Merge branch 'eugenp:master' into master

libraries-security/pom.xml

@@ -78,6 +78,29 @@
             <artifactId>sshd-core</artifactId>
             <version>${apache-mina.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.xacml4j</groupId>
+            <artifactId>xacml-core</artifactId>
+            <version>${xacml4j.version}</version>
+            <exclusions>
+                <exclusion>
+                    <groupId>org.slf4j</groupId>
+                    <artifactId>slf4j-log4j12</artifactId>
+                </exclusion>
+            </exclusions>
+        </dependency>
+        <dependency>
+            <groupId>org.xacml4j</groupId>
+            <artifactId>xacml-test</artifactId>
+            <version>${xacml4j.version}</version>
+            <scope>test</scope>
+            <exclusions>
+                <exclusion>
+                    <groupId>org.slf4j</groupId>
+                    <artifactId>slf4j-log4j12</artifactId>
+                </exclusion>
+            </exclusions>
+        </dependency>
     </dependencies>
 
     <properties>
@@ -90,6 +113,7 @@
         <jsch.version>0.1.55</jsch.version>
         <apache-mina.version>2.5.1</apache-mina.version>
         <spring-security-oauth2.version>2.4.0.RELEASE</spring-security-oauth2.version>
+        <xacml4j.version>1.4.0</xacml4j.version>
     </properties>
 
 </project>

libraries-security/src/main/java/com/baeldung/scribejava/ScribejavaApplication.java

@@ -2,9 +2,11 @@ package com.baeldung.scribejava;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.boot.web.servlet.ServletComponentScan;
 
 
 @SpringBootApplication
+@ServletComponentScan
 public class ScribejavaApplication {
 
     public static void main(String[] args) {

libraries-security/src/main/java/com/baeldung/scribejava/controller/RBACController.java

@@ -0,0 +1,27 @@
+package com.baeldung.scribejava.controller;
+
+import java.io.IOException;
+
+import javax.annotation.security.DeclareRoles;
+import javax.annotation.security.RolesAllowed;
+import javax.servlet.ServletException;
+import javax.servlet.annotation.HttpConstraint;
+import javax.servlet.annotation.ServletSecurity;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+@WebServlet(name="rbac", urlPatterns = {"/protected"})
+@DeclareRoles("USER")
+@ServletSecurity(
+    @HttpConstraint(rolesAllowed = "USER")
+)
+public class RBACController  extends HttpServlet {
+
+    private static final long serialVersionUID = 1L;
+
+    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
+        resp.getWriter().println("Hello, USER");
+    }
+}

libraries-security/src/test/java/com/baeldung/xacml4j/NightlyWithdrawalPolicyUnitTest.java

@@ -0,0 +1,233 @@
+package com.baeldung.xacml4j;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.Test;
+import org.xacml4j.v20.Xacml20TestUtility;
+import org.xacml4j.v30.Attribute;
+import org.xacml4j.v30.Categories;
+import org.xacml4j.v30.Category;
+import org.xacml4j.v30.CompositeDecisionRule;
+import org.xacml4j.v30.Decision;
+import org.xacml4j.v30.Entity;
+import org.xacml4j.v30.RequestContext;
+import org.xacml4j.v30.ResponseContext;
+import org.xacml4j.v30.Result;
+import org.xacml4j.v30.XacmlPolicyTestSupport;
+import org.xacml4j.v30.pdp.PolicyDecisionPoint;
+import org.xacml4j.v30.pdp.PolicyDecisionPointBuilder;
+import org.xacml4j.v30.spi.combine.DecisionCombiningAlgorithmProviderBuilder;
+import org.xacml4j.v30.spi.function.FunctionProviderBuilder;
+import org.xacml4j.v30.spi.pip.PolicyInformationPointBuilder;
+import org.xacml4j.v30.spi.repository.InMemoryPolicyRepository;
+import org.xacml4j.v30.spi.repository.PolicyRepository;
+import org.xacml4j.v30.types.DoubleExp;
+import org.xacml4j.v30.types.StringExp;
+import org.xacml4j.v30.types.TimeExp;
+
+public class NightlyWithdrawalPolicyUnitTest extends XacmlPolicyTestSupport {
+    
+    private static final String POLICY_SET = "xacml4j/NightlyWithdrawalsPolicy.xml";
+
+    @Test
+    public void testWhenNightlyWithdrawalOver500_thenFail() throws Exception {
+
+        PolicyDecisionPoint pdp = buildPDP(POLICY_SET);
+
+        // Action category
+        Attribute actionAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:action:action-id")
+            .value(StringExp.of("withdrawal"))
+            .build();
+        Entity actionEntity = Entity.builder()
+            .attribute(actionAttribute)
+            .build();
+        Category actionCategory = Category.builder(Categories.ACTION)
+            .entity(actionEntity)
+            .build();
+
+        // Environment Category
+        Attribute timeAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:environment:current-time")
+            .includeInResult(false)
+            .value(TimeExp.of("21:00:00"))
+            .build();
+        Entity timeEntity = Entity.builder()
+            .attribute(timeAttribute)
+            .build();
+
+        Category environmentCategory = Category.builder(Categories.ENVIRONMENT)
+            .entity(timeEntity)
+            .build();
+
+        // ATM category
+        Attribute amountAttribute = Attribute.builder("urn:baeldung:atm:withdrawal:amount")
+            .value(DoubleExp.of("1200.00"))
+            .build();
+        Entity atmEntity = Entity.builder()
+            .attribute(amountAttribute)
+            .build();
+
+        Category atmCategory = Category.builder(Categories.parse("urn:baeldung:atm:withdrawal"))
+            .entity(atmEntity)
+            .build();
+
+        RequestContext request = RequestContext.builder()
+            .attributes(actionCategory, environmentCategory, atmCategory)
+            .build();
+
+        ResponseContext response = pdp.decide(request);
+        assertNotNull(response);
+        assertTrue("Shoud have at least one result", response.getResults() != null && !response.getResults()
+            .isEmpty());
+
+        Result result = response.getResults()
+            .iterator()
+            .next();
+        assertTrue("Evaluation should succeed", result.getStatus()
+            .isSuccess());
+        assertEquals("Should DENY withdrawal", Decision.DENY, result.getDecision());
+
+    }
+
+    @Test
+    public void testWhenNightlyWithdrawalUnder500_thenSuccess() throws Exception {
+
+        PolicyDecisionPoint pdp = buildPDP(POLICY_SET);
+
+        // Action category
+        Attribute actionAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:action:action-id")
+            .includeInResult(false)
+            .value(StringExp.of("withdrawal"))
+            .build();
+        Entity actionEntity = Entity.builder()
+            .attribute(actionAttribute)
+            .build();
+        Category actionCategory = Category.builder(Categories.ACTION)
+            .entity(actionEntity)
+            .build();
+
+        // Environment Category
+        Attribute timeAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:environment:current-time")
+            .includeInResult(false)
+            .value(TimeExp.of("21:00:00"))
+            .build();
+        Entity timeEntity = Entity.builder()
+            .attribute(timeAttribute)
+            .build();
+        Category environmentCategory = Category.builder(Categories.ENVIRONMENT)
+            .entity(timeEntity)
+            .build();
+
+        // ATM category
+        Attribute amountAttribute = Attribute.builder("urn:baeldung:atm:withdrawal:amount")
+            .value(DoubleExp.of("499.00"))
+            .build();
+        Entity atmEntity = Entity.builder()
+          .attribute(amountAttribute)
+          .build();
+        Category atmCategory = Category.builder(Categories.parse("urn:baeldung:atm:withdrawal"))
+          .entity(atmEntity)
+          .build();
+
+        RequestContext request = RequestContext.builder()
+          .attributes(actionCategory, environmentCategory, atmCategory)
+          .build();
+
+        ResponseContext response = pdp.decide(request);
+        assertNotNull(response);
+        assertTrue("Shoud have at least one result", 
+          response.getResults() != null && !response.getResults().isEmpty());
+
+        Result result = response.getResults().iterator().next();
+        assertTrue("Evaluation should succeed", result.getStatus().isSuccess());
+        assertEquals("Should PERMIT withdrawal", Decision.PERMIT, result.getDecision());
+
+    }
+
+    @Test
+    public void testWhenBusinessHoursWithdrawalOver500_thenSuccess() throws Exception {
+
+        PolicyDecisionPoint pdp = buildPDP(POLICY_SET);
+
+        // Action category
+        Attribute actionAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:action:action-id")
+          .includeInResult(false)
+          .value(StringExp.of("withdrawal"))
+          .build();
+        Entity actionEntity = Entity.builder()
+          .attribute(actionAttribute)
+          .build();
+        Category actionCategory = Category.builder(Categories.ACTION)
+          .entity(actionEntity)
+          .build();
+
+        // Environment Category
+        Attribute timeAttribute = Attribute.builder("urn:oasis:names:tc:xacml:1.0:environment:current-time")
+          .includeInResult(false)
+          .value(TimeExp.of("12:00:00"))
+          .build();
+        Entity timeEntity = Entity.builder()
+          .attribute(timeAttribute)
+          .build();
+        Category environmentCategory = Category.builder(Categories.ENVIRONMENT)
+          .entity(timeEntity)
+          .build();
+
+        // ATM category
+        Attribute amountAttribute = Attribute.builder("urn:baeldung:atm:withdrawal:amount")
+          .value(DoubleExp.of("2000.00"))
+          .build();
+        Entity atmEntity = Entity.builder()
+          .attribute(amountAttribute)
+          .build();
+
+        Category atmCategory = Category.builder(Categories.parse("urn:baeldung:atm:withdrawal"))
+          .entity(atmEntity)
+          .build();
+
+        RequestContext request = RequestContext.builder()
+          .attributes(actionCategory, environmentCategory, atmCategory)
+          .build();
+
+        ResponseContext response = pdp.decide(request);
+        assertNotNull(response);
+        assertTrue("Shoud have at least one result", response.getResults() != null && !response.getResults()
+          .isEmpty());
+
+        Result result = response.getResults()
+          .iterator()
+          .next();
+        assertTrue("Evaluation should succeed", result.getStatus().isSuccess());
+        assertEquals("Should PERMIT withdrawal", Decision.PERMIT, result.getDecision());
+
+    }
+
+    private PolicyDecisionPoint buildPDP(String... policyResources) throws Exception {
+        PolicyRepository repository = new InMemoryPolicyRepository("tes-repository", FunctionProviderBuilder.builder()
+          .defaultFunctions()
+          .build(),
+        DecisionCombiningAlgorithmProviderBuilder.builder()
+            .withDefaultAlgorithms()
+            .create());
+
+        List<CompositeDecisionRule> policies = new ArrayList<CompositeDecisionRule>(policyResources.length);
+        for (String policyResource : policyResources) {
+            CompositeDecisionRule policy = repository.importPolicy(Xacml20TestUtility.getClasspathResource(policyResource));
+            log.info("Policy: {}", policy);
+            policies.add(policy);
+        }
+
+        return PolicyDecisionPointBuilder.builder("testPdp")
+          .policyRepository(repository)
+          .pip(PolicyInformationPointBuilder.builder("testPip")
+          .defaultResolvers()
+          .build())
+          .rootPolicy(policies.get(0))
+          .build();
+    }
+
+}

libraries-security/src/test/resources/xacml4j/NightlyWithdrawalsPolicy.xml

@@ -0,0 +1,137 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Policy xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17" 
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
+  xsi:schemaLocation="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17 http://docs.oasis-open.org/xacml/3.0/xacml-core-v3-schema-wd-17.xsd" 
+  PolicyId="urn:baeldung:atm:WithdrawalPolicy"
+  Version="1.0" 
+  RuleCombiningAlgId="urn:oasis:names:tc:xacml:1.0:rule-combining-algorithm:deny-overrides">
+    <Description>
+       Withdrawal policy example
+    </Description>
+    <Target/>
+    <Rule RuleId="urn:oasis:names:tc:baeldung:WithDrawalPolicy:Rule1" Effect="Deny">
+        <Description>
+Deny withdrawals over $500 between 20:00 and 08:00
+        </Description>
+        <Target>
+            <AnyOf>
+                <AllOf>
+                    <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">withdrawal</AttributeValue>
+                        <AttributeDesignator 
+                            DataType="http://www.w3.org/2001/XMLSchema#string" 
+                            MustBePresent="true" 
+                            Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action" 
+                            AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id"/>
+                    </Match>
+                </AllOf>
+            </AnyOf>
+        </Target>
+        <Condition>
+            <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">
+                <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:not">
+                    <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-in-range">
+                        <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-one-and-only">
+                            <AttributeDesignator
+                                DataType="http://www.w3.org/2001/XMLSchema#time"
+                                MustBePresent="true"
+                                Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
+                                AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time"/>
+                        </Apply>
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">08:00:00</AttributeValue>
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">20:00:00</AttributeValue>
+                    </Apply>
+                </Apply>
+                <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:double-greater-than">
+                    <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:double-one-and-only">
+                        <AttributeDesignator
+                            DataType="http://www.w3.org/2001/XMLSchema#double"
+                            MustBePresent="true"
+                            Category="urn:baeldung:atm:withdrawal"
+                            AttributeId="urn:baeldung:atm:withdrawal:amount"/>
+                    </Apply>
+                    <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#double">500.00</AttributeValue>
+                </Apply>
+            </Apply>
+        </Condition>
+    </Rule>
+    <Rule RuleId="urn:oasis:names:tc:baeldung:WithDrawalPolicy:Rule2" Effect="Permit">
+        <Description>
+Permit withdrawals under $500 between 20:00 and 08:00
+        </Description>
+        <Target>
+            <AnyOf>
+                <AllOf>
+                    <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">withdrawal</AttributeValue>
+                        <AttributeDesignator 
+                            DataType="http://www.w3.org/2001/XMLSchema#string" 
+                            MustBePresent="true" 
+                            Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action" 
+                            AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id"/>
+                    </Match>
+                </AllOf>
+            </AnyOf>
+        </Target>
+        <Condition>
+            <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:and">
+                <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:not">
+                    <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-in-range">
+                        <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-one-and-only">
+                            <AttributeDesignator
+                                DataType="http://www.w3.org/2001/XMLSchema#time"
+                                MustBePresent="true"
+                                Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
+                                AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time"/>
+                        </Apply>
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">08:00:00</AttributeValue>
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">20:00:00</AttributeValue>
+                    </Apply>
+                </Apply>
+                <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:double-less-than-or-equal">
+                    <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:double-one-and-only">
+                        <AttributeDesignator
+                            DataType="http://www.w3.org/2001/XMLSchema#double"
+                            MustBePresent="true"
+                            Category="urn:baeldung:atm:withdrawal"
+                            AttributeId="urn:baeldung:atm:withdrawal:amount"/>
+                    </Apply>
+                    <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#double">500.00</AttributeValue>
+                </Apply>
+            </Apply>
+        </Condition>
+    </Rule>
+    <Rule RuleId="urn:oasis:names:tc:baeldung:WithDrawalPolicy:Rule3" Effect="Permit">
+        <Description>
+Permit withdrawals of any value between 08:00 and 20:00
+        </Description>
+        <Target>
+            <AnyOf>
+                <AllOf>
+                    <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
+                        <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">withdrawal</AttributeValue>
+                        <AttributeDesignator 
+                            DataType="http://www.w3.org/2001/XMLSchema#string" 
+                            MustBePresent="true" 
+                            Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action" 
+                            AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id"/>
+                    </Match>
+                </AllOf>
+            </AnyOf>
+        </Target>
+        <Condition>
+            <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-in-range">
+                <Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:time-one-and-only">
+                    <AttributeDesignator
+                        DataType="http://www.w3.org/2001/XMLSchema#time"
+                        MustBePresent="true"
+                        Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
+                        AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time"/>
+                </Apply>
+                <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">08:00:00</AttributeValue>
+                <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">20:00:00</AttributeValue>
+            </Apply>
+        </Condition>
+    </Rule>
+    
+</Policy>

libraries-security/src/test/resources/xacml4j/Request.xml

@@ -0,0 +1,30 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Request 
+    xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17 http://docs.oasis-open.org/xacml/3.0/xacml-core-v3-schema-wd-17.xsd"
+    CombinedDecision="true"
+    ReturnPolicyIdList="false">
+    
+    <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action">
+        <Attribute 
+            IncludeInResult="false" 
+            AttributeId="urn:oasis:names:tc:xacml:3.0:attribute-category:action">
+            <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">withdrawal</AttributeValue>
+        </Attribute>
+    </Attributes>
+    <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action">
+        <Attribute 
+            IncludeInResult="false" 
+            AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time">
+            <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">21:00:00</AttributeValue>
+        </Attribute>
+    </Attributes>
+    <Attributes Category="urn:baeldung:atm:withdrawal">
+        <Attribute 
+            IncludeInResult="false" 
+            AttributeId="urn:baeldung:atm:withdrawal:amount">
+            <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#decimal">1200</AttributeValue>
+        </Attribute>
+    </Attributes>
+</Request>

libraries-security/src/test/resources/xacml4j/Response.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Response xmlns="urn:oasis:names:tc:xacml:2.0:context:schema:os" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:oasis:names:tc:xacml:2.0:context:schema:os http://docs.oasis-open.org/xacml/access_control-xacml-2.0-context-schema-os.xsd">
+	<Result>
+		<Decision>NotApplicable</Decision>
+		<Status>
+			<StatusCode Value="urn:oasis:names:tc:xacml:1.0:status:ok"/>
+		</Status>
+	</Result>
+</Response>

maven-modules/maven-parent-pom-resolution/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles:
+
+- [Understanding the "relativePath" Tag - Maven Parent POM Resolution At A Glance](https://www.baeldung.com/maven-relativepath)

maven-modules/maven-parent-pom-resolution/pom.xml

@@ -0,0 +1,28 @@
+<?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</groupId>
+    <artifactId>maven-parent-pom-resolution</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <packaging>pom</packaging>
+
+    <modules>
+        <module>project-a</module>
+    </modules>
+
+    <!-- to detect the POM hierarchy, just type "mvn dependency:display-ancestors" -->
+    <build>
+        <pluginManagement>
+            <plugins>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-dependency-plugin</artifactId>
+                    <version>3.2.0</version>
+                </plugin>
+            </plugins>
+        </pluginManagement>
+    </build>
+
+</project>

maven-modules/maven-parent-pom-resolution/project-a/pom.xml

@@ -0,0 +1,20 @@
+<?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>
+    <artifactId>project-a</artifactId>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>maven-parent-pom-resolution</artifactId>
+        <version>1.0-SNAPSHOT</version>
+        <!-- The parent pom is resolved to ../pom.xml -->
+    </parent>
+    <packaging>pom</packaging>
+
+    <modules>
+        <module>project-b</module>
+        <module>project-c</module>
+    </modules>
+
+</project>

maven-modules/maven-parent-pom-resolution/project-a/project-b/pom.xml

@@ -0,0 +1,15 @@
+<?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>
+    <artifactId>project-b</artifactId>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>project-a</artifactId>
+        <version>1.0-SNAPSHOT</version>
+        <!-- The parent pom is resolved to project a's pom.xml -->
+    </parent>
+    <packaging>pom</packaging>
+
+</project>

maven-modules/maven-parent-pom-resolution/project-a/project-c/pom.xml

@@ -0,0 +1,20 @@
+<?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>
+    <artifactId>project-c</artifactId>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>project-b</artifactId>
+        <version>1.0-SNAPSHOT</version>
+        <relativePath>../project-b/pom.xml</relativePath>
+        <!-- The parent pom is resolved to project a's pom.xml -->
+    </parent>
+    <packaging>pom</packaging>
+
+    <modules>
+        <module>project-d</module>
+    </modules>
+
+</project>

maven-modules/maven-parent-pom-resolution/project-a/project-c/project-d/pom.xml

@@ -0,0 +1,17 @@
+<?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>
+    <artifactId>project-d</artifactId>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>project-a</artifactId>
+        <version>1.0-SNAPSHOT</version>
+        <!-- removing relativePath won't work even if project-a is the aggregator project -->
+        <!-- it only works in IntelliJ IDEA when project-a is registered as a Maven Project -->
+        <relativePath>../../pom.xml</relativePath>
+    </parent>
+    <packaging>pom</packaging>
+
+</project>

maven-modules/pom.xml

@@ -36,6 +36,7 @@
         <module>host-maven-repo-example</module>
         <module>plugin-management</module>
         <module>maven-surefire-plugin</module>
+        <module>maven-parent-pom-resolution</module>
     </modules>
 
 </project>