在说这个插件是什么前我们先思考这么一个问题：当我们开发人员进入项目组进行开发前，要准备开发环境，而领导总是会强调工具的统一，编译环境的统一。比如要求所有开发人员使用JDK1.8进行开发。
开发人员接下来就是去下载指定版本的JDK，然后开始开发。但是如果开发人员的机器配置比较多，有好几个版本的JDK，而他虽然下载了JDK1.8，但是忘记配置环境变量，很有可能他用了JDK1.6进行的编译。
问题有了，该如何解决？ Maven Enforcer plugin就是来解决这类问题。Enforcer可以在项目validate时，对项目环境进行检查。
2.Maven Enforcer plugin怎么用？
Enforcer配置后默认会在validate后执行enforcer:enforce,然后对项目环境进行检查。拿上面对JDK的校验为例，我们在pom.xml中配置插件。
<?xml version="1.0" encoding="UTF-8"?> <project> ... <build> <plugins> <plugin> <artifactId>maven-enforcer-plugin</artifactId> <version>1.4.1</version> <executions> <execution> <id>default-cli</id> <goals> <goal>enforce</goal> </goals> <phase>validate</phase> <configuration> <rules> <requireJavaVersion> <message> <![CDATA[You are running an older version of Java. This application requires at least JDK ${java.version}.]]> </message> <version>[${java.version}.0,)</version> </requireJavaVersion> </rules> </configuration> </execution> </executions> </plugin> </plugins> </build> <properties> <java.version>1.8</java.version> </properties> </project>
我本地配置没有JDK1.8，按照我的期望，执行maven命令时应该会失败。让我们来执行命令 mvn validate 会打出如下日志告知我们JDK版本过低。
qyfmac$ mvn validate
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO]
[INFO] AppFuse Modular Application
[INFO] AppFuse Modular Application - Core
[INFO] AppFuse Modular Application - Web (Spring MVC)
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building AppFuse Modular Application 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-versions) @ stock ---
[WARNING] Rule 1: org.apache.maven.plugins.enforcer.RequireJavaVersion failed with message:
You are running an older version of Java. This application requires at least JDK 1.8.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] AppFuse Modular Application ........................ FAILURE [ 0.987 s]
[INFO] AppFuse Modular Application - Core ................. SKIPPED
[INFO] AppFuse Modular Application - Web (Spring MVC) ..... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.886 s
[INFO] Finished at: 2015-09-22T15:37:04+08:00
[INFO] Final Memory: 16M/156M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce (enforce-versions) on project stock: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
其实执行命令 mvn enforcer:enforce 也可以达到上面的效果。区别就是validate是maven全局命令，enforcer:enforce是只执行这个插件的命令。
这里要注意一点，执行enforcer:enforce时，id必须是default-cli（具体原因请参阅:http://stackoverflow.com/questions/24827194/maven-enforcer-plugin-missing-or-invalid-rules）,否则会报
The parameters 'rules' for goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce are missing or invalid
3.Maven Enforcer plugin标签详解
现在对这个插件的配置讲解一下。
<plugin> <artifactId>maven-enforcer-plugin</artifactId> <version>1.4.1</version> <executions> <execution> <id>default-cli</id> －－一个执行实例的id <goals> <goal>enforce</goal> －－执行的命令 </goals> <phase>validate</phase> －－执行的阶段 <configuration> <rules> －－规则 <requireJavaVersion> －－JDK的版本 <message> －－失败后提示消息 <![CDATA[You are running an older version of Java. This application requires at least JDK ${java.version}.]]> </message> <version>[${java.version}.0,)</version> －－JDK版本规则 </requireJavaVersion> </rules> </configuration> </execution> </executions> </plugin>
Enforcer的rules除了控制JDK版本，还有很多其他规则，甚至可以通过它的接口自定义。
- alwaysFail - Always fail... used to test plugin configuration.
- alwaysPass - Always passes... used to test plugin configuration.
- banDistributionManagement - enforces that project doesn't have distributionManagement.
- bannedDependencies - enforces that excluded dependencies aren't included.
- bannedPlugins - enforces that specific plugins aren't included in the build.
- bannedRepositories - enforces to not include banned repositories.
- banTransitiveDependencies - enforces that project doesn't have transitive dependencies.
- dependencyConvergence - ensure all dependencies converge to the same version.
- evaluateBeanshell - evaluates a beanshell script.
- reactorModuleConvergence - enforces that a multi module build follows best practice.
- requireActiveProfile - enforces one or more active profiles.
- requireEnvironmentVariable - enforces the existence of an environment variable
- requireFilesDontExist - enforces that the list of files does not exist.
- requireFilesExist - enforces that the list of files does exist.
- requireFilesSize - enforces that the list of files exists and is within a certain size range.
- requireJavaVersion - enforces the JDK version.
- requireMavenVersion - enforces the Maven version.
- requireNoRepositories - enforces to not include repositories.
- requireOS - enforces the OS / CPU Architecture.
- requirePluginVersions - enforces that all plugins have a specified version.
- requirePrerequisite - enforces that prerequisites have been specified.
- requireProperty - enforces the existence and values of properties.
- requireReleaseDeps - enforces that no snapshots are included as dependencies.
- requireReleaseVersion - enforces that the artifact is not a snapshot.
- requireSameVersions - enforces that specific dependencies and/or plugins have the same version.
- requireUpperBoundDeps - ensures that every (transitive) dependency is resolved to it's specified version or higher.
You may also create and inject your own custom rules by following the maven-enforcer-rule-api instructions.
我们再写一个例子。比如除了需要限制JDK外，我们还要限定maven版本，项目不得包含TestNG依赖，操作系统必须是mac os x 64位,项目版本号在执行install时必须是正式版本。
直接上代码：
<?xml version="1.0" encoding="UTF-8"?> <project> ... <build> <plugins> <plugin> <artifactId>maven-enforcer-plugin</artifactId> <version>1.4.1</version> <executions> <execution> <id>default-cli</id> <goals> <goal>display-info</goal> <goal>enforce</goal> </goals> <phase>validate</phase> <configuration> <rules> <requireMavenVersion> <message> <![CDATA[You are running an older version of Maven. This application requires at least Maven ${maven.version}.]]> </message> <version>[${maven.version},)</version> </requireMavenVersion> <requireJavaVersion> <message> <![CDATA[You are running an older version of Java. This application requires at least JDK ${java.version}.]]> </message> <version>[${java.version}.0,)</version> </requireJavaVersion> <bannedDependencies> <!--是否检查传递性依赖(间接依赖)--> <searchTransitive>true</searchTransitive> <excludes> <!--groupId[:artifactId][:version][:type][:scope][:classifier]-->
<exclude>org.testng:testng</exclude> </excludes> <message>don't use TestNG,must use JUnit</message> </bannedDependencies> <requireOS> <name>mac os x</name> <family>mac</family> <arch>x86_64</arch> <version>10.10.3</version> </requireOS> <requireProperty> <property>project.version</property> <message>"Project version must be specified."</message> <regex>.*(\d|-SNAPSHOT)$</regex> <regexMessage>"Project version must end in a number or -SNAPSHOT."</regexMessage> </requireProperty> </rules> </configuration> </execution> <execution> <id>enforce-install</id> <goals> <goal>enforce</goal> </goals> <phase>install</phase> <configuration> <rules> <requireProperty> <property>project.version</property> <message>"Project version must be specified."</message> <regex>.*(\d)$</regex> <regexMessage>"Project version must end in a number."</regexMessage> </requireProperty> </rules> </configuration> </execution> </executions> </plugin> </plugins> </build> <dependencies> ... <!--测试放开--> <!--<dependency>--> <!--<groupId>org.testng</groupId>--> <!--<artifactId>testng</artifactId>--> <!--<version>6.1.1</version>--> <!--<scope>test</scope>--> <!--</dependency>--> </dependencies> <properties> <java.version>1.7</java.version> <maven.version>3.3.3</maven.version> </properties> </project>
这里要说一下requireOS了，不支持表达式，必须准确的写一个版本号，个人认为这个不好，不过可以把version注释掉，不校验操作系统的版本。
如果某些情况下不检查环境，可以在maven命令上加一个 -Denforcer.skip=true 来跳过enforcer插件执行。
例如:mvn clean validate -Denforcer.skip=true
更多神奇的配置方式还请参阅官方网站 ：http://maven.apache.org/enforcer/maven-enforcer-plugin/index.html。