The manifest file

Here is a typical manifest file generated by the maven-jar-plugin-1.8: 

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.6.5
Created-By: 1.4.2_10-b03 (Sun Microsystems Inc.)
Built-By: lukas
Maven-Version: 1.1-beta-3-SNAPSHOT

Name: org/apache/maven
Extension-name: org.apache.maven
Specification-Title: Create jar files
Specification-Vendor: Apache Software Foundation
Specification-Version: 1.8
Implementation-Title: org.apache.maven
Implementation-Vendor: Apache Software Foundation
Implementation-Version: 1.8-SNAPSHOT

Note that the structure was different and probably not compatible in older versions of the plugin, see MPJAR-36.

Default attributes

AttributeDefault value
Created-By The java version used to generate the manifest file
Built-By ${user.name}
Maven-Version ${maven.application.version}
Class-Path See the maven.jar.manifest.classpath.add property.
Main-Class See the maven.jar.mainclass property.
Extension-List See the maven.jar.manifest.extensions.add property.
Name ${pom.package} with "." (dot) replaced by "/" (slash)
Extension-name ${pom.package}
Specification-Title ${pom.shortDescription} (trimmed to 46 characters)
Specification-Vendor ${pom.organization.name}
Specification-Version This is constructed from ${pom.currentVersion} in the following way: first any occurence of the string "-SNAPSHOT" is chopped off, the rest is then tokenized with a delimiter "." (dot), and up to three tokens are kept. Note that according to Sun's specification this must be a number to compare, so if your ${pom.currentVersion} contains any non-digit character (apart from "."), the resulting manifest will not be valid. You should then override this default (see below).
Implementation-Title ${pom.package}
Implementation-Vendor ${pom.organization.name}
Implementation-Version ${pom.currentVersion}

Overriding default values

Any of the above default values can be overridden by specifying a custom manifest file via the maven.jar.manifest property. Note that values in manifest sections (separated from the main section by a blank line) can only be overridden by also specifying the section name. For instance, even if you only want to override the Specification-Version in the example above, you need at least the following in your custom manifest: 

Name: org/apache/maven
Specification-Version: 1.8.0