How to send an integer over a socket to a Java application using python?

Question

I have a client in Python that sends data (preceded by a data length message):

s = socket.socket()
s.connect((host, port))
data = 'hello world'
s.sendall('%16s' % len(data)) #send data length
s.sendall(data) #send data
s.close()

And a server in Java that receives the data. The server uses DataInputStream.readInt() to read the data length before reading the data. However I seem to be getting really large numbers returned by readInt(). What is the problem?

Answer 1

Java expects the binary representation of your integer. You can use the struct module to generate binary representations.

In your case, this would be:

import struct
s.sendall(struct.pack('i', len(data)))

Also make sure you use the correct byte order. Java could be expecting network byte order.

Answer 2

As @mensi says, absent any other processing Java expects to receive the binary representation of the data, which differs from what Python is sending. A common solution to this sort of issue is to serialize your data -- that is, to translate it into a format more suitable for network transmission, and reconstitute the data on the receiving side.

A common serialization format for which both Python and Java have support is JSON. Recent versions of Python have the json module as part of the standard library.