在我們平時的開發過程中，常常會遇到引入各種不同的 jar 包，然后引發的 Maven 依賴沖突，今天我們來學習下如何使用 Maven 命令檢測 pom.xml 中的重復依賴項。

為什么要檢測重復的依賴關系

在pom.xml中, 經常引入各種不同的jar 包， 又會依賴其他的jar。特別是一些常用的工具庫，比較容易出現版本沖突，例如，讓我們看下這個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 >

從上面的代碼中，commons-lang3 被引用了兩次，而且版本號也不一樣?，F在我們就來看看如何使用Maven命令來檢測這些重復的依賴關系。

依賴樹命令

讓我們在終端運行 mvn dependency:tree的命令，看看輸出結果

$ 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
...

我們可以看到，commons-lang3 jar的3.11版和 3.12 版同時被引入進來了，出現這種情況是因為Maven選擇了pom.xml中后來出現的依賴。

依賴關系analyze-duplicate命令

現在讓我們運行 mvn dependency:analyze-duplicate，看看輸出輸出結果。

$ 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
。。。

我們注意到，WARNING和INFO日志語句都提到了重復依賴的存在。

如果存在重復的依賴，則構建失敗

在上面的例子中，我們看到了如何檢測重復的依賴關系，但BUILD仍然是成功的，但這可能導致使用了不正確的 jar 版本。

使用[Maven Enforcer Plugin]（https://maven.apache.org/enforcer/maven-enforcer-plugin/index.html），我們可以確保在存在重復依賴的情況下構建不成功。

我們需要在pom.xml中加入這個Maven插件，并加入banDuplicatePomDependencyVersions規則。

< 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 >

現在，該規則約束了我們的Maven構建。

$ 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.

刪除重復的依賴關系

只要確定了重復的依賴關系，我們就需要在 pom.xml中刪除它們，只保留那些我們項目使用的唯一依賴關系。

總結

本文中，我們學習了如何使用mvn dependency:tree和mvn dependency:analyze-duplicate命令檢測Maven中的重復依賴，還學習了如何使用Maven Enforcer插件，通過應用內置規則使包含重復依賴的構建失敗。