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

Since Maven 3 was released in 2010, the Java build ecosystem has evolved dramatically—modularization, parallel builds, and cloud‑native containerization have become standard, leaving Maven feeling outdated.

Maven 4 addresses these accumulated pain points. Although a final GA release date is still pending, Maven 4 has already reached its fifth release candidate (RC5), indicating near‑final stability.

POM Model Upgrade: From 4.0.0 to 4.1.0

The POM model version is now 4.1.0 :

<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>

Backward compatible : Maven 4 can still build POMs that declare version 4.0.0.

New capabilities only apply when the modelVersion is 4.1.0.

The modelVersion element can be omitted; Maven will infer it from the schema.

Now is the right time to explore and prepare for upgrading.

Build POM / Consumer POM Separation: Solving "POM Pollution"

In Maven 3, a published POM contains plugin configuration, build details, parent references, and all properties, forcing downstream consumers to parse a lot of irrelevant information.

Maven 4 introduces flattening of the consumer POM:

No plugin configuration.

No parent POM reference.

No unused dependencies.

Only real transitive dependencies remain.

All properties are resolved to concrete values.

Enable it with:

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

Maven 3 required the external flatten-maven-plugin ; Maven 4 provides this as a native capability.

New Artifact Types: Explicit Classpath / Module‑Path Control

Previously, Maven inferred the classpath or module‑path based on the presence of module-info.class. Maven 4 adds explicit types:

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

Developers can now declare exactly where a dependency belongs, improving clarity for Java 9+ module systems.

Annotation Processor Types

Maven 4 adds dedicated processor types:

processor

classpath-processor

modular-processor

Example with Lombok:

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

This cleanly separates API classpath from processor classpath.

Modules Renamed to Subprojects: Aligning with Java 9

Java 9 introduced a module system that conflicted with Maven’s <modules>. Maven 4 deprecates modules and replaces it with subprojects:

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

Additional improvements:

Parent inference when <parent/> is empty.

Automatic discovery of subprojects without explicit listing.

Unified build timestamps.

Secure publishing: if any subproject fails, the whole release is aborted.

Tree‑Based Lifecycle: True Parallel Builds

Maven 3’s lifecycle is linear, making parallel builds inefficient for large multi‑module projects. Maven 4 introduces a Tree‑based Lifecycle where each subproject advances independently, starting as soon as its dependencies are ready. This yields significant speed gains for big builds.

Enable it with:

mvn -b concurrent verify

Small Configuration Enhancements

1. Conditional Profile Expressions

Profiles can now use full expressions instead of simple OS or JDK checks:

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

2. Unified sources Model

Maven 3 required separate sourceDirectory and testSourceDirectory. Maven 4 consolidates them:

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

This model better supports multiple source directories, multiple Java versions, and projects without plugin configuration.

Official Upgrade Tool

Maven 4 provides mvnup to analyze existing projects and suggest executable migration steps:

mvnup check   # generate a report
mvnup apply   # apply automatic fixes

The tool inspects POM files, plugins, and project structure, then outputs concrete upgrade actions.

The migration assistance, combined with the new native features, makes moving from Maven 3 to Maven 4 a semantic and practical upgrade.

Source: xxy Open‑Source Community