Dependency Conflict Resolution

About

A dependency conflict occurs in Maven when multiple dependencies in a project bring different versions of the same library (transitive dependencies), leading to:

• Version Mismatches → Different versions of the same dependency in the classpath.

• Compilation/Runtime Issues → Unexpected behavior due to incompatible versions.

• ClassNotFoundException / NoSuchMethodError → When an older version is used instead of the expected one.

Maven resolves these conflicts automatically, but understanding the resolution mechanism helps avoid issues.

Maven does not allow multiple versions of the same library in the classpath simultaneously.

Why ?

• Java’s classloading mechanism does not support multiple versions of the same class in the same classpath.

• If two dependencies bring different versions of the same library, Maven picks one but warns about the conflict.

• The other version is ignored, which can cause runtime errors if the selected version lacks required methods/classes.

How Do Dependency Conflicts Occur?

Consider a project with dependencies:

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.17.1</version>
</dependency>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

What Happens Internally?

• spring-boot-starter-web also depends on log4j-core, but at version 2.14.1.

• Now, two versions (2.17.1 & 2.14.1) exist in the dependency tree.

• Maven must decide which version to use.

What Happens If Two Versions Are Declared?

Example: Conflicting Dependencies

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.15.0</version>
</dependency>

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.3</version>
</dependency>

Maven resolves this conflict automatically, choosing only one version based on:

1. Direct Dependency Preference – If one version is declared explicitly, it is preferred.

2. Nearest-Wins Strategy – If both are transitive dependencies, Maven picks the closest version.

To check which version is used:

mvn dependency:tree

Only one version appears in the resolved dependency tree.

Maven’s Conflict Resolution Strategy

1. Nearest-Wins Strategy (Dependency Mediation)

• Maven chooses the nearest dependency version in the dependency tree.

• If a dependency appears multiple times, the closest one to the project wins.

Example: Nearest-Wins in Action

<dependencies>
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>lib-A</artifactId>
        <version>1.2.0</version>
    </dependency>

    <dependency>
        <groupId>com.example</groupId>
        <artifactId>lib-B</artifactId>
        <version>1.0.0</version>
    </dependency>
</dependencies>

If lib-B internally depends on lib-A:1.0.0, but the project declares lib-A:1.2.0, Maven will use 1.2.0 because it is closer to the project.

Issue: This may cause incompatibilities if the newer version is not backward compatible.

2. Dependency Tree Analysis

Use the following command to check all dependencies and their versions:

mvn dependency:tree

Example output:

[INFO] com.example:my-project:jar:1.0.0
[INFO] ├── org.apache.logging.log4j:log4j-core:jar:2.17.1 (direct)
[INFO] ├── org.springframework.boot:spring-boot-starter-web:jar:3.1.0
[INFO] │   └── org.apache.logging.log4j:log4j-core:jar:2.14.1 (conflict)

Solution: Manually enforce log4j-core:2.17.1 to resolve the conflict.

3. Forcing a Specific Version (Dependency Management)

If Maven picks an older or undesired version, explicitly override it using <dependencyManagement>:

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>2.17.1</version>
        </dependency>
    </dependencies>
</dependencyManagement>

This ensures all modules in a multi-module project use the same version.

4. Using Exclusions to Remove Unwanted Dependencies

If a dependency is causing conflicts indirectly, we can exclude it:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
        </exclusion>
    </exclusions>
</dependency>

This removes log4j-core from spring-boot-starter-web.

Be careful! Excluding dependencies may break functionality.

5. Using a BOM (Bill of Materials)

BOMs standardize versions across projects:

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>3.1.0</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

It ensures consistent versions across modules.

Can We Force Two Versions in the Same Project?

Not directly in the same classpath! But some workarounds exist:

Solution 1: Shading (Relocating Dependencies)

• Used in fat JARs (e.g., with Maven Shade Plugin) to bundle different versions under separate namespaces.

• Example: If two different libraries require different jackson-databind versions, we can relocate one version.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>shade</goal>
            </goals>
            <configuration>
                <relocations>
                    <relocation>
                        <pattern>com.fasterxml.jackson</pattern>
                        <shadedPattern>shaded.com.fasterxml.jackson</shadedPattern>
                    </relocation>
                </relocations>
            </configuration>
        </execution>
    </executions>
</plugin>

This moves jackson-databind:2.13.3 under shaded.com.fasterxml.jackson, avoiding conflicts.

Solution 2: Isolating Dependencies in Different Classloaders

• Used in OSGi, Java Modules (JPMS), or custom classloading strategies.

• Example: Spring Boot’s parent-first and child-first classloading strategies.