Bug 26699 Create Jar fails with no warning if jar read-only
Once *.jardesc has been created, if I invoke "Create JAR" from the context menu and the jar file already exists and is read-only, the progress dialog flashes and nothing more happens. Please add check for this condition and at least an error message should be displayed. Even better, ask something like "Jar file read-only, overwrite?" Y/N and overwrite the file on yes (similar to what the option "warn if jar file exists" does).
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/jarpackager/JarFileExportOperation.java
if (isInJavaProject) {
				try {
					isInOutputFolder= jProject.getOutputLocation().isPrefixOf(resource.getFullPath());
				} catch (JavaModelException ex) {
					isInOutputFolder= false;
				}
			}
			
			exportClassFiles(progressMonitor, pkgRoot, resource, jProject, destinationPath);
			exportResource(progressMonitor, pkgRoot, isInJavaProject, resource, destinationPath, isInOutputFolder);
			progressMonitor.worked(1);
			ModalContext.checkCanceled(progressMonitor);
		} else if (element instanceof IPackageFragment)
			exportPackageFragment(progressMonitor, (IPackageFragment)element);
		else
			exportContainer(progressMonitor, (IContainer)resource);
	}
	private void exportPackageFragment(IProgressMonitor progressMonitor, IPackageFragment pkgFragment) throws java.lang.InterruptedException {
		Object[] children;
		try {
			children= pkgFragment.getChildren();
			for (int i= 0; i < children.length; i++)
				exportElement(children[i], progressMonitor);
			children= pkgFragment.getNonJavaResources();
			for (int i= 0; i < children.length; i++)
				exportElement(children[i], progressMonitor);
		} catch (CoreException e) {
			
			addWarning(JarPackagerMessages.getFormattedString("JarFileExportOperation.errorDuringExport", pkgFragment.toString()), e); 
		}