The Build Lifecycle

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Introduction to the Build Lifecycle


Build Lifecycle Basics

Maven 2.0 is based around the central concept of a build lifecycle. What this means is that the process for building and distributing a particular artifact (project) is clearly defined.

For the person building a project, this means that it is only necessary to learn a small set of commands to build any Maven project, and the POM will ensure they get the results they desired.

There are three built-in build lifecycles: default, clean and site. The default lifecycle handles your project deployment, the clean lifecycle handles project cleaning, while the site lifecycle handles the creation of your project's site documentation.

A Build Lifecycle is Made Up of Phases

Each of these build lifecycles is defined by a different list of build phases, wherein a build phase represents a stage in the lifecycle.

For example, the default lifecycle has the following build phases (for a complete list of the build phases, refer to the Lifecycle Reference):

   * validate - validate the project is correct and all necessary information is available
   * compile - compile the source code of the project
   * test - test the compiled source code using a suitable unit testing framework. These tests should not require the code be packaged or deployed
   * package - take the compiled code and package it in its distributable format, such as a JAR.
   * integration-test - process and deploy the package if necessary into an environment where integration tests can be run
   * verify - run any checks to verify the package is valid and meets quality criteria
   * install - install the package into the local repository, for use as a dependency in other projects locally
   * deploy - done in an integration or release environment, copies the final package to the remote repository for sharing with other developers and projects.

These build phases (plus the other build phases not shown here) are executed sequentially to complete the default lifecycle. Given the build phases above, this means that when the default lifecycle is used, Maven will first validate the project, then will try to compile the sources, run those against the tests, package the binaries (e.g. jar), run integration tests against that package, verify the package, install the verifed package to the local repository, then deploy the installed package in a specified environment.

To do all those, you only need to call the last build phase to be executed, in this case, deploy:

mvn deploy

That is because if you call a build phase, it will execute not only that build phase, but also every build phase prior to the called build phase. Thus, doing

mvn integration-test

will do every build phase before it (validate, compile, package, etc.), before executing integration-test.

There are more commands that are part of the lifecycle, which will be discussed in the following sections.

It should also be noted that the same command can be used in a multi-module scenario (i.e. a project with one or more subprojects). For example:

mvn clean install

This command will traverse into all of the subprojects and run clean, then install (including all of the prior steps).


A Build Phase is Made Up of Goals

However, even though a build phase is responsible for a specific step in the build lifecycle, the manner in which it carries out those responsibilities may vary. And this is done by declaring the goals bound to those build phases.

A goal represents a specific task (finer than a build phase) which contributes to the building and managing of a project. It may be bound to zero or more build phases. A goal not bound to any build phase could be executed outside of the build lifecycle by direct invocation. The order of execution depends on the order in which the goal(s) and the build phase(s) are invoked. For example, consider the command below. The clean and package arguments are build phases while the dependency:copy-dependencies is a goal.

mvn clean dependency:copy-dependencies package

If this were to be executed, the clean phase will be executed first (meaning it will run all preceeding phases of the clean lifecycle, plus the clean phase itself), and then the dependency:copy-dependencies goal, before finally executing the package phase (and all its preceeding build phases of the default lifecycle).

Moreover, if a goal is bound to one or more build phases, that goal will be called in all those phases.

Furthermore, a build phase can also have zero or more goals bound to it. If a build phase has no goals bound to it, that build phase will not execute. But if it has one or more goals bound to it, it will execute all those goals (Note: As of Maven 2.0.5, multiple goals bound to a phase are executed in the same order as they are declared in the POM).


Setting Up Your Project to Use the Build Lifecycle

The build lifecycle is simple enough to use, but when you are constructing a Maven build for a project, how do you go about assigning tasks to each of those build phases?

Packaging

The first, and most common way, is to set the packaging for your project via the equally named POM element <packaging>. Some of the valid packaging values are jar, war, ear and pom. If no packaging value has been specified, it will default to jar.

Each packaging contains a list of goals to bind to a particular phase. For example, the jar packaging will bind the following goals to build phases of the default lifecycle. process-resources resources:resources compile compiler:compile process-test-resources resources:testResources test-compile compiler:testCompile test surefire:test package jar:jar install install:install deploy deploy:deploy

This is an almost standard set of bindings; however, some packagings handle them differently. For example, a project that is purely metadata (packaging value is pom) only binds goals to the install and deploy phases (for a complete list of goal-to-build-phase bindings of some of the packaging types, refer to the Lifecycle Reference).

Note that for some packaging types to be available, you may also need to include a particular plugin in your <build> section of your POM (as described in the next section). One example of a plugin that requires this is the Plexus plugin, which provides a plexus-application and plexus-service packaging.

Plugins

The second way to add goals to phases is to configure plugins in your project. Plugins are artifacts that provide goals to Maven. Furthermore, a plugin may have one or more goals wherein each goal represents a capability of that plugin. For example, the Compiler plugin has two goals: compile and testCompile. The former compiles the source code of your main code, while the later compiles the source code of your test code.

As you will see in the later sections, plugins can contain information that indicates which lifecycle phase to bind a goal to. Note that adding the plugin on its own is not enough information - you must also specify the goals you want to run as part of your build.

The goals that are configured will be added to the goals already bound to the lifecycle from the packaging selected. If more than one goal is bound to a particular phase, the order used is that those from the packaging are executed first, followed by those configured in the POM. Note that you can use the <executions> element to gain more control over the order of particular goals.

For example, the Modello plugin binds by default its goal modello:java to the generate-sources phase (Note: The modello:java goal generates Java source codes). So to use the Modello plugin and have it generate sources from a model and incorporate that into the build, you would add the following to your POM in the <plugins> section of <build>:

...
<plugin>
  <groupId>org.codehaus.modello</groupId>
  <artifactId>modello-maven-plugin</artifactId>
  <version>1.0-alpha-18</version>
  <executions>
    <execution>
      <configuration>
        <model>maven.mdo</model>
        <modelVersion>4.0.0</modelVersion>
      </configuration>
      <goals>
        <goal>java</goal>
      </goals>
    </execution>
  </executions>
</plugin>
...

You might be wondering why that <executions> element is there. That is so that you can run the same goal multiple times with different configuration if needed. Separate executions can also be given an ID so that during inheritance or the application of profiles you can control whether goal configuration is merged or turned into an additional execution.

When multiple executions are given that match a particular phase, they are executed in the order specified in the POM, with inherited executions running first.

Now, in the case of modello:java, it only makes sense in the generate-sources phase. But some goals can be used in more than one phase, and there may not be a sensible default. For those, you can specify the phase yourself. For example, let's say you have a goal display:time that echos the current time to the commandline, and you want it to run in the process-test-resources phase to indicate when the tests were started. This would be configured like so:

...
<plugin>
  <groupId>com.mycompany.example</groupId>
  <artifactId>maven-touch-plugin</artifactId>
  <version>1.0</version>
  <executions>
    <execution>
      <phase>process-test-resources</phase>
      <goals>
        <goal>timestamp</goal>
      </goals>
    </execution>
  </executions>
</plugin>
...