MavenSupportSpec

Summary

A lot of Java applications use Maven (and associated maven plugins) as their build system. In order to package this software as appropriate Debian packages, a lot of changes need to be applied to the usual Maven process, in particular to avoid the online dependency resolving feature of Maven. This document presents a solution to this problem and details the minimal work needed before we can start packaging a minimal Maven project.

Release Note

The infrastructure making it possible to package Maven-based Java applications like Geronimo or Glassfish in main or universe is now in place.

Rationale

Most recent complex Java applications leverage ASF's Maven system and artifact repository to ease the dependency resolving and building of their software. Unfortunately the usual Maven process downloads/updates needed artifacts (JARs), Maven plugins and build/runtime dependency information (.pom files) from the Maven repositories on the Internet into a local repository cache and works from there. This is incompatible with the way our buildd system works, and with the idea of repeatable builds in general. Given the power of the Maven plugins-based build system, working around Maven to build those complex packages without it is almost impossible.

Use Cases

  • Alan in an Ubuntu developer. He wants to package Geronimo in universe, but Geronimo uses Maven. He is able to use the Maven packaging method and the already-packaged Java libraries and Maven plugins to write a fully-offline and repeatable package, compatible with our buildd system.
  • Betty is a Java developer. She wants to use Maven to build her own Java software. Fortunately, this method doesn't modify Maven in any way so it doesn't prevent her from using Maven the usual way, setting up a local repository that downloads and updates files from the online Maven repositories.

Assumptions

Software using Maven to build is usually very large and requires lots of dependencies. Even once the basic blocks described in this document are in place, packaging any large Java software will require lots of external packaging work to have all the required Java libraries available as proper Debian packages to build-depend from. So keep in mind that this might just be the visible tip of the iceberg.

Design

Why we can't just call maven in debian/rules

  • Online dependency download/update: Maven downloads needed build/runtime dependency information (.pom files) and resulting artifacts (Java libraries and Maven plugins) from Maven repositories on the Internet. For each item already present in the local repository, it checks if the version is up to date. We need to provide all those dependencies as part of the source or as packaged build dependencies, and disable Maven freshness check.
  • Use of a specific repository: Maven doesn't use the libraries installed in /usr/share/java, it only uses its own local repository. Since the required Java libraries build dependencies will be pulled as packages, Maven needs to make use of those.

Solution

The proposed method is based on two principles:

  • Reconstruct at build-time a complete Maven local repository in debian/mvn_repo by recreating dynamically all required POM files and replacing every JAR with a symbolic link to the corresponding system-installed library
  • Call mvn in offline mode to avoid any superfluous online freshness check and use the "-s" option to make it use the mvn_repo directory as its repository.

Helper tools

Helper tools can be used to ease the process. They are separated in two sets, one used by the packager and one used at build-time.

maven-packaging-support

Maven packaging support is a set of tools that help the packager to build the repository description file. Two tools would be provided:

  • mvn-go-offline: downloads all necessary dependencies in a local Maven repository
  • mvn-debianize: walks through the repository built by mvn-go-offline, asks questions on where to find the corresponding system libraries, and builds the repository description file

The resulting repository description file is then shipped as part of the debian/ directory in the source package.

maven-helper

Maven helper is a set of tools that are used at build time to reconstruct the repository and call Maven with the relevant options. It is used as a build dependency of the package, and provides three helpers for debian/rules:

  • mh_clean: used in the clean rule, cleans potential intermediary files
  • mh_prepare_repo: used in a configure rule, reconstructs POM files and system links in the local Maven repository from the repository description file
  • mh_mvn: used in the build rule, calls mvn with the right options to prevent offline downloading and force usage of the rebuilt repository

Potential issues

  • Maven relies on specific versions of libraries: in the .pom files, each artifact can depend on a very specific version of a given JAR, which is usually different from the one in the Ubuntu repositories. Extra care in testing is needed to double-check that using a slightly different JAR version doesn't make it break at build or at runtime.

Implementation

maven-packaging-support

This set of tools needs a few yet-unpackaged Java libraries before it can be delivered as a proper Debian package.

See work in progress: https://launchpad.net/maven-packaging-support

maven-helper

This set of tools is already packaged as a Debian package.

See work in progress: https://code.launchpad.net/~ludovicc/maven-debhelper/main

PPA: https://launchpad.net/~ludovicc/+archive

Packaging of basic Maven project dependencies

Simply calling the classic Maven targets (clean, build, package...) requires some dependencies to be present, so to demonstrate Maven packaging we need to provide at least the mimimal build dependencies required by even the simplest helloWorld maven project.

Maven dependencies for a helloWorld project

(in bold, non-existing packages)

Target

Artifacts

clean

maven-clean-plugin maven-file-management maven-shared-io plexus-utils

compile

maven-resources-plugin maven-compiler-plugin plexus-compiler-api plexus-compiler-javac plexus-utils plexus-compiler-manager

test

junit surefire-junit surefire-api surefire-booter maven-surefire-plugin plexus-utils

package

commons-lang maven-archiver maven-jar-plugin plexus-archiver plexus-utils plexus-io

Bootstrapping

Fortunately we don't really need maven to build those missing basic blocks. They can be built using ant with minimal effort. Here is a breakdown of the order in which the new packages can be created. maven-plugin-testing-harness.jar is optional, needed only to enable testing in the maven plugins build.

JAR

Runtime & Compile depends

Testing depends

Level 0 (can be done using current archive)

surefire-api.jar

commons-lang plexus-utils

jmock

plexus-archiver.jar

plexus-component-api plexus-container-default plexus-utils

junit

plexus-io.jar

plexus-component-api plexus-container-default

junit

maven-shared-io.jar

classworlds junit maven wagon-provider-api plexus-utils plexus-container-default

easymock

maven-plugin-testing-harness.jar

maven junit

Level 1 (needs level 0)

surefire-booter.jar

surefire-api plexus-utils

jmock

surefire-junit.jar

surefire-api

jmock

file-management.jar

classworlds maven maven-shared-io wagon-provider-api plexus-container-default plexus-utils

junit

maven-archiver.jar

maven plexus-archiver plexus-utils

maven-resources-plugin.jar

maven

maven-plugin-testing-harness

maven-compiler-plugin.jar

classworlds junit maven plexus-compiler-api plexus-compiler-manager plexus-container-default plexus-utils plexus-compiler-javac

jsch commons-cli doxia-sink-api maven-plugin-testing-harness wagon-file wagon-http-lightweight wagon-provider-api wagon-ssh plexus-interactivity-api

Level 2 (needs level 1)

maven-surefire-plugin.jar

maven surefire-booter

jmock

maven-jar-plugin.jar

commons-lang maven-archiver plexus-io plexus-utils

maven-plugin-testing-harness

maven-clean-plugin.jar

classworlds junit file-management maven-shared-io maven wagon-provider-api plexus-container-default plexus-utils

jsch commons-cli doxia-sink-api maven-plugin-testing-harness wagon-file wagon-http-lightweight wagon-ssh plexus-archiver plexus-interactivity-api

If this spec is accepted, packaging-needed bugs would be filed (first Level 0 ones, then Level 1...)

Directions for packagers

Lots of dependencies are Maven-specific, in particular the Maven plugins. It might make sense not to ship those libraries into /usr/share/java but rather in /usr/share/maven2/plugins. Since we use links from mvn_repo, from our point of view the location doesn't matter, and it would avoid crowding the /usr/share/java directory with JARs that can only be called by Maven.

Test/Demo Plan

We should provide Debian packaging for a minimal helloWorld Maven project.

Outstanding Issues

None.

Discussion

I noticed the following tools which may be of some use too ? :

Hope this helps -- OlivierBerger

afaict those tools make installable debian binary packages but cannot be used to create packaging compliant with the buildd rules (no access to the Internet, all must be built from source and existing dependencies, no binaries in source) and therefore probably can't be used -- ThierryCarrez

Alternative solutions (for reference)

JPackage patchset to Maven

The JPackage patchset is implemented in Fedora. It patches maven so that, when called with specific options, it supports using JARs installed in /usr/share/java directly. The POM files are shipped together with the Java libraries. A depmap.xml file converts Maven artifact names to system file locations. It is built from depmap fragments shipped with each Java library.

The advantage of this solution is to ship Maven-related information inside the Java library packages themselves, and rely on that system to automatically find the required JARs. However, it requires a heavy patch on Maven that must be maintained, a basic infrastructure that is very costly to put in place (refresh every JAR), and the system might be brittle because it doesn't freeze the POM information in a state that is necessarily compatible with the build you are trying to make (debugging of dependency issues is more complicated).

Packaging Aware Maven Proxy

In this approach, we would run a Maven proxy that would solve all dependency mapping and let Maven build a repository at build-time that downloads artifacts from the system itself. Maven would be run with a settings file that point to the local proxy, and the maven proxy would be launched prior to launching a maven build process.

The proxy would need to have the following features:

  • Generate pom.xml files upon request according to debian packaging dependencies
  • Map request artifact versions to existing artifact versions
  • Serve jar files from /usr/share/java
  • Block access to the internet

The advantages of this approach is that it doesn't require any Maven patching. However, building the maven proxy is complex, and starting a localhost daemon in parallel to a build might not be the best solution for buildds.


CategorySpec

JavaTeam/Specs/MavenSupportSpec (last edited 2009-01-07 18:40:18 by onkarshinde)