What Maven 4 Brings: New POM Model, Flattened Consumer POMs, and Parallel Build Power

POM Model Upgrade: 4.0.0 → 4.1.0

Maven 4 moves the POM model version from 4.0.0 to 4.1.0, enabling backward compatibility while allowing new features that are only effective for the 4.1.0 model. The modelVersion element can be omitted because Maven can infer it from the schema.

<project xmlns="http://maven.apache.org/POM/4.1.0"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.1.0 http://maven.apache.org/xsd/maven-4.1.0.xsd">
  <modelVersion>4.1.0</modelVersion>
</project>

Build POM / Consumer POM Separation

This is Maven 4’s most disruptive change: the POM that is published to repositories (Build POM) no longer contains plugin configuration, build details, parent references, or unused dependencies, while the Consumer POM contains only the real transitive dependencies and resolved properties. This flattening eliminates “POM pollution” for downstream users.

Backward compatible – Maven 4 can still build projects using a 4.0.0 POM.

New capabilities apply only to the 4.1.0 model.

Enable flattening with:

mvn clean install -Dmaven.consumer.pom.flatten=true

In Maven 3 this required the external Flatten Maven Plugin ; Maven 4 provides it natively.

New Artifact Types: Explicit Classpath / Module Path Control

Maven 3 inferred the classpath or module path based on the presence of module-info.class. Maven 4 introduces explicit types so developers can declare where a dependency belongs.

<type>classpath-jar</type>
<type>module-jar</type>

Example with Lombok:

<dependency>
  <groupId>org.projectlombok</groupId>
  <artifactId>lombok</artifactId>
  <version>${lombok.version}</version>
  <type>classpath-processor</type>
</dependency>

Modules Renamed to Subprojects

To ease the transition to Java 9 modules, Maven 4 deprecates the modules element and replaces it with subprojects, which are automatically discovered.

<subprojects>
  <subproject>project-a</subproject>
  <subproject>project-b</subproject>
</subprojects>

Tree‑Based Lifecycle for Parallel Builds

Maven 3’s lifecycle is linear, making parallel builds inefficient. Maven 4 introduces a tree‑based lifecycle where each subproject progresses independently once its dependencies are ready, dramatically speeding up large multi‑module builds.

mvn -b concurrent verify

Enhanced Configuration: Conditional Profiles

Profiles can now use full expression language instead of simple OS or JDK checks.

<condition>
  exists('${project.basedir}/src/**/*.xsd') && length(${user.name}) > 5
</condition>

Unified sources Model

Maven 4 replaces the separate sourceDirectory and testSourceDirectory tags with a unified sources element that can declare multiple source entries, each with its own scope and directory.

<sources>
  <source>
    <scope>main</scope>
    <directory>my-custom-dir/foo</directory>
  </source>
  <source>
    <scope>test</scope>
    <directory>my-custom-dir/bar</directory>
  </source>
</sources>

Official Upgrade Tool (mvnup)

The mvnup command analyses the existing POM, plugins, and project structure, then generates actionable migration suggestions.

mvnup check   # only generate a report
mvnup apply   # automatically apply the changes