Question

I have a java program that will send(over TCP) packets to a c++ program. Only two types of objects will be sent (struct A or struct B). I need to serialize the two objects and send them to the c++ program, such that when de-serialized inside the c++ program they will be of the type struct A or struct B, depending on what has been sent. How can I achieve this without any external java packages?

            struct A
            {
                unsigned int field1; 
                unsigned int field2; 
            }

            struct B
            {
                unsigned int length;
                struct A list[GLOBALLENGTH];
            }

GLOBALLENGTH is a static and globel int.

Please note that I dont need a general solution, as I will strictly be dealing with the mentioned types above. Also, I dont have access to the c++ program(it is a blackbox), both operate in little indian.

Answer 1

It sounds like the c++ program expects a raw dump of the data in each structure, with no padding, in little-endian order, with the fields in the declared order, using 32-bit integers. If all that’s right, here’s one way to write bytes for consumption by the c++ program:

private static final int A_SIZE = 8;
private static final int B_SIZE = 4 + GLOBALLENGTH * 8;

public byte[] writeA(A a) {
    byte[] bytes = new byte[A_SIZE];
    writeA(a, bytes, 0);
    return bytes;
}

private void writeA(A a, byte[] bytes, int offset) {
    ByteBuffer buff = ByteBuffer.wrap(bytes, offset, A_SIZE);
    buff.order(ByteOrder.LITTLE_ENDIAN);
    buff.putInt(a.field1).putInt(a.field2);
}

public byte[] writeB(B b) {
    byte[] bytes = new byte[B_SIZE];
    ByteBuffer buff = ByteBuffer.wrap(bytes, 0, 4);
    buff.order(ByteOrder.LITTLE_ENDIAN);
    buff.putInt(b.length);
    int offset = 4;
    for (A a : b.list) {
        writeA(a, bytes, offset);
        offset += A_SIZE;
    }
    return bytes;
}

You can then call the appropriate public method (writeA(A) or writeB(B)) and send the bytes to the server.