Maven

A build tool for Java plus a package manager

The short version:

• Maven adds package management to the functions that you get with Ant.
• By package management, I mean that instead of you having to manage the process of obtaining all the .jar files for third party library that your project may depend on (and the .jars that those files depend on, and so on…), maven manages that for you.
• Maven is built on a different philosophy than Ant, explained below.

Similarities with Ant

• Both Ant and Maven provide a way to manage the process of compiling, making jars, producing javadoc, running your code.
• Both use XML as the format
• Built in Java, so its ostensibly portable like Java (Makefiles are very Unix centric)
• Both are maintained by the Apache Software Foundation

Superficial differences between Maven and Ant:

• pom.xml instead of build.xml
• command to run the tool is mvn instead of ant
• The directory structures for Ant is somewhat flexible, but Maven has definite expectations.
  • With Ant, you can decide how to organize your directories, and configure your build.xml file accordingly
  • With Maven, you need to do things Maven’s way, or you’ll be sorry. We’ll ok, so strictly speaking that’s not true, according to the Maven FAQ, but it certainly is easier to just organize things according to the Maven defaults.
• The pom.xml syntax is a bit more complicated than the build.xml syntax (okay, that’s a personal opinion)
  • But, you get more power, so it’s perhaps worth it?
• Maven will manage a lot more things for you than Ant does, including downloading and caching the .jar files your application needs.
  • You still have to specify what .jars those are in dependency declarations in the pom.xml file.

The big difference: imperative vs. declarative.

Ant’s build.xml is imperative.

• The creator of the build.xml file, has to understand both what you want to do (e.g. compile some Java code), and how to do it (e.g. with the javac task.
• The build.xml author creates targets that specify both what you want to do, and how.

Maven’s pom.xml is declarative.

• The pom.xml is a way to configure dependencies, and specify which plugins are being used, and how they are configured.
• The plugins provide goals, which are the things that the programmer wants to accomplish (like compiling, making a jar, publishing javadoc, etc.)
• In a pom.xml, you say only what you want to do, and specify a bit of configuration information, but Maven decides how to accomplish each goal, using it’s plugins.
• The plug-ins all use a set of common conventions and expectations, so that, in principle, there isn’t that much configuration you have to do for most common tasks.

The biggest advantage of the declarative style is when you have dependencies. If you know you depend on something, say junit or apache-commons or batik-svggen, you just grab the bit of code that “declares” the dependency, and it will automatically go get the .jar file for you, and put it in the right place in your classpath for compiling, creating javadoc, testing, making a jar, etc.

The biggest drawback is that if/when you are trying to do something very specific like put a file in a specific directory (e.g. putting your javadoc under docs, because that’s where github-pages expects them to be), you may find yourself fighting with Maven a bit. Maven wants to put things where it wants to put them, and expects you as the developer to just submit to its will.

References:

• Apache Maven home page
• Apache Maven FAQ
• Apache Maven in 5 minutes

Remove custom files (e.g. logs) in mvn clean

• If you want *.log from the logs directory (or any other custom fileset or directory) to be deleted when you do mvn clean, here is how you can accomplish that: just “configure the maven-clean-plugin” in your pom.xml (as explained here). Be sure this goes in the build plugsin (not the reporting plugins, for example.)

   <!-- also remove logs on maven clean -->
      <plugin>
        <artifactId>maven-clean-plugin</artifactId>
        <version>3.1.0</version>
        <configuration>
          <filesets>
            <fileset>
              <directory>logs</directory>
              <includes>
                <include>**/*.log</include>
              </includes>
            </fileset>
          </filesets>
        </configuration>
      </plugin>