Infatti
Ho risolto in questo modo
codice:
DataInputStream is=new DataInputStream(jar.getInputStream(jar.getEntry(nome)));
                        DataOutputStream oos=new DataOutputStream(new FileOutputStream(tmpFile));
byte read[]=new byte[1];
while(is.read(read)!=-1)
   oos.write(read);
oos.flush();
is.close();
oos.close();