Java compiler level does not match the version of the installed Java project facet

I have created a New Dynamic Project under Eclipse Helios Version, where my JRE Version is set to 1.6. I have added Maven capabilities to the Web Application by clicking on Configure → Convert to Maven Project.

After adding this, a build error appeared in the Eclipse Problems view:

Java compiler level does not match the version of the installed Java project facet.
Unknown Faceted Project Problem (Java Version Mismatch)

Please tell me how to resolve this error (I want to have my JRE version as 1.6 only).

If your project is not a Maven project, right-click on your project and choose Properties to open the Project Properties dialog.

There is a Project Facets item on the left, select it, look for the Java facet on the list, choose which version you want to use for the project and apply.

Assuming that you are using the m2e plugin in Eclipse, you'll need to specify the source and target versions as 1.6 for maven-compiler-plugin. m2e uses these values to determine the project's Java compiler level. A snippet of the POM is shown below:

<build>
  <plugins>
    <plugin>
      <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>1.6</source>
          <target>1.6</target>
        </configuration>
    </plugin>
  </plugins>
</build>

Alternatively, you can specify the maven.compiler.source and maven.compiler.target properties with values of 1.6, that happen to be the equivalent:

<properties>
    <maven.compiler.target>1.6</maven.compiler.target>
    <maven.compiler.source>1.6</maven.compiler.source>
</properties>