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AboutPOMFile.md

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Everything About POM File

  • POM stands for Project Object Model. It is an XML representation of a Maven project held in a file named pom.xml.

  • A project contains configuration files, as well as the developers involved and the roles they play, the defect tracking system, the organization and licenses, the URL of where the project lives, the project's dependencies, and all of the other little pieces that come into play to give code life. It is a one-stop-shop for all things concerning the project. In fact, in the Maven world, a project does not need to contain any code at all, merely a pom.xml.

  • The POM contains all necessary information about a project, as well as configurations of plugins to be used during the build process.

  • A Project Object Model or POM is the fundamental unit of work in Maven. It is an XML file that contains information about the project and configuration details used by Maven to build the project. It contains default values for most projects. Examples for this is the build directory, which is target; the source directory, which is src/main/java; the test source directory, which is src/test/java; and so on. When executing a task or goal, Maven looks for the POM in the current directory. It reads the POM, gets the needed configuration information, then executes the goal.

  • Some of the configuration that can be specified in the POM are the project dependencies, the plugins or goals that can be executed, the build profiles, and so on. Other information such as the project version, description, developers, mailing lists and such can also be specified.

  • The POM is independent of any language that's why maven can be use for other languages as well as long as plugin are available in maven to support those language.

Super POM

  • The "super pom.xml" is a concept in Maven that refers to the default parent POM (Project Object Model) that all Maven projects inherit from.

  • When you create a new Maven project, Maven automatically sets up the project's POM file with a parent element that references the super POM. This means that any configuration or settings defined in the super POM are inherited by your project's POM file, unless overridden.

  • The super POM defines a set of default values and settings for Maven projects, such as the default version of the Java language to use, the default packaging type, and the default repositories to use for downloading dependencies. These default values can be overridden in your project's POM file if needed.

  • The super POM is required in Maven because it provides a base set of configurations that ensure consistency across Maven projects. It also allows for a standardized way of defining common settings and plugins across multiple projects.

  • In summary, the super POM is an essential part of Maven's inheritance model and provides a base set of configurations that are shared across all Maven projects.

  • you can technically edit the super POM, but it's not recommended as it could affect all the Maven projects that inherit from it.

Minimal POM

  • A minimal POM (Project Object Model) is the most basic form of a POM file in Maven. It is the bare minimum required for Maven to recognize a project as a valid Maven project and perform basic tasks such as building, packaging, and deploying.

  • A minimal POM file typically includes the following elements:

  1. project : This is the root element of the POM file and defines the basic information about the project, such as its group ID, artifact ID, and version.

  2. modelVersion : This element specifies the version of the POM file format being used. In the case of Maven 2 and Maven 3, this is usually set to "4.0.0".

  3. groupId : This element specifies the unique identifier for the group or organization that the project belongs to.

  4. artifactId : This element specifies the unique identifier for the project itself.

  5. version : This element specifies the version number of the project.

  6. packaging : This element specifies the type of artifact that the project produces, such as "jar" (default), "war", "pom", etc.

  7. dependencies : This element specifies the dependencies that the project relies on.

  • Example
<project>
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.mycompany.app</groupId>
  <artifactId>my-app</artifactId>
  <version>1</version>
</project

Project Inheritance

  • In Maven, project inheritance allows you to define a parent-child relationship between Maven projects by specifying a parent POM for a child project. This enables you to share common configurations, dependencies, and plugins across multiple Maven projects, reducing redundancy and simplifying project management.

  • By using project inheritance, you can easily manage dependencies and configurations across multiple Maven projects, reduce duplication of code and settings, and maintain consistency across projects.

  • The Super POM is one example of project inheritance, however you can also introduce your own parent POMs by specifying the parent element in the POM, as demonstrated in the following examples. Here<parent></parent> tags is used.

  • The packaging type of parent POM is pom.

  • Example

  • Directory Structure

.
 |-- my-module
 |   \-- pom.xml
 \-- parent
     \-- pom.xml
  • POM File
<project>
  <modelVersion>4.0.0</modelVersion>

  <parent>
    <groupId>com.mycompany.app</groupId>
    <artifactId>my-app</artifactId>
    <version>1</version>
    <relativePath>../parent/pom.xml</relativePath>
  </parent>

  <artifactId>my-module</artifactId>
</project>

Project Aggregation

  • In project aggregation, we have a parent project that aggregates multiple child projects. The parent project contains a POM file that defines common settings, such as dependency management and build configuration, that are shared by all of the child projects.

  • Each child project also has its own POM file that references the parent POM file. This establishes a parent-child relationship between the parent project and its child projects.

  • Now, when we build the parent project, it also builds all of its child projects. This means that all of the child projects inherit the common settings defined in the parent POM file.

  • In summary, project aggregation allows us to manage multiple related projects as a single entity, while still maintaining a parent-child relationship between the parent project and its child projects. The parent project defines common settings that are shared by all of the child projects, and the child projects inherit these settings from the parent project.

  • In project aggregation, you can define all the dependencies in the parent POM and don't define any dependencies in the child POMs. Instead, you define the modules for each child project in the parent POM and inherit the parent POM in the child projects. This way, the child projects can inherit all the common configurations and dependencies from the parent POM without having to define them again in their own POMs.

  • Project inheritance is a specific type of project aggregation, where there is a single parent-child relationship between projects. When you have more than one project that share common settings and dependencies, you can use project aggregation to group them together under a common parent project.

  • Project aggregation can be seen as a higher-level concept that includes project inheritance as a specific use case. It allows you to manage multiple projects together, share common settings and dependencies, and simplify management. Project inheritance, on the other hand, is useful when you have a specific parent-child relationship between two projects, and you want to inherit some settings and dependencies from the parent project.

  • Project inheritance is like a multi-level inheritance, where each child project inherits settings and dependencies from its parent project, and the parent project can also inherit settings and dependencies from its own parent project.

  • On the other hand, project aggregation is like multiple inheritance , grouping multiple projects together under a common parent project. Each child project still has its own independent settings and dependencies, but they can also share common settings and dependencies defined in the parent project.

  • So project inheritance is a hierarchical relationship between parent and child projects, while project aggregation is more like a logical grouping of projects that share some common functionality or configuration.

  • To defined multi child project we used <module></module> tag. The parent pom has a pom packaging type.

  • The purpose of defining modules in the parent POM is to specify which child projects belong to the multi-module build. This allows the parent POM to manage the build for all the child projects and ensure that they are built in the correct order.

  • By specifying the modules in the parent POM, you are essentially creating a hierarchy of projects with a clear parent-child relationship. The child projects inherit the dependencies and plugins defined in the parent POM, and they can also override or add to these settings if needed.

  • Defining modules is an essential part of the project aggregation process because it allows the parent POM to manage the build for all the child projects and ensure that they are built in the correct order. Without this hierarchy, it would be difficult to manage the dependencies and plugins for all the child projects, and the build process would become more complex.

Project Interpolation and Variables

  • Project Interpolation is the ability of Maven to replace variables (also called properties) in POM files and other configuration files with their corresponding values at runtime. Variables can be defined in different ways, such as in the POM file itself, in the Maven settings file, or passed as command line arguments.

  • Variables are useful for defining values that are used throughout a project, such as the version number of a project, the name of an organization, or the path to a particular file. By using variables, you can avoid hard-coding values in multiple places and make it easier to maintain consistency across the project.

  • In Maven, variables are defined in the <properties> section of the POM file. This section contains key-value pairs, where the key is the variable name and the value is the variable's value. Once defined, variables can be referenced anywhere in the POM using the ${variableName} syntax.

  • Example

<properties>
  <project.version>1.0.0</project.version>
</properties>

<dependencies>
  <dependency>
    <groupId>com.example</groupId>
    <artifactId>my-project</artifactId>
    <version>${project.version}</version>
  </dependency>
</dependencies>

Categories in POM

1. Relationships

  • POM relationships is its handling of project relationships, this includes dependencies (and transitive dependencies), inheritance, and aggregation (multi-module projects).

  • POM relationships are important because they allow developers to manage dependencies and other project configurations in a centralized way. By defining common configuration in a parent POM, developers can easily share and reuse that configuration across multiple child projects. This helps to maintain consistency and reduce redundancy in Maven projects

  1. Maven Coordinates : In Maven, a project is identified using a unique set of coordinates, which are called Maven coordinates. Maven coordinates are required because they provide a standard way of identifying and referencing a project or module, which is essential for dependency management. When a project has dependencies on other projects or modules, it needs to know their Maven coordinates in order to download and use them. Maven coordinates consist of three parts:

A. Group ID: A unique identifier for a group of related projects. Typically, this is based on the organization name or domain name of the project.Note that the dot-notated groupId does not have to correspond to the package structure that the project contains.

B. Artifact ID: The name of the project or module.

C. Version: The version of the project or module.

  1. Multi-Module: Project Aggregation

  2. Inheritance: Project Inheritance

  3. Dependencies : The cornerstone of the POM is its dependency list. Most projects depend on others to build and run correctly.

2. General Project Information

  1. General : Tells about project information.

  1. license : In Maven, a project license is an important piece of metadata that describes the legal terms and conditions under which the project and its dependencies are made available to end-users. It is required to provide a clear understanding of the rights and restrictions associated with using the project. A license is typically specified in the POM file of a Maven project using the element. It contains information such as the license name, URL, and distribution terms. Maven supports a wide range of licenses, such as Apache License, GPL, MIT License, and many others.

  1. Organization : Provide organization name

  1. Developers : In Maven, developers refer to the individuals or organizations who contribute to a project. The developers section in the POM file is used to provide information about the people who have contributed to the project, such as their names, email addresses, and roles. The developers section is used for documentation purposes and provides information about who has contributed to the project, so it's useful for others who want to know more about the project and its contributors. Additionally, it can be used for communication purposes as well, such as to contact the developers in case of issues or to ask for contributions.

  1. Contributers : In Maven, contributors refer to the individuals or entities that have contributed to the development of a project. They may have contributed code, documentation, bug reports, or other forms of support. The contributors section in the project's POM file is used to list the contributors and their respective roles in the project.

3. Build Settings

  • In Maven, the build settings define how the project should be built, packaged, and deployed. These settings are specified in the project's POM (Project Object Model) file and include information such as the project's source code location, the project's output directory, the plugins that should be used during the build process, and other configuration settings.

  • Build settings are important because they allow you to specify how your project should be built in a standardized way. This makes it easier for other developers to build and contribute to your project, as they don't need to understand the specifics of how your build process works. It also allows you to automate your build process and easily deploy your application to different environments.

  • Inside <build></build> all build related configuration are mentioned in it.

  1. Directories

  • The source code directory : This specifies the location of your project's source code.

  • The output directory: This specifies where the compiled class files and other build artifacts should be placed.

  • The project dependencies: This specifies the other Maven artifacts that your project depends on.

  • The plugins to use during the build: This specifies the plugins that should be used during the build process to perform tasks such as compiling code, generating documentation, and packaging the project.

  1. Extension : In Maven, extensions are used to enhance or modify the build process beyond the standard functionality provided by Maven plugins. Extensions can provide additional capabilities to the build process, such as custom packaging types, custom lifecycles, or custom repository layouts.

  • Extensions can be used to integrate third-party tools or libraries that are not available as Maven plugins, or to extend the functionality of existing plugins. They can be developed by the community or by the project team itself to suit specific needs.

  • Example, if there are artifacts which is not present in the central repository of maven and present in other central repository , we can use extension to get those artifacts.

  1. Resources : In Maven, resources refer to any non-Java files such as configuration files, properties files, XML files, images, etc. that are required for a project. These resources are usually placed in a specific folder in the project structure, typically under src/main/resources or src/test/resources.

  • Maven allows you to include resources in your project by adding them to the resources section in the POM file. You can specify the location of the resource files, and Maven will automatically copy them to the appropriate output directory during the build process.

  • Resources can be useful for storing configuration files, properties files, and other data files that are required for your application to run. By including them in your Maven project, you ensure that they are always available to your application, regardless of the environment in which it is deployed.

  1. Reporting : In Maven, reporting is a feature that allows you to generate various types of reports about your project, such as code coverage reports, test reports, dependency reports, and many others. These reports provide insights into different aspects of your project and can be used for various purposes, such as monitoring project health, identifying areas for improvement, and communicating project status to stakeholders.

  • Reporting in Maven is achieved through the use of plugins that are specifically designed to generate reports. These plugins are executed during the build process and produce reports in various formats, such as HTML, XML, or PDF.

  • The reporting feature is useful because it allows you to easily generate and share project reports with your team or stakeholders, without having to manually gather and analyze data. Additionally, the reports can be configured to suit your specific needs, allowing you to focus on the aspects of your project that are most important to you.

4. Build Environment

  • The Build Environment in Maven refers to the configuration of the environment in which the Maven build runs. This includes various aspects such as the Java version, the operating system, the build tool version, etc.

  • Maven provides several mechanisms to configure the build environment, such as the section of the POM file, the use of profiles, and the use of system properties. These mechanisms can be used to specify various settings such as the Java compiler version, the target Java runtime version, the location of external tools, and so on.

  • The Environment for production and development will be different , so accordingly thier build configuration will also vary.

  1. Issue Management : Issue management is a feature in Maven that helps to track and manage issues related to a project. Issues can be anything related to the project such as bugs, feature requests, or other improvements.

  • The <issueManagement> element in the POM file is used to specify the issue tracking system that the project uses. It contains the URL of the issue tracking system and the name of the project in the system.

  • For example, if a project uses JIRA/BugZilla for issue tracking, the <issueManagement> section of the POM file can be configured.

  1. CI management : CI (Continuous Integration) Management is a process of automating the build and testing of code changes on a continuous basis. In Maven, it refers to the integration of the project with a CI tool to facilitate the continuous integration process.

  • CI Management is required to ensure that the code changes made by developers are continuously integrated and tested in a shared environment. This helps to identify issues early on in the development process and allows for faster feedback, making it easier to fix issues before they become more difficult and costly to address.

  • There are several CI tools available that can be integrated with Maven, including Jenkins, Travis CI, and CircleCI. These tools can be used to automate the build and testing process and can be configured to run builds automatically whenever code changes are pushed to the repository.

  1. Mailing List : Maven's mailing list configuration is only used for subscribing to a mailing list or a group of people interested in a particular topic related to the project. Once subscribed, users can send and receive emails related to the topic of the mailing list. However, it does not send automatic mails to the users.
  • A mailing list is a way for the end-users of an application to communicate with the development team, or with other users of the application. It can be used for support requests, bug reports, feature requests, and general discussion about the application.

  1. SCM : Source Code Management

  1. Pre-Requisite : This section allows use to defined certain pre-requisite conditions which is not met then the build will failed.

  1. Repositories : This stores the artifect which is fetch from the maven central repositories if its not available in the local repositories. This can be useful when artifacts are not in central maven repositories.

  1. Distributed Management : Distributed management in the Maven environment refers to the ability to manage dependencies and artifacts across multiple repositories and servers. This is important when dealing with large-scale projects that require the use of a variety of dependencies and libraries.
  • In a distributed environment, different teams or individuals may be responsible for maintaining different parts of a project, and each team may have its own repository or server for storing artifacts. Maven allows for the coordination of these different repositories and servers, allowing for seamless integration and sharing of dependencies between teams.

  1. Profiles : Maven Profiles are seggration of builds specific to the environments like test build in local system and deploying build in production , these seems to have different profiles.

More about Plugins

  • In a Maven plugin's <execution> element, you specify the goals that you want to execute during the build, and any necessary configuration for those goals in the <configuration> element.

  • The goals element specifies the Maven plugin's goal(s) that should be executed during the build. For example, in the json-schema-generator-maven-plugin ( under Java-JSON-Schema folder ) example, the generate-json-schemas goal is executed during the prepare-package phase.

  • The configuration element allows you to pass configuration parameters to the goal. These parameters can be used to control the behavior of the goal, such as specifying which classes should have JSON schemas generated for them.

  • So, in summary, you define a specific configuration of a Maven plugin by specifying its <execution> element. Within the <execution> element, you specify the goals that should be executed, and provide any necessary configuration for those goals in the <configuration> element.

  • In some cases, the plugin may have sensible defaults that work well without any configuration specified, so omitting the <configuration> element will work fine. In other cases, the plugin may require certain configuration to be specified, and will fail if that configuration is not provided.

  • For example, in the json-schema-generator-maven-plugin example we looked at earlier, the <configuration> element includes an <includes> element specifying which classes should have JSON schemas generated for them. If you were to omit this <configuration> element, the plugin would likely fail, as it wouldn't know which classes to generate schemas for.

  • It's always a good idea to check the documentation for the plugin you're using to see if any configuration is required or recommended. If in doubt, it's often better to provide some configuration (even if it's just the minimum required) rather than omitting it entirely, to avoid any unexpected failures during the build.

  • A plugin can have multiple goals, and each goal can be executed separately. In Maven, you can execute a specific goal of a plugin using the syntax mvn <plugin-name>:<goal-name>.

  • For example, if you have a plugin called my-plugin with two goals, goal-1 and goal-2, you can execute goal-1 with the command mvn my-plugin:goal-1, and execute goal-2 with mvn my-plugin:goal-2.

  • In a Maven build, you can specify which goals should be executed as part of an execution block for a plugin. However, you can also execute goals outside of an execution block by running them directly from the command line.

  • Maven automatically assigns a default phase for a plugin based on the plugin's packaging and lifecycle bindings.

  • The default phase bindings for each plugin can be found in the plugin's documentation or in the Maven Central Repository. You can also override the default phase binding for a plugin by explicitly specifying a phase element in the plugin's configuration.

  • For example, If you don't explicitly specify a phase for the maven-surefire-plugin, it will be executed during the test phase by default. This is because the maven-surefire-plugin is primarily used to execute tests and the test phase is where all the tests are executed.

  • Similarly, other plugins might also have default phases assigned to them, but it's a good practice to explicitly mention the phase in which you want a plugin to execute to avoid any confusion or unexpected behavior.

  • The targetPackage configuration option in Maven plugins is used to specify the package name under which the generated Java classes should be placed. It is used by plugins that generate Java code from other sources such as XSD or JSON schema files.

  • On the other hand, the outputDirectory configuration option specifies the directory where the compiled classes should be placed. This directory is usually target/classes by default, but can be changed to any other directory using the outputDirectory configuration option.

  • <packageName> and <targetPackage serve the same purpose of specifying the package name for the generated classes, but <packageName> is used within the jaxb2-maven-plugin (XSD-to-Java) configuration while <targetPackage> is used within the schema's (JSON-Schema-To-Java) binding file.