Rise of Automated Builds and Deployments

A build tool such as Ant is focused solely onpreprocessing, compilation, packaging, testing, anddistribution.Maven is a project management tool whichprovides a superset of features found in a buildtool.Maven provides build capabilities, and can runreports, generate a web site, and facilitatecommunication among members of a workingteam.Maven is a project management tool whichencompasses a project object model, a set ofstandards, a project lifecycle, a dependencymanagement system, and logic for executing plugingoals at defined phases in a lifecycle.

Maven incorporates convention over configurationconcept by providing sensible default behavior forprojects.Source code is assumed to be in${basedir}/src/main/javaResources are assumed to be in${basedir}/src/main/resourcesTests are assumed to be in ${basedir}/src/testThe byte code is compiled to${basedir}/target/classes and then a distributableJAR file is created in ${basedir}/target.

Intelligence of Maven is implemented in theplugins which are retrieved from the MavenRepository.The fact that Maven retrieves bothdependencies and plugins from the remoterepository allows for universal reuse of buildlogic.Maven has abstracted common build tasks intoplugins which are maintained centrally andshared universally.Maven maintains a model of a project withdescription of the attributes of the project.

Dependency Management: A project is defined by aunique set of coordinates consisting of a groupidentifier, an artifact identifier, and a version, enablingthe coordinates to declare dependencies.Remote Repositories: Coordinates defined in the MavenProject Object Model (POM) can be used to createrepositories of Maven artifacts.Universal Reuse of Build Logic: Plugins contain logic thatworks with the descriptive data and configurationparameters defined in Project Object Model (POM).Tool Portability / Integration: Maven has standardizedthe Project description maintained in the IDE, and whileeach IDE continues to maintain custom projectfiles, they can be easily generated from the model.Easy Searching and Filtering of Project Artifacts: Toolslike Nexus allow you to index and search the contents ofa repository using the information stored in the POM.

Create a simple pom.xml as follows:4.0.0org.sonatype.mavenbookmy-project1.0Place your source code in${basedir}/src/main/javaRun mvn install from the command line.Run mvn site and then find an index.html file intarget/site that contains links to JavaDoc and a fewreports about your source code.

Ant doesnt have formal conventions like acommon project directory structure or defaultbehavior.Ant is proceduralAnt doesnt have a lifecycle, with goals andsequence of tasks defined for each goal manually.Maven has conventions. It knows where yoursource code is because you followed theconvention.Maven is declarative. A pom.xml file is created andput into the source in the default directory.Maven has a lifecycle which was invoked whenyou executed mvn install. The command tellsMaven to execute a series of sequential lifecyclephases until it reached the install lifecycle phase.

Once Maven is unpacked to the installation directory, the twoenvironment variables PATH and M2_HOME need to beset.To set these environment variables from the command-line, type inthe following Windows commands:set M2_HOME=c:Program Filesapache-maven-2.2.1set PATH=%PATH%;%M2_HOME%binCorresponding Linux Commands:cd /usr/localln -s apache-maven-2.2.1 mavenexport M2_HOME=/usr/local/mavenexport PATH=${M2_HOME}/bin:${PATH}Check the version by running mvn -v from the command-line.~/.m2/settings.xml contains user-specific configuration forauthentication, repositories and other information.~/.m2/repository/ contains the local Maven repository.

To start a new Maven project, use the MavenArchetype plugin from the command line.$ mvn archetype:generate -DgroupId=org.sonatype.mavenbook.simple -DartifactId=simple -DpackageName=org.sonatype.mavenbook -Dversion=1.0-SNAPSHOTarchetype:generate is called a Maven goal.The -Dname=value pairs are arguments that arepassed to the goal and take the form of -Dproperties.The plugin is the prefix archetype, and the goal isgenerate.The project is installed using the mvn installcommand.

The Maven Archetype plugin creates a directory simple/that matches the artifactId, known as the projects basedirectory.Every Maven project has a Project Object Model (POM)in pom.xml which describes the project, configuresplugins, and declares dependencies.All the projects source code and resources are placedunder src/main.In a Java project, Java classes are placed insrc/main/java and classpath resources are placed insrc/main/resources.The test cases for the project are located in src/test.All tests are placed in src/test/java, and classpathresources for tests are located in src/test/resources.

The first few elements in POMgroupId, artifactId, packaging,versionare known as the Maven coordinates which uniquelyidentify a project.name and url are descriptive elements of the POM providing ahuman readable name and associating the project with a website.The dependencies element defines a single, test-scopeddependency on a unit testing frameworkMaven always executes against an effective POM, acombination of settings from the projects pom.xml, all parentPOMs, a super-POM defined within Maven, user-definedsettings, and active profiles.All projects ultimately extend the super-POM, which defines aset of sensible default configuration settings.The "effective" POM can be seen (with the contents of theprojects POM interpolated with the contents of all parentPOMs, user settings, and any active profiles) using thecommand:$ mvn help:effective-pom

A Maven Plugin is a collection of one or moregoals.Maven also provides for the ability to definecustom plugins.Maven plugins can be simple core plugins likethe Jar plugin, which contains goals for creatingJAR files, Compiler plugin, which contains goalsfor compiling source code and unit tests, or theSurefire plugin, which contains goals forexecuting unit tests and generating reports.

A goal is a specific task that may be executed as astandalone goal or along with other goals as partof a larger build.A goal is a unit of work in Maven.The compile goal in the Compiler plugin, compilesall of the source code for a project.Goals are configured via configuration propertiesthat can be used to customize behavior.When referring to a plugin goal, we frequently usethe shorthand notation, pluginId:goalId similar toarchetype:generate.Goals define parameters that can define sensibledefault values.

The command mvn install doesnt specify a plugingoal but specifies a Maven lifecycle phase.A phase is a step in what Maven calls the buildlifecycle which is an ordered sequence of phasesinvolved in building a project.Maven can support a number of differentlifecycles, but the one thats most often used is thedefault Maven lifecycle, which begins with a phaseto validate the basic integrity of the project andends with a phase that involves deploying a projectto production.Plugin goals can be attached to a lifecycle phase.As Maven moves through the phases in a lifecycle, itwill execute the goals attached to each particularphase.Each phase may have zero or more goals bound to it

When mvn install reached the package phase, itexecuted the jar goal in the Jar plugin.Since the simple Quickstart project has (by default) a jarpackaging type, the jar:jar goal is bound to the packagephase.The preceding goals are executed as Maven stepsthrough the phases preceding package in the Mavenlifecycle; executing a phase will first execute allpreceding phases in order, ending with the phasespecified on the command line.Each phase corresponds to zero or more goals, and asno plugin configuration or customization is performed.

When Maven executes a goal, each goal has accessto the information defined in a projects POM.Goals execute in the context of a POM.Goals are actions we wish to take upon aproject, and a project is defined by a POM.The POM names the project, provides a set ofunique identifiers (coordinates) for a project, anddefines the relationships between this project andothers through dependencies, parents, andprerequisites.Maven coordinates define a set of identifiers whichcan be used to uniquely identify a project, adependency, or a plugin in a Maven POM.

The combined identifiers: thegroupId, artifactId, version and packaging make up theprojects maven coordinates.Maven pinpoints a project via its coordinates when oneproject relates to another, either as a dependency, aplugin, or a parent project reference.groupId: Thegroup, company, team, organization, project, or othergroup. It begins with reverse domain name of org.artifactId: A unique identifier under groupId thatrepresents a single project.version: A specific release of a project.packaging: The type of project, defaulting tojar, describing the packaged output produced by aproject. This is not a part of a projects unique identifier.

A repository is a collection of project artifactsstored in a directory structure that closelymatches a projects Maven coordinates.The standard for a Maven repository is to storean artifact in the following directory relative tothe root of the repository:////-..Maven always looks up for the artifact in thelocal repository before downloading it from theremote Maven repository.

Maven downloads POM files for dependency inaddition to artifacts on order to support transitivedependencies.Maven adds all the dependencies of the library tothe projects dependencies implicitly resolvingconflicts with default behavior.The POM file declares dependencies on otherartifacts which are called transitive dependencies.Maven also provides for different dependencyscopes limiting its availability for particular goals.The provided scope tells Maven that a dependencyis needed for compilation, but should not bebundled with the output of a build.

Create a basic skeleton of project using MavenArchetype using mvn archetype:generate command.Configure the Maven Compiler plugin to target Java 5using the POM.maven-compiler-plugin1.51.5

Add Organizational, Legal, and DeveloperInformation to the pom.xml.Add project dependencieslog4jlog4j1.2.14The http://repository.sonatype.org contains somedependency libraries.

Add new packages (directories) in projectssource code location stored in src/main/java.Add resources to the default package (or in rootof the classpath) such as src/main/resources.The program is executed using the Exec pluginfrom the Codehaus Mojo project.mvn installmvn exec:java -Dexec.mainClass=org.sonatype.mavenbook.weather.MainAdd unit tests to the src/test/java folder.

The Exec plugin allows you to execute Java classes andother scripts.It is not a core Maven plugin, but it is available from theMojo project hosted by Codehaus.Description of the plugin can be found by:mvn help:describe -Dplugin=exec DfullTo find out what is on the classpath, the MavenDependency plugin can be used to print out a list ofresolved dependencies.mvn dependency:resolvemvn dependency:treeTo see the full dependency trail, including rejectedartifacts:mvn install -X

A test-scoped dependency is a dependency that isavailable on the classpath only during test compilationand test execution.If the project has war or ear packaging, a test-scopeddependency would not be included in the projectsoutput archive.To add a test-scoped dependency, add the dependencyelement to the projects dependencies section in thePOM file.run mvn dependency:resolve to see the dependencyelement listed as a dependency with scope test.Add the test resources in the src/test/resourcesdirectory.The test phase run for the mvn package or mvn installcommands, along with mvn test which runs all thelifecycle phases up test phase.

When Maven encounters a build failure, its default behavior is tostop the current build.To continue building a project even when the Surefire pluginencounters failed test cases, the testFailureIgnore configurationproperty of the Surefire plugin should be set to true.mvn test -Dmaven.test.failure.ignore=trueorg.apache.maven.pluginsmaven-surefire-plugintruetrueTo skip the unit tests, the maven.test.skip properties should be set totrue.mvn install -Dmaven.test.skip=true

The Maven Assembly plugin is a plugin to create arbitrarydistributions for applications.It can be used to assemble the output of any project in any format bydefining a custom assembly descriptor. [...]maven-assembly-pluginjar-with-dependencies[...]Command to build the assembly: mvn install assembly:assembly