Bug 39720 Adding external jar to the Build Path of an existing Java project [build path]
When adding an external jar file to an existing Java project, the "Setting Build Path" error dialog is displayed. The dialog displays the text "An error occured while setting the build path. Reason: /usr does not exist". The jar to be added in this case is located in the /usr/local/bin/ directory. I am working with the release 2.1 build for linux.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/CPListElement.java
IPath path= curr.getPath();
		IWorkspaceRoot root= project.getProject().getWorkspace().getRoot();
		
		IResource res= null;
		boolean isMissing= false;
		URL javaDocLocation= null;
		switch (curr.getEntryKind()) {
			case IClasspathEntry.CPE_CONTAINER:
				res= null;
				try {
					isMissing= (JavaCore.getClasspathContainer(path, project) == null);
				} catch (JavaModelException e) {
					isMissing= true;
				}
				break;
			case IClasspathEntry.CPE_VARIABLE:
				IPath resolvedPath= JavaCore.getResolvedVariablePath(path);
				res= null;
				isMissing=  root.findMember(resolvedPath) == null && !resolvedPath.toFile().isFile();
				javaDocLocation= JavaUI.getLibraryJavadocLocation(resolvedPath);
				break;
			case IClasspathEntry.CPE_LIBRARY:
				res= root.findMember(path);
				if (res == null) {
					if (!ArchiveFileFilter.isArchivePath(path)) {
						if (root.getWorkspace().validatePath(path.toString(), IResource.FOLDER).isOK()) {
							res= root.getFolder(path);
						}
					}
					isMissing= !path.toFile().isFile();