Bug 28510 java build path inconsistency [build path]
20021216 select rt.jar in the package explorer open properties it allows you to set the javadoc location but for source attachment you are sent somewhere else it's inconsistent - on the java build path they both look sort of the same (they sit under the same node in the tree)
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/CPListElement.java
isMissing= !path.toFile().isFile(); 
				}
				javaDocLocation= JavaUI.getLibraryJavadocLocation(path);
				break;
			case IClasspathEntry.CPE_SOURCE:
				res= root.findMember(path);
				if (res == null) {
					if (root.getWorkspace().validatePath(path.toString(), IResource.FOLDER).isOK()) {
						res= root.getFolder(path);
					}
					isMissing= true;
				}
				break;
			case IClasspathEntry.CPE_PROJECT:
				res= root.findMember(path);
				isMissing= (res == null);
				break;
		}
		CPListElement elem= new CPListElement(project, curr.getEntryKind(), path, res);
		elem.setExported(curr.isExported());
		elem.setAttribute(SOURCEATTACHMENT, curr.getSourceAttachmentPath());
		elem.setAttribute(JAVADOC, javaDocLocation);
		elem.setAttribute(OUTPUT, curr.getOutputLocation());
		elem.setAttribute(EXCLUSION, curr.getExclusionPatterns()); 
		if (project.exists()) {
			elem.setIsMissing(isMissing);
		}
		return elem;
	}	
}