Users trying to convert Java archives (.jar) to Android packages (.apk) usually find it very difficult to actually achieve jar to apk conversion. Such conversion is usually attempted for the purpose to port old Java based game apps to Android.
Although you can find some web based jar to apk converters, this works only with Java apps that had been written with Android Java libraries in mind. As for older apps, these would need to be manually rewritten by a programmer, so they will use Android Java libraries instead the generic Java ones. This is needed mainly because Java for Android is somewhat limited and does not support all the stuff from desktop platforms.Open JAR file Open APK file
Online JAR to APK web service