概述
在这个教程中，我们将学习如何使用Maven命令检测pom.xml中的重复依赖项。我们还将了解如何使用Maven Enforcer插件在发现重复依赖时停止构建。
为什么检测重复依赖？
pom.xml中存在重复依赖的风险在于，项目构建路径可能不会应用到目标库的最新版本。例如，考虑以下的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>
如图所示，有两个对同一库（commons-lang3）的依赖，尽管两个依赖的版本不同。
接下来，我们将学习如何使用Maven命令来检测这些重复依赖。
依赖树命令
现在，让我们在终端中运行mvn dependency:tree命令并查看输出。
$ mvn dependency:tree
[INFO] Scanning for projects...
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.baeldung: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.baeldung: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.baeldung: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
[INFO] Finished at: 2021-12-20T09:45:20+05:30
[INFO] ------------------------------------------------------------------------
这里，我们看到警告，指出pom.xml中存在重复依赖。此外，我们注意到虽然项目中存在更高版本（3.12.0）的commons-lang3.jar，但Maven选择了较晚出现在pom.xml中的3.11版本。这是因为Maven选择了最后出现的依赖。
依赖analyze-duplicate命令
现在，运行mvn dependency:analyze-duplicate命令并查看输出。
$ mvn dependency:analyze-duplicate
[INFO] Scanning for projects...
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.baeldung: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.baeldung: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
[INFO] Finished at: 2021-12-20T09:54:02+05:30
[INFO] ------------------------------------------------------------------------
在这里，我们注意到警告和信息日志都提到了重复依赖的存在。
如果存在重复依赖，构建失败
在上述示例中，我们已经了解了如何检测重复依赖，但构建仍然成功，这可能导致使用了不正确的jar版本。
通过使用Maven Enforcer插件，我们可以确保在存在重复依赖时构建失败。
为此，我们需要将此Maven插件添加到pom.xml中，并添加规则banDuplicatePomDependencyVersions：
<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，规则绑定到我们的Maven构建：
$ mvn verify
[INFO] Scanning for projects...
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.baeldung: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.baeldung: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] ------------------------------------------------------------------------
[INFO] Total time: 0.537 s
[INFO] Finished at: 2021-12-20T09:55:46+05:30
[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.
删除重复依赖
一旦识别出重复依赖，最简单的方法是从pom.xml中删除它们，只保留项目实际使用的独特依赖。
总结
在这篇文章中，我们学习了如何使用mvn dependency:tree和mvn dependency:analyze-duplicate命令在Maven中检测重复依赖。我们还了解到如何使用Maven Enforcer插件通过内置规则在包含重复依赖的构建中失败。