What is the preferred way to transfer objects between server and client?

Good afternoon. I am developing a server-client application with Glassfish + ObjectDB (native mode) on the server and an Android application on the client side. What is the prefreable way (in terms of traffic and security) for sending data stored in Java objects in ObjectDB to an Android app? (Data must be encrypted.) I am thinking about:

• transmit serializable objects through an input / output stream.
• transmit data in XML / JSON format.

Or maybe another way?

thanks for reference.