[Explicit]

Nice, mine differs slightly:

Code:

public static void dumpJar(List<ClassNode> classes, Manifest manifest, String absolutePath) {
   try (JarOutputStream jos = new JarOutputStream(new FileOutputStream(new File(absolutePath)), manifest)) {
       for (ClassNode cn : classes) {
           ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);

           jos.putNextEntry(new JarEntry(cn.name + ".class"));
           cn.accept(writer);
           jos.write(writer.toByteArray());
       }
   } catch (IOException e) {
       e.printStackTrace();
   }
}

Gotta have a manifest to make it runnable xD