Using C struct in Java

Question

I have to code a Java program that will receive messages from network and display their content to the user. The problem is that the messages that I receive are simply binar

Answer 1

There is a library called Preon that was designed to help you with this type of task: Preon site Basically, they try to keep all the logic for reading your pojo's from the binary stream in annotations tied to each field in your pojo.

An example from their docs where you control the size of the int you are reading:

class Rectangle
{
  @BoundNumber(size="16") private int x1;
  @BoundNumber(size="16") private int y1;
  @BoundNumber(size="16") private int x2;
  @BoundNumber(size="16") private int y2;
}

or to specify endianness:

class Rectangle
{
  @BoundNumber(byteOrder=LittleEndian) private int x1;
  @BoundNumber(byteOrder=LittleEndian) private int y1;
  @BoundNumber(byteOrder=LittleEndian) private int x2;  
  @BoundNumber(byteOrder=LittleEndian) private int y2;
}

You can even use mini-equations with references to values in previous fields to specify size / length, etc.

@BoundList(size="width * height") byte[] pixels;
@BoundNumber(size="nrBits * 2") int value;

Oh, and they also offer conditional logic, all in annotations.

Answer 2

There are several libraries that help in this area. One of the simplest to use (annotation driver) is certainly Preon

Answer 3

java.nio has ByteBuffer, which supports flipping byte order when reading and writing, on-the-fly if necessary.

Answer 4

Swig will handle a lot of the tedious repetitive work for you in terms of mapping the C structs to Java objects. Check out the Swig/Java manual and the entry on wrapping C structures.

Answer 5

You can use DataInputStream to load data from any InputStream. As long as you know the exact layout of your structs, this should be sufficient.