Develop Reference

Java's equivalence to the Python's recv() network function

I have a client implemented in Java, and a server implemented in client.
The client sends a message (string) such as "nesting:1:2" to the server, the server decodes the meaning of the input to create and send back binary data to the client.
This is the Python server code.
class MyTCPHandler(SocketServer.BaseRequestHandler):
def handle(self):
# self.data has the data
(name, index, n) = self.data.split(":")
m = int(n)
i = int(index)
size = sizes.sizes[name][i]
# creates the binary data
bf = [0x41] * size * m
key = ''.join(chr(x) for x in bf)
self.request.send(key) #
if __name__ == "__main__":
HOST = socket.gethostbyname(socket.gethostname())
PORT = 9999
server = SocketServer.TCPServer((HOST, PORT), MyTCPHandler)
server.serve_forever()
The client Python code is as follows:
af, socktype, proto. canonname, sa = res
s = socket(af, socktype, proto)
s.connect(sa)
s.sendall('nostring:1:5')
data = s.recv(1024)
s.close()
I tried to find the equivalent to the s.recv() method, I found read() method. So, I tried this method where byte[] buffer = new byte[157*10]; is declared outside the method.
public byte[] receive() throws IOException {
InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
socket = new Socket(serverAddr, SERVER_PORT);
in = socket.getInputStream();
in.read(buffer);
in.close();
socket.close();
return this.buffer;
}
The issue is that the in.read(buffer) never returns until the server disconnects.
What might be wrong? This is the full source code for the client.

I didn't try, but to my mind, the socket should not be closed between the send and receive action, one socket means one connection to the server.
The python server will most likely try to answer on the given socket, which will be closed, and the java client will wait on another.
Try something like that: creating the socket in the constructor and closing it at the end of the receive (if you are sure you might call the send/receive pair only one time)
import java.net.*;
import java.io.*;
class GpcSocket {
private Socket socket;
// default value
private int SERVER_PORT = 9999;
private String SERVER_IP = "192.168.3.100";
OutputStream out = null;
InputStream in = null;
byte[] buffer = new byte[157*10];
public GpcSocket(String serverIP, int serverPort) {
this.SERVER_PORT = serverPort;
this.SERVER_IP = serverIP;
InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
socket = new Socket(serverAddr, SERVER_PORT);
}
public int send(byte[] str) throws IOException {
out = socket.getOutputStream();
out.write(str);
out.flush();
// out.close();
return str.length;
}
public byte[] receive() throws IOException {
in = socket.getInputStream();
in.read(buffer);
in.close();
socket.close()
return this.buffer;
}
}

How to keep Socket Connection until Server Process it?

I am writing Socket program , Here Client Sends a String through Stream , Server Process it and writes back to Client. My problem is, after Server process the String , it Writes back to Stream but in client It can't able to read the Stream its showing exception as Exception in while: java.net.SocketException: socket closed Here is my code,
Client ,
public void run() {
while (true) {
try {
// Open your connection to a server, at port 1231
s1 = new Socket("localhost", 1231);
OutputStream s1out = s1.getOutputStream();
DataOutputStream dos = new DataOutputStream(s1out);
InputStream in=s1.getInputStream();
DataInputStream dis=new DataInputStream(in);
String s = br.readLine();
dos.writeUTF(s);
dos.flush();
dos.close();
System.out.println(dis.readUTF());//it is the String from Server after processing
dis.close();
} catch (IOException ex) {
// Logger.getLogger(SimpleClient.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("Exception in while: " + ex);
}
}
In Server
public void run()
{
while(true){
try {
System.out.println("Waiting for connect to client");
s1=serverSocket.accept();
s1In = s1.getInputStream();
dis = new DataInputStream(s1In);
out=s1.getOutputStream();
dos=new DataOutputStream(out);
String clientData=dis.readUTF();
//processing task String
dos.writeUTF("Bus Registered Successfully");
dos.flush();
}
}
Here I am not able to read Bus Registered Successfully at client side . How to Solve this.?

Well there are many things not right in your program. But first let me answer your question ... you are closing the socket just after writing the stream ... so server throws exception, just remove dos.close(); just after the dos.flush();. It will run fine.
Now back to the programming practices ...
1) Server should accept the connection in a while(true) loop and then make a new thread. So following statement should not be the part of run method.
System.out.println("Waiting for connect to client");
s1=serverSocket.accept();
s1In = s1.getInputStream();
dis = new DataInputStream(s1In);
out=s1.getOutputStream();
dos=new DataOutputStream(out);
2) There is no need of run method in client. Because Every new client will be a new program that has its own variables and socket.

A quick look shows me that the reason the socket is closed is because you used dos.close().
Closing a DataInputStream (or PrintStream, or any similiar stream) will close the underlying socket.
Just take out dos.close().
You can also move the dos.close() to the very end of the try block. As a general rule, don't close anything related to the socket until you're done with the socket.

Empty Socket InputStream

I'm trying to build a simple web server in Java as practice but I'm running into a very strange issue. When I attempt to read the InputStream from the Socket, sometimes there is no data to be read.
The process I'm following is this:
I create a ServerSocket, listening on port 80, and call accept() to get a Socket. I then send a request from my browser (Firefox) to localhost, which triggers the accept() to return the Socket.
Sometimes, it will read the HTTP request perfectly. Other times, it fails to read any data (read() returns a -1).
Here is some sample code to illustrate what I'm doing, without any exception handling thrown in:
ServerSocket serv = new ServerSocket(80);
while (true)
{
Socket con = ServerSocket.accept();
InputStream input = con.getInputStream();
bytes[] bytes = new bytes[4000000]; // for simplicity, I figured I'd
// just make the array huge for now
int bytesRead = input.read(bytes);
if (bytesRead > 0)
{
StringBuffer sBuffer = new StringBuffer(bytesRead);
for (int i = 0; i < bytesRead; i++)
{
sBuffer.append((char) bytes[i]);
}
System.out.println(sBuffer.toString());
}
}
EDIT: I've also tried using a BufferedInputStream and BufferedReader to not avail.

One socket can receive more packets. So you should create a new thread for every socket. Something like:
while(true){
Socket socket = serv.accept();
Client client = new Client(socket);
new Thread(client).start();
}
Then you need to create the Client class which implements the runable interface.
Socket socket;
public Client(Socket socket){
this.socket = socket;
}
public void run(){
while(socket.isConnected()){
//here reads the packets from the inputstream
}
}

You should always loop when reading data from an InputStream. there is no guarantee that all the data will be returned from a single read() call (loop until you receive -1).

Java socket server blocking

I have some problems with my server socket. I create a DatagramSocket to chat between a server and a client.
public static void main (String[] args) throws IOException {
byte[] send = new byte[1024];
byte[] receive = new byte[1024];
BufferedReader entree;
DatagramSocket serverSocket = null;
InetAddress ip;
InetAddress ipDest;
int port;
try {
serverSocket = new DatagramSocket(8888);
} catch (SocketException e) {
e.printStackTrace();
}
while (true) {
DatagramPacket recu = new DatagramPacket(receive, receive.length);
serverSocket.receive(recu);
String sentence = new String(recu.getData());
ipDest = recu.getAddress();
port = recu.getPort();
System.out.println("Reçu:"+sentence);
entree = new BufferedReader(new InputStreamReader(System.in));
String chaine = entree.readLine();
send = chaine.getBytes();
DatagramPacket dp = new DatagramPacket(send, send.length, ipDest, port);
serverSocket.send(dp);
send = new byte[1024];
receive = new byte[1024];
}
But I use new BufferedReader(new InputStreamReader(System.in)) get the next stuff to send, and it is blocking. So, I cannot receive what's comming from the client and print it.
How can I arrange this ?
Merci, eo

Trying to do non-blocking reads on System.in in Java is an exercise in futility. There's no portable way to do it, so Java doesn't support it.
Even if you create a separate thread and do a blocking read there, you'll have the problem of that thread being non-interruptible. See: Java: how to abort a thread reading from System.in
Basically, you either need to use a platform specific library (JNI) (JCurses for linux, for example), or use a GUI.
Edit to add: What you can do is move your socket reading to a different thread, as that is interruptible.

Java - Socket - Freeze

i've a minimal server which wait until a client connect ,then he start a thread which will send a reply back to the client, the problem is the reply.
This is the code of the server:
int port = 1234;
ServerSocket servSock = null;
servSock = new ServerSocket(port);
while (true) {
Socket link = servSock.accept();
serverThread st = new serverThread(link);
st.start();
}
This is the code of the run() method of the thread,the one which send the answer back, sk is the socket "link" passed by parameter in the server code
public void run() {
String dato = "";
InputStream i = null;
try {
i = sk.getInputStream();
} catch (IOException ex) {
Logger.getLogger(serverThread.class.getName()).log(Level.SEVERE, null, ex);
}
Scanner input = new Scanner(i);
//i receive the data sent
while (input.hasNext()) {
dato += input.nextLine();
}
// then i send a reply
DataOutputStream outputStream=new DataOutputStream(sk.getOutputStream());
outputStream.writeInt(1);
outputStream.close();
Client side ( only the code which should receive the answer from the server) :
Socket link;
int valid = 0;
String url="localhost";
int port=1234;
link = new Socket(InetAddress.getByName(url), port);
//i've to send some data to the server
PrintWriter output = new PrintWriter(link.getOutputStream(), true);
String a = new String(Base64.encode(mex));
output.println(createXml(tag, a));
output.flush();
//then i need to receive an answer from the server
DataInputStream answerI = new DataInputStream(link.getInputStream());
while(answerI.available()!=0)// but answerI.available() is always equal 0
valid = answerI.readInt();
answerI.close();
output.close ();
link.close();
With this code the istruction valid = answerI.readInt(); is not reached.
Without the while cycle, the client freeze at line : valid = answerI.readInt();
Can anyone help me?
Thank you in advance

I'm guessing that the server is blocked in a call to input.hasNext(). This will return false when the socket is closed, and its InputStream returns -1 to signal the end of the stream. However, the socket is still open. The client can send another line; the Scanner is blocking to see what the client's next move will be.
There are ways to shutdown "half" of a socket, so that the server can tell that the client has closed its sending channel but can still receive a response.
However, this approach is complicated. I suggest a change to the protocol so that the server can determine when it is allowed to respond.

In this protocol you don't need the reply if it is always '1'. Just close the socket. The client should block in a read() which will return -1 when the server closes the socket.