After a fresh checkout of hadoop-common I do 'ant compile eclipse'.
I open eclipse, set ANT_HOME and build the project.
At that point the following error appears:
The type com.sun.javadoc.RootDoc cannot be resolved. It is indirectly referenced from required .class files ExcludePrivateAnnotationsJDiffDoclet.java /common/src/java/org/apache/hadoop/classification/tools line 1 Java Problem
The solution is to add the "tools.jar" from the JDK to the buildpath/classpath.
This should be fixed in the build.xml.