Bukama commented on code in PR #598: URL: https://github.com/apache/maven-site/pull/598#discussion_r1898709642
########## content/markdown/whatsnewinmaven4.md: ########## @@ -0,0 +1,474 @@ +# What's New in Maven 4? + +Maven is over 20 years old and is one of the most used build tools in the Java world. +Throughout the years, one important rule has been maintaining the highest backward compatibility possible, especially +with its [POM-schema with Model version 4.0.0][2], used not only for the build itself but also by consumers. +This made Maven more than a tool; it became a whole ecosystem with many dependencies on the POM, especially the Maven +Central repository, other build tools, and IDEs. +But this stable schema comes at a price - the lack of flexibility. + +> "With the Maven build schema preserved in amber, we can’t evolve much: we’ll stay forever with Maven 3 minor releases, +> unable to implement improvements that we imagine will require seriously updating the POM schema…" +> — <cite>[Hervé Boutemy (in Javaadvent 2021)][1]</cite> + +Maven 4 will prepare for changes which are impossible nowadays, like a completely new build schema. + +Another pain point of Maven 3 is a codebase with a lot of deprecated, convoluted, non-performant, and duplicated code +which costs the volunteers who maintain Maven a lot of time. +This not only means that the Maven codebase contains old Java code that can be optimized nowadays but also old +dependencies and poor API design of its own APIs, especially for Maven plugins. +Therefore, Maven 4 will also be a maintenance release. + +This article presents and explains major changes brought by Maven 4, grouped into several topics. + +## POM Changes + +### Build-POM and Consumer-POM + +As written in the introduction, Model version 4.0.0 is used not only for the build but also by consumers of the +artifact. +However, several contents of the POM are only necessary for the build while others, like the dependencies, are also +needed by the consumer. +Maven 4 will therefore differentiate between a "Build-POM" and a "Consumer-POM". +As the names suggest, the "Build-POM" will contain all information needed to build the artifact, e.g., used plugins and +their configuration, while the "Consumer-POM", which is created during the Maven build, will not contain those. +This POM will only keep what is really needed to use the artifact, e.g., dependency information. + +**Note**: See below for a comparison of the content of both POMs. + +### Model version 4.1.0 + +With now having two types of POM, Maven 4 can already make additions to the Build-POM as it will only be used by Maven ( +and of course IDEs). +Therefore, with Maven 4, a new Model version 4.1.0 is introduced. +This version introduces some new elements and attributes, while others are marked as deprecated. +To not break the ecosystem, this version is only available for the Build-POM, while the Consumer-POM will still use +version 4.0.0. +This means Maven will generate the Consumer-POM during the build. + +**Note**: Maven 4 will of course continue to build your model version 4.0.0 project! +There is no need to update your POMs to 4.1.0 as long as you don't want to make use of the new features. +But as with every software update - it's suggested to update them to 4.1.0, e.g., to avoid problems when deprecated +features are removed in future updates. + +### Modules are now subprojects + +From the early days of Maven 1 until today, all build information is stored in the POM, short for "Project Object +Model". +Together with build folders and other files, the wording "Maven project" is used. +However, for projects containing multiple parts, e.g., an API and a client, each of those parts was called a "module" +and listed in the `<modules>` section of the POM, leading to the terms "multi-module project". +This wording introduced some inconsistency, especially as projects without any `<modules>` section are often called " +single-module". +Since the introduction of the [Java Platform Module System][3] in Java 9, the term "module" has raised additional +confusion. + +Maven 4 gets rid of this by naming "modules" as what they are - subprojects. +Model version 4.1.0 contains a new element `<subprojects>` analogous to the now deprecated, but still usable, element +`<modules>`. + +**Note**: It's suggested to use the terms `multi-project setup` and `single-project setup` to differentiate between a +Maven project with or without subprojects. + +### New packaging type: bom + +Maven 4 introduces a dedicated packaging type to provide a [Bill of Material BOM][4] called "bom" to differentiate more +precisely between "parent POMs" and dependency-managing BOMs. +While the new type is only available with Model Version 4.1.0, the final outcome is a full Maven 3 compatible (model +4.0.0) POM file! +For an example, see the link above or +the [live coding by Maven maintainer Karl Heinz Marbaise at IntelliJ IDEA Conf 2024][5]. + +**Note**: With Maven 4, it's also possible to exclude dependencies that are declared by BOMs using the existing +`<exclusions>` element. +Also note that in Maven 4, importing BOMs with a classifier is now possible. +Therefore, the Maven team suggests that project BOMs should be generated as classified artifacts, using the +`<bomClassifier>` element. +This means that an imported BOM must **not** come from the same reactor as the current build but be available outside +the project before the build (in other words, "you should import only external BOMs") or it may break your build (as Review Comment: "may", as it's not 100%, but chances are good - that's how I understood the linked issue and what Tamas said to me. -- This is an automated message from the Apache Git Service. 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