java.lang.UnsupportedClassVersionError: com/sun/javadoc/Type : Unsupported major.minor version 51.0

java.lang.UnsupportedClassVersionError: com/sun/javadoc/Type : Unsupported major.minor version 51.0 - This exception will occur when you mess up with different java versions in ant build script.

eg: If Java 7 set in Windows / Linux environment variables (JAVA_HOME & PATH) but trying to build the project in eclipse which is configured to Java 6.

Resolutions:
1. Update environment variables to set Java 6
or
2. Add Java 7 to Eclipse's Installed JREs and select it.