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十年PHP架构师的成长之路，程序员必备
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Maven avoids the need to discover and specify the libraries that your own dependencies require by including transitive dependencies automatically. This feature is facilitated by reading the project files of your dependencies from the remote repositories specified. In general, all dependencies of those projects are used in your project, as are any that the project inherits from its parents, or from its dependencies, and so on. There is no limit to the number of levels that dependencies can be gathered from. A problem arises only if a cyclic dependency is discovered. With transitive dependencies, the graph of included libraries can quickly grow quite large. For this reason, there are additional features that limit which dependencies are included: Dependency mediation - this determines what version of an artifact will be chosen when multiple versions are encountered as dependencies. Maven picks the “nearest definition”. That is, it uses the version of the closest dependency to your project in the tree of dependencies. You can always guarantee a version by declaring it explicitly in your project's POM. Note that if two dependency versions are at the same depth in the dependency tree, the first declaration wins. “nearest definition” means that the version used will be the closest one to your project in the tree of dependencies. Consider this tree of dependencies: In text, dependencies for A, B, and C are defined as A -> B -> C -> D 2.0 and A -> E -> D 1.0, then D 1.0 will be used when building A because the path from A to D through E is shorter. You could explicitly add a dependency to D 2.0 in A to force the use of D 2.0, as shown here: Dependency management - this allows project authors to directly specify the versions of artifacts to be used when they are encountered in transitive dependencies or in dependencies where no version has been specified. In the example in the preceding section a dependency was directly added to A even though it is not directly used by A. Instead, A can include D as a dependency in its dependencyManagement section and directly control which version of D is used when, or if, it is ever referenced. Dependency scope - this allows you to only include dependencies appropriate for the current stage of the build. This is described in more detail below. Excluded dependencies - If project X depends on project Y, and project Y depends on project Z, the owner of project X can explicitly exclude project Z as a dependency, using the “exclusion” element. Optional dependencies - If project Y depends on project Z, the owner of project Y can mark project Z as an optional dependency, using the “optional” element. When project X depends on project Y, X will depend only on Y and not on Y's optional dependency Z. The owner of project X may then explicitly add a dependency on Z, at her option. (It may be helpful to think of optional dependencies as “excluded by default.”) Although transitive dependencies can implicitly include desired dependencies, it is a good practice to explicitly specify the dependencies your source code uses directly. This best practice proves its value especially when the dependencies of your project change their dependencies. For example, assume that your project A specifies a dependency on another project B, and project B specifies a dependency on project C. If you are directly using components in project C, and you don't specify project C in your project A, it may cause build failure when project B suddenly updates/removes its dependency on project C. Another reason to directly specify dependencies is that it provides better documentation for your project: one can learn more information by just reading the POM file in your project, or by executing mvn dependency:tree. Maven also provides dependency:analyze plugin goal for analyzing the dependencies: it helps making this best practice more achievable.
