防止在多模块Maven中找到“未找到插件”

在多模块Maven项目的子模块上定义Maven插件会给我们“找不到插件”错误。尤其是如果我们有一个多模块项目，并且只想在一个特定模块中应用Maven插件，则此错误会经常发生。

假设我们有一个看起来像这样的多模块root pom。

<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.jdriven.blog</groupId><artifactId>maven-plugin-multimodule</artifactId><version>0.1-SNAPSHOT</version><packaging>pom</packaging><modules><module>module1</module> <!-- Module1 is a regular jar --><module>module2</module> <!-- Module2 has tomcat7 plugin configured --></modules>
</project>

本能地将插件（例如tomcat7）添加到此特定模块module2 ，就像这样。

<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"><modelVersion>4.0.0</modelVersion><artifactId>maven-plugin-multimodule-module2</artifactId><packaging>war</packaging><parent><groupId>com.jdriven.blog</groupId><artifactId>maven-plugin-multimodule</artifactId><version>0.1-SNAPSHOT</version><relativePath>../</relativePath></parent><build><plugins><plugin><groupId>org.apache.tomcat.maven</groupId><artifactId>tomcat7-maven-plugin</artifactId><version>2.2</version><configuration><!-- This is where our specific configuration goes --></configuration></plugin></plugins></build>
</project>

在多模块root pom上运行命令mvn tomcat7:help时，出现以下错误：

[ERROR] No plugin found for prefix 'tomcat7' in the current project 
and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] 
available from the repositories

解决方案非常简单：我们在多模块root pom的pluginManagement部分中指定插件。

<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.jdriven.blog</groupId><artifactId>maven-plugin-multimodule</artifactId><version>0.1-SNAPSHOT</version><packaging>pom</packaging><modules><module>module1</module> <!-- Module1 is a regular jar --><module>module2</module> <!-- Module2 has tomcat7 plugin configured --></modules><build><!-- In the multi-module root pom, use the pluginManagement to define the version of the maven-plugin --><pluginManagement><plugins><plugin><groupId>org.apache.tomcat.maven</groupId><artifactId>tomcat7-maven-plugin</artifactId><version>2.2</version></plugin></plugins></pluginManagement></build></project>

并且在我们特定的模块module2我们清除了插件的版本，因为它已在多模块root pom（父代）中定义。

<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"><modelVersion>4.0.0</modelVersion><artifactId>maven-plugin-multimodule-module2</artifactId><packaging>war</packaging><parent><groupId>com.jdriven.blog</groupId><artifactId>maven-plugin-multimodule</artifactId><version>0.1-SNAPSHOT</version><relativePath>../</relativePath></parent><build><plugins><plugin><groupId>org.apache.tomcat.maven</groupId><artifactId>tomcat7-maven-plugin</artifactId><!-- No version needed here, it is already defined in the multi-module root pom --><configuration><!-- This is where our specific configuration goes --></configuration></plugin></plugins></build></project>

现在，何时可以运行命令mvn tomcat7:help而不会出现任何错误。我们准备在子模块上配置插件。