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]（），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-dependencies: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed.

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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