Java and Binary data in the context of sockets

Question

Java newbie here. Are there any helper functions to serialize data in and out of byte arrays? I am writing a Java package that implements a network protocol. So I have to write some typical variables like a version (1byte), sequence Number (long) and binary data (bytes) in a loop. How do I do this in Java? Coming from C I am thinking of creating a byte array of the required size and then since there is no memcpy() I am converting the long into a temporary byte array and then copying it into the actual byte array. It seems so inefficient and also really error prone. Is there a class I could use to marshall and unmarshall parameters to a byte array?

Also why does all the Socket classes only deals with char[] and not byte[]? A socket by definition has to deal with binary data also. How is this done in Java?

I am sure what I am missing is the Java mindset. Appreciate it if some one can point it to me.

EDIT: I did look at DataOutputStream and DataInputStream but I cannot convert the bytes to a String not to a byte[] which means the information might be lost in the conversion to write to a socket.

• Pav

Answer 1

Have a look at DataInputStream, DataOutputStream, ObjectInputStream and ObjectOutputStream. Check first if the layout of the data is acceptable to you. Also, Serialization.

Answer 2

Sockets neither deal with char[] nor with byte[] but with InputStream and OutputStream which are used to read and write bytes.

Answer 3

If you are sending the data over a socket, then you don't need a temporary byte array at all; you can wrap the socket's OutputStream with DataOutputStream or ObjectOutputStream and just write what you want to write.

There might be an aspect I've missed that means you do actually need temporary byte arrays. If so, look at ByteArrayOutputStream. Also, there's no memcpy(), sure, but there is System.arraycopy.

Answer 4

As above, DataInputStream and DataOutputStream are exactly what you are looking for. Re your comment about String, if you're planning to use Java Strings over the wire, you're not designing a network protocol, youre designing a Java protocol. There are readUTF() and writeUTF() if you're sure the other end is Java or if you can code the other end to understand these formats. Or you can send as bytes along with the appropriate charset, or predefine the charset for the entire protocol if that makes sense.