不会还在为jar包冲突发愁吧！！
The following article is from Java技术指北 Author 指北君
来源丨Java技术指北（ID：javanorth）
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: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed.
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。
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