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Initial commit: Maven training examples
This repository contains various Maven issue examples and their fixes for training purposes. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <[email protected]>
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‎README.md‎

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# Maven Issues Demonstration
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This repository demonstrates the most common Maven issues with working examples and fixes.
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## Issues Covered
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### 1. [Wrong Scopes](issues/issue1-wrong-scopes/)
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- Test libraries in production JARs
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- Server-provided libraries included
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- Runtime dependencies at compile time
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- **Examples**: JUnit as compile, Servlet API missing provided scope
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### 2. [Wrong Dependency Management](issues/issue2-dependency-management/)
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- Missing `<dependencyManagement>` section
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- Version conflicts between related libraries
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- Inconsistent versions across modules
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- **Examples**: Mixed Spring versions, missing SLF4J version
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### 3. [Transitive Dependency Conflicts](issues/issue3-transitive-conflicts/)
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- Different libraries bringing conflicting versions
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- Runtime ClassNotFoundException errors
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- API breaking changes
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- **Examples**: SLF4J 1.7 vs 2.0, Guava version conflicts
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### 5. [Duplicate Dependencies](issues/issue5-duplicate-deps/)
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- Same artifact with different versions
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- Multiple libraries doing same thing
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- Bloated classpath and JAR size
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- **Examples**: Multiple JSON libraries, logging implementations
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### 6. [Plugin Version Management](issues/issue6-plugin-versions/)
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- Missing plugin versions (uses Maven defaults)
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- Outdated plugin versions
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- Inconsistent behavior across environments
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- **Examples**: Compiler plugin without version, old JAR plugin
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### 8. [Circular Dependencies](issues/issue8-circular-deps/)
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- Modules depending on each other in circles
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- Build failures and compilation loops
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- Architecture problems
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- **Examples**: Module A ↔ Module B dependency cycle
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### 11. [Parent POM Issues](issues/issue11-parent-pom/)
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- Direct dependencies forcing inheritance
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- Missing dependency/plugin management
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- Version conflicts between parent/children
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- **Examples**: Forced Spring inheritance, hardcoded Java 8
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### 12. [Multi-module Coordination](issues/issue12-multi-module/)
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- Wrong module build order
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- Version drift between sibling modules
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- Hardcoded inter-module versions
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- **Examples**: Dependencies built before their dependencies
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## How to Use
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Each issue directory contains:
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- `pom.xml` - Problematic configuration
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- `pom-fixed.xml` or `fixed/` - Correct solution
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- `README.md` - Detailed explanation and commands
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### Running Examples
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```bash
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# Try to build the problematic examples (they will fail/conflict)
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cd issues/issue1-wrong-scopes && mvn clean compile
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# Build the fixed versions (they work correctly)
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cd issues/issue1-wrong-scopes/fixed && mvn clean compile
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```
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### Useful Maven Commands for Analysis
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```bash
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# Show dependency tree
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mvn dependency:tree
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# Find conflicts
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mvn dependency:analyze
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# Check for updates
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mvn versions:display-dependency-updates
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mvn versions:display-plugin-updates
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# Validate configuration
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mvn validate
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# Show effective POM
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mvn help:effective-pom
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```
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## Key Takeaways
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1. **Use Dependency Management**: Central version control without forced inheritance
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2. **Correct Scopes**: test, provided, runtime, compile appropriately
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3. **Plugin Versions**: Always specify plugin versions explicitly
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4. **BOM Imports**: Use framework BOMs for consistent versions
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5. **Multi-module Order**: Build foundation modules first
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6. **Avoid Circles**: Design clear dependency hierarchies
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7. **Version Consistency**: Use properties and `${project.version}`
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## Common Patterns for Fixes
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- Replace `<dependencies>` with `<dependencyManagement>` in parent POMs
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- Add `<exclusions>` to resolve transitive conflicts
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- Use `${project.version}` for inter-module dependencies
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- Import BOMs with `<type>pom</type><scope>import</scope>`
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- Add `<pluginManagement>` for consistent plugin versions
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- Extract common interfaces to break circular dependencies

‎issue1-wrong-scopes/README.md‎

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# Issue 1: Wrong Dependency Scopes
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## Problem
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Dependencies have incorrect scopes, causing:
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- Test libraries included in production JAR (bloating size)
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- Server-provided libraries included (conflicts)
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- Runtime-only dependencies available at compile time
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## Examples in pom.xml
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### Wrong Scopes:
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1. **JUnit** - Missing scope (defaults to `compile`) → Should be `test`
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2. **Servlet API** - Missing scope → Should be `provided`
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3. **Mockito** - `compile` scope → Should be `test`
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4. **MySQL Driver** - `compile` scope → Should be `runtime`
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## Fix
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```xml
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<!-- Correct scopes -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.2</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>javax.servlet</groupId>
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<artifactId>javax.servlet-api</artifactId>
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<version>4.0.1</version>
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<scope>provided</scope>
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</dependency>
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```
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## Impact
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- **Bloated JARs**: Test libraries in production
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- **ClassPath conflicts**: Server vs bundled versions
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- **Security**: Unnecessary libraries exposed

‎issue1-wrong-scopes/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
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http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>com.example</groupId>
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<artifactId>wrong-scopes-demo</artifactId>
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<version>1.0.0</version>
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<packaging>jar</packaging>
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<properties>
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<maven.compiler.source>11</maven.compiler.source>
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<maven.compiler.target>11</maven.compiler.target>
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</properties>
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<dependencies>
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<!-- WRONG: JUnit should be test scope, not compile -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.2</version>
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<!-- Missing scope - defaults to compile, wrong for test library -->
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</dependency>
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<!-- WRONG: Servlet API should be provided scope (server provides it) -->
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<dependency>
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<groupId>javax.servlet</groupId>
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<artifactId>javax.servlet-api</artifactId>
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<version>4.0.1</version>
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<!-- Missing scope - defaults to compile, should be provided -->
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</dependency>
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<!-- WRONG: Mockito should be test scope -->
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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-core</artifactId>
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<version>4.6.1</version>
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<scope>compile</scope> <!-- Explicitly wrong scope -->
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</dependency>
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<!-- WRONG: JDBC driver should be runtime scope -->
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<dependency>
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<groupId>mysql</groupId>
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<artifactId>mysql-connector-java</artifactId>
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<version>8.0.29</version>
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<scope>compile</scope> <!-- Should be runtime -->
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</dependency>
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<!-- CORRECT: Business logic dependency -->
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<dependency>
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<groupId>org.apache.commons</groupId>
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<artifactId>commons-lang3</artifactId>
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<version>3.12.0</version>
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<scope>compile</scope>
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</dependency>
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</dependencies>
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</project>

‎issue11-parent-pom/README.md‎

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# Issue 11: Parent POM Issues
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## Problem
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Poorly designed parent POMs causing:
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- Forced inheritance of unwanted dependencies
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- Version conflicts between parent and children
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- Inflexible plugin configurations
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- Inconsistent project structure
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## Issues in Parent POM
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### Parent POM Problems:
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1. **Direct dependencies**: Forces ALL children to inherit Spring
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2. **Missing properties**: No encoding, Java version standardization
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3. **No dependency management**: Versions scattered across children
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4. **Direct plugins**: Hardcoded Java 8, inflexible configuration
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5. **Missing distribution management**: Where to deploy artifacts?
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### Child POM Problems:
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1. **Missing relativePath**: Can't find parent reliably
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2. **Version inconsistencies**: child1 uses parent 1.0.0, child2 uses 1.0.1
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3. **Unmanaged dependencies**: commons-lang3 version in child, not parent
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4. **Dependency conflicts**: Spring 5.3.21 vs 5.2.0.RELEASE
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5. **Multiple test frameworks**: JUnit 4 + JUnit 5
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### Build Issues:
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```bash
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# Children can't find parent
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mvn clean compile
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# [ERROR] Non-resolvable parent POM
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# Version conflicts
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mvn dependency:tree
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# Shows conflicting Spring versions
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```
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## Solution (see fixed/ directory)
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### Proper Parent POM Structure:
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#### 1. **Use dependencyManagement** (not dependencies):
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```xml
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<dependencyManagement>
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<dependencies>
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<!-- Children choose what they need -->
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<dependency>
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<groupId>org.springframework</groupId>
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<artifactId>spring-framework-bom</artifactId>
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<version>${spring.version}</version>
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<type>pom</type>
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<scope>import</scope>
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</dependency>
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</dependencies>
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</dependencyManagement>
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```
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#### 2. **Use pluginManagement** (not plugins):
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```xml
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<build>
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<pluginManagement>
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<plugins>
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<!-- Children can override if needed -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<version>${maven-compiler-plugin.version}</version>
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</plugin>
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</plugins>
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</pluginManagement>
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</build>
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```
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#### 3. **Essential properties**:
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```xml
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<properties>
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<maven.compiler.source>11</maven.compiler.source>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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<spring.version>5.3.23</spring.version>
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</properties>
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```
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#### 4. **Multi-module structure**:
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```xml
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<modules>
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<module>../child1</module>
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<module>../child2</module>
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</modules>
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```
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### Benefits of Fixed Version:
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- **Optional inheritance**: Children choose what they need
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- **Consistent versions**: BOM imports prevent conflicts
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- **Flexibility**: Children can override when necessary
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- **Proper build order**: Multi-module builds work correctly
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- **Version management**: Centralized but not forced

‎issue11-parent-pom/child1/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
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http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<!-- WRONG: Incorrect parent reference -->
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<parent>
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<groupId>com.example.parent</groupId>
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<artifactId>parent-pom</artifactId>
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<version>1.0.0</version>
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<!-- MISSING: relativePath should point to ../parent/pom.xml -->
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</parent>
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<artifactId>child1</artifactId>
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<!-- WRONG: Inherits parent version but should it? -->
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<!-- MISSING: Own version number -->
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<!-- PROBLEM: Forced to inherit Spring dependency it doesn't need -->
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<!-- PROBLEM: Forced to use Java 8 when it needs Java 11 -->
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<dependencies>
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<!-- Child needs different dependencies but gets parent ones too -->
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<dependency>
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<groupId>org.apache.commons</groupId>
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<artifactId>commons-lang3</artifactId>
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<version>3.12.0</version> <!-- Should be managed by parent -->
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</dependency>
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<!-- WRONG: Duplicating parent's test dependency -->
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<version>4.13.2</version>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<!-- Child cannot override parent's compiler plugin easily -->
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</project>

‎issue11-parent-pom/child2/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
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http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<!-- WRONG: Different parent version! -->
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<parent>
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<groupId>com.example.parent</groupId>
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<artifactId>parent-pom</artifactId>
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<version>1.0.1</version> <!-- Different version than child1! -->
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<relativePath>../parent/pom.xml</relativePath>
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</parent>
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<artifactId>child2</artifactId>
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<version>2.0.0</version> <!-- INCONSISTENT: Different versioning strategy -->
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<dependencies>
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<!-- FORCED: Gets Spring dependency it doesn't want -->
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<!-- spring-core inherited from parent -->
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<!-- Child2 needs web features but parent doesn't provide version management -->
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<dependency>
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<groupId>org.springframework</groupId>
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<artifactId>spring-web</artifactId>
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<version>5.2.0.RELEASE</version> <!-- CONFLICT: Different Spring version! -->
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</dependency>
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<!-- REDUNDANT: Gets JUnit 4 from parent but needs JUnit 5 -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.8.2</version>
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<scope>test</scope>
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</dependency>
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<!-- Now has both JUnit 4 and 5! -->
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</dependencies>
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<!-- PROBLEM: Stuck with Java 8 compiler from parent -->
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<!-- But this module needs newer Java features -->
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</project>

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