Rules_jvm_external: Setting Dependency Scope In Pom.xml

by Alex Johnson 56 views

Are you using rules_jvm_external to manage your Java dependencies and struggling with the scope of dependencies in your generated pom.xml? You're not alone! This article dives into how to control the dependency scope, specifically when exporting a Java library to Maven using Bazel and rules_jvm_external. We'll explore a common issue where dependencies are incorrectly marked with the "runtime" scope instead of the desired "compile" scope, and how to rectify this.

The Challenge: Incorrect Dependency Scopes

When working with Bazel and rules_jvm_external, exporting Java libraries to Maven often involves using the java_export rule. This is incredibly useful for sharing your libraries with other projects within your organization or the wider community. However, a common pitfall arises when the generated pom.xml incorrectly assigns the "runtime" scope to dependencies that should have the "compile" scope. Let's break down why this matters.

The scope of a dependency in Maven dictates when that dependency is needed. The two most common scopes are:

  • Compile: Dependencies with this scope are required for compiling, testing, and running your project. They are essential for the core functionality of your library or application.
  • Runtime: Dependencies with this scope are only needed at runtime. They are not required for compilation but are necessary when the application is executed. Think of things like JDBC drivers or specific logging implementations.

When dependencies that are crucial for compilation are marked as "runtime," it can lead to compilation failures in projects that consume your library. This is because the compiler won't have access to the necessary classes and resources during the build process.

So, how do you ensure that your dependencies are correctly scoped when using rules_jvm_external and java_export? The following sections will guide you through the process and provide practical solutions.

Diving into java_export and pom.xml Generation

Let's examine a typical scenario where this issue arises. Imagine you have a java_export rule defined in your BUILD file like this:

java_export(
 name = "mylib",
 srcs = [
 "src/main/java/.../file1.java",
 ],
 maven_coordinates = "my.company:my.lib:0.0.1-SNAPSHJOT",
 visibility = ["//visibility:public"],
 deps = [
 "@maven//:io_grpc_grpc_all",
 "@maven//:org_slf4j_slf4j_api",
 ],
 runtime_deps = [
 "@maven//:org_slf4j_slf4j_simple",
 ]
)

This rule defines a Java library named mylib and specifies its source files, Maven coordinates, visibility, and dependencies. The deps attribute lists dependencies required for compilation and runtime, while the runtime_deps attribute lists dependencies only needed at runtime.

When you run the Bazel command to publish this library to Maven:

MAVEN_USER="..." MAVEN_PASSWORD="..." bazel run --stamp --define "maven_repo=https://artifactory.my.company/..." //lib/java/mylib:mylib.publish

Bazel will generate a pom.xml file based on the information in your java_export rule. This pom.xml file is crucial because it tells Maven how to build and use your library. However, the generated pom.xml might contain unexpected scope assignments.

In the scenario described, dependencies pulled in by @maven//:io_grpc_grpc_all (such as grpc-java) might be marked with the "runtime" scope in the generated pom.xml. This is often not the desired behavior, as these dependencies are typically required for compilation.

Understanding the root cause of this issue requires a deeper look into how rules_jvm_external handles dependency resolution and scope mapping. We'll explore this in the next section.

Unraveling Dependency Resolution and Scope Mapping

To effectively address the incorrect scope issue, it's essential to understand how rules_jvm_external determines the scope of dependencies. The rules_jvm_external tool relies on information from the Maven artifacts themselves (specifically, their pom.xml files) to determine the scope of a dependency.

When rules_jvm_external fetches a Maven artifact, it parses the artifact's pom.xml file and extracts dependency information. This information includes the artifact's group ID, artifact ID, version, and scope. The scope is a crucial piece of metadata that dictates how the dependency should be handled during the build process.

The challenge arises because the scope information in the upstream Maven artifacts might not always align with your project's needs. For instance, a library might declare a dependency with a "runtime" scope, but your project might require that dependency during compilation. This discrepancy can lead to the issues we've discussed.

Furthermore, transitive dependencies (dependencies of your direct dependencies) inherit their scope from the upstream pom.xml files. This means that if a transitive dependency is marked with "runtime" in its pom.xml, it will also be treated as a runtime dependency in your project, unless you explicitly override it.

So, how can you override the default scope mapping and ensure that your dependencies have the correct scope? The key lies in understanding the mechanisms provided by rules_jvm_external for customizing dependency attributes. We'll explore these mechanisms in detail in the following sections, providing you with the tools and knowledge to fine-tune your dependency management.

Solutions: Overriding Dependency Scopes

Now that we understand the problem and the underlying mechanisms, let's explore the solutions. The primary way to control dependency scopes in rules_jvm_external is through the jvm_import rule and the artifact attribute.

The jvm_import rule allows you to declare a Maven artifact and explicitly specify its attributes, including the scope. This provides a powerful way to override the default scope derived from the upstream pom.xml file.

Here's how you can use jvm_import to set the scope to "compile" for a specific dependency:

jvm_import(
 name = "grpc_java_override",
 artifacts = ["io.grpc:grpc-java:1.44.1"],
 scope = "compile",
)

In this example, we're creating a jvm_import rule named grpc_java_override. The artifacts attribute specifies the Maven artifact we want to override (in this case, io.grpc:grpc-java:1.44.1). The crucial part is the scope = "compile" attribute, which explicitly sets the scope of this dependency to "compile".

To use this override, you need to replace the original dependency in your java_export rule with the jvm_import target. Let's modify our previous example:

java_export(
 name = "mylib",
 srcs = [
 "src/main/java/.../file1.java",
 ],
 maven_coordinates = "my.company:my.lib:0.0.1-SNAPSHJOT",
 visibility = ["//visibility:public"],
 deps = [
 ":grpc_java_override",
 "@maven//:org_slf4j_slf4j_api",
 ],
 runtime_deps = [
 "@maven//:org_slf4j_slf4j_simple",
 ]
)

Notice that we've replaced @maven//:io_grpc_grpc_all with :grpc_java_override in the deps list. This tells Bazel to use our overridden dependency definition instead of the default one.

By using jvm_import and explicitly setting the scope, you can ensure that your dependencies have the correct scope in the generated pom.xml file. This will prevent compilation issues and ensure that your library functions as expected in consuming projects.

Best Practices for Managing Dependency Scopes

While jvm_import provides a powerful mechanism for overriding dependency scopes, it's important to use it judiciously. Overriding scopes should be the exception, not the rule. Here are some best practices for managing dependency scopes in your Bazel projects:

  • Understand the Default Scopes: Before overriding any scopes, take the time to understand why a dependency has a particular scope in its upstream pom.xml. The scope might be intentional and reflect the library's design.
  • Minimize Overrides: Overriding scopes can make your build configuration more complex and harder to maintain. Only override scopes when absolutely necessary, such as when a dependency is incorrectly scoped and causing compilation issues.
  • Document Overrides: When you do override a scope, add a comment explaining why you're doing so. This will help others (and your future self) understand the reasoning behind the override.
  • Centralize Overrides: Consider creating a central location for your jvm_import rules that override scopes. This makes it easier to manage and maintain your overrides.
  • Consider Alternatives: Before overriding a scope, explore alternative solutions. For example, you might be able to refactor your code to avoid needing a dependency at compile time.

By following these best practices, you can effectively manage dependency scopes in your Bazel projects and ensure that your libraries are built and used correctly.

Conclusion: Mastering Dependency Management with rules_jvm_external

Controlling dependency scopes is crucial for building robust and reliable Java libraries with Bazel and rules_jvm_external. By understanding how rules_jvm_external handles dependency resolution and scope mapping, and by using the jvm_import rule effectively, you can ensure that your dependencies have the correct scope in the generated pom.xml file.

Remember to use scope overrides judiciously, document your changes, and consider alternative solutions before overriding. By following the best practices outlined in this article, you can master dependency management and build high-quality Java libraries with confidence.

For further reading and a deeper understanding of Maven scopes, explore the official Maven documentation on dependency scope.