Maven Expert

maven-expert

Keyword: maven | Platforms: gemini,claude,codex

Apache Maven Build Tool Expert Skill - The foundation of Java dependency management and project structure.

Core Mandates

• BOM Management: Always prefer BOM (Bill of Materials) to manage versions (e.g., quarkus-bom, jackson-bom) to avoid dependency hell.
• Dependency Scope: Rigorously use compile, provided, runtime, and test scopes for cleaner artifacts.
• Transitive Discipline: Use mvn dependency:tree to identify and exclude conflicting transitive dependencies.
• Reproducible Builds: Lock down plugin versions in <pluginManagement>.

pom.xml Examples

Quarkus Project with BOM

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>my-service</artifactId>
    <version>1.0.0-SNAPSHOT</version>

    <properties>
        <quarkus.platform.version>3.20.1</quarkus.platform.version>
        <maven.compiler.release>21</maven.compiler.release>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <surefire-plugin.version>3.5.2</surefire-plugin.version>
    </properties>

    <dependencyManagement>
        <dependencies>
            <!-- Quarkus BOM - manages ALL Quarkus dependency versions -->
            <dependency>
                <groupId>io.quarkus.platform</groupId>
                <artifactId>quarkus-bom</artifactId>
                <version>${quarkus.platform.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <dependencies>
        <!-- No version needed - managed by BOM -->
        <dependency>
            <groupId>io.quarkus</groupId>
            <artifactId>quarkus-rest</artifactId>
        </dependency>
        <dependency>
            <groupId>io.quarkus</groupId>
            <artifactId>quarkus-arc</artifactId>
        </dependency>
        <dependency>
            <groupId>io.quarkus</groupId>
            <artifactId>quarkus-hibernate-orm-panache</artifactId>
        </dependency>

        <!-- Test dependencies -->
        <dependency>
            <groupId>io.quarkus</groupId>
            <artifactId>quarkus-junit5</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.rest-assured</groupId>
            <artifactId>rest-assured</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>io.quarkus.platform</groupId>
                <artifactId>quarkus-maven-plugin</artifactId>
                <version>${quarkus.platform.version}</version>
                <extensions>true</extensions>
                <executions>
                    <execution>
                        <goals>
                            <goal>build</goal>
                            <goal>generate-code</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Multi-Module Parent POM

<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>parent</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <modules>
        <module>common</module>
        <module>service-user</module>
        <module>service-order</module>
    </modules>

    <properties>
        <quarkus.platform.version>3.20.1</quarkus.platform.version>
        <maven.compiler.release>21</maven.compiler.release>
    </properties>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>io.quarkus.platform</groupId>
                <artifactId>quarkus-bom</artifactId>
                <version>${quarkus.platform.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
            <!-- Internal module versions -->
            <dependency>
                <groupId>com.example</groupId>
                <artifactId>common</artifactId>
                <version>${project.version}</version>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <!-- Plugin versions locked here, NOT in child modules -->
    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.13.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <version>3.5.2</version>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

Dependency Scope Decision Tree

Is this library needed at compile time AND runtime?
  YES → compile (default, usually omit <scope>)
Is it provided by the runtime container (Quarkus, app server)?
  YES → provided (e.g., jakarta.servlet-api, quarkus internals)
Is it only needed at runtime, not compiled against?
  YES → runtime (e.g., JDBC drivers, SLF4J implementations)
Is it only for tests?
  YES → test
Is it needed only at build time for annotation processing?
  YES → provided + annotation processor config in compiler plugin

Version Conflict Resolution Workflow

Step 1: Identify the conflict

# Show full dependency tree
mvn dependency:tree -Dincludes=com.fasterxml.jackson

# Output shows conflicting versions:
# [INFO] +- io.quarkus:quarkus-rest-jackson:jar:3.20.1:compile
# [INFO] |  \- com.fasterxml.jackson.core:jackson-databind:jar:2.18.2:compile
# [INFO] +- some-other-lib:jar:1.0:compile
# [INFO] |  \- com.fasterxml.jackson.core:jackson-databind:jar:2.14.0:compile  ← CONFLICT!

Step 2: Analyze with verbose output

# Show why a specific version was chosen
mvn dependency:tree -Dverbose -Dincludes=jackson-databind

# Find unused or undeclared deps
mvn dependency:analyze

Step 3: Resolve

<!-- Option A: Exclude transitive dependency -->
<dependency>
    <groupId>some-other-lib</groupId>
    <artifactId>some-other-lib</artifactId>
    <version>1.0</version>
    <exclusions>
        <exclusion>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </exclusion>
    </exclusions>
</dependency>

<!-- Option B: Force version via BOM (preferred) -->
<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>com.fasterxml.jackson</groupId>
            <artifactId>jackson-bom</artifactId>
            <version>2.18.2</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<!-- Option C: Direct declaration wins (Maven nearest-first rule) -->
<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.18.2</version>
    </dependency>
</dependencies>

Resolution Priority

Common Errors & Fixes

"package X does not exist" after mvn compile

# Cause: Missing dependency or wrong scope
mvn dependency:tree | grep "the-missing-package"

# Fix: Add missing dependency or change scope from test/provided to compile

"Non-resolvable parent POM"

<!-- Cause: relativePath not set or wrong -->
<parent>
    <groupId>com.example</groupId>
    <artifactId>parent</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <relativePath>../pom.xml</relativePath>  <!-- Must be correct -->
</parent>

<!-- If parent is from remote repo, set empty: -->
<relativePath/>

Tests pass locally but fail on CI

<!-- Cause: Surefire fork reuse + static state pollution -->
<plugin>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <reuseForks>false</reuseForks>  <!-- Isolate test classes -->
        <!-- OR: control fork count -->
        <forkCount>1C</forkCount>  <!-- 1 fork per CPU core -->
    </configuration>
</plugin>

"Could not find artifact" in private registry

<!-- settings.xml (~/.m2/settings.xml) -->
<settings>
    <servers>
        <server>
            <id>private-repo</id>
            <username>${env.MAVEN_USER}</username>
            <password>${env.MAVEN_TOKEN}</password>
        </server>
    </servers>
</settings>

<!-- pom.xml -->
<repositories>
    <repository>
        <id>private-repo</id>  <!-- Must match server id -->
        <url>https://nexus.example.com/repository/maven-releases/</url>
    </repository>
</repositories>

Slow builds

# Parallel build (1 thread per CPU core)
mvn install -T 1C

# Skip tests when iterating
mvn install -DskipTests

# Build only specific module + its dependencies
mvn install -pl service-user -am

# Offline mode (skip remote checks)
mvn install -o

Maven-to-Bazel Migration

• Dependency Extraction: Identify external dependencies for maven_install in rules_jvm_external.
• Pom-to-Build: Mapping Maven <groupId>:<artifactId> to Bazel @maven//:group_artifact targets.
• Resource Management: Translating Maven's src/main/resources convention to Bazel resources attributes.

Migration Mapping Table

Optimization & Plugins

• Multi-module Projects: Efficiently managing parent-child relationships and <relativePaths>.
• Essential Plugins: Config and optimization for maven-compiler-plugin, maven-surefire-plugin, and maven-shade-plugin.
• Profiles: Using -P profiles for environment-specific configurations (dev, staging, prod).

Expert Tips

• Avoid <version>LATEST</version> or <version>RELEASE</version>; it breaks build reproducibility.
• Use mvn dependency:analyze to find unused declared dependencies.
• Prefer provided scope for libraries that should be part of the runtime container (like Quarkus-core during augmentation).
• Use mvn versions:display-dependency-updates to check for outdated dependencies.
• Always use <dependencyManagement> in parent POM, never hardcode versions in child modules.

🌐 Maven Knowledge Sources

Directive: Use web_fetch to analyze complex POM structures, dependency conflict resolution strategies, or Maven-to-Bazel migration patterns.

• Maven Dependency Mechanism: Introduction to Dependency Mechanism - Nearest-first, scope, and mediation.
• BOM Import Guide: Introduction to the Bill of Materials - Managing versions centrally.
• Maven-to-Bazel Migration: Migrating from Maven to Bazel - Strategy and tools.
• Quarkus Maven Tooling: Quarkus Maven Guide - Extension management and dev mode.

References

• Apache Maven Official Documentation
• rules_jvm_external (Bazel Maven support)
• Quarkus Official Guides

Skill Interoperability

The maven-expert 📦 skill acts as a source for dependency management and project orchestration, supporting:

• rules-quarkus 🔧: Facilitates the migration of Maven-based projects to Bazel.