Apache Maven
Encyclopedia
Maven is a build automation
and software comprehension tool. While primarily used for Java
programming, it can also be used to build and manage projects written in C#, Ruby
, Scala, and other languages. Maven serves a similar purpose to the Apache Ant
tool, but it is based on different concepts and works in a profoundly different manner. Maven is hosted by the Apache Software Foundation
, where it was formerly part of the Jakarta Project
.
Maven uses a construct known as a Project Object Model (POM) to describe the software project being built, its dependencies on other external modules and components, and the build order. It comes with pre-defined targets for performing certain well-defined tasks such as compilation of code and its packaging.
Maven dynamically downloads Java
libraries and Maven plug-ins from one or more repositories. Maven provides built-in support for retrieving files from the Maven 2 Central Repository and other Maven repositories, and can upload artifacts to specific repositories after a successful build. A local cache of downloaded artifacts acts as the primary means of synchronizing the output of projects on a local system.
Maven is built using a plugin-based architecture that allows it to make use of any application controllable through standard input. Theoretically, this would allow anyone to write plugins to interface with build tools (compilers, unit test tools, etc.) for any other language. In reality, support and use for languages other than Java has been minimal. Currently a plugin for the .NET framework exists and is maintained, and a C
/C++
native plugin is maintained for Maven 2 maven-native and maven-nar
4.0.0
com.mycompany.app
my-app
1.0
junit
junit
3.8.1
test
This POM only defines a unique identifier for the project (coordinates) and its dependency on the JUnit
framework. However, that is already enough for building the project and running the unit tests associated with the project. Maven accomplishes this by embracing the idea of Convention over Configuration
, that is, Maven provides good default values for the project's configuration. The directory structure of a normal idiomatic Maven project has the following directory entries:
Then the command
mvn package
will compile all the Java files, run any tests, and package the deliverable code and resources into
Using Maven itself, the user provides only configuration for the project, while the configurable plug-ins do the actual work of compiling the project, cleaning target directories, running unit tests, generating API documentation and so on. In general, users should not have to write plugins themselves. Contrast this with Ant
and make in which one writes imperative procedures for doing the aforementioned tasks.
Larger projects should be divided into several modules, or sub-projects, each with its own POM. One can then write a root POM through which one can compile all the modules with a single command. POMs can also inherit configuration from other POMs. All POMs inherit from the Super POM by default. The Super POM provides default configuration, such as default source directories, default plugins, and so on.
mvn [plugin-name]:[goal-name]
For example, a Java project can be compiled with the compiler-plugin's compile-goal by running
There are Maven plugins for building, testing, source control management, running a web server, generating Eclipse
project files, and much more. Plugins are introduced and configured in a-section of a
However, it would be cumbersome if one would have to run several goals manually in order to build, test and package a project:
mvn compiler:compile
mvn surefire:test
mvn jar:jar
Maven's lifecycle-concept handles this issue.
1. process-resources
2. compile
3. process-test-resources
4. test-compile
5. test
6. package
7. install
8. deploy
Goals provided by plugins can be associated with different phases of the lifecycle. For example, by default, the goal "compiler:compile" is associated with the compile-phase, while the goal "surefire:test" is associated with the test-phase. When the command
mvn test
is executed, Maven will run all the goals associated with each of the phases up to the test-phase. So it will run the "resources:resources"-goal associated with the process-resources-phase, then "compiler:compile", and so on until it finally runs the "surefire:test"-goal.
Maven also has standard lifecycles for cleaning the project and for generating a project site. If cleaning were part of the default lifecycle, the project would be cleaned every time it was built. This is clearly undesirable, so cleaning has been given its own lifecycle.
Thanks to standard lifecycles, one should be able to build, test and install every Maven-project using the
-library simply has to declare Hibernate's project coordinates in its POM. Maven will automatically download the dependency and the dependencies that Hibernate itself needs (called transitive dependencies) and store them in the user's local repository. Maven 2 Central Repository is used by default to search for libraries, but one can configure the repositories used (e.g. company-private repositories) in POM.
There are search engines such as Maven Central, which can be used to find out coordinates for different open-source libraries and frameworks.
Projects developed on a single machine can depend on each other through the local repository. The local repository is a simple folder structure which acts both as a cache for downloaded dependencies and as a centralized storage place for locally built artifacts. The Maven command
This design constraint is more like how an IDE handles projects and it provides many benefits, such as a succinct project definition and the possibility of automatic integration of a Maven project with other development tools such as IDEs, build servers, etc.
The downside is that it requires a user to first understand what a project is from the Maven point of view and how Maven works with projects, because what happens when one executes a phase in Maven is not immediately obvious just from examining the Maven project file. This required structure is also often a barrier in migrating a mature project to Maven because it is usually hard to adapt from other approaches.
In Ant, projects do not really exist from the tool's technical perspective. Ant works with XML build scripts defined in one or more files. It processes targets from these files and each target executes tasks. Each task performs a technical operation such as running a compiler or copying files around. Tasks are executed primarily in the order given by their defined dependency on other tasks. Thus, Ant is a tool that chains together tasks and executes them based on inter-dependencies and other Boolean conditions.
The benefits provided by Ant are also numerous. It has an XML language optimized for clearer definition of what each task does and on what it depends. Also, all the information about what will be executed by an Ant target can be found in the Ant script.
A developer not familiar with Ant would normally be able to determine what a simple Ant script does just by examining the script. This is not usually true for Maven.
However, even an experienced developer that is new to a project using Ant cannot infer what the higher level structure of an Ant script is and what it does without examining the script in detail. Depending on the script's complexity, this can quickly become a daunting challenge. With Maven, a developer who previously worked with other Maven projects can quickly examine the structure of a never before seen Maven project and execute the standard Maven work-flows against it while already knowing what to expect as an outcome.
It is possible to use Ant scripts that are defined and behave in a uniform manner for all projects in a working group or an organization. However, when the number and complexity of projects rises, it is also very easy to stray from the initially desired uniformity. With Maven this is less of a problem because the tool always imposes a certain way of doing things.
Note that it is also possible to extend and configure Maven in a way that departs from the Maven way of doing things.
s exist to provide integration of Maven with the IDE's build mechanism and source editing tools, allowing Maven to compile projects from within the IDE, and also to set the classpath for code completion, highlighting compiler errors, etc. Examples of popular IDEs supporting development with Maven include:
These add-ons also provide the ability to edit the POM or use the POM to determine a project's complete set of dependencies directly within the IDE.
Some built-in features of IDEs are forfeited when the IDE no longer performs compilation. For example, Eclipse's JDT has the ability to recompile a single java source file after it has been edited. Many IDEs work with a flat set of projects instead of the hierarchy of folders preferred by Maven. This complicates the use of SCM systems in IDEs when using Maven.
project. In July 2004, Maven was released as the critical first milestone, v1.0. Maven 2 was declared v2.0 in October 2005 after about 6 months in beta cycles. Maven 3.0 was released in October 2010 being mostly backwards compatible with Maven 2.
Maven 3.0 has reworked the core Project Builder infrastructure such that the POMs file-based representation is now decoupled from its in-memory object representation. This has expanded the possibility for Maven 3.0 add-ons to leverage non-XML based project definition files. Languages suggested include Ruby
(already in private prototype by Jason van Zyl), YAML
, and Groovy. Experimental work for a YAML-based POM definition file (requires an external conversion script to be executed) has been piloted by Don Brown of Atlassian
.
Special attention has been paid to ensuring compatibility between Maven 2 and 3. For most projects, an upgrade to Maven 3 won't require any adjustments of their project structure. The first beta of Maven 3 saw the introduction of a parallel build feature which leverages a configurable number of cores on a multi-core machine and is especially suited for large multi-module projects.
Build Automation
Build automation is the act of scripting or automating a wide variety of tasks that software developers do in their day-to-day activities including things like:* compiling computer source code into binary code* packaging binary code* running tests...
and software comprehension tool. While primarily used for Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
programming, it can also be used to build and manage projects written in C#, Ruby
Ruby (programming language)
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
, Scala, and other languages. Maven serves a similar purpose to the Apache Ant
Apache Ant
Apache Ant is a software tool for automating software build processes. It is similar to Make but is implemented using the Java language, requires the Java platform, and is best suited to building Java projects....
tool, but it is based on different concepts and works in a profoundly different manner. Maven is hosted by the Apache Software Foundation
Apache Software Foundation
The Apache Software Foundation is a non-profit corporation to support Apache software projects, including the Apache HTTP Server. The ASF was formed from the Apache Group and incorporated in Delaware, U.S., in June 1999.The Apache Software Foundation is a decentralized community of developers...
, where it was formerly part of the Jakarta Project
Jakarta Project
The Jakarta Project creates and maintains open source software for the Java platform. It operates as an umbrella project under the auspices of the Apache Software Foundation, and all of Jakarta products are released under the Apache License.-Subprojects:...
.
Maven uses a construct known as a Project Object Model (POM) to describe the software project being built, its dependencies on other external modules and components, and the build order. It comes with pre-defined targets for performing certain well-defined tasks such as compilation of code and its packaging.
Maven dynamically downloads Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
libraries and Maven plug-ins from one or more repositories. Maven provides built-in support for retrieving files from the Maven 2 Central Repository and other Maven repositories, and can upload artifacts to specific repositories after a successful build. A local cache of downloaded artifacts acts as the primary means of synchronizing the output of projects on a local system.
Maven is built using a plugin-based architecture that allows it to make use of any application controllable through standard input. Theoretically, this would allow anyone to write plugins to interface with build tools (compilers, unit test tools, etc.) for any other language. In reality, support and use for languages other than Java has been minimal. Currently a plugin for the .NET framework exists and is maintained, and a C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
/C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
native plugin is maintained for Maven 2 maven-native and maven-nar
Example
Maven projects are configured using a Project Object Model, which is stored in apom.xmlXMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
-file. Here's a minimal example:This POM only defines a unique identifier for the project (coordinates) and its dependency on the JUnit
JUnit
JUnit is a unit testing framework for the Java programming language. JUnit has been important in the development of test-driven development, and is one of a family of unit testing frameworks collectively known as xUnit that originated with SUnit....
framework. However, that is already enough for building the project and running the unit tests associated with the project. Maven accomplishes this by embracing the idea of Convention over Configuration
Convention over Configuration
Convention over configuration is a software design paradigm which seeks to decrease the number of decisions that developers need to make, gaining simplicity, but not necessarily losing flexibility....
, that is, Maven provides good default values for the project's configuration. The directory structure of a normal idiomatic Maven project has the following directory entries:
| Directory name | Purpose |
|---|---|
| project home | Contains the pom.xml and all subdirectories. |
| src/main/java | Contains the deliverable Java sourcecode for the project. |
| src/main/resources | Contains the deliverable resources for the project, such as property files. |
| src/test/java | Contains the testing classes (JUnit or TestNG test cases, for example) for the project. |
| src/test/resources | Contains resources necessary for testing. |
Then the command
mvn package
will compile all the Java files, run any tests, and package the deliverable code and resources into
target/my-app-1.0.jar (assuming the artifactId is my-app and the version is 1.0.)Using Maven itself, the user provides only configuration for the project, while the configurable plug-ins do the actual work of compiling the project, cleaning target directories, running unit tests, generating API documentation and so on. In general, users should not have to write plugins themselves. Contrast this with Ant
Apache Ant
Apache Ant is a software tool for automating software build processes. It is similar to Make but is implemented using the Java language, requires the Java platform, and is best suited to building Java projects....
and make in which one writes imperative procedures for doing the aforementioned tasks.
Project Object Model
A Project Object Model (POM) provides all the configuration for a single project. General configuration covers the project's name, its owner and its dependencies on other projects. One can also configure individual phases of the build process, which are implemented as plugins. For example, one can configure the compiler-plugin to use Java version 1.5 for compilation, or specify packaging the project even if some unit test fails.Larger projects should be divided into several modules, or sub-projects, each with its own POM. One can then write a root POM through which one can compile all the modules with a single command. POMs can also inherit configuration from other POMs. All POMs inherit from the Super POM by default. The Super POM provides default configuration, such as default source directories, default plugins, and so on.
Plugins
Most of Maven's functionality is in plugins. A plugin provides a set of goals that can be executed using the following syntax:mvn [plugin-name]:[goal-name]
For example, a Java project can be compiled with the compiler-plugin's compile-goal by running
mvn compiler:compile.There are Maven plugins for building, testing, source control management, running a web server, generating Eclipse
Eclipse (software)
Eclipse is a multi-language software development environment comprising an integrated development environment and an extensible plug-in system...
project files, and much more. Plugins are introduced and configured in a
pom.xml file. Some basic plugins are included in every project by default, and they have sensible default settings.However, it would be cumbersome if one would have to run several goals manually in order to build, test and package a project:
mvn compiler:compile
mvn surefire:test
mvn jar:jar
Maven's lifecycle-concept handles this issue.
Build lifecycles
Build lifecycle is a list of named phases that can be used to give order to goal execution. One of Maven's standard lifecycles is the default lifecycle, which includes the following phases, in this order:1. process-resources
2. compile
3. process-test-resources
4. test-compile
5. test
6. package
7. install
8. deploy
Goals provided by plugins can be associated with different phases of the lifecycle. For example, by default, the goal "compiler:compile" is associated with the compile-phase, while the goal "surefire:test" is associated with the test-phase. When the command
mvn test
is executed, Maven will run all the goals associated with each of the phases up to the test-phase. So it will run the "resources:resources"-goal associated with the process-resources-phase, then "compiler:compile", and so on until it finally runs the "surefire:test"-goal.
Maven also has standard lifecycles for cleaning the project and for generating a project site. If cleaning were part of the default lifecycle, the project would be cleaned every time it was built. This is clearly undesirable, so cleaning has been given its own lifecycle.
Thanks to standard lifecycles, one should be able to build, test and install every Maven-project using the
mvn install-command.Dependencies
The example-section hinted at Maven's dependency-handling mechanism. A project that needs the HibernateHibernate (Java)
Hibernate is an object-relational mapping library for the Java language, providing a framework for mapping an object-oriented domain model to a traditional relational database...
-library simply has to declare Hibernate's project coordinates in its POM. Maven will automatically download the dependency and the dependencies that Hibernate itself needs (called transitive dependencies) and store them in the user's local repository. Maven 2 Central Repository is used by default to search for libraries, but one can configure the repositories used (e.g. company-private repositories) in POM.
There are search engines such as Maven Central, which can be used to find out coordinates for different open-source libraries and frameworks.
Projects developed on a single machine can depend on each other through the local repository. The local repository is a simple folder structure which acts both as a cache for downloaded dependencies and as a centralized storage place for locally built artifacts. The Maven command
mvn install builds a project and places its binaries in the local repository. Then other projects can utilize this project by specifying its coordinates in their POMs.Maven compared with Ant
The fundamental difference between Maven and Ant is that Maven's design regards all projects as having a certain structure and a set of supported task work-flows (e.g. getting resources from source control, compiling the project, unit testing, etc.). While most software projects in effect support these operations and actually do have a well-defined structure, Maven requires that this structure and the operation implementation details be defined in the POM file. Thus, Maven relies on a convention on how to define projects and on the list of work-flows that are generally supported in all projects.This design constraint is more like how an IDE handles projects and it provides many benefits, such as a succinct project definition and the possibility of automatic integration of a Maven project with other development tools such as IDEs, build servers, etc.
The downside is that it requires a user to first understand what a project is from the Maven point of view and how Maven works with projects, because what happens when one executes a phase in Maven is not immediately obvious just from examining the Maven project file. This required structure is also often a barrier in migrating a mature project to Maven because it is usually hard to adapt from other approaches.
In Ant, projects do not really exist from the tool's technical perspective. Ant works with XML build scripts defined in one or more files. It processes targets from these files and each target executes tasks. Each task performs a technical operation such as running a compiler or copying files around. Tasks are executed primarily in the order given by their defined dependency on other tasks. Thus, Ant is a tool that chains together tasks and executes them based on inter-dependencies and other Boolean conditions.
The benefits provided by Ant are also numerous. It has an XML language optimized for clearer definition of what each task does and on what it depends. Also, all the information about what will be executed by an Ant target can be found in the Ant script.
A developer not familiar with Ant would normally be able to determine what a simple Ant script does just by examining the script. This is not usually true for Maven.
However, even an experienced developer that is new to a project using Ant cannot infer what the higher level structure of an Ant script is and what it does without examining the script in detail. Depending on the script's complexity, this can quickly become a daunting challenge. With Maven, a developer who previously worked with other Maven projects can quickly examine the structure of a never before seen Maven project and execute the standard Maven work-flows against it while already knowing what to expect as an outcome.
It is possible to use Ant scripts that are defined and behave in a uniform manner for all projects in a working group or an organization. However, when the number and complexity of projects rises, it is also very easy to stray from the initially desired uniformity. With Maven this is less of a problem because the tool always imposes a certain way of doing things.
Note that it is also possible to extend and configure Maven in a way that departs from the Maven way of doing things.
IDE integration
Add-ons to several popular Integrated Development EnvironmentIntegrated development environment
An integrated development environment is a software application that provides comprehensive facilities to computer programmers for software development...
s exist to provide integration of Maven with the IDE's build mechanism and source editing tools, allowing Maven to compile projects from within the IDE, and also to set the classpath for code completion, highlighting compiler errors, etc. Examples of popular IDEs supporting development with Maven include:
- EclipseEclipse (software)Eclipse is a multi-language software development environment comprising an integrated development environment and an extensible plug-in system...
- NetBeansNetBeansNetBeans refers to both a platform framework for Java desktop applications, and an integrated development environment for developing with Java, JavaScript, PHP, Python, Groovy, C, C++, Scala, Clojure, and others...
- IntelliJ IDEA
- JBuilderJBuilderJBuilder is an integrated development environment for the programming language Java, from Borland, and then CodeGear. Codegear was purchased by Embarcadero Technologies in 2008....
- JDeveloperJDeveloperJDeveloper is a freeware IDE supplied by Oracle Corporation. It offers features for development in Java, XML, SQL and PL/SQL, HTML, JavaScript, BPEL and PHP...
(version 11.1.2) - MyEclipseMyEclipseMyEclipse is a commercially available Java EE and Ajax IDE created and maintained by the company Genuitec, a founding member of the Eclipse Foundation....
These add-ons also provide the ability to edit the POM or use the POM to determine a project's complete set of dependencies directly within the IDE.
Some built-in features of IDEs are forfeited when the IDE no longer performs compilation. For example, Eclipse's JDT has the ability to recompile a single java source file after it has been edited. Many IDEs work with a flat set of projects instead of the hierarchy of folders preferred by Maven. This complicates the use of SCM systems in IDEs when using Maven.
History
Maven, created by Sonatype's Jason van Zyl, began as a subproject of Apache Turbine in 2002. In 2003, it was voted on and accepted as a top level Apache Software FoundationApache Software Foundation
The Apache Software Foundation is a non-profit corporation to support Apache software projects, including the Apache HTTP Server. The ASF was formed from the Apache Group and incorporated in Delaware, U.S., in June 1999.The Apache Software Foundation is a decentralized community of developers...
project. In July 2004, Maven was released as the critical first milestone, v1.0. Maven 2 was declared v2.0 in October 2005 after about 6 months in beta cycles. Maven 3.0 was released in October 2010 being mostly backwards compatible with Maven 2.
Future
Maven 3.0 information began trickling out in 2008. After eight alpha releases, the first beta version of Maven 3.0 was released in April 2010.Maven 3.0 has reworked the core Project Builder infrastructure such that the POMs file-based representation is now decoupled from its in-memory object representation. This has expanded the possibility for Maven 3.0 add-ons to leverage non-XML based project definition files. Languages suggested include Ruby
Ruby (programming language)
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
(already in private prototype by Jason van Zyl), YAML
YAML
YAML is a human-readable data serialization format that takes concepts from programming languages such as C, Perl, and Python, and ideas from XML and the data format of electronic mail . YAML was first proposed by Clark Evans in 2001, who designed it together with Ingy döt Net and Oren Ben-Kiki...
, and Groovy. Experimental work for a YAML-based POM definition file (requires an external conversion script to be executed) has been piloted by Don Brown of Atlassian
Atlassian
Atlassian is a software company based in Sydney, Australia which makes business enterprise software, targeted at software developers. On 1 September 2010, the World Economic Forum announced the company as a Technology Pioneer for 2011.- Products :...
.
Special attention has been paid to ensuring compatibility between Maven 2 and 3. For most projects, an upgrade to Maven 3 won't require any adjustments of their project structure. The first beta of Maven 3 saw the introduction of a parallel build feature which leverages a configurable number of cores on a multi-core machine and is especially suited for large multi-module projects.
See also
- Apache ContinuumApache ContinuumApache Continuum, a partner to Apache Maven, is a continuous integration server, which runs builds on a configurable schedule. Much like CruiseControl, Continuum emails developers when the build is broken, requesting that the culprit fix the problem...
, a continuous integration server which integrates tightly with Maven - Apache Archiva, a repository manager commonly used with Maven
- Apache AntApache AntApache Ant is a software tool for automating software build processes. It is similar to Make but is implemented using the Java language, requires the Java platform, and is best suited to building Java projects....
- Apache JellyApache JellyApache Jelly is a Java and XML based scripting and processing engine for turning XML into executable code. Jelly is a component of Apache Commons.Custom XML languages are commonly created to perform some kind of processing action...
, a tool for turning XML into executable code - Apache IvyApache IvyApache Ivy is a transitive relation dependency manager. It is a sub-project of the Apache Ant project, with which Ivy works to resolve project dependencies. An external XML file defines project dependencies and lists the resources necessary to build a project...
, alternative dependency management tool for Java - GradleGradleGradle is a project automation tool that builds upon the concepts of Apache Ant and Apache Maven and introduces a Groovy based DSL instead of the more traditional XML form of declaring the project configuration....
a build tool based on convention over configuration - Sonatype Nexus, a repository manager commonly used with Maven
- HudsonHudson (software)Hudson is a continuous integration tool written in Java, which runs in a servlet container, such as Apache Tomcat or the GlassFish application server. It supports SCM tools including CVS, Subversion, Git and Clearcase and can execute Apache Ant and Apache Maven based projects, as well as arbitrary...
- List of build automation software
Further reading
- Available for free as PDF download or online reading: http://www.sonatype.com/documentation/books
External links
- The Maven 2 tutorial: A practical guide for Maven 2 users - tutorial at Codehaus.org
- The Complete Reference, a printed and free online reading book.
- Building Web Applications with Maven 2
- The Maven 2 POM demystified - article at JavaWorldJavaWorldJavaWorld is an online technology website focused on Java technologies, founded in 1996. Since 2006, JavaWorld has become a subsidiary of Network World...
- Verify the dependencies in your POM
- Maven history
- Maven for PHP