Every non-trivial Java application has many dependencies on other resources, especially JAR files. We must keep track of which JARs we are utilizing and their versions as the project develops. Systems may undergo several releases during their development lifecycle, and among those releases, dependency lists may change. New JARs may be added, or old versions may be replaced with newer ones. We must trace all these changes as part of our codebase. It is not considered good practice to keep those JARs in the source control system alongside our source codebase. It is preferred to keep those JARs in a separate repository, and dependent JARs are brought from that location into the development environment when required. Hence, we only need to keep dependency information— which JARs we depend on and their versions— together with our system’s codebase. This information is used to initialize our development and deployment environment during the build or deployment processes.
Maven has significant support for dependency management, and in Maven 2.0, it is advanced with transitive dependency management. With transitive dependency, you don’t need to discover and specify libraries that our own dependencies require. For example, it is sufficient to specify Hibernate as our sole dependency, without specifying the libraries that Hibernate depends on. Maven 2.0 also introduces dependency scopes, such as compile-time, runtime, or testing-only dependencies, building upon the concept of transitive dependencies.
Maven 2.0 also offers dependency tasks for Ant as a separate JAR. Unfortunately, it does not currently work in the Turkish locale. We yearn for such a mechanism in our current project, and a quick search revealed several other solutions similar to Maven’s. One such solution is Ribomation’s Dependencies Download Ant Task, and the other is Dependencies Ant Task in HttpUnit.org. Both utilize Maven’s ibiblio repository as the default external repository.
I have successfully configured the second solution. The concepts are similar to Maven. It manages a set of dependencies, and each missing dependency is downloaded from a remote repository. The default is Maven Central Repository, and unnecessary repeated downloads are avoided using a local cache. The following lines briefly explain how to install and use the dependency task:
Download the ZIP archive and extract the ant-dependencies.jar into your Ant lib directory. Include the typedef in your build.xml file as follows:
<typedef resource="dependencies.properties" />
<dependencies pathId="base.classpath" fileSetId="web-inf.lib" >
<dependency group="junit" version="3.8.1" />
<dependency group="xerces" version="2.2.1" artifact="xmlParserAPIs"/>
<dependency group="xerces" version="2.6.0" artifact="xercesImpl"/>
</dependencies>
The dependencies element defines a set containing dependency child elements. Each dependency child element defines files required in our project. The group attribute defines the external project group that creates the dependency. Each project we depend on may consist of several JARs. Therefore, any dependency may also have an artifact attribute, indicating which JAR file is required in the specified project group. If the project group consists of a single JAR, there is no need to add the artifact attribute, as it defaults to the group. We also need to specify which version of those JAR files our project depends on.
<javac srcdir="${src}" destdir="${classes}">
<classpath refid="base.classpath" />
</javac>
<copy todir="${web-inf}/lib" flatten="true">
<fileset refid="web-inf.lib"/>
</copy>
Missing JARs are downloaded into the local cache, which is ~/.maven/repository by default. If you want to change it, you must set the ant.dependencies.cache system property before running Ant. Please note that this is not a normal Ant property; it is a Java system property. Therefore, you should pass it to the JRE as a parameter with the -D option. We can also change the default remote repository, which is ibiblio, by setting the repositoryList attribute of the dependency task. It takes comma-separated HTTP URLs as remote repository values. Another way to change the external repository is to use the ant.remote.repository system property, but it is deprecated.
You may want to create your own external repository in your intranet and eliminate the need for connecting to a remote
repository over the Internet. If you examine the Maven Central Repository, you will see that it is enough to create a
directory structure, with each project group having its own folder with the same name as the group, and a jars folder in
each group folder to keep JAR files. Each JAR file is in the form of **