What happens if a starter dependency includes conflicting versions of libraries with other dependencies in the project?

When a Spring Boot starter dependency introduces a library version that differs from another dependency in the project, Spring Boot's dependency management mechanism helps resolve the conflict by selecting a single compatible version. It uses a predefined Bill of Materials (BOM) and the dependency resolution rules of Maven or Gradle to ensure that the application uses consistent and compatible library versions.

Key Points: • Spring Boot manages dependency versions through its BOM (Bill of Materials). • Maven or Gradle resolves version conflicts and selects a single version of each library. • Spring Boot prioritizes tested and compatible dependency versions. • Dependency conflicts can still occur if versions are manually overridden. • Tools such as Maven Dependency Tree help identify and troubleshoot conflicts.

Why Do Dependency Conflicts Occur?

Consider a project with:

Dependency A

Uses:

Jackson 2.15

Dependency B

Uses:

Jackson 2.18

Now the application has two different versions of the same library.

Questions arise:

• Which version should be used? • Which classes should be loaded? • Will the libraries remain compatible?

This situation is called a dependency conflict.

How Spring Boot Handles It

Spring Boot provides a dependency management BOM.

Developer Adds Dependencies | Spring Boot BOM | Version Resolution | Single Compatible Version Selected | Application Build

The BOM contains tested versions of commonly used libraries and ensures consistency across the project.

Example:

spring-boot-starter-web

may include:

• Jackson • Tomcat • Spring MVC • Validation API

Even if multiple libraries require Jackson, Spring Boot selects a compatible version based on its dependency management configuration.

Maven Dependency Resolution

Maven generally follows the "nearest dependency" rule.

Example:

Project | |-- Library A | | | |-- Jackson 2.15 | |-- Library B | |-- Jackson 2.18

Maven resolves the conflict and chooses one version.

When Spring Boot Dependency Management is active, the version defined in the BOM usually takes precedence.

Example: Suppose a project contains:

spring-boot-starter-web

and

a third-party library requiring an older Jackson version.

Spring Boot may override the older version and use the version defined in its BOM to maintain compatibility across the application.

How to Check Dependency Conflicts

Maven Command:

mvn dependency:tree

Output shows:

• Direct dependencies • Transitive dependencies • Version conflicts

This is one of the most commonly used commands for troubleshooting dependency issues.

Can Conflicts Still Cause Problems?

Yes.

Problems may occur when:

• Developers manually override versions. • Third-party libraries require incompatible versions. • Old libraries are mixed with newer Spring Boot versions.

Possible issues:

• ClassNotFoundException • NoSuchMethodError • Bean initialization failures • Application startup errors

How to Resolve Conflicts

1. Use Spring Boot managed versions whenever possible.

2. Analyze dependencies:

mvn dependency:tree

3. Exclude unwanted dependencies.

Example:

<dependency>
    <groupId>com.example</groupId>
    <artifactId>sample-lib</artifactId>

    <exclusions>
        <exclusion>
            <groupId>
                com.fasterxml.jackson.core
            </groupId>
            <artifactId>
                jackson-databind
            </artifactId>
        </exclusion>
    </exclusions>
</dependency>

4. Upgrade incompatible libraries.

Benefits of Spring Boot Dependency Management

• Reduces version conflicts • Ensures library compatibility • Simplifies dependency upgrades • Improves application stability • Reduces maintenance effort

Real-World Example

An e-commerce application uses:

• Spring Boot • Hibernate • Jackson • Kafka • Redis

Each library has its own transitive dependencies.

Without dependency management:

• Version mismatches become common. • Integration issues increase.

With Spring Boot BOM:

• Compatible versions are selected automatically. • The application remains stable and easier to maintain.

Interview Tip: A concise interview answer is:

"When multiple dependencies bring different versions of the same library, Spring Boot uses its dependency management BOM along with Maven or Gradle dependency resolution rules to select a single compatible version. This helps prevent version conflicts, ensures library compatibility, and reduces runtime issues. Developers can analyze and resolve conflicts using tools such as mvn dependency:tree."