ClassNotFoundException error when trying to run java project using maven

In order to execute a java project using maven, I put on the terminal this two commands:

To build project:

mvn package

To run project:

mvn exec:java

The build always execute with success, but every time I try to run the project, I receive this error:

java.lang.ClassNotFoundException: com.pipa.api.Application
    at java.net.URLClassLoader.findClass (URLClassLoader.java:471)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:588)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:521)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:281)
    at java.lang.Thread.run (Thread.java:834)

Do you know what may be happening?

This is my Application.java file, with main function inside

package com.pipa.api;

import com.pipa.api.handlers.FetchUserPositionHandler;
import com.pipa.api.handlers.HighScoreHandler;
import com.pipa.api.handlers.ScoreRegisterHandler;
import com.sun.net.httpserver.HttpServer;

import java.io.IOException;
import java.net.InetSocketAddress;

public class Application {

    public static void main(String[] args) throws IOException {
        int serverPort = 8000;
        HttpServer server = HttpServer.create(new InetSocketAddress(serverPort), 0);

        server.createContext("/", new FetchUserPositionHandler());

        server.createContext("/highscorelist", new HighScoreHandler());

        server.createContext("/score", new ScoreRegisterHandler());

        server.setExecutor(null);
        server.start();
    }
}

This is my pom.xml

<groupId>com.pipa.httpserver</groupId>
    <artifactId>pipa</artifactId>
    <version>1.0-SNAPSHOT</version>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.4.0</version>
                <configuration>
                    <mainClass>com.pipa.api.Application</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

I finally did this works. I discover that I need to put main / java right after src, on my project folder structure, following the pattern that maven uses. I did not notice, but even that my build command was working, my .jar file was been generated empty.