Maven - error cannot find or load main class

Question

I tried to build this example : https://github.com/oltzen/JavaCppExample with Maven (mvn clean install) on Linux. After the successful build, when I run : java de.oltzen.javacppexample.Abc : it says 'could not load or find the main class'

The video tutorial (https://www.youtube.com/watch?v=LZrrqZLhtmw) uses Eclipse and it runs the program with Run as .. Java Application

Is the POM file missing something ?

I tried to add this plugin in POM but it did not work:

          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <mainClass>de.oltzen.javacppexample.Abc</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>

I am executing from JavaCppExample/target/classes/ :-

The /classes folder contains package folders : de/oltzen/javacppexample/ The last folder contains the class file Abc.class

So I run the command :
java de.oltzen.javacppexample.Abc

The /target folder contains : 1) classes [folder containing the package] 2) JavaCppExample.jar 3) maven-archiver 4) maven-status

Please help

Try cleaning the project couple of times in eclipse or restart the eclipse and then try again. — Smile, May 13 '20 at 04:56
@SamuelAudet No, just build and execute it. whether simply with main class or with Jar file — Nitin, May 13 '20 at 06:24
Please update the question with the folder in which you are running `java de.oltzen.javacppexample.Abc` and what all files are present in this folder. — Smile, May 13 '20 at 07:35
If you are able to generate a jar file without any error, then you use "java -jar /relative_path/fileName.jar" — YogendraR, May 13 '20 at 08:22
@YogendraR I have tried but same issue. I have executed from /target/classes/ folder — Nitin, May 13 '20 at 08:26
@YogendraR . I ran it from target folder. java -jar . It says no main manifest attribute, in JavaCppExample.jar — Nitin, May 13 '20 at 08:47
open your jar file and see if there is manifest.mf file that file should contain main class attribute then only it will work — YogendraR, May 13 '20 at 08:50
@YogendraR there is no manifest file. However I tried to specify it in the POM file but then it said :Exception in thread "main" java.lang.NoClassDefFoundError: org/bytedeco/javacpp/Pointer. My class is extending this third party JavaCPP class - Pointer — Nitin, May 13 '20 at 08:55
I didn't know that you are using third party libraries as well. To make it as a runnable jar, make it fat/uber jar. you can use maven-assembly-plugin, go through the maven docs — YogendraR, May 13 '20 at 09:00
Add the dependency to pom which has `org/bytedeco/javacpp/Pointer` class — Smile, May 13 '20 at 09:03

score 1 · Answer 1 · answered May 13 '20 at 11:10

I added the following plugins for maven copy dependencies and executed java -jar javaCppExample.jar [from /target folder]and it worked. Thanks everyone !

[Simply build using mvn clean install]

<plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <encoding>UTF-8</encoding>
                    <target>1.8</target>
                    <source>1.8</source>
                </configuration>
            </plugin>


            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <executions>
                    <execution>
                        <id>copy-dependencies</id>
                        <phase>prepare-package</phase>
                        <goals>
                            <goal>copy-dependencies</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>${project.build.directory}/lib/</outputDirectory>
                            <overWriteReleases>false</overWriteReleases>
                            <overWriteSnapshots>false</overWriteSnapshots>
                            <overWriteIfNewer>true</overWriteIfNewer>
                        </configuration>
                    </execution>
                </executions>
            </plugin>


            <plugin>
                <!--  Build an executable JAR -->
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.0.2</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <classpathPrefix>lib/</classpathPrefix>
                            <mainClass>de.oltzen.javacppexample.Abc</mainClass>
                        </manifest>
                    </archive>

                </configuration>
            </plugin>

Motivated Mind · Answer 2 · 2020-05-13T09:53:24.253

If you are only looking to run your prgramm via maven then use exec-maven-plugin.

          <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>1.6.0</version>
            <executions>
              <execution>
                <goals>
                  <goal>java</goal>
                </goals>
              </execution>
            </executions>
            <configuration>
              <mainClass>de.oltzen.javacppexample.Abc</mainClass>
            </configuration>
          </plugin>

Explore more here => https://www.mojohaus.org/exec-maven-plugin/index.html

If you want to build & run as jar then use assembly plugin

         <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-assembly-plugin</artifactId>
            <version>3.1.0</version>
            <configuration>
              <appendAssemblyId>true</appendAssemblyId>
              <descriptors>
                <descriptor>${project.basedir}/assembly/assembly.xml</descriptor>
              </descriptors>
              <descriptorRefs>
                <descriptorRef>jar-with-dependencies</descriptorRef>
              </descriptorRefs>
              <archive>
                <manifest>
                  <mainClass>de.oltzen.javacppexample.Abc</mainClass>
                </manifest>
              </archive>
            </configuration>
            <executions>
              <execution>
                <id>make-assembly</id>
                <phase>install</phase>
                <goals>
                  <goal>single</goal>
                </goals>
              </execution>
            </executions>
          </plugin>

Explore more about Maven assembly plugin to customize to your needs.

First solution by Adonis worked here: https://stackoverflow.com/questions/42470641/error-could-not-find-or-load-main-class-maven-project-out-of-eclipse — Nitin, May 13 '20 at 11:05

Maven - error cannot find or load main class

2 Answers2