Bug 13799 JAR exporter exports class files from output folder
There was a problem with the output folder not being detected as such.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/jarpackager/JarFileExportOperation.java
} finally {
			try {
				if (fJarWriter != null)
					fJarWriter.close();
			} catch (IOException ex) {
				String message= JarPackagerMessages.getFormattedString("JarFileExportOperation.unableToCloseJarFile", ex.getLocalizedMessage()); 
				addError(message, ex);
				throw new InvocationTargetException(ex, message);
			}
			progressMonitor.done();
		}
	}
	
	protected boolean preconditionsOK() {
		if (!fJarPackage.areClassFilesExported() && !fJarPackage.areJavaFilesExported()) {
			addError(JarPackagerMessages.getString("JarFileExportOperation.noExportTypeChosen"), null); 
			return false;
		}
		if (fJarPackage.getElements() == null || fJarPackage.getElements().length == 0) {
			addError(JarPackagerMessages.getString("JarFileExportOperation.noResourcesSelected"), null); 
			return false;
		}
		if (fJarPackage.getJarLocation() == null) {
			addError(JarPackagerMessages.getString("JarFileExportOperation.invalidJarLocation"), null); 
			return false;
		}
		if (!fJarPackage.isManifestAccessible()) {
			addError(JarPackagerMessages.getString("JarFileExportOperation.manifestDoesNotExist"), null); 
			return false;
		}