How does Maven's dependency transitivity work? How to resolve dependency conflicts?

Maven's dependency transitivity refers to the mechanism where a project automatically acquires dependencies of its dependencies when it depends on another project. This mechanism simplifies dependency management but can also lead to dependency conflicts and version inconsistency issues.

Dependency Transitivity Rules:

1. Shortest path first: If multiple versions of a dependency exist, Maven will select the version with the shortest dependency path. For example: A→B→C(v1.0) and A→D→C(v2.0), if the A→B path is shorter, v1.0 is selected.
2. Declaration order first: When dependency path lengths are equal, Maven will select the version declared first in pom.xml.
3. Dependency scope affects transitivity: Only dependencies with compile scope are transitive, dependencies with test and provided scopes are not transitive.

Dependency Conflict Resolution:

1. Use <exclusions> tag to exclude unwanted transitive dependencies:

xml
<dependency>
    <groupId>com.example</groupId>
    <artifactId>example-lib</artifactId>
    <version>1.0.0</version>
    <exclusions>
        <exclusion>
            <groupId>org.conflict</groupId>
            <artifactId>conflict-lib</artifactId>
        </exclusion>
    </exclusions>
</dependency>

1. Use <dependencyManagement> to uniformly manage dependency versions:

xml
<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.conflict</groupId>
            <artifactId>conflict-lib</artifactId>
            <version>2.0.0</version>
        </dependency>
    </dependencies>
</dependencyManagement>

1. Directly declare the required version to override the transitive dependency version.

Best Practices:

• Regularly use mvn dependency:tree command to view the dependency tree and identify potential conflicts
• Use mvn dependency:analyze to analyze unused and declared dependencies
• Use dependencyManagement in parent POM to uniformly manage versions
• Avoid using SNAPSHOT version dependencies unless in a development environment
• For large projects, consider using BOM (Bill of Materials) to manage dependency versions