• Project Object Model
• Life Cycle Stages
• Packaging Type
• Goals
• Project and Modules
• Build Plugins
• POM is declarative, not procedural.
• Local Repository, Central Repository
• Reuse component projects Locally
• Build Profile.
• Implicit Target (Jar) Filenames
• Dependency deduction and Parallel Execution
• Dependency versioning

POM - Project Object Model is expressed in pom.xml file. Object oriented representation of projects. Declarative instead of procedural.

Build lifecycle has following stages: validate, compile, test, package, verify, install and deploy.

Ultimately you want to express the dependency tree and specify actions for specific goals.

You can have a single POM file (in simple project) or collection of directories with their own POM files. Each POM file represents a Project. POM files can inherit from another. POM files inheritance form a tree structure (not graph). But implicit dependencies between projects is a graph.

A project can aggregate multiple component sub-projects called modules in pom.xml

Each POM file associated with packaging type which is any one of following:

• POM (just plain POM file only)
• jar is the default. It implicitly immediately defines implementation for goals in various default lifecycles.
• Other goals are war and ear
• maven-plugin packaging type is used to bundle implementation for custom goal (like compile, install, etc)
• You can define your own packaging type, but usually you will never need to do that. See http://stackoverflow.com/questions/1427722/how-do-i-create-a-new-packaging-type-for-maven

Defining your own goal involves writing maven plugin (Mojo) which is packaged as maven-plugin.

You list the plugins (standard or custom plugins) that you use in POM under <build><plugins>. They are by default inherited i.e. custom goals and actions apply to child POMs as well. To break that inheritance, explicitly mark them as: <inherited>false</inherited>

Similarly, the parent <dependencies> also inherited to children. (You can't disable it).

Use mvn dependency:tree to dump dependencies.

For creating destination jar file it uses project artifactId, etc for target file name. You can customize it though using <finalName> property. If you want to generate 2 separate Jar files from single project, you need to use maven-assembly-plugin.

Build profile is an important concept. You can trigger various profiles as per different conditions like:

• You can directly activate profile like below::
  mvn groupId:artifactId:goal -P profile-1,profile-2 mvn compile -P myDebugEnv

• Define as <activeProfiles> in POM xml file itself or in

• Define <activation> condition in POM xml e.g. when provided jdk version is 1.6 or os is Windows. or when a specific file is missing or present. etc.