How to re-sign an existing android system app after modifing it

 as the mm will automatic sign the apk files with a debug signing .so i can't sign apk file created by use the mm command ..and I find I can't build the android system project immediately with ant because the hide api of googles ..at the end ..I solve this problem with two steps.

 first I build the modified source code(android source code imported as a java project) and then i use the ant to build it with the template ant tasks provide by android sdk (android ant build template task file is in <SDK>/platforms/<target_platform>/templates/android_rules.xml .and then i commented the task compile content :javac and just copied the classes build by eclipse to the destination fold the bin folder..