Sending Data from iOS to java socket server

Question

Hi I have the next issue, why when I send data like this

uint8_t buffer[11] = "I send this";

NSInteger nwritten = [outputStream write:buffer maxLength:sizeof(buffer)];

if (-1 == nwritten) {
    NSLog(@"Error writing to stream %@: %@", outputStream, [outputStream streamError]);
}else{
    NSLog(@"Wrote %i bytes to stream %@.", nwritten, outputStream);
}

in the other side in the java socket server

connection = new Socket(Constants.HOST, Constants.CHAT_LISTENER_PORT);
_in = new DataInputStream(connection.getInputStream());
String data = _in.readUTF();

But nothing appear like as if it had not sent anything.

I read to much and I found the problem is from platforms because java byte works with big-endians and iOS with little-endians but I don't found information about how do to this.

uint8_t buffer[11] = "I send this"; to big-Endians format

Please heellppp, thanks.

Sorry my English is very but there is nothing in spanish :/ thaks.

Answer 1

On the server side in the next Line in java code:

DataInputStream.readUTF();

this .readUTF() expect a UTF java native type what is not the same with objective-c type, and the solution was send the string codifying in UTF native java like this.

NSString *msg = @"initChat_";
NSString *messageToSend  = [NSString stringWithFormat:@"%@", msg];
NSData *data = [self convertToJavaUTF8:messageToSend];

int dataLenght = [data length];

int num = [outputStream write:(const uint8_t *)[data bytes] maxLength:dataLenght];


if (-1 == num) {
    NSLog(@"Error writing to stream %@: %@", outputStream, [outputStream streamError]);
}else{
    NSLog(@"Wrote %i bytes to stream %@.", num, outputStream);
}

And the Magic come from:

- (NSData*) convertToJavaUTF8 : (NSString*) str {
NSUInteger len = [str lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
Byte buffer[2];    
buffer[0] = (0xff & (len >> 8));
buffer[1] = (0xff & len);
NSMutableData *outData = [NSMutableData dataWithCapacity:2];
[outData appendBytes:buffer length:2];        
[outData appendData:[str dataUsingEncoding:NSUTF8StringEncoding]];
return outData;}

Answer 2

At your request string end add \\n

For example uint8_t buffer[] = "I send this\\n" ,it may work.

Answer 3

The swift4 version of the answer.

extension String {
    func javaUTF8() -> Data? {
        guard let data = self.data(using: .utf8) else {
            return nil
        }
        let length = self.lengthOfBytes(using: .utf8)
        var buffer = [UInt8]()
        buffer.append(UInt8(0xff & (length >> 8)))
        buffer.append(UInt8(0xff & length))
        var outdata = Data()
        outdata.append(buffer, count: buffer.count)
        outdata.append(data)
        return outdata
    }
}

you can use it, as below.

let data = "hello word".javaUTF8()

Answer 4

readUTF expects a specific format, you need to send the length as a big-endian 16-bit unsigned short, followed by the text UTF encoded. I suspect its taking the first bytes as the length which works out to a large number so it is waiting for this many characters to be read.

UTF-8 is not big endian or little endian, so this is not a problem for you. If you are sending 16-bit, 32-bit or 64-bit data values, then you could have a problem with endianness.