Bug 103927 [build path] External jar Dialog cannot set both Javadoc Location and Java Source Attachment in one iteration
Add an external jar to the project. From the context menu of the jar select properties. Select Javadoc location, browse to the desired location. Select Java Source Attachment. Browse to an external file. Press OK. From the context menu of the jar select properties again. Javadoc location is empty. (At this point it can be entered again without problems)
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/BuildPathSupport.java
public static final String JRE_PREF_PAGE_ID= "org.eclipse.jdt.debug.ui.preferences.VMPreferencePage"; 
	
	private BuildPathSupport() {
		super();
	}
	
	/**
	 * Finds a source attachment for a new archive in the existing classpaths.
	 * @param elem The new classpath entry
	 * @return A path to be taken for the source attachment or <code>null</code>
	 */
	public static IPath guessSourceAttachment(CPListElement elem) {
		if (elem.getEntryKind() == IClasspathEntry.CPE_CONTAINER) {
			return null;
		}
		IJavaProject currProject= elem.getJavaProject(); 
		try {
			
			IJavaModel jmodel= JavaCore.create(ResourcesPlugin.getWorkspace().getRoot());
			IJavaProject[] jprojects= jmodel.getJavaProjects();
			for (int i= 0; i < jprojects.length; i++) {