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
IFile file= (IFile)iter.next();
					if (!resource.isLocal(IResource.DEPTH_ZERO))						
						file.setLocal(true , IResource.DEPTH_ZERO, progressMonitor);
					IPath classFilePath= baseDestinationPath.append(file.getName());
					progressMonitor.subTask(JarPackagerMessages.getFormattedString("JarFileExportOperation.exporting", classFilePath.toString())); 
					fJarWriter.write(file, classFilePath);
				}
			} catch (CoreException ex) {
				addToStatus(ex);
			}
		}
	}
	/**
	 * Exports the resources as specified by the JAR package.
	 */
	protected void exportSelectedElements(IProgressMonitor progressMonitor) throws InterruptedException {
		fExportedClassContainers= new HashSet(10);
		int n= fJarPackage.getElements().length;
		for (int i= 0; i < n; i++)
			exportElement(fJarPackage.getElements()[i], progressMonitor);
	}
	/**
	 * Returns an iterator on a list with files that correspond to the
	 * passed file and that are on the classpath of its project.
	 *
	 * @param	file			the file for which to find the corresponding classpath resources
	 * @param	pathInJar		the path that the file has in the JAR (i.e. project and source folder segments removed)
	 * @param	javaProject		the javaProject that contains the file
	 * @return	the iterator over the corresponding classpath files for the given file
	 */