Bug 123786 Following bug 109980, the compliance 6.0 should be added to the compiler's compliance list.
If the JRE/JDK used is a 1.6/6.0 VM, then the compliance 6.0 should be available. Compliance 1.6 implies source level 1.6 and target level 1.6.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/BuildPathSupport.java
IJavaProject currProject= elem.getJavaProject(); 
		try {
			
			IJavaModel jmodel= JavaCore.create(ResourcesPlugin.getWorkspace().getRoot());
			IJavaProject[] jprojects= jmodel.getJavaProjects();
			for (int i= 0; i < jprojects.length; i++) {
				IJavaProject curr= jprojects[i];
				if (!curr.equals(currProject)) {
					IClasspathEntry[] entries= curr.getRawClasspath();
					for (int k= 0; k < entries.length; k++) {
						IClasspathEntry entry= entries[k];
						if (entry.getEntryKind() == elem.getEntryKind() && entry.getPath().equals(elem.getPath())) {
							IClasspathAttribute[] attributes= entry.getExtraAttributes();
							for (int n= 0; n < attributes.length; n++) {
								IClasspathAttribute attrib= attributes[n];
								if (IClasspathAttribute.JAVADOC_LOCATION_ATTRIBUTE_NAME.equals(attrib.getName())) {
									return attrib.getValue();
								}
							}
						}
					}
				}
			}
		} catch (JavaModelException e) {
			JavaPlugin.log(e.getStatus());
		}
		return null;
	}
	
	private static class UpdatedClasspathContainer implements IClasspathContainer {