Maven-11: 从命令行调用插件

mvn -h显示mvn命令帮助：

usage: mvn [options] [<goal(s)>] [<phase(s)>]

Options:
 -am,--also-make                        If project list is specified, also
                                        build projects required by the
                                        list
 -amd,--also-make-dependents            If project list is specified, also
                                        build projects that depend on
                                        projects on the list
 -B,--batch-mode                        Run in non-interactive (batch)
                                        mode
 -b,--builder <arg>                     The id of the build strategy to
                                        use.
 -C,--strict-checksums                  Fail the build if checksums don't
                                        match
 -c,--lax-checksums                     Warn if checksums don't match
 -cpu,--check-plugin-updates            Ineffective, only kept for
                                        backward compatibility
 -D,--define <arg>                      Define a system property
 -e,--errors                            Produce execution error messages
 -emp,--encrypt-master-password <arg>   Encrypt master security password
 -ep,--encrypt-password <arg>           Encrypt server password
 -f,--file <arg>                        Force the use of an alternate POM
                                        file (or directory with pom.xml).
 -fae,--fail-at-end                     Only fail the build afterwards;
                                        allow all non-impacted builds to
                                        continue
 -ff,--fail-fast                        Stop at first failure in
                                        reactorized builds
 -fn,--fail-never                       NEVER fail the build, regardless
                                        of project result
 -gs,--global-settings <arg>            Alternate path for the global
                                        settings file
 -h,--help                              Display help information
 -l,--log-file <arg>                    Log file to where all build output
                                        will go.
 -llr,--legacy-local-repository         Use Maven 2 Legacy Local
                                        Repository behaviour, ie no use of
                                        _remote.repositories. Can also be
                                        activated by using
                                        -Dmaven.legacyLocalRepo=true
 -N,--non-recursive                     Do not recurse into sub-projects
 -npr,--no-plugin-registry              Ineffective, only kept for
                                        backward compatibility
 -npu,--no-plugin-updates               Ineffective, only kept for
                                        backward compatibility
 -nsu,--no-snapshot-updates             Suppress SNAPSHOT updates
 -o,--offline                           Work offline
 -P,--activate-profiles <arg>           Comma-delimited list of profiles
                                        to activate
 -pl,--projects <arg>                   Comma-delimited list of specified
                                        reactor projects to build instead
                                        of all projects. A project can be
                                        specified by [groupId]:artifactId
                                        or by its relative path.
 -q,--quiet                             Quiet output - only show errors
 -rf,--resume-from <arg>                Resume reactor from specified
                                        project
 -s,--settings <arg>                    Alternate path for the user
                                        settings file
 -T,--threads <arg>                     Thread count, for instance 2.0C
                                        where C is core multiplied
 -t,--toolchains <arg>                  Alternate path for the user
                                        toolchains file
 -U,--update-snapshots                  Forces a check for missing
                                        releases and updated snapshots on
                                        remote repositories
 -up,--update-plugins                   Ineffective, only kept for
                                        backward compatibility
 -V,--show-version                      Display version information
                                        WITHOUT stopping build
 -v,--version                           Display version information
 -X,--debug                             Produce execution debug output

goal和phase分别指插件目标和生命周期阶段。

我们知道，可以通过mvn命令激活生命周期阶段，从而执行那些绑定在生命周期阶段上的插件目标。但Maven还支持直接从命令行调用插件目标。Maven支持这种方式是因为有些任务不适合绑定在生命周期上，例如maven-help-plugin:describe,我们不需要在构建项目的时候去描述插件信息。又如maven-dependency-plugin:tree，我们也不需要在构建项目的时候去显示依赖树。因此这些插件目标应该通过如下方式使用：

$ mvn help:describe -Dplugin=compiler

$ mvn dependency:tree

不过，这里还有一个疑问，describe是maven-help-plugin的目标没错，但冒号前面的help是什么呢？它既不是groupId,也不是artifactId，Maven是如何根据该信息找到对应版本插件的呢？同理，为什么不是maven-dependency-plugin:tree而是dependency:tree呢？

解答该疑问之前，可以先尝试一下如下的命令：

$ mvn org.apache.maven.plugins:maven-help-plugin:2.1:describe -Dplugin=compiler

$ mvn org.apache.maven.plugins:maven-dependency-plugin:2.1:tree

这两条命令就比较容易理解了。它们的效果与之前的两条命令基本是一样的，但显然前面的命令更简洁，更容易记忆和使用。为了达到该目的，Maven引入了目标前缀的概念。help是maven-help-plugin的目标前缀，dependency是maven-dependency-plugin的目标前缀，有了插件前缀，Maven就能找到对应的artifactId。不过仅有artifactId，而没有groupId和version，Maven是无法精确定位到某个插件的。那么，Maven是如何做到的呢？我们将在后面的文章详细剖析这个问题。