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Apache Maven

Apache Maven is a project management and build tool for Java developers. It handles several tasks including:

  1. Dependency management: when you use versioned libraries, Maven can pull the appropriate versions from the cloud.
  2. Build management: Maven can compile your program and all dependencies.
  3. Project metadata: Maven can assign information about your project when it compiles your program(s).
  4. Program execution: Maven will allow you to run your code and/or test cases.

Maven Projects

A Maven project is specified in a file called pom.xml in the root directory of the project.

By default, the project is laid out as follows:

  • src/ represents the source files to be compiled

    • src/main/java represents Java source

    • src/main/resources represents files that are copied into the same directory as the Java project at compile time

    • src/test/java represents source files that are only used as test cases during mvn test -- typically these are JUnit tests.

    • src/testresources represents files that are copied into the same directory as the Java project at test time.

  • target/ represents the compiled code. Normally this is created as needed. You can run mvn clean to delete the target.
  • ~/.m2/repository is where Maven downloads versioned dependencies.

Maven Goals

When you run Maven, you typically do this with a goal. Some common goals include:

  • compile -- compile main source
  • test-compile -- compile test cases
  • test -- run test cases
  • exec (with additional options) -- run source code
  • package -- generate a jar package of your code (and optionally all dependencies)
  • install -- copies the jar file into ~/.m2/repository after building and creating it

What Happens When You Run mvn compile

When you run mvn compile, all of the libraries your program depends upon should be downloaded to a directory called .m2 in your home directory. Then your program should get compiled to target/classes inside your project directory. Any files in src/main/resources will also be copied into target.

When you run mvn exec:java, it will run the main class (from the pom.xml) from the target/classes directory, also adding all dependencies to the class path.

What Happens When You Run mvn test

When you run mvn test-compile, similarly to how mvn compile generates classes in target/classes, you will get additional classes in target/test-classes. (These will be your src/test/java classes.) Again, anything from src/test/resources will get copied in.

When you run mvn test, this will run all test cases from target/test-classes. Due to a special Maven plugin, the results will get summarized in target/surefire-reports.

What Happens When You Run mvn clean

If you run mvn clean, this will delete the target subdirectory and all of its content.

What Happens When You Run mvn exec:java@goal

When you run mvn exec, Maven first tries to determine what to run (often, Java, as in mvn exec:java). Next it looks in the pom.xml file for the exec-maven-plugin. Within the related XML, there should be a definition of the goal (named as the ID) and the mainClass. You can see this illustrated here:

    <plugin>
      <groupId>org.codehaus.mojo</groupId>
      <artifactId>exec-maven-plugin</artifactId>
      <version>3.1.0</version>
      <executions>
        <execution>
          <id>loader</id>
          <goals>
            <goal>java</goal>
          </goals>
        <configuration>
          <mainClass>edu.upenn.cis.nets2120.hw1.LoadData</mainClass>
          <arguments>
          </arguments>
        </configuration>
        </execution>
      </executions>
    </plugin>