Developing with Eclipse and Maven

The latest release of m2eclipse contains a POM editor which provides some dependency analysis tools. These tools promise to change the way people maintain and monitor a project's transitive dependencies. One of the main attractions to Maven is the fact that it manages a project's dependencies. If you are writing an application which depends on the Spring Framework's Hibernate3 integration, all you need to do is depend on the spring-hibernate3 artifact from the Central Maven Repository. Maven then reads this artifact's POM and adds all of the necessary transitive dependencies. While this is a great feature that attracts people to using Maven in the first place, it can often become confusing with a project starts to depend on tens of dependencies, each with tens of transitive dependencies.

Problems start to happen when you depend on a project with a poorly crafted POM which fails to flag dependencies as optional, or when you start encountering conflicts between transitive dependencies. If one of your requirements is to exclude a dependency like commons-logging or the servlet-api, or if you need to find out why a certain dependency is showing up under a specific scope you will frequently need to invoke the dependency:tree and dependency:resolve goals from the command-line to track down the offending transitive dependencies.

This is where the POM editor in m2eclipse comes in handy. If you open a project with many dependencies, you can open the Dependency Tree tab and see a two-column display of dependencies as shown in Figure 7.4, “Dependency Tree Tab of the POM Editor”. The left-side of the panel displays a tree of dependencies. The first level of the tree consists of direct dependencies from your project, and each subsequent level lists the dependencies of each dependency. The left-hand side is a great way to figure out how a specific dependency made its way into your project's resolved dependencies. The right-hand side of this panel displays the resolved dependencies. This is the list of effective dependencies after all conflicts and scopes have been applied, and it is the effective list of dependencies that your project will use for compilation, testing, and packaging.

The feature which makes the Dependency Tree tab so valuable is that it can be used as an investigative tool to figure out how a specific dependency made it into the list of resolved dependencies. Searching and filtering functionality available in the editor makes it really easy to search and browse trough the project dependencies. You can use “Search” entry field from the editor tool-bar and “Sort” and “Filter” actions from “Dependency Hierarchy” and “Resolved Dependencies” sections to navigate trough dependencies. Figure 7.5, “Locating Dependencies in the Dependency Tree” shows what happens when you click on commons-logging in the "Resolved Dependencies" list. When filtering is enabled in “Dependencies Hierarchy” section, clicking on a resolved dependency filters the hierarchy on the left-hand side of the panel to show all of the node which contributed to the resolved dependency. If you are trying to get rid of a resolved dependency, you can use this tool to find out what dependencies (and what transitive dependencies) are contributing the artifact to your resolved dependencies. In other words, if you are trying to get rid of something like commons-logging from your dependency set, the Dependency Tree tab is the tool you will likely want to use.

m2eclipse also provides you with the ability to view your project's dependencies as a graph. Figure 7.6, “Viewing the Dependencies of a Project as a Graph” shows the dependencies of idiom-core. The top-most box is the idiom-core project and the other dependencies are shown below it. Direct dependencies are linked from the top box and the transitive dependencies are linked from those. You can select a specific node in the graph to highlight the linked dependencies, or you can use the Search field at the top of the page to find matching nodes.

Note that “open folder” icon on each graph node indicates that the corresponding artifact is present in the Eclipse workspace and “jar” icon indicates that the node's artifact is referenced from the Maven repository.

The graph presentation can be changed by right clicking in the editor. You can choose to show artifact ids, group ids, versions, scopes, or if you want to wrap node text or show icons. Figure 7.7, “Radial Layout of Dependency Graph” shows the same graph from Figure 7.6, “Viewing the Dependencies of a Project as a Graph” with a radial layout.