Either json or Soap to exchange data in my project?
Depending on your needs, you could use both.  For example, using XML bindings you get the (de)serialization of the data going across the wire for free.  That is, if you're going to be POSTing lots of data to your web-service, and want to avoid calling the equivalent of "request.getParameter" for each parameter and building your own objects and creating/registering different servlets for each endpoing, the bindings can save in development time.  And for the response, you can have the payload be defined as a String and return JSON text, which gives you the benefits of that compact, javascript-friendly of that notation.
I am doing a car rental software, in which a front end back end are there, where the back end will do the accounting part. I have to send some data like customer name, amount, currency etc. to account engine to prepare the ledgers. I am confused whether to use json or soap for information exchange between front and back ends. ur suggestions are precious. thank u..