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
private void buildProjects(IProgressMonitor progressMonitor) {
		Set builtProjects= new HashSet(10);
		Object[] elements= fJarPackage.getElements();
		for (int i= 0; i < elements.length; i++) {
			IProject project= null;
			Object element= elements[i];
			if (element instanceof IResource)
				project= ((IResource)element).getProject();
			else if (element instanceof IJavaElement)
				project= ((IJavaElement)element).getJavaProject().getProject();
			if (project != null && !builtProjects.contains(project)) {
				try {
					project.build(IncrementalProjectBuilder.INCREMENTAL_BUILD, progressMonitor);
				} catch (CoreException ex) {
					String message= JarPackagerMessages.getFormattedString("JarFileExportOperation.errorDuringProjectBuild", project.getFullPath()); 
					addError(message, ex);
				} finally {
					
					builtProjects.add(project);
				}
			}
		}
	}
	/**
	 * Tells whether the given resource (or its children) have compile errors.
	 * The method acts on the current build state and does not recompile.
	 * 
	 * @param resource the resource to check for errors
	 * @return <code>true</code> if the resource (and its children) are error free
	 * @throws import org.eclipse.core.runtime.CoreException if there's a marker problem