluationContext;
+
+import com.baeldung.ruleengine.model.Order;
+
+
+public class SpelRule {
+
+    private final String expression;
+    private final String description;
+
+    public SpelRule(String expression, String description) {
+        this.expression = expression;
+        this.description = description;
+    }
+
+    public boolean evaluate(Order order) {
+        ExpressionParser parser = new SpelExpressionParser();
+        StandardEvaluationContext context = new StandardEvaluationContext(order);
+        context.setVariable("order", order);
+        return parser.parseExpression(expression)
+            .getValue(context, Boolean.class);
+    }
+
+    public String getDescription() {
+        return description;
+    }
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/model/Customer.java

@@ -0,0 +1,26 @@
+package com.baeldung.ruleengine.model;
+
+import lombok.AccessLevel;
+import lombok.Getter;
+import lombok.Setter;
+import lombok.experimental.FieldDefaults;
+
+@Setter
+@Getter
+@FieldDefaults(level = AccessLevel.PRIVATE)
+public class Customer {
+
+    String name;
+    int loyaltyPoints;
+    boolean firstOrder;
+
+    public Customer() {
+    }
+
+    public Customer(String name, int loyaltyPoints, boolean firstOrder) {
+        this.name = name;
+        this.loyaltyPoints = loyaltyPoints;
+        this.firstOrder = firstOrder;
+    }
+
+}

rule-engines-modules/simple-rule-engine/src/main/java/com/baeldung/ruleengine/model/Order.java

@@ -0,0 +1,25 @@
+package com.baeldung.ruleengine.model;
+
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+import lombok.experimental.FieldDefaults;
+
+@Getter
+@Setter
+public class Order {
+
+    private Double amount;
+    private Customer customer;
+
+    public Order() {
+    }
+
+    public Order(Double amount, Customer customer) {
+        this.amount = amount;
+        this.customer = customer;
+    }
+}

rule-engines-modules/simple-rule-engine/src/main/resources/logback.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

rule-engines-modules/simple-rule-engine/src/test/java/com/baeldung/ruleengine/RuleEngineUnitTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.ruleengine;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+
+import com.baeldung.ruleengine.model.Customer;
+import com.baeldung.ruleengine.model.Order;
+
+public class RuleEngineUnitTest {
+
+    @Test
+    void whenTwoRulesTriggered_thenBothDescriptionsReturned() {
+        Customer customer = new Customer("Max", 550, true);
+        Order order = new Order(600.0, customer);
+
+        RuleEngine engine = new RuleEngine(List.of(new LoyaltyDiscountRule(), new FirstOrderHighValueSpecialDiscountRule()));
+
+        List<String> results = engine.evaluate(order);
+
+        assertEquals(2, results.size());
+        assertTrue(results.contains("Loyalty Discount Rule: Customer has more than 500 points"));
+        assertTrue(results.contains("First Order Special Discount Rule: First Time customer with high value order"));
+    }
+
+    @Test
+    void whenNoRulesTriggered_thenEmptyListReturned() {
+        Customer customer = new Customer("Max", 50, false);
+        Order order = new Order(200.0, customer);
+
+        RuleEngine engine = new RuleEngine(List.of(new LoyaltyDiscountRule(), new FirstOrderHighValueSpecialDiscountRule()));
+
+        List<String> results = engine.evaluate(order);
+
+        assertTrue(results.isEmpty());
+    }
+}

rule-engines-modules/simple-rule-engine/src/test/java/com/baeldung/ruleengine/SpelRuleUnitTest.java

@@ -0,0 +1,34 @@
+package com.baeldung.ruleengine;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.baeldung.ruleengine.model.Customer;
+import com.baeldung.ruleengine.model.Order;
+
+public class SpelRuleUnitTest {
+
+    @Test
+    void whenLoyalCustomer_thenEligibleForDiscount() {
+        Customer customer = new Customer("Bob", 730, false);
+        Order order = new Order(200.0, customer);
+
+        SpelRule rule = new SpelRule(
+            "#order.customer.loyaltyPoints > 500",
+            "Loyalty discount rule"
+        );
+        assertTrue(rule.evaluate(order));
+    }
+
+    @Test
+    void whenFirstOrderHighAmount_thenEligibleForSpecialDiscount() {
+        Customer customer = new Customer("Bob", 0, true);
+        Order order = new Order(800.0, customer);
+
+        SpelRule approvalRule = new SpelRule(
+            "#order.customer.firstOrder and #order.amount > 500",
+            "First-time customer with high order gets special discount"
+        );
+        assertTrue(approvalRule.evaluate(order));
+    }
+}

rule-engines-modules/simple-rule-engine/src/test/resources/logback.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>