大家好，我是指北君。
在我们平时的开发过程中，常常会遇到引入各种不同的 jar 包，然后引发的 Maven 依赖冲突，今天我们来学习下如何使用 Maven 命令检测 pom.xml 中的重复依赖项。
为什么要检测重复的依赖关系在pom.xml中, 经常引入各种不同的jar 包， 又会依赖其他的jar。特别是一些常用的工具库，比较容易出现版本冲突，例如，让我们看下这个pom.xml。
< project > [ ... ] < dependencies > < dependency > < groupId > org .apache .commons groupId > < artifactId > commons - lang3 artifactId > < version > 3.12 .0 version > dependency > < dependency > < groupId > org .apache .commons groupId > < artifactId > commons - lang3 artifactId > < version > 3.11 version > dependency > dependencies > [ ... ] project >
从上面的代码中，commons-lang3 被引用了两次，而且版本号也不一样。现在我们就来看看如何使用Maven命令来检测这些重复的依赖关系。
依赖树命令让我们在终端运行 mvn dependency:tree的命令，看看输出结果
$ mvn dependency : tree [ INFO ] Scanning for projects... [ WARNING ] [ WARNING ] Some problems were encountered while building the effective model for com .javanorth : maven - duplicate - dependencies : jar : 0 .0 .1 - SNAPSHOT [ WARNING ] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique : org .apache .commons : commons - lang3 : jar - > version 3.12 .0 vs 3.11 @ line 14 , column 15 [ WARNING ] [ WARNING ] It is highly recommended to fix these problems because they threaten the stability of your build. [ WARNING ] [ WARNING ] For this reason , future Maven versions might no longer support building such malformed projects. [ WARNING ] [ INFO ] [ INFO ] -------------< com.javanorth:maven-duplicate-dependencies >-------------- [ INFO ] Building maven - duplicate - dependencies 0.0 .1 - SNAPSHOT [ INFO ] --------------------------------[ jar ]--------------------------------- [ INFO ] [ INFO ] --- maven-dependency-plugin:2.8:tree (default-cli) @ maven-duplicate-dependencies --- [ WARNING ] The artifact xml - apis : xml - apis : jar : 2.0 .2 has been relocated to xml - apis : xml - apis : jar : 1.0 .b2 [ INFO ] com .javanorth : maven - duplicate - dependencies : jar : 0.0 .1 - SNAPSHOT [ INFO ] \ - org .apache .commons : commons - lang3 : jar : 3.11 : compile [ INFO ] ------------------------------------------------------------------------ [ INFO ] BUILD SUCCESS [ INFO ] ------------------------------------------------------------------------ [ INFO ] Total time : 1.136 s ...
我们可以看到，commons-lang3 jar的3.11版和 3.12 版同时被引入进来了，出现这种情况是因为Maven选择了pom.xml中后来出现的依赖。
依赖关系analyze-duplicate命令现在让我们运行 mvn dependency:analyze-duplicate，看看输出输出结果。
$ mvn dependency : analyze - duplicate [ INFO ] Scanning for projects... [ WARNING ] [ WARNING ] Some problems were encountered while building the effective model for com .javanorth : maven - duplicate - dependencies : jar : 0 .0 .1 - SNAPSHOT [ WARNING ] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique : org .apache .commons : commons - lang3 : jar - > version 3.12 .0 vs 3.11 @ line 14 , column 15 [ WARNING ] [ WARNING ] It is highly recommended to fix these problems because they threaten the stability of your build. [ WARNING ] [ WARNING ] For this reason , future Maven versions might no longer support building such malformed projects. [ WARNING ] [ INFO ] [ INFO ] -------------< com.javanorth:maven-duplicate-dependencies >-------------- [ INFO ] Building maven - duplicate - dependencies 0.0 .1 - SNAPSHOT [ INFO ] --------------------------------[ jar ]--------------------------------- [ INFO ] [ INFO ] --- maven-dependency-plugin:2.8:analyze-duplicate (default-cli) @ maven-duplicate-dependencies --- [ WARNING ] The artifact xml - apis : xml - apis : jar : 2.0 .2 has been relocated to xml - apis : xml - apis : jar : 1.0 .b2 [ INFO ] List of duplicate dependencies defined in < dependencies /> in your pom .xml : o org .apache .commons : commons - lang3 : jar [ INFO ] ------------------------------------------------------------------------ [ INFO ] BUILD SUCCESS [ INFO ] ------------------------------------------------------------------------ [ INFO ] Total time : 0.835 s 。。。
我们注意到，WARNING和INFO日志语句都提到了重复依赖的存在。
如果存在重复的依赖，则构建失败在上面的例子中，我们看到了如何检测重复的依赖关系，但BUILD仍然是成功的，但这可能导致使用了不正确的 jar 版本。
使用[Maven Enforcer Plugin]（https://maven.apache.org/enforcer/maven-enforcer-plugin/index.html），我们可以确保在存在重复依赖的情况下构建不成功。
我们需要在pom.xml中加入这个Maven插件，并加入banDuplicatePomDependencyVersions规则。
< project > [ ... ] < build > < plugins > < plugin > < groupId > org .apache .maven .plugins groupId > < artifactId > maven - enforcer - plugin artifactId > < version > 3.0 .0 version > < executions > < execution > < id > no - duplicate - declared - dependencies id > < goals > < goal > enforce goal > goals > < configuration > < rules > < banDuplicatePomDependencyVersions /> rules > configuration > execution > executions > plugin > plugins > build > [ ... ] project >
现在，该规则约束了我们的Maven构建。
$ mvn verify [ INFO ] Scanning for projects... [ WARNING ] [ WARNING ] Some problems were encountered while building the effective model for com .javanorth : maven - duplicate - dependencies : jar : 0 .0 .1 - SNAPSHOT [ WARNING ] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique : org .apache .commons : commons - lang3 : jar - > version 3.12 .0 vs 3.11 @ line 14 , column 14 [ WARNING ] [ INFO ] -------------< com.javanorth:maven-duplicate-dependencies >-------------- [ INFO ] Building maven - duplicate - dependencies 0.0 .1 - SNAPSHOT [ INFO ] --------------------------------[ jar ]--------------------------------- [ INFO ] [ INFO ] --- maven-enforcer-plugin:3.0.0:enforce (no-duplicate-declared-dependencies) @ maven-duplicate-dependencies --- [ WARNING ] Rule 0 : org .apache .maven .plugins .enforcer .BanDuplicatePomDependencyVersions failed with message : Found 1 duplicate dependency declaration in this project : - dependencies .dependency [ org .apache .commons : commons - lang3 : jar ] ( 2 times ) [ INFO ] ------------------------------------------------------------------------ [ INFO ] BUILD FAILURE [ INFO ] ------------------------------------------------------------------------ [ ERROR ] Failed to execute goal org .apache .maven .plugins : maven - enforcer - plugin : 3.0 .0 : enforce ( no - duplicate - declared - dependencies ) on project maven - duplicate - dependencie s :删除重复的依赖关系
只要确定了重复的依赖关系，我们就需要在 pom.xml中删除它们，只保留那些我们项目使用的唯一依赖关系。
总结本文中，我们学习了如何使用mvn dependency:tree和mvn dependency:analyze-duplicate命令检测Maven中的重复依赖，还学习了如何使用Maven Enforcer插件，通过应用内置规则使包含重复依赖的构建失败。
原文地址：https://mp.weixin.qq.com/s/tOCefHhOLcS3pkBm05z6eQ
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