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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.
The source has to 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. The WAR has Artifacts (JAR and WAR files) are similarly required to be stored in a network server called Artifactory. Then the Jenkins 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 WAR artifact from Artifactory to the server where CAS will run and applies last minute changes JBoss deploy directory plugging in values from properties files or operator parameters into text configuration files to set database URLs and passwords. Eclipse cannot build the WAR artifact, and Maven has no editors or debuggers. So you use both Eclipse and Maven to develop applications.Yale standards require the use of Maven to build every application. The Jenkins Build Job runs Maven and expects to end with a WAR file in Artifactory. Eclipse could be replaced by Netbeans or Intellij or some other IDE (or a text editor). So any Yale software has to be a Maven project with a pom.xml file and the directory structure mandated by Maven conventions (Java source is in src/main/java and Web files are in src/main/webapp). The developer has to check this project into Subversion. How you edit it is up to you, but Eclipse is the recommended toolpasswords 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. However, since the desktop developer typically does not have access to Artifactory, the local Maven repository (typically in the ".m2" subdirectory of your home directory) is used instead of Artifactory during this phase of development.
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 project 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 Eclipse has a built in Maven support component called M2E that can read the Maven 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 and create equivalent Eclipse configuration files. Since Eclipse edits and compiles, this mostly means that the .classpath Eclipse configuration file is created so that Java source in 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 yearis compiled using all the JAR files defined as a dependency in pom.xml and downloaded to your local Maven repository. Eclipse is not a tool for building final artifacts, so M2E does not support special Maven processing steps for running preprocessors or filtering files copied to the WAR.
Eclipse is an extensible development environment to which you can add additional "plugin" function. Maven is an extensible build environment with its own system of plugins. However, Eclipse plugins are not Maven plugins. Each new release of Eclipse gets a bit smarter about Maven, but there are limits. For example, before Eclipse Luna you had to manually add Eclipse AspectJ support (AJDT) to easily handle the AspectJ elements of CAS compiling. After Luna, M2E detects the AspectJ references in the Maven POM and automatically triggers the installation of AJDT into Eclipse. However, Eclipse will complain automatically that there are things in the pom.xml file it does not understand and cannot duplicate. That is typically not a problem.
If you need some Maven processing that Eclipse doesn't do (for example, to run a preprocessor to generate Java source from some WSDL) then after Eclipse creates its project (with errors due to the missing source) you run a batch "mvn install" on the project to perform the extra steps. Then you have to go back and manually define the generated source directory to Eclipse as a Java Source directory and the errors go away.
Eclipse has the ability to run Maven commands on projects. You can do a "mvn install" from Eclipse instead of from the command line. However, Eclipse doesn't do any Maven processing automatically. Instead, M2E understands Maven POM files and configures Eclipse to do Eclipse processing that is roughly the same thing Maven would do when it compiles the project source. For everything else you have to do real "batch" Maven processing using real Maven.
The Jenkins Build job checks out the CAS Server project from SVN and does a real batch "mvn install" on it. Then it copies the generated artifacts to Artifactory. With Eclipse you can define a Run Configuration named "Maven Build" that also does a "mvn install" on the cas-server project in your Eclipse workspace. This does the same processing, but the artifacts are stored only in your local Maven repository and are not copied to Artifactory.
The Jenkins Install job checks out the CAS Server "Installer" job from SVN. At Yale, an Installer job is a Maven project that runs an Ant build.xml script. The script copies the artifact to the JBoss deploy directory. In Jenkins the artifact comes from Artifactory, but if you check the Installer Maven project out as an Eclipse project in your Workspace and do a "mvn install" with it, the artifact comes from your local Maven repository where the Maven Build Run Configuration put it.
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.