Eclipse is an IDE (Integrated Development Environment) with editors, debuggers, and panels to view type hierarchies and show all uses of particular classes and variables. It is an excellent tool to edit and debug software, including J2EE applications. With Eclipse you can check out the current version of source from Subversion, make changes to the code, test the changes, and check the modifications back into Subversion.
Maven is a batch tool for building and packaging production "artifacts". An artifact may be the WAR file for a Web application, or the EAR file for a J2EE application (containing WARS and EJBs), or it may just be a component library packaged as a JAR file. In order to create the artifact, Maven will compile any source, copy data files, and even download over the internet any referenced public Java JAR libraries. Maven "pulls" all these pieces together to create the production deploy file. At Yale, artifacts are stored in a special Maven related Web server called Artifactory.
Eclipse is not good at building artifacts, and Maven has no editor or debugger. So they are complimentary tools that handle different phases of the development process.
Yale conventions require that the source to any Yale software project be stored in Subversion. Artifacts (JAR and WAR files) are similarly required to be stored in a network server called Artifactory. Build and installation have to be managed by the Jenkins tool which runs what are loosely called "jobs". A Jenkins Build job runs the Maven project to check the source out of SVN, creates the artifact, and stores it in Artifactory. The Jenkins Install job copies the artifact from Artifactory to the JBoss deploy directory plugging in values from properties files or operator parameters into text configuration files to set database passwords or other last minute text values.
During development, Eclipse not only provides an editor and debugger, but its built in support for Maven substitutes for the Jenkins Build and Install job.
Eclipse has Maven support, but it does not require that you use it. You do not have to put your source files in the Maven standard src/main/java subdirectory but can configure any subdirectory as Java source. Similarly, you can manually configure any JAR file anywhere on disk to be used to find Java classes during compile. The Eclipse project is configured with a .project file, a .classpath file, and a .settings directory. Maven is configured with a pom.xml file.
It is a Yale convention that the Eclipse configuration files are typically not checked into Subversion (this is not a hard rule, but you will typically find it is true). This means that if you start with an empty Eclipse workspace and check the project out, then the Eclipse support for Maven (a component called "M2E") has to read the pom.xml file, apply the Maven conventions, and configure the Eclipse .project and .classpath values. Eclipse can operate on Java source in any directory, but it is a Maven convention that the source be located in the src/main/java directory. Eclipse will be told to compile source files from the same directories where Maven expects source, and to put the output class files in the same directory where Maven would have put its class files. M2E will also download JAR libraries listed as dependencies into the Maven repository and will create an Eclipse Build Path that uses these dependency JARs to resolve references during compilation.
Maven has a large library of special purpose or extended function "plugin" modules that can be added to do extra processing steps. It also has special configuration parameters that are specified in the Plugins section of the pom.xml file. M2E does not guarantee that it will understand and duplicate the function of every plugin or every configuration parameter. It gets a bit smarter in each release, but you still may have to go back and manually tweak some parameters, such as the version of Java (1.6, 1.7. 1.8) that you want the compiler to use when processing the source.
If your project uses special Maven steps to generate Java source (from WSDL for a Web Service) then you have gone over the border beyond what M2E can do automatically in Eclipse. At this point you have to run Maven once in batch mode ("mvn install") to get the extra processing steps that M2E cannot handle, and then you may need to manually configure the generated source files as a new source directory to Eclipse.
For example, CAS uses the "WAR Overlay" technique supported by Maven batch. You can Google for the term, but WAR Overlay is triggered when the pom.xml that generates a WAR file lists another WAR file as a dependency. CAS has a vanilla cas-server-webapp project that generates a template WAR file with no Yale code or customizations. The CAS development convention is to build a second WAR project that contains only Yale code and customizations, but does not duplicate all the HTML and Spring XML files in the first project that contain no Yale changes. Because the Yale project lists the vanilla WAR as a dependency, when Maven batch runs the second project it automatically includes all the files from the vanilla WAR, but it replaces vanilla files with Yale customized files that have the same name and it adds any new Yale files.
Eclipse does not understand the "WAR Overlay" type of processing. It sees two projects that each produce a WAR file. It builds the vanilla WAR correctly, but then when it runs the Yale project it creates a WAR file that has only the Yale files in it and does not merge the Yale modifications on top of the vanilla files. So the WAR it generates is incomplete and cannot run successfully. Eclipse cannot build CAS, so you have to run Maven in batch ("mvn install") on its own or under Eclipse to get the full WAR Overlay processing and generate the correct Yale artifact.
However, although Eclipse does not really know how to build the CAS WAR, it does correctly configure the dependencies, the Build Path of JAR files that the Yale code needs to compile correctly, and provides an editor with autocomplete and class hierarchy and all the rest of the tools that make editing and debugging easy. That is the part that M2E does correctly, and that is the only part that is really necessary.
Every new release of Eclipse changes the set of features that are automatically included and those that have to be manually installed. Furthermore, the Maven integration of M2E gets a bit smarter in each release. Generally this makes things simpler and there are fewer extra steps to get a useful development environment. However, a "cookbook" style list of specific instructions will not be exactly correct in the next release, so these instructions will be a bit more descriptive of the how and why so you can adapt if the exact steps change from year to year.
This cannot be a serious introduction to either Maven as a comprehensive project building utility or Eclipse as an IDE. There are books that cover the intricate details of each. However, it will cover a bit more than the two page tutorial that you find with either system. Move though the material sequentially and skip pages that cover material with which you are already familiar, although the objective here will be to provide more explanation than you normally get about how things work. By design, each major topic has its own URL that you can bookmark or link to.