Maven – Project import error when using Eclipse

<build>
  <plugins>
    <plugin>
      <artifactId>maven-assembly-plugin</artifactId>
      <configuration>
        <archive>
          <manifest>
            <mainClass>fully.qualified.MainClass</mainClass>
          </manifest>
        </archive>
        <descriptorRefs>
          <descriptorRef>jar-with-dependencies</descriptorRef>
        </descriptorRefs>
      </configuration>
    </plugin>
  </plugins>
</build>

and you run it with

mvn clean compile assembly:single

Compile goal should be added before assembly:single or otherwise the code on your own project is not included.

See more details in comments.

Commonly this goal is tied to a build phase to execute automatically. This ensures the JAR is built when executing mvn install or performing a deployment/release.

<plugin>
  <artifactId>maven-assembly-plugin</artifactId>
  <configuration>
    <archive>
      <manifest>
        <mainClass>fully.qualified.MainClass</mainClass>
      </manifest>
    </archive>
    <descriptorRefs>
      <descriptorRef>jar-with-dependencies</descriptorRef>
    </descriptorRefs>
  </configuration>
  <executions>
    <execution>
      <id>make-assembly</id> <!-- this is used for inheritance merges -->
      <phase>package</phase> <!-- bind to the packaging phase -->
      <goals>
        <goal>single</goal>
      </goals>
    </execution>
  </executions>
</plugin>

I want to import existing maven project into eclipse. I found 2 ways to do it, one is through running from command line mvn eclipse:eclipse and another is to install maven eclipse plugin from eclipse. What is the difference between the both and which one is preferable?

The maven-eclipse-plugin is a Maven plugin and has always been there (one of the first plugin available with Maven 1, one of the first plugin migrated to Maven 2). It has been during a long time the only decent way to integrateimport an existing maven project with Eclipse. Actually, it doesn't provide real integration, it just generates the .project and .classpath files (it has also WTP support) from a Maven project. I've used this plugin during years and was very happy with it (and very unsatisfied at this time by Eclipse plugins for Maven like m2eclipse).

The m2eclipse plugin is one of the Eclipse plugins for Maven. It's actually the first and most mature of the projects aimed at integrating Maven within the Eclipse IDE (this has not always been the case, it was not really usable ~2 years ago, see the feedback in Mevenide vs. M2Eclipse, Q for Eclipse/IAM). But, even if I do not use things like creating a Maven project from Eclipse or the POM editor or other fancy wizards, I have to say that this plugin is now totally usable, provides very smooth integration, has nice features... In other words, I finally switched to it :) I'd now recommend it to any user (advanced or beginners).

If I install maven eclipse plugin through the eclipse menu Help -> Install New Software, do I still need to modify my pom.xml to include the maven eclipse plugin in the plugins section?

This question is a bit confusing but the answer is no. With the m2eclipse plugin installed, just right-click the package explorer and Import... > Maven projects to import an existing maven project into Eclipse.