Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class

【背景】spring-boot项目,打包成可执行jar,项目内有两个带有main方法的类并且都使用了@SpringBootApplication注解(或者另一种情形:你有两个main方法并且所在类都没有使用@SpringBootApplication注解),pom.xml如下

Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<version>1.5.3.RELEASE</version>
<executions>
<execution>
<goals>
<goal>repackage</goal>
</goals>
</execution>
</executions>
</plugin>
Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class

【问题】

  • 执行mvn clean package,报错如下(说点不相关的,使用install同理。因为spring-boot:repackage目标(goal)(下文会说)被绑定在package构建阶段(phases),而package阶段在install阶段之前,指定构建阶段之前的阶段都会执行。详细参见:Introduction to the Build Lifecycle

  [ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:1.5.3.RELEASE:repackage (default) on project webapps-api-bid: Execution default of goal org.springframework.boot:spring-boot-maven-plugin:1.5.3.RELEASE:repackage failed: Unable to find a single main class from the following candidates [com.xx.api.main.ApiBidMain, com.xx.webapps.api.main.WebappsApiBidMain]

  • 执行mvn clean package spring-boot:repackage,报错如下,不如上面日志详细

  [ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:1.5.3.RELEASE:repackage (default) on project webapps-api-bid: Execution default of goal org.springframework.boot:spring-boot-maven-plugin:1.5.3.RELEASE:repackage failed: Unable to find main class

【解决】

  Note:参考官网描述,没有指定<mainClass>或者继承了spring-boot-starter-parent并且<start-class>属性未配置时,会自动寻找签名是public static void main(String[] args)的方法... 所以插件懵逼了,两个妹子和谁在一起呢...

  • [推荐] 通用解决方法<configuration>下配置mainClass,指定程序入口。
Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<version>1.5.3.RELEASE</version>
<configuration>
<mainClass>com.xx.webapps.api.main.WebappsApiBidMain</mainClass>
</configuration>
<executions>
<execution>
<goals>
<goal>repackage</goal>
</goals>
</execution>
</executions>
</plugin>
Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class

  Spring Boot Maven Plugin提供了几个目标(goal),我们在<executions>标签里配置的<goal>repackage</goal>对应spring-boot:repackage这个目标。

  • repackage: create a jar or war file that is auto-executable. It can replace the regular artifact or can be attached to the build lifecyle with a separate classifier.
  • run: run your Spring Boot application with several options to pass parameters to it.
  • start and stop: integrate your Spring Boot application to the integration-test phase so that the application starts before it.

  The plugin rewrites your manifest, and in particular it manages the Main-Class and Start-Classentries, so if the defaults don't work you have to configure those there (not in the jar plugin). The Main-Class in the manifest is actually controlled by the layout property of the boot plugin

  [译] 该插件重写了清单文件(MANIFEST.MF,也就是jar里面的清单文件),此文件管理着主类(Main-Class)和开始类(Start-Class)入口。清单文件中的Main-Class由layout控制

  这里的Start-Class就是我们配置的<mainClass>,而Main-Class受layout属性的控制,别被名字搞乱了(是不是很诡异?看看解决方法二就明白为啥如此诡异了).... 来张图直观的感受下,对应使用上面xml配置打包后的清单文件(MANIFEST.MF):

  Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class

  layout属性默认不需要配置,插件会自动推断。不同的layout属性清单文件里面的Main-Class也会相应的不同。比如layout不配置或者配置为JAR对应的Main-Class是JarLauncher,layout配置为WAR对应的Main-Class是WarLauncher。

  • [有限制条件] 解决方法二如果你的pom继承自spring-boot-starter-parent(注意此前提),也可以直接在<properties>配置<start-class>(其实这里的start-class直接对应清单文件里的Start-Class):
<properties>
<start-class>com.xx.webapps.api.main.WebappsApiBidMain</start-class>
</properties>
  • 解决方法三:打包的的时候注释掉其他的@SpringBootApplication... 或者你有两处main方法并且都没有使用@SpringBootApplication注解,注释掉一个main方法..... 这就是第三种解决方法233333

【随便说说】

  说说spring-boot:repackage这个目标。Spring Boot Maven Plugin这个插件包含一系列目标(goal),我们在<executions>标签里配置的<goal>repackage</goal>对应spring-boot:repackage这个目标,看下官方介绍

  spring-boot:repackage repackages your jar/war to be executable.

  Repackages existing JAR and WAR archives so that they can be executed from the command line using java -jar. With layout=NONE can also be used simply to package a JAR with nested dependencies (and no main class, so not executable).

  简单点说,这货重新打包个可执行的jar/war,可以在命令行使用-jar执行。如果指定layout为NONE那就没有主类只是打个普通的jar(不可执行),一般不会这么做。

  一般情况,这个目标会打一个新的jar/war,并把maven默认打的jar/war添加.original后缀,在target目录下可以看到:

  Spring Boot Maven Plugin打包异常及三种解决方法:Unable to find main class

【参考】

1.https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#build-tool-plugins-maven-packaging

2.https://docs.spring.io/spring-boot/docs/2.0.0.BUILD-SNAPSHOT/maven-plugin//repackage-mojo.html

3.https://*.com/questions/23217002/how-do-i-tell-spring-boot-which-main-class-to-use-for-the-executable-jar

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