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Document POM inheritance for banDuplicateClasses using combine.children (#341)
--------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: slachiewicz <6705942+slachiewicz@users.noreply.github.com>
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‎src/it/banduplicate-classes-inheritance/README.md‎

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# Test: banDuplicateClasses Configuration Inheritance
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This integration test demonstrates how to properly inherit and extend `banDuplicateClasses` configuration from a parent POM.
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## Problem
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When defining `ignoreClasses` or `dependencies` in a parent POM, child POMs cannot simply add more entries without using special Maven configuration combination attributes. By default, Maven replaces arrays/lists during POM inheritance rather than merging them.
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## Solution
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Use the `combine.children="append"` attribute on collection elements (`<ignoreClasses>` and `<dependencies>`) in the child POM to append to the parent's configuration.
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### Parent POM Example
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```xml
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.*</ignoreClass>
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</ignoreClasses>
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<dependencies>
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<dependency>
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<groupId>commons-logging</groupId>
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<artifactId>commons-logging</artifactId>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.impl.WeakHashtable*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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```
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### Child POM Example
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```xml
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses combine.children="append">
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<ignoreClass>some.other.Class</ignoreClass>
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</ignoreClasses>
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<dependencies combine.children="append">
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<dependency>
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<groupId>org.slf4j</groupId>
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<artifactId>jcl-over-slf4j</artifactId>
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<ignoreClasses>
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<ignoreClass>org.slf4j.*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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```
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### Result
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The child POM's effective configuration will include entries from both parent and child:
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**ignoreClasses:**
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- `org.apache.commons.logging.*` (from parent)
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- `some.other.Class` (from child)
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**dependencies:**
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- commons-logging configuration (from parent)
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- jcl-over-slf4j configuration (from child)
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## Maven Configuration Combination
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The `combine.children="append"` attribute is a standard Maven feature for controlling how configurations are merged during POM inheritance.
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**Important Notes:**
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- The attribute must be placed on the collection element in the **child** POM (not the parent)
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- Works for both `<ignoreClasses>` and `<dependencies>` elements
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- Without this attribute, child configuration will **replace** parent configuration
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See [Maven POM Reference](https://maven.apache.org/pom.html#Plugins) for more details on configuration combination.

‎src/it/banduplicate-classes-inheritance/child/pom.xml‎

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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>org.codehaus.mojo.extra-enforcer-rules.it</groupId>
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<artifactId>banduplicate-classes-inheritance-parent</artifactId>
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<version>1.0-SNAPSHOT</version>
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</parent>
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<artifactId>banduplicate-classes-inheritance-child</artifactId>
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<name>Test banDuplicateClasses inheritance - Child</name>
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<build>
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<plugins>
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses combine.children="append">
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<ignoreClass>some.other.Class</ignoreClass>
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</ignoreClasses>
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<dependencies combine.children="append">
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<dependency>
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<groupId>org.slf4j</groupId>
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<artifactId>jcl-over-slf4j</artifactId>
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<ignoreClasses>
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<ignoreClass>org.slf4j.*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</project>

‎src/it/banduplicate-classes-inheritance/invoker.properties‎

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invoker.goals = enforcer:enforce

‎src/it/banduplicate-classes-inheritance/pom.xml‎

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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>org.codehaus.mojo.extra-enforcer-rules.it</groupId>
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<artifactId>banduplicate-classes-inheritance-parent</artifactId>
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<version>1.0-SNAPSHOT</version>
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<packaging>pom</packaging>
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<name>Test banDuplicateClasses inheritance - Parent</name>
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<modules>
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<module>child</module>
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</modules>
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<dependencies>
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<dependency>
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<groupId>commons-logging</groupId>
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<artifactId>commons-logging</artifactId>
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<version>1.1.1</version>
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</dependency>
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<dependency>
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<groupId>org.slf4j</groupId>
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<artifactId>jcl-over-slf4j</artifactId>
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<version>1.6.1</version>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<version>@enforcerPluginVersion@</version>
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<dependencies>
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<dependency>
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<groupId>@project.groupId@</groupId>
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<artifactId>@project.artifactId@</artifactId>
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<version>@project.version@</version>
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</dependency>
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</dependencies>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.*</ignoreClass>
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</ignoreClasses>
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<dependencies>
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<dependency>
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<groupId>commons-logging</groupId>
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<artifactId>commons-logging</artifactId>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.impl.WeakHashtable*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</project>

‎src/it/banduplicate-classes-inheritance/verify.groovy‎

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// Verify that the build succeeded
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File buildLog = new File(basedir, "build.log")
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assert buildLog.exists()
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String log = buildLog.text
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// The build should succeed
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assert !log.contains("BUILD FAILURE"), "Build should have succeeded"
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// Verify that the child's configuration includes both parent and child ignoreClasses
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// Look for the child module's configuration section
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def childConfig = log.find(/(?s)Building Test banDuplicateClasses inheritance - Child.*?end configuration/)
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assert childConfig != null, "Could not find child configuration in build log"
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// Check that both ignore patterns are present in the child's effective configuration
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assert childConfig.contains("org.apache.commons.logging.*"), "Parent's ignoreClass pattern should be inherited"
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assert childConfig.contains("some.other.Class"), "Child's ignoreClass pattern should be present"
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// Check that dependencies from both parent and child are present
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assert childConfig.contains("commons-logging"), "Parent's dependency configuration should be inherited"
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assert childConfig.contains("jcl-over-slf4j"), "Child's dependency configuration should be present"
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println "Test passed: banDuplicateClasses correctly combined configuration from parent and child POMs"
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println " - Parent's ignoreClass pattern 'org.apache.commons.logging.*' was inherited"
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println " - Child's ignoreClass pattern 'some.other.Class' was added"
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println " - Parent's dependency configuration for 'commons-logging' was inherited"
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println " - Child's dependency configuration for 'jcl-over-slf4j' was added"

‎src/site/apt/banDuplicateClasses.apt.vm‎

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+---+
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* Inheriting Configuration from Parent POMs
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When you want to extend <<<banDuplicateClasses>>> configuration from a parent POM in a child POM, you must use Maven's <<<combine.children="append">>> attribute on the collection elements (<<<ignoreClasses>>> and <<<dependencies>>>) to append to the parent's configuration rather than replacing it.
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Without this attribute, Maven will replace the parent's configuration entirely.
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** Parent POM Example
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+---+
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.*</ignoreClass>
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</ignoreClasses>
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<dependencies>
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<dependency>
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<groupId>commons-logging</groupId>
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<artifactId>commons-logging</artifactId>
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<ignoreClasses>
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<ignoreClass>org.apache.commons.logging.impl.WeakHashtable*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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+---+
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** Child POM Example
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+---+
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<plugin>
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<artifactId>maven-enforcer-plugin</artifactId>
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<configuration>
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<rules>
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<banDuplicateClasses>
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<ignoreClasses combine.children="append">
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<ignoreClass>some.other.Class</ignoreClass>
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</ignoreClasses>
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<dependencies combine.children="append">
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<dependency>
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<groupId>org.slf4j</groupId>
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<artifactId>jcl-over-slf4j</artifactId>
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<ignoreClasses>
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<ignoreClass>org.slf4j.*</ignoreClass>
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</ignoreClasses>
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</dependency>
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</dependencies>
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</banDuplicateClasses>
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</rules>
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</configuration>
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</plugin>
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+---+
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Note: The <<<combine.children="append">>> attribute must be placed on the collection element in the child POM (not the parent POM).
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See {{{https://maven.apache.org/pom.html#Plugins}Maven POM Reference}} for more details on configuration combination.
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* Trademarks
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Apache, Apache Maven, Maven and the Apache feather logo are trademarks of The Apache Software Foundation.

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