Problem

You are using Eclipse and making a Google App Engine(GAE) project, save changes to some file, and Eclipse barfs out the following error:

Class not found exception for com/google/appengine/tools/enhancer/Enhance when starting eclipse

Solution

The problem is likely that your project is not using JRE1.7 or above. To fix the problem:

  1. Install JRE1.7 following the steps from Installing Java Runtime Environment 1.7 (JRE 1.7)
  2. Ensure Eclipse is using JRE1.7 by following the steps at Updating Eclipse to use a specific Java Runtime Environment (JRE) by Default
  3. Ensure the specific project is using JRE1.7 by right-clicking the project in Eclipse -> Properties -> Follow the steps from Updating an Eclipse project to use a specific Java Runtime Environment (JRE)