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在我們平時(shí)的開發(fā)過程中，常常會(huì)遇到引入各種不同的 jar 包，然后引發(fā)的 Maven 依賴沖突，今天我們來學(xué)習(xí)下如何使用 Maven 命令檢測(cè) pom.xml 中的重復(fù)依賴項(xiàng)。
為什么要檢測(cè)重復(fù)的依賴關(guān)系
在pom.xml中, 經(jīng)常引入各種不同的jar 包， 又會(huì)依賴其他的jar。特別是一些常用的工具庫(kù)，比較容易出現(xiàn)版本沖突，例如，讓我們看下這個(gè)pom.xml。
[...]
org.apache.commons
commons-lang3
3.12.0
org.apache.commons
commons-lang3
3.11
[...]
從上面的代碼中，commons-lang3 被引用了兩次，而且版本號(hào)也不一樣。現(xiàn)在我們就來看看如何使用Maven命令來檢測(cè)這些重復(fù)的依賴關(guān)系。
依賴樹命令
讓我們?cè)诮K端運(yùn)行 mvn dependency:tree的命令，看看輸出結(jié)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(shí)被引入進(jìn)來了，出現(xiàn)這種情況是因?yàn)镸aven選擇了pom.xml中后來出現(xiàn)的依賴。
依賴關(guān)系analyze-duplicate命令
現(xiàn)在讓我們運(yùn)行 mvn dependency:analyze-duplicate，看看輸出輸出結(jié)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 inin 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(yǔ)句都提到了重復(fù)依賴的存在。
如果存在重復(fù)的依賴，則構(gòu)建失敗
在上面的例子中，我們看到了如何檢測(cè)重復(fù)的依賴關(guān)系，但BUILD仍然是成功的，但這可能導(dǎo)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(fù)依賴的情況下構(gòu)建不成功。
我們需要在pom.xml中加入這個(gè)Maven插件，并加入banDuplicatePomDependencyVersions規(guī)則。
[...]
org.apache.maven.plugins
maven-enforcer-plugin
3.0.0
no-duplicate-declared-dependencies
enforce
[...]
現(xiàn)在，該規(guī)則約束了我們的Maven構(gòu)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:
刪除重復(fù)的依賴關(guān)系
只要確定了重復(fù)的依賴關(guān)系，我們就需要在 pom.xml中刪除它們，只保留那些我們項(xiàng)目使用的唯一依賴關(guān)系。
總結(jié)
本文中，我們學(xué)習(xí)了如何使用mvn dependency:tree和mvn dependency:analyze-duplicate命令檢測(cè)Maven中的重復(fù)依賴，還學(xué)習(xí)了如何使用Maven Enforcer插件，通過應(yīng)用內(nèi)置規(guī)則使包含重復(fù)依賴的構(gòu)建失敗。
文章標(biāo)題：不會(huì)還在為Jar包沖突發(fā)愁吧?。?
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