Maven by Example
3.5. Core Concepts

To execute a single Maven plugin goal, we used the syntax mvn archetype:generate, where archetype is the identifier of a plugin and generate is the identifier of a goal. When Maven executes a plugin goal, it prints out the plugin identifier and goal identifier to standard output:

$ mvn archetype:generate -DgroupId=org.sonatype.mavenbook.simple \
-DartifactId=simple \
-Dpackage=org.sonatype.mavenbook
...
[INFO] [archetype:generate]
[INFO] artifact org.apache.maven.archetypes:maven-archetype-quickstart: \
checking for updates from central
...

A Maven Plugin is a collection of one or more goals. Examples of Maven plugins can be simple core plugins like the Jar plugin, which contains goals for creating JAR files, Compiler plugin, which contains goals for compiling source code and unit tests, or the Surefire plugin, which contains goals for executing unit tests and generating reports. Other, more specialized Maven plugins include plugins like the Hibernate3 plugin for integration with the popular persistence library Hibernate, the JRuby plugin which allows you to execute ruby as part of a Maven build or to write Maven plugins in Ruby. Maven also provides for the ability to define custom plugins. A custom plugin can be written in Java, or a plugin can be written in any number of languages including Ant, Groovy, beanshell, and, as previously mentioned, Ruby.

A goal is a specific task that may be executed as a standalone goal or along with other goals as part of a larger build. A goal is a “unit of work” in Maven. Examples of goals include the compile goal in the Compiler plugin, which compiles all of the source code for a project, or the test goal of the Surefire plugin, which can execute unit tests. Goals are configured via configuration properties that can be used to customize behavior. For example, the compile goal of the Compiler plugin defines a set of configuration also passed the package parameter to the generate goal as org.sonatype.mavenbook. If we had omitted the packageName parameter, the package name would have defaulted to org.sonatype.mavenbook.simple.

Goals define parameters that can define sensible default values. In the archetype:generate example, we did not specify what kind of archetype the goal was to create on our command line; we simply passed in a groupId and an artifactId. Not passing in the type of artifact we wanted to create caused the generate goal to prompt us for input, the generate goal stopped and asked us to choose an archetype from a list. If you had run the archetype:create goal instead, Maven would have assumed that you wanted to generate a new project using the default maven-archetype-quickstart archetype. This is our first brush with convention over configuration. The convention, or default, for the create goal is to create a simple project called Quickstart. The create goal defines a configuration property archetypeArtifactId that has a default value of maven-archetype-quickstart. The Quickstart archetype generates a minimal project shell that contains a POM and a single class. The Archetype plugin is far more powerful than this first example suggests, but it is a great way to get new projects started fast. Later in this book, we’ll show you how the Archetype plugin can be used to generate more complex projects such as web applications, and how you can use the Archetype plugin to define your own set of projects.

The core of Maven has little to do with the specific tasks involved in your project’s build. By itself, Maven doesn’t know how to compile your code or even how to make a JAR file. It delegates all of this work to Maven plugins like the Compiler plugin and the Jar plugin, which are downloaded on an as-needed basis and periodically updated from the central Maven repository. When you download Maven, you are getting the core of Maven, which consists of a very basic shell that knows only how to parse the command line, manage a classpath, parse a POM file, and download Maven plugins as needed. By keeping the Compiler plugin separate from Maven’s core and providing for an update mechanism, Maven makes it easier for users to have access to the latest options in the compiler. In this way, Maven plugins allow for universal reusability of common build logic. You are not defining the compile task in a build file; you are using a Compiler plugin that is shared by every user of Maven. If there is an improvement to the Compiler plugin, every project that uses Maven can immediately benefit from this change. (And, if you don’t like the Compiler plugin, you can override it with your own implementation.)

The second command we ran in the previous section Maven lifecycle, which begins with a phase to validate the basic integrity of the project and ends with a phase that involves deploying a project to production. Lifecycle phases are intentionally vague, defined solely as validation, testing, or deployment, and they may mean different things to different projects. For example, in a project that produces a Java archive, the package phase produces a JAR; in a project that produces a web application, the package phase produces a WAR.

Plugin goals can be attached to a lifecycle phase. As Maven moves through the phases in a lifecycle, it will execute the goals attached to each particular phase. Each phase may have zero or more goals bound to it. In the previous section, when you ran mvn install, you might have noticed that more than one goal was executed. Examine the output after running mvn install and take note of the various goals that are executed. When this simple example reached the package phase, it executed the jar goal in the Jar plugin. Since our simple Quickstart project has (by default) a jar packaging type, the jar:jar goal is bound to the package phase.

We know that the package phase is going to create a JAR file for a project with jar packaging. But what of the goals preceding it, such as compiler:compile and surefire:test? These goals are executed as Maven steps through the phases preceding package in the

To summarize, when we executed mvn install, Maven executes all phases up to the install phase, and in the process of stepping through the lifecycle phases it executes all goals bound to each phase. Instead of executing a Maven lifecycle goal you could achieve the same results by specifying a sequence of plugin goals as follows:

mvn resources:resources \
    compiler:compile \
    resources:testResources \
    compiler:testCompile \
    surefire:test \
    jar:jar \
    install:install

It is much easier to execute lifecycle phases than it is to specify explicit goals on the command line, and the common lifecycle allows every project that uses Maven to adhere to a well-defined set of standards. The lifecycle is what allows a developer to jump from one Maven project to another without having to know very much about the details of each particular project’s build. If you can build one Maven project, you can build them all.

The Archetype plugin created a project with a file named pom.xml. This is the Project Object Model (POM), a declarative description of a project. When Maven executes a goal, each goal has access to the information defined in a project’s POM. When the jar:jar goal needs to create a JAR file, it looks to the POM to find out what the JAR file’s name is. When the compiler:compile goal compiles Java source code into bytecode, it looks to the POM to see if there are any parameters for the compile goal. Goals execute in the context of a POM. Goals are actions we wish to take upon a project, and a project is defined by a POM. The POM names the project, provides a set of unique identifiers (coordinates) for a project, and defines the relationships between this project and others through dependencies, parents, and prerequisites. A POM can also customize plugin behavior and supply information about the community and developers involved in a project.

Maven coordinates define a set of identifiers which can be used to uniquely identify a project, a dependency, or a plugin in a Maven POM. Take a look at the following POM.

We’ve highlighted the Maven coordinates for this project: the groupId, artifactId, version and packaging. These combined identifiers make up a project’s coordinates. There is a fifth, seldom-used coordinate named classifier which we will introduce later in the book. You can feel free to ignore classifiers for now. Just like in any other coordinate system, a set of Maven coordinates is an address for a specific point in "space". Maven pinpoints a project via its coordinates when one project relates to another, either as a dependency, a plugin, or a parent project reference. Maven coordinates are often written using a colon as a delimiter in the following format: groupId:artifactId:packaging:version. In the above pom.xml file for our current project, its coordinates are represented as mavenbook:my-app:jar:1.0-SNAPSHOT.

The packaging format of a project is also an important component in the Maven coordinates, but it isn’t a part of a project’s unique identifier. A project’s groupId:artifactId:version make that project unique; you can’t have a project with the same three groupId, artifactId, and version identifiers.

These four elements become the key to locating and using one particular project in the vast space of other “Mavenized” projects . Maven repositories (public, private, and local) are organized according to these identifiers. When this project is installed into the local Maven repository, it immediately becomes locally available to any other project that wishes to use it. All you must do is add it as a dependency of another project using the unique Maven coordinates for a specific artifact.

When you run Maven for the first time, you will notice that Maven downloads a number of files from a remote Maven repository. If the simple project was the first time you ran Maven, the first thing it will do is download the latest release of the Resources plugin when it triggers the resources:resource goal. In Maven, artifacts and plugins are retrieved from a remote repository when they are needed. One of the reasons the initial Maven download is so small (1.5 MiB) is due to the fact that Maven doesn’t ship with much in the way of plugins. Maven ships with the bare minimum and fetches from a remote repository when it needs to. Maven ships with a default remote repository location (http://repo1.maven.org/maven2) which it uses to download the core Maven plugins and dependencies.

Often you will be writing a project which depends on libraries that are neither free nor publicly distributed. In this case you will need to either setup a custom repository inside your organization’s network or download and install the dependencies manually. The default remote repositories can be replaced or augmented with references to custom Maven repositories maintained by your organization. There are multiple products available to allow organizations to manage and maintain mirrors of the public Maven repositories.

What makes a Maven repository a Maven repository? A repository is a collection of project artifacts stored in a directory structure that closely matches a project’s Maven coordinates. You can see this structure by opening up a web browser and browsing the central Maven repository at http://repo1.maven.org/maven2/. You will see that an artifact with the coordinates org.apache.commons:commons-email:1.1 is available under the directory /org/apache/commons/commons-email/1.1/ in a file named commons-email-1.1.jar. The standard for a Maven repository is to store an artifact in the following directory relative to the root of the repository:

/<groupId>/<artifactId>/<version>/<artifactId>-<version>.<packaging>

Maven downloads artifacts and plugins from a remote repository to your local machine and stores these artifacts in your local Maven repository. Once Maven has downloaded an artifact from the remote Maven repository it never needs to download that artifact again as Maven will always look for the artifact in the local repository before looking elsewhere. On Windows XP, your local repository is likely in C:\Documents and Settings\USERNAME\.m2\repository, and on Windows Vista, your local repository is in C:\Users\USERNAME\.m2\repository. On Unix systems, your local Maven repository is available in ~/.m2/repository. When you build a project like the simple project you created in the previous section, the install phase executes a goal which installs your project’s artifacts in your local Maven repository.

In your local repository, you should be able to see the artifact created by our simple project. If you run the mvn install command, Maven will install our project’s artifact in your local repository. Try it.

$ mvn install
...
[INFO] [install:install]
[INFO] Installing .../simple-1.0-SNAPSHOT.jar to \
~/.m2/repository/com/sonatype/maven/simple/1.0-SNAPSHOT/ \
simple-1.0-SNAPSHOT.jar
...

As you can see from the output of this command, Maven installed our project’s JAR file into our local Maven repository. Maven uses the local repository to share dependencies across local projects. If you develop two projects—project A and project B—with project B depending on the artifact produced by project A, Maven will retrieve project A’s artifact from your local repository when it is building project B. Maven repositories are both a local cache of artifacts downloaded from a remote repository and a mechanism for allowing your projects to depend on each other.

In this chapter’s simple example, Maven resolved the coordinates of the JUnit dependency—+junit:junit:3.8.1+—to a path in a Maven repository /junit/junit/3.8.1/junit-3.8.1.jar. The ability to locate an artifact in a repository based on Maven coordinates gives us the ability to define dependencies in a project’s POM. If you examine the simple project’s pom.xml file, you will see that there is a section which deals with dependencies, and that this section contains a single dependency—JUnit.

A more complex project would contain more than one dependency, or it might contain dependencies that depend on other artifacts. Support for transitive dependencies is one of Maven’s most powerful features. Let’s say your project depends on a library that, in turn, depends on 5 or 10 other libraries (Spring or Hibernate, for example). Instead of having to track down all of these dependencies and list them in your pom.xml explicitly, you can simply depend on the library you are interested in and Maven will add the dependencies of this library to your project’s dependencies implicitly. Maven will also take care of working out conflicts between dependencies, and provides you with the ability to customize the default behavior and exclude certain transitive dependencies.

Let’s take a look at a dependency which was downloaded to your local repository when you ran the previous example. Look in your local repository path under ~/.m2/repository/junit/junit/3.8.1/. If you have been following this chapter’s examples, there will be a file named junit-3.8.1.jar and a junit-3.8.1.pom file in addition to a few checksum files which Maven uses to verify the authenticity of a downloaded artifact. Note that Maven doesn’t just download the JUnit JAR file, Maven also downloads a POM file for the JUnit dependency. The fact that Maven downloads POM files in addition to artifacts is central to Maven’s support for transitive dependencies.

When you install your project’s artifact in the local repository, you will also notice that Maven publishes a slightly modified version of the project’s pom.xml file in the same directory as the JAR file. Storing a POM file in the repository gives other projects information about this project, most importantly what dependencies it has. If Project B depends on Project A, it also depends on Project A’s dependencies. When Maven resolves a dependency artifact from a set of Maven coordinates, it also retrieves the POM and consults the dependencies POM to find any transitive dependencies. These transitive dependencies are then added as dependencies of the current project.

A dependency in Maven isn’t just a JAR file; it’s a POM file that, in turn, may declare dependencies on other artifacts. These dependencies of dependencies are called transitive dependencies, and they are made possible by the fact that the Maven repository stores more than just bytecode; it stores metadata about artifacts.

Maven Resolves Transitive Dependencies. image::figs/web/simple-project_depgraph.png" width="[]

In the previous figure, project A depends on projects B and C. Project B depends on project D, and project C depends on project E. The full set of direct and transitive dependencies for project A would be projects B, C, D, and E, but all project A had to do was define a dependency on B and C. Transitive dependencies can come in handy when your project relies on other projects with several small dependencies (like Hibernate, Apache Struts, or the Spring Framework). Maven also provides you with the ability to exclude transitive dependencies from being included in a project’s classpath.

Maven also provides for different dependency scopes. The simple project’s pom.xml contains a single dependency—junit:junit:jar:3.8.1+—with a scope of +test. When a dependency has a scope of test, it will not be available to the compile goal of the Compiler plugin. It will be added to the classpath for only the compiler:testCompile and surefire:test goals.

When you create a JAR for a project, dependencies are not bundled with the generated artifact; they are used only for compilation. When you use Maven to create a WAR or an EAR file, you can configure Maven to bundle dependencies with the generated artifact, and you can also configure it to exclude certain dependencies from the WAR file using the provided scope. The provided scope tells Maven that a dependency is needed for compilation, but should not be bundled with the output of a build. This scope comes in handy when you are developing a web application. You’ll need to compile your code against the Servlet specification, but you don’t want to include the Servlet API JAR in your web application’s WEB-INF/lib directory.

Another important feature of Maven is its ability to generate documentation and reports. In your simple project’s directory, execute the following command:

$ mvn site

This will execute the site lifecycle phase. Unlike the default build lifecycle that manages generation of code, manipulation of resources, compilation, packaging, etc., this lifecycle is concerned solely with processing site content under the src/site directories and generating reports. After this command executes, you should see a project web site in the target/site directory. Load target/site/index.html and you should see a basic shell of a project site. This shell contains some reports under “Project Reports” in the lefthand navigation menu, and it also contains information about the project, the dependencies, and developers associated with it under “Project Information.” The simple project’s web site is mostly empty, since the POM contains very little information about itself beyond its Maven coordinates, a name, a URL, and a single test dependency.

On this site, you’ll notice that some default reports are available. A unit test report communicates the success and failure of all unit tests in the project. Another report generates Javadoc for the project’s API. Maven provides a full range of configurable reports, such as the Clover report that examines unit test coverage, the JXR report that generates cross-referenced HTML source code listings useful for code reviews, the PMD report that analyzes source code for various coding problems, and the JDepend report that analyzes the dependencies between packages in a codebase. You can customize site reports by configuring which reports are included in a build via the pom.xml file.