5 Maven Tips

I have been working with Maven for 3 years now and over that time I have learned some tips and tricks that help me work faster with Maven. In this article, I am going to talk about 5 of those tips.

1. Maven rf option
Most of us work in a multi-module environment and it happens very often that a build fails at some module. It's a big pain to rerun the entire the build. To save you from going through this pain, Maven has an option called rf (i.e. resume) from which you can resume your build from the module where it failed. So, if your build failed at myproject-commons you can run the build from this module by typing:


mvn -rf myproject-commons clean install



2. Maven pl option
This next Maven option helps you build specified reactor projects instead of building all projects. For example, if you need to build only two of your modules myproject-commons and myproject-service, you can type:

mvn -pl myproject-commons,myproject-service clean install

This command will only build commons and service projects.


3. Maven Classpath ordering
In Maven, dependencies are loaded in the order in which they are declared in the pom.xml. As of version 2.0.9, Maven introduced deterministic ordering of dependencies on the classpath. The ordering is now preserved from your pom, with dependencies added by inheritence added last. Knowing this can really help you while debugging NoClassDefFoundError. Recently, I faced NoClassDefFoundError: org/objectweb/asm/CodeVisitor while working on a story where cglib-2.1_3.jar was getting loaded before cglib-nodep-2.1_3.jar. And as cglib-2.1_3.jar does not have this CodeVisitor class I was getting the error.


4. Maven Classifiers
We all know about qualifiers groupId, artifactId, version that we use to describe a dependency but there is  fourth qualifier called classifier. It allows distinguishing two artifacts that belong to the same POM but were built differently, and is appended to the filename after the version. For example, if you want to build two separate jars, one for Java 5 and another for Java 6, you can use classifier. It lets you locate your dependencies with the further level of granularity.


<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<scope>test</scope>
			<classifier>jdk16</classifier>
		</dependency>

5. Dependency Version Ranges
   Have you ever worked with a library that releases too often when you want that you should always work with the latest without changing the pom. It can be done using dependency version changes. So, if you want to specify that you should always work with the version above a specified version, you will write:

<version>[1.1.0,)</version>

This line means that the version should always be greater than or equal to 1.1.0. You can read more about dependency version ranges at this link.