Why Your Java Modules Fail: The 2025 Package Name Trap

Introduction: The Silent Build Killer

You’ve done everything right. You migrated your application to a modern Java version like 21 or beyond. You’ve embraced the Java Platform Module System (JPMS) to create a more secure, reliable, and maintainable codebase. Your build runs, your tests pass, and then, at launch, your application crashes with a cryptic java.lang.module.ResolutionException. Welcome to the 2025 Package Name Trap.

As we move deeper into the modular era of Java, a subtle but devastating problem is becoming more common: the split package. This issue, often buried deep within your dependency tree, can halt development and leave even experienced engineers scratching their heads. This article will dissect this trap, show you how to diagnose it with precision, and provide robust strategies to ensure your modular applications are resilient and future-proof.

A Quick Refresher: What is the Java Platform Module System (JPMS)?

Introduced in Java 9, JPMS (also known as Project Jigsaw) was designed to bring modularity to the Java platform. Its primary goals are:

• Strong Encapsulation: Modules can explicitly declare which of their packages are exported (publicly available) and which are kept internal. This prevents accidental reliance on implementation details.
• Reliable Configuration: The module system verifies that all required dependencies are present at launch time, preventing the dreaded NoClassDefFoundError at runtime.
• Improved Scalability: Allows for the creation of custom, minimal runtime images containing only the necessary platform modules for your application.

A cornerstone principle of JPMS is its strict enforcement of package ownership: a package must belong to exactly one module. This simple rule is the source of both its power and the perilous trap we're about to explore.

The Trap Unveiled: Split Packages Explained

What is a Split Package?

A split package occurs when two or more modules on your module path attempt to define types within the same package name. For example, if both library-x.jar and library-y.jar contain classes in the package com.framework.utils, that package is considered “split” across two modules.

The module system cannot tolerate this ambiguity. It needs a single, authoritative source for every package to maintain reliable configuration and strong encapsulation. When it detects a split, it aborts with a ResolutionException.

Why It’s a “Trap” for 2025

This isn’t a new problem, but it’s a trap that will ensnare more developers by 2025 for several reasons:

1. Incremental Adoption: More libraries are being modularized, but the ecosystem is still a mix of modular JARs and traditional, non-modular JARs (which become “automatic modules”). This hybrid environment is a breeding ground for conflicts.
2. Transitive Dependencies: The conflict is rarely in your direct dependencies. It’s often hidden several layers deep in your transitive dependency tree, making it hard to spot.
3. Legacy Patterns: Older libraries sometimes packaged common utility classes (like those from Apache Commons or Guava) directly into their JARs, a practice that is disastrous in a modular world.

A Concrete Example of Failure

Imagine your project's module-info.java looks like this:

module com.mycompany.app {
    requires com.framework.analytics;
    requires com.framework.security;
}

Let’s say the com.framework.analytics module depends on an old, non-modular JAR, common-utils-1.0.jar. Meanwhile, the com.framework.security module has its own, slightly different version of the same utility classes. Both contain classes in the package org.apache.commons.lang3.

When the JVM constructs the module graph, it sees:

• Automatic module common.utils (from the JAR) exports org.apache.commons.lang3.
• Module com.framework.security also contains (and might export) org.apache.commons.lang3.

The result? A fatal error at startup: java.lang.module.ResolutionException: package org.apache.commons.lang3 is in module common.utils and module com.framework.security.

Diagnosing the Split Package Trap

Finding the source of a split package is half the battle. Fortunately, the JDK provides powerful tools for this.

Reading the Stack Trace

The exception message is your first and best clue. It will explicitly name the package and the two modules that claim it. Don't be intimidated by the long trace; focus on the root cause message:

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.module.ResolutionException: package com.google.common.base is in module com.google.common and module guava

This message clearly states that the package com.google.common.base is being provided by two different modules: com.google.common and guava.

Essential Diagnostic Tools

Your primary weapon against split packages is jdeps, the Java Dependency Analyzer. To find splits, you can use the --check command.

First, run a dependency tree command with your build tool (e.g., mvn dependency:tree or gradle dependencies) to identify all the JARs in your project. Then, point jdeps at your module path and main module:

jdeps --module-path 'libs/*' --check com.mycompany.app

If a split package exists, jdeps will fail with a clear report identifying the conflicting modules and the package they share, allowing you to find the problem before you even try to run your application.

Comparison: Classpath vs. Module Path

To fully grasp the trap, it's essential to understand the fundamental shift from the old classpath to the new module path.

Future-Proofing Your Modules: How to Escape the Trap

Once you've diagnosed a split package, you need to fix it. Here are the solutions, from most to least desirable.

Solution 1: Upgrade and Consolidate Dependencies

This is the cleanest and most correct solution. Investigate the conflicting modules identified by jdeps or the exception stack trace.

1. Identify the Common Dependency: In our example, both modules are trying to provide org.apache.commons.lang3.
2. Standardize the Version: Use your build tool (Maven's <dependencyManagement> or Gradle's dependency constraints) to force all transitive dependencies to resolve to a single, modern, and fully modular version of the common library (e.g., org.apache.commons:commons-lang3).
3. Exclude Transitive Offenders: If a library is incorrectly bundling another library's classes, use your build tool's exclusion mechanism to remove the transitive dependency.

Solution 2: Leverage jlink for Early Detection

If you are distributing a standalone application, using jlink to create a custom runtime image is an excellent practice. jlink builds a self-contained image with your application code, its dependencies, and only the necessary JDK modules.

The key benefit here is that the jlink process performs the same rigorous module validation as the JVM at launch time. If there is a split package or any other resolution issue, jlink will fail during your build, forcing you to fix it long before it reaches a user's machine.

Solution 3: The Last Resort - Shading and Relocating

Sometimes, you're stuck with a legacy library that you cannot upgrade or modify, and it's causing a split package. In these desperate cases, you can turn to “shading.”

Tools like the maven-shade-plugin or Gradle's Shadow Plugin can bundle a dependency's classes into your own JAR and, crucially, relocate its packages to a new, private path. For example, you could relocate com.google.common.base to com.mycompany.shaded.com.google.common.base.

Use with extreme caution. Shading can increase your JAR size, complicate debugging, and may violate the license of the library you are shading. It's a powerful tool for isolating problematic dependencies, but it should always be your last resort after exhausting all other options.

Conclusion: Build Resilient Modules for the Future

The 2025 Package Name Trap is not a flaw in the module system; it's a feature that exposes pre-existing flaws in our dependency graphs. By enforcing strict package ownership, JPMS forces us to confront the bad practices that led to JAR Hell. As Java's modular ecosystem matures, encountering and resolving split packages will become a standard part of a developer's workflow.

By understanding the cause, using tools like jdeps to diagnose issues early, and employing a clear strategy for resolution—prioritizing dependency consolidation over complex workarounds—you can build robust, reliable, and truly modular Java applications that are ready for 2025 and beyond.