what do you mean by open it? to run it you open it in your browser and it should run with the java plug-in fine. or install the JRE and that should run it.
but if by "open" you mean, see what the source code is, then that's not possible. a .jar file is made when the source code is compiled and it is now in machine code and can't be read like source code can be. sometimes you can "decompile" it but not always
a bit more detail and can possibly help you figure it out.