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Helidon's Application Pom

PR 1022 introduced Helidon application parent poms. These are poms to be used by Helidon applications. This PR also re-structured the Helidon project poms. This document gives an overview of those changes.

The Problem

One of our core project principles is transparency. We prefer transparency over magic black boxes. Because of this, the pom files used in our quickstart sample applications (and maven archetypes) used self contained pom files. No parent. Everything was there for all to see.

We liked this, and customers that were well versed in Maven liked this. But beginners, and those people less knowledgeable of the intricacies of Maven, often tripped over details in the pom file when they had to inevitably modify it. All the details of plugin configuration and dependency management tended to obscure the most important part of the pom -- the list of dependencies.

This led to mistakes, like adding a dependency to the dependency management section (not the dependencies section). And once a mistake was made in the pom, errors tended to compound leading to frustration.

The Solution

To address this we decided to bite the bullet and introduce Helidon application poms for SE and MP. This is a trade-off. It helps newbies by simplifying the pom they deal with and reducing the chance of making a mistake. But it obscures the details of what's going on and makes things more complicated for customers that want to use their own parent poms.

We decided this was a reasonable tradeoff because it's important for the initial experience with Helidon to be frictionless and pleasant. Also customers that want their own parent pom are usually knowledgeable about Maven and can navigate our new pom structure.

One other benefit: this simplifies our examples. Now they all inherit an application pom parent. And we can manage plugins and versions in one place and ensure consistency across all examples.

The Design

The poms and their relationship are:

io.helidon:helidon-parent (parent/pom.xml)
└── io.helidon:helidon-bom (bom/pom.xml)
    └── io.helidon:helidon-dependencies (dependencies/pom.xml)
        └── io.helidon.applications:helidon-applications-project (applications/pom.xml)
            ├── io.helidon.applications:helidon-se (applications/mp/pom.xml)
            └── io.helidon.applications:helidon-mp (applications/se/pom.xml)

And what they do (starting from the bottom):

  • helidon-mp: the parent pom used by Helidon MP applications. It contains plugin configuration specific to Helidon MP applications, like jandex. It inherits from helidon-applications-project.
  • helidon-se: the parent pom used by Helidon SE applications. It contains plugin configuration specific to Helidon SE applications, like native image supportin the helidon-maven-plugin. It also inherits from helidon-applications-project.
  • helidon-applications-project: contains all the boiler plate for Helidon applications. For example, this configures the maven-dependency-plugin for copying runtime dependencies into the target/lib directory, and the maven-jar-plugin to create the application jar and set the classPath and mainClass attributes in the jar manifest. It inherits from helidon-dependencies
  • helidon-dependencies: dependency management for all third party dependencies used by both Helidon applications and the Helidon project itself. This helps ensure the same versions are used by Helidon applications and the Helidon project. Inherits from helidon-bom.
  • helidon-bom: dependency management for Helidon jars. This is our project bom. Inherits from helidon-parent
  • helidon-parent: basic Maven stuff like plugin management for basic plugins, repository configuration, etc.

Use by Helidon Project

All Helidon examples and stand-alone test apps use either helidon-se or helidon-mp as their parent. This ensure consistency across all our Helidon applications.

The Helidon archetypes generate projects that use either helidon-se or helidon-mp as their parent. So customer applications will typically use one of these as their parent pom.

The helidon-project pom (top level pom.xml), uses helidon-dependenices as its parent. This ensures common dependency and plugin management across the Helidon project and applications.

What about Stand Alone Poms?

Customers that want to build Helidon applications as part of their Maven project might not want to use our parent pom. The assumption is that these customers will be knowledgeable about Maven. To help these customers we have included a couple stand-alone examples. You can find those under examples/quickstarts/helidon-standalone-quickstart-*