Maven Issues Demonstration

This repository demonstrates the most common Maven issues with working examples and fixes.

Issues Covered

1. Wrong Scopes

• Test libraries in production JARs
• Server-provided libraries included
• Runtime dependencies at compile time
• Examples: JUnit as compile, Servlet API missing provided scope

2. Wrong Dependency Management

• Missing <dependencyManagement> section
• Version conflicts between related libraries
• Inconsistent versions across modules
• Examples: Mixed Spring versions, missing SLF4J version

3. Transitive Dependency Conflicts

• Different libraries bringing conflicting versions
• Runtime ClassNotFoundException errors
• API breaking changes
• Examples: SLF4J 1.7 vs 2.0, Guava version conflicts

4. Duplicate Dependencies

• Same artifact with different versions
• Multiple libraries doing same thing
• Bloated classpath and JAR size
• Examples: Multiple JSON libraries, logging implementations

5. Plugin Version Management

• Missing plugin versions (uses Maven defaults)
• Outdated plugin versions
• Inconsistent behavior across environments
• Examples: Compiler plugin without version, old JAR plugin

6. Circular Dependencies

• Modules depending on each other in circles
• Build failures and compilation loops
• Architecture problems
• Examples: Module A ↔ Module B dependency cycle

7. Parent POM Issues

• Direct dependencies forcing inheritance
• Missing dependency/plugin management
• Version conflicts between parent/children
• Examples: Forced Spring inheritance, hardcoded Java 8

8. Multi-module Coordination

• Wrong module build order
• Version drift between sibling modules
• Hardcoded inter-module versions
• Examples: Dependencies built before their dependencies

How to Use

Each issue directory contains:

• pom.xml - Problematic configuration
• pom-fixed.xml or fixed/ - Correct solution
• README.md - Detailed explanation and commands

Running Examples

# Try to build the problematic examples (they will fail/conflict)
cd issue1-wrong-scopes && mvn clean compile

# Build the fixed versions (they work correctly)
cd issue1-wrong-scopes/fixed && mvn clean compile

Useful Maven Commands for Analysis

# Show dependency tree
mvn dependency:tree

# Find conflicts
mvn dependency:analyze

# Check for updates
mvn versions:display-dependency-updates
mvn versions:display-plugin-updates

# Validate configuration
mvn validate

# Show effective POM
mvn help:effective-pom

Key Takeaways

1. Use Dependency Management: Central version control without forced inheritance
2. Correct Scopes: test, provided, runtime, compile appropriately
3. Plugin Versions: Always specify plugin versions explicitly
4. BOM Imports: Use framework BOMs for consistent versions
5. Multi-module Order: Build foundation modules first
6. Avoid Circles: Design clear dependency hierarchies
7. Version Consistency: Use properties and ${project.version}

Common Patterns for Fixes

• Replace <dependencies> with <dependencyManagement> in parent POMs
• Add <exclusions> to resolve transitive conflicts
• Use ${project.version} for inter-module dependencies
• Import BOMs with <type>pom</type><scope>import</scope>
• Add <pluginManagement> for consistent plugin versions
• Extract common interfaces to break circular dependencies