BAEL-803: Backward Chaining with Drools - new example (#2921)

* spring beans DI examples

* fix-1: shortening examples

* List of Rules Engines in Java

* BAEL-812: Openl-Tablets example added

* BAEL-812: artifacts names changed

* BAEL-812: moving rule-engines examples to rule-engines folder

* BAEL-812: removing evaluation article files

* BAEL-812: folder renamed

* BAEL-812: folder renamed

* BAEL-812: pom.xml - parent added

* BAEL-1027: Introduction to GraphQL - initial commit

* BAEL-781: Explore the new Spring Cloud Gateway

* BAEL-781: Fix UserService.java

* BAEL-781: Fix user-service pom.xml

* BAEL-781: remove eureka-server from the example

* BAEL-781: modifying example

* BAEL-803: Backward Chaining wih Drools

* BAEL-803: pom.xml

* BAEL-803: Backward Chaining with Drools - new example

drools/backward-chaining/src/main/java/com/baeldung/drools/BackwardChaining.java

@@ -0,0 +1,33 @@
+package com.baeldung.drools;
+
+import org.kie.api.KieServices;
+import org.kie.api.runtime.KieContainer;
+import org.kie.api.runtime.KieSession;
+
+import com.baeldung.drools.model.Fact;
+import com.baeldung.drools.model.Result;
+
+public class BackwardChaining {
+    public static void main(String[] args) {
+        Result result = new BackwardChaining().backwardChaining();
+        System.out.println(result.getValue());
+        result.getFacts().stream().forEach(System.out::println);
+    }
+
+    public Result backwardChaining() {
+        Result result = new Result();
+        KieServices ks = KieServices.Factory.get();
+        KieContainer kContainer = ks.getKieClasspathContainer();
+        KieSession ksession = kContainer.newKieSession("ksession-backward-chaining");
+        ksession.setGlobal("result", result);
+        ksession.insert(new Fact("Asia", "Planet Earth"));
+//        ksession.insert(new Fact("China", "Asia"));
+        ksession.insert(new Fact("Great Wall of China", "China"));
+
+        ksession.fireAllRules();
+
+        return result;
+
+    }
+
+}

drools/backward-chaining/src/main/java/com/baeldung/drools/model/Fact.java

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+package com.baeldung.drools.model;
+
+import org.kie.api.definition.type.Position;
+
+public class Fact {
+
+    @Position(0)
+    private String element;
+
+    @Position(1)
+    private String place;
+
+    public Fact(String element, String place) {
+        this.element = element;
+        this.place = place;
+    }
+
+    public String getElement() {
+        return element;
+    }
+
+    public void setElement(String element) {
+        this.element = element;
+    }
+
+    public String getPlace() {
+        return place;
+    }
+
+    public void setPlace(String place) {
+        this.place = place;
+    }
+
+    @Override
+    public int hashCode() {
+        final int prime = 31;
+        int result = 1;
+        result = prime * result + ((element == null) ? 0 : element.hashCode());
+        result = prime * result + ((place == null) ? 0 : place.hashCode());
+        return result;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj)
+            return true;
+        if (obj == null)
+            return false;
+        if (getClass() != obj.getClass())
+            return false;
+        Fact other = (Fact) obj;
+        if (element == null) {
+            if (other.element != null)
+                return false;
+        } else if (!element.equals(other.element))
+            return false;
+        if (place == null) {
+            if (other.place != null)
+                return false;
+        } else if (!place.equals(other.place))
+            return false;
+        return true;
+    }
+
+    @Override
+    public String toString() {
+        return "Fact{" + "element='" + element + '\'' + ", place='" + place + '\'' + '}';
+    }
+}

drools/backward-chaining/src/main/java/com/baeldung/drools/model/Result.java

@@ -0,0 +1,31 @@
+package com.baeldung.drools.model;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Result {
+    private String value;
+    private List<String> facts = new ArrayList<>();
+
+    public String getValue() {
+        return value;
+    }
+
+    public void setValue(String value) {
+        this.value = value;
+    }
+
+    public List<String> getFacts() {
+        return facts;
+    }
+
+    public void setFacts(List<String> facts) {
+        this.facts = facts;
+    }
+    
+    public void addFact(String fact) {
+        this.facts.add(fact);
+    }
+    
+    
+}

drools/backward-chaining/src/main/resources/META-INF/maven/pom.properties

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+groupId=com.baeldung.drools
+artifactId=DroosBackwardChaining
+version=1.0.0-SNAPSHOT

drools/backward-chaining/src/main/resources/backward_chaining/rules.drl

@@ -0,0 +1,34 @@
+package com.baeldung
+
+import com.baeldung.drools.model.Fact;
+
+global com.baeldung.drools.model.Result result;
+
+dialect  "mvel"
+
+query belongsTo(String x, String y)
+  Fact(x, y;)
+  or
+  (Fact(z, y;) and belongsTo(x, z;))
+end
+
+rule "Great Wall of China BELONGS TO Planet Earth"
+when
+    belongsTo("Great Wall of China", "Planet Earth";)
+then
+	result.setValue("Decision one taken: Great Wall of China BELONGS TO Planet Earth");
+end
+
+rule "Great Wall of China DOES NOT BELONG TO of Planet Earth"
+when
+    not belongsTo("Great Wall of China", "Planet Earth";)
+then
+    result.setValue("Decision two taken: Great Wall of China DOES NOT BELONG TO Planet Earth");
+end
+
+rule "print all facts"
+when
+    belongsTo(element, place;)
+then
+    result.addFact(element + " IS ELEMENT OF " + place);
+end

drools/backward-chaining/src/test/java/com/baeldung/test/BackwardChainingTest.java

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+package com.baeldung.test;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.kie.api.KieServices;
+import org.kie.api.runtime.KieContainer;
+import org.kie.api.runtime.KieSession;
+
+import com.baeldung.drools.model.Fact;
+import com.baeldung.drools.model.Result;
+
+import static junit.framework.TestCase.assertEquals;
+
+@RunWith(value = JUnit4.class)
+public class BackwardChainingTest {
+    private Result result;
+    private KieServices ks;
+    private KieContainer kContainer;
+    private KieSession ksession;
+
+    @Before
+    public void before() {
+        result = new Result();
+        ks = KieServices.Factory.get();
+        kContainer = ks.getKieClasspathContainer();
+        ksession = kContainer.newKieSession("ksession-backward-chaining");
+        ksession.setGlobal("result", result);
+    }
+
+    @Test
+    public void whenWallOfChinaIsGiven_ThenItBelongsToPlanetEarth() {
+
+        ksession.setGlobal("result", result);
+        ksession.insert(new Fact("Asia", "Planet Earth"));
+        ksession.insert(new Fact("China", "Asia"));
+        ksession.insert(new Fact("Great Wall of China", "China"));
+
+        ksession.fireAllRules();
+        
+        // Assert Decision one
+        assertEquals(result.getValue(), "Decision one taken: Great Wall of China BELONGS TO Planet Earth");
+    }
+
+    @Test
+    public void whenChinaIsNotGiven_ThenWallOfChinaDoesNotBelongToPlanetEarth() {
+        ksession.insert(new Fact("Asia", "Planet Earth"));
+        // ksession.insert(new Location("China", "Asia")); // not provided to force Decision two
+        ksession.insert(new Fact("Great Wall of China", "China"));
+        
+        ksession.fireAllRules();
+        
+        // Assert Decision two
+        assertEquals(result.getValue(), "Decision two taken: Great Wall of China DOES NOT BELONG TO Planet Earth");
+    }
+}