How can in run my spring boot application with "java -cp example.jar com.example.MyMainClass"

Question

I tried to change the Spring Boot class loader.. but it is not working, only print this message:

Error: Could not find or load main class com.example.MyMainClass

I have tried this solution: https://github.com/dkublik/spring-boot-loader-play

Answer 1

If you are using Spring Boot 2.1.5.RELEASE, the following should work:

java -cp "yourfile.jar" org.springframework.boot.loader.JarLauncher

Please, note that the spring-boot-maven-plugin repackages the original jar file at maven's package build phase, which results in a jar with a different internal file structure. In this new/repackaged jar, you can find the classes from your project inside BOOT-INF/classes/, which differs from the original jar, where you find them in the first level of the file structure. The plugin keeps the original jar file as yourfile.jar.original inside the target folder.

Also, i suggest to take a look at the file META-INF/MANIFEST.MF inside the repackaged jar, which reflects the description above. If you open this file you should see something similar to the following:

Manifest-Version: 1.0
Implementation-Title: yourproject
Implementation-Version: 1.0.0-SNAPSHOT
Start-Class: com.example.MyMainClass
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Build-Jdk-Spec: 1.8
Spring-Boot-Version: 2.1.5.RELEASE
Created-By: Maven Archiver 3.4.0
Main-Class: org.springframework.boot.loader.JarLauncher

Please, note that the value of the Main-Class header from repackaged jars will always be org.springframework.boot.loader.JarLauncher, defining this as the entry point of your jar. Later on, Spring will look for the Start-Class header, thus loading your project through IoC (Inversion of Control).

For more info:

Packaging Programs in JAR Files

Spring Boot Maven Plugin

Inversion of Control and Dependency Injection in Spring

Answer 2

Try to create a fat jar. This way you can run your application with

java -cp example.jar

<plugin>
 <groupId>org.apache.maven.plugins</groupId>
 <artifactId>maven-shade-plugin</artifactId>
   <executions>
      <execution>
       <phase>package</phase>
        <goals>
          <goal>shade</goal>
         </goals>
         </execution>
        </executions>
       <configuration>
          <finalName>example</finalName>
      </configuration>
 </plugin>

Answer 3

Wouldn't:

java -jar application.jar

work for you?