Dependencies declared in your project’s pom.xml file often have their own dependencies . Such dependencies are called transitive dependencies . Take the example of Hibernate Core. For it to function properly, it requires JBoss Logging, dom4j,
javaassist, and so forth. The Hibernate Core declared in your pom.xml file is considered a direct dependency, and dependencies such as dom4j and javaassist are considered your project’s transitive dependencies. A key benefit of Maven is that it automatically deals with transitive dependencies and includes them in your project.
Figure 3-3 provides an example of transitive dependencies. Notice that transitive dependencies can have their own dependencies. As you might imagine, this can quickly get complex, especially when multiple direct dependencies pull different versions of the same JAR file.
Maven uses a technique known as dependency mediation to resolve version conflicts. Simply stated, dependency mediation allows Maven to pull the dependency that is closest to the project in the dependency tree. In Figure 3-3, there are two versions of dependency B: 0.0.8 and 1.0.0. In this scenario, version 0.0.8 of dependency B is included in the project, because it is a direct dependency and closest to the tree. Now look at the three versions of dependency F: 0.1.3, 1.0.0, and 2.2.0. All three dependencies are at the same depth. In this scenario, Maven will use the first-found dependency, which would be 0.1.3, and not the latest 2.2.0 version. If you want Maven to use the latest 2.2.0 version of artifact F, you need to explicitly add that version dependency to pom.xml file.
Figure 3-3 Transitive dependencies
