套接字Java将对象发送到C ++服务器

Question

I am using sockets and I have one c++ server and one java client.

My c++ server is waiting for a c++ struct like this one:

struct Testing{

    double  a;
    double  b;
    double  c;

};

And my server code looks like this:

int recvStruct(int fd)
{
    struct Testing test;
    int ibytes = sizeof(struct Testing);

    if (ibytes!=read(fd,&test,ibytes))
    {
        printf("Error read\r\n");
        return 0;
    }
    else
    {
        dosomething();

Then in my java client, i created one class:

public class Testing{
    double a,b,c;

public Testing(double a, double b, double c){
    this.a = a;
    this.b = b;
    this.c = c;
    }
}

and for the last, my client java code is this one:

try {
        Socket socket = new Socket("ipaddress", 12345);

        OutputStream os= socket.getOutputStream();
        Testing object = new Object(1,2,3);
        os.writeObject(object);
        socket.close();

 } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        /* e.printStackTrace();
        response = "UnknownHostException: " + e.toString();*/
 } catch (IOException e) {
        // TODO Auto-generated catch block
        /*e.printStackTrace();
        response = "IOException: " + e.toString();*/
 } finally {
        if (socket != null) {
             try {
                 socket.close();
             } catch (IOException e) {
                 // TODO Auto-generated catch block
                /* e.printStackTrace();*/
             }
 }

My problem is that i always get that error read message.

Thank you.

Answer 1

Your code just assumes that your C++ Testing class consists of the exact same number of bytes, in the exact same order, as the result of Java's OutputStream.writeObject function.

OutputStream.writeObject isn't even a function to serialise the component bytes of its members; it does much more than that :

The class of the object, the signature of the class, and the values of the non-transient and non-static fields of the class and all of its supertypes are written.

In conclusion, given that there is no relationship between the two whatsoever, success is extremely unlikely.

Instead, serialise to your own known format, then deserialise into the language structure you require. Research the bolded terms for more information.

Answer 2

If your struct is a sequence of 3 double, assuming in C++ a double is 8 byte as in Java, you should send a sequence of 24 bytes. To convert each double I would use:

long bits = Double.doubleToLongBits(someDouble);
byte[] bytes = new byte[8];
for(int offset = 0, shift = 7*8; 
        offset < bytes.length; 
        offset++, shift-=8)
        bytes[offset] = (byte)(bits >>> shift);