Maven Error: Could not find or load main class

Question

I'm using a Java Maven program and I don't know what to enter as the <mainClass>. I've tried all kinds of things based off of numerous stackoverflow questions, but they are not solving the error.

Each time it says:

Maven Error: Could not find or load main class ...

I have this written inside my pom.xml (minus the ???)

  <build>
  ...
  <plugins>
  ...
    <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>2.5.3</version>
        <configuration>
            <descriptors>
                <descriptor>src/main/assembly/jar-with-dependencies.xml</descriptor>
            </descriptors>
            <archive>
                <manifest>
                    <mainClass> ??? </mainClass>
                </manifest>
            </archive>
        </configuration>
        <executions>
            <execution>
            <id>make-assembly</id>
            <phase>package</phase>
            <goals>
                <goal>single</goal>
            </goals>
            </execution>
        </executions>
    </plugin>
  ...
  </plugins>
  ...
  </build>

How do I fix these errors?

Answer 1

I got it too, for me the problem got resolved after deleting the m2 folder (C:\Users\username.m2) and updating the maven project.

Answer 2

The first thing i would suggest is to use the correct configuration for predefined descriptors.

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>2.5.3</version>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
        </configuration>
        [...]
</project>

To configure the main class you need to know the package and name of the class you would like to use which should be given into <mainClass>...</mainClass> parameter.

Furthermore i recommend to stop using Maven 2 and move to Maven 3 instead.

Answer 3

For me the problem was nothing to do with Maven but to do with how I was running the .jar. I wrote some code and packaged it as a .jar with Maven. I ran it with

java target/gs-maven-0.1.0.jar

and got the error in the OP. Actually you need the -jar option:

java -jar target/gs-maven-0.1.0.jar

Answer 4

add this to your pom.xml file:

<configuration>
   <transformers>
       <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
       <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
         <mainClass>sample.HelloWorldApplication</mainClass>
       </transformer>
   </transformers>
</configuration>

and add the class name of your project (full path) along with the package name like "com.packageName.className" which consists of the main method having "run" method in it. And instead of your "???" write ${mainClass} which will automatically get the className which you have mentioned above.

Then try command mvn clean install and mvn -jar "jar_file_name.jar" server "yaml_file_name.yml"

I hope it will work normally and server will start at the specified port.

Answer 5

I got this error using Maven, and I discovered the solution.

Error: Could not find or load main class com.mycompany.testapifactory.Main

I'm using java JDK version 1.7 on Linux, my pom.xml file was the default generated by Netbeans and I was using these commands to compile, which do work fine with a normal hello-world java application:

mvn clean compile
java -jar target/TestAPIFactory-1.0-SNAPSHOT.jar com.mycompany.testapifactory.Main

What happened:

It turns out my problem was that my Main method was extending something Exotic like this:

public class Main extends SomeExoticLibraryClass{
    public static void main(String[] args){
        //...
    }
}

It was this extending of the main class that caused the above error.

TLDR solution:

Make sure your main class isn't extending any 3rd party classes. Refactor those out and away into their own classes. That error message is awful, and requires process of elimination to find out what to do.

Answer 6

specify the main class location in pom under plugins

<build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <archive>
                        <index>true</index>
                        <manifest>
                            <mainClass>com.example.hadoop.wordCount.WordCountApp</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>