I'm a bit of a beginner programmer so it's possible this is quite obvious and I'm overlooking the answer. But on to the question.
I have a two-part program (its a little more complicated than this example, but the situation is the same). The program has multiple messages fired between the client and the server. I have a PrintWriter on the server-side to send messages to the client, and on the client, I have a BufferedReader to read the messages sent.
When this example is run, I'm given two lines as output. The first message is both messages, and the second is NULL. What I am wondering is if there is a way to basically halt the server until I am ready for the second message, so that I can do something on the client-side before the second message is sent.
I am hoping to not use Thread.Sleep, as I would rather the Server wait around until the Client says it is ready.
This is the client:
public class Client{
public void run(){
Socket socket = null;
InputStream in = null;
BufferedReader reader = null;
try{
socket = new socket("LocalHost",1234);
in = socket.getInputStream();
reader = new BufferedReader(new InputStreamReader(in));
}
String messageFromServer = "";
try{
messageFromServer=reader.readLine();
}
System.out.println(messageFromServer);
String messageFromServer = "";
try{
messageFromServer=reader.readLine();
}
System.out.println(messagefromServer);
//close everything
}
}
This is the server:
public class Server{
public void run(){
ServerSocket server = null;
Socket client = null;
try{
server = new ServerSocket(1234);
client = server.accept();
}
PrintWriter writer = null;
OutputStream out = null;
try{
out = client.getOutputStream();
writer = new PrintWriter(out, true);
}
writer.write("Hi I'm a server");
//do some stuff that takes some time, user input, etc. etc.
writer.write("I'm still a server");
//close everything
}
Thanks :)
The problem with the way you currently have you code is the fact that you are using a BufferedReader, but the server is not terminating it's messages with a new line.
When you close the writer, the client is reaching the EOF or EOS and unblocking the read so it appears that both strings are being sent at once...
If you do something like...
writer.write("Hi I'm a server\n");
// This will force the message to be written to the client and picked up ;)
writer.flush();
writer.write("I'm still a server\n");
writer.flush();
Then you will get the messages seperatly...
You can use ObjectInputStream to read Objects instead of Strings.
This way you will read only one Message(String in your case) every call to ObjectInputStream.readObject();
BTW you can read the first message, "do something" and then read the second message. you don't have to read all of the sent messages at once.
If there are no other messages, then your thread will be blocked when trying to read an object from the ObjectInputStream.
Use it like:
ObjectInputStream inputStream = new ObjectInputStream( socket.getInputStream() )
Related
I am trying to send data to one of my servers and receive an ACK back from it. However, the processing gets hung up when waiting for a response from the server. I know for a fact that there is a connection because I can see the data reaching the server. I also know that the server is outputting data correctly because my C# client is receiving data back from the server. I will note that this client is running on a centOS virtual machine. The server is a remote windows machine. I wouldn't imagine that there would be an issue due to the virtual environment because I am able to use an SNMP java client (SNMP4j package) to make calls to a remote server. I believe my server is outputting raw binary too, but I would expect to see some kind of output either way.
// A Java program for a Client
import java.net.*;
import java.io.*;
public class Client
{
// initialize socket and input output streams
private Socket socket = null;
private DataInputStream input = null;
private DataOutputStream out = null;
private DataInputStream serveroutput= null;
// constructor to put ip address and port
public Client(String address, int port)
{
// establish a connection
try
{
socket = new Socket(address, port);
System.out.println("Connected");
// takes input from terminal
input = new DataInputStream(System.in);
// sends output to the socket
out = new DataOutputStream(socket.getOutputStream());
serveroutput = new DataInputStream(socket.getInputStream());
}
catch(UnknownHostException u)
{
System.out.println(u);
}
catch(IOException i)
{
System.out.println(i);
}
// string to read message from input
String line = "";
// keep reading until "Over" is input
while (!line.equals("Over"))
{
try
{
line = input.readLine();
out.writeUTF(line);
System.out.println(serveroutput.readLine())
}
catch(IOException i)
{
System.out.println(i);
}
}
// close the connection
try
{
input.close();
out.close();
socket.close();
}
catch(IOException i)
{
System.out.println(i);
}
}
Could be great if you would share the otherside codes. (sorry cannot comment yet)
Try use something else over writeUTF(), simply maybe a PrintStream, as mentioned by #marquis-of-lorne (read|write)UTF may be confusing by the peer.
Also this might be a good practice to flush() out the output from both side when there is nothing else to send to make sure data is sent completely.
You may also try BufferedReader over InputDataStream as you are trying to read lines. readLine() from InputDataStream is deprecated.
I'm currently working on a small chat-program. The 2 classes, I have a problem with are the classes containing the clientside and the serverside of a socket. I want them to interact, sending a string from the server to the client, but the string isn't received apparently. I also don't get an error. Can someone explain, why this happens or how to fix it?
Codesnippet from the client:
try {
BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
while(true) {
System.out.println("WAITING");
System.out.println(br.readLine());
}
} catch(Exception ex) { ex.printStackTrace(); }
Codesnippet from the server:
// Method of the code throws Exception
System.out.println("Waiting for someone to connect.");
Socket socket = serverSocket.accept();
System.out.println("Someone connected.");
OutputStream os = socket.getOutputStream();
PrintWriter pw = new PrintWriter(os, true);
while(true) {
System.out.println("WRITING");
pw.write("hi");
}
The output is currently:
server:
WRITING
WRITING
WRITING
WRITING
WRITING
...
client:
WAITING //only once
The client expects a complete line:
br.readLine()
and blocks until the line is complete.
But the server only sends "hi" repeatedly, without ever sending any EOL character:
pw.write("hi");
So the line never ends, and the client keeps blocking.
Use pw.println("hi");
The server buffer is not getting flushed. If you change pw.write(...) to pw.println(...) it should fix the issue - println(...) auto-flushes the buffer
I'm trying to send strings to a server from a client but it doesn't seem to be reading from the input stream.
Client
Scanner scanner = new Scanner(System.in);
Socket connection = new Socket("localhost", 13)
OutputStream out = connection.getOutputStream();
while(true) {
String message = scanner.nextLine();
IOUtils.write(message, out, "UTF-8");
out.flush();
}
Server
ServerSocket server = new ServerSocket(localhost,13);~
Socket connection = server.accept();
InputStream in = connection.getInputStream();
StringWriter writer = new StringWriter();
while(true) {
try {
IOUtils.copy(in, writer);
System.out.println(writer.toString());
} catch(IOException io) {}
}
It reads if I close the stream from the client's outputstream but I am trying to send multiple messages from the client to the server. Could someone please help
You seem to think that each time you call flush() at client-side, the server will know it and be able to know that this is the end of a message. That's not the case. IOUtils.copy() reads everything from a stream of bytes. While the stream end hasn't been reached, copy() won't return.
You can see streams as two sides of a long tube. If you pour 10 buckets of water in the at the end of the tube, all you'll get at the other side is a continuous flow of water.
If you need multiple separate messages, then you need to design a protocol allowing to separate messages, and read until the end of a message has been reached. It could be based on separators for example. Or you could send the length of the message followed by the message itself, to let the server know how many bytes it must read to get the next message.
I'm trying to learn sockets using Java and I sucessfully sent data to a ServerSocket running in my own machine. When I try to read from this socket using readline (so I can just echo my own sent message) my program hangs and won't return.
Here's the code:
public static void main(String[] args) throws UnknownHostException, IOException {
TCPClient cli = new TCPClient("127.0.0.1", "15000");
try {
cli.ostream.writeUTF("Teste");
String echo = cli.istream.readLine(); //it hangs in this line
System.out.println(echo);
}
TCPClient is a class I defined so I can test my program on a simpler interface before using swing on my homwework. here's the code:
public class TCPClient {
public DataOutputStream ostream = null;
public BufferedReader istream = null;
public TCPClient(String host, String port) throws UnknownHostException {
InetAddress ip = InetAddress.getByName(host);
try {
Socket socket = new Socket(host, Integer.parseInt(port));
ostream = new DataOutputStream(socket.getOutputStream());
istream = new BufferedReader(new InputStreamReader(socket.getInputStream()));
} catch (IOException ex) {
Logger.getLogger(TCPClient.class.getName()).log(Level.SEVERE, null, ex);
}
}
My server is pretty simple. After the connection is estabilished, it enters in this loop and stays here until I close the client (because of the infinite loop). Afterwards, some exception handling returns it to the point before the connection started.
while(true){
String msg = istream.readLine();
System.out.println("Arrived on server: " + msg); //just works on debug
ostream.writeUTF("ACK: " + msg);
ostream.flush();
}
I don't see what am I missing.
PS: the wierd stuff is that if I debug the server, I can see the message arriving there (I can print it, for example), but this isn't possible if I just run this code. Does this have some concurrency relation I'm overlooking?
thx
The problem is that readLine tries reading a line. It will not return the line until it's sure that the end of line has been reached. This means that it expects either a newline character, or the end of the communication. Since the server doesn't send any newline char and doesn't close the stream, the client waits indefinitely.
cli.ostream.writeUTF("Teste");
Shouldn't this be containing a new line? Otherwise read method will be waiting for new line I think.
Also as suggested you can try flushing the ostream after writing to it.
writeUTF() doesn't write a line. See the Javadoc. writeUTF() is for use with readUTF(). And Readers are for use with Writers. So change the DataOutputStream to a BufferedWriter and call write() and then newline().
Okay this is a revised question from earlier today, I have included code to help explain the problem. I am sending two messages from the client to the server. The server then picks the messages up and processes them. The server finally attempts to send a message back to the client(please note in the server code "testmessage"), it is here I am having problems. Either I am not recieving the message at the client side or sending it incorrectly from the server side.
public class ClientConnection {
String address, language, message;
int portNumber;
Socket clientSocket = null;
public ClientConnection(String lan, String mes, String add, int pn) throws IOException{
address = add;
portNumber = pn;
language = lan;
message = mes;
}
public String createAndSend() throws IOException{
// Create and connect the socket
Socket clientSocket = null;
clientSocket = new Socket(address, portNumber);
PrintWriter pw = new PrintWriter(clientSocket.getOutputStream(),true);
BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
// Send first message - Message is being correctly received
pw.write(language+"\n");
pw.flush();
// Send off the data
// Send the second message - Message is being correctly received
pw.write(message);
pw.flush();
pw.close();
// Send off the data
// NOTE: Either I am not receiving the message correctly or I am not sending it from the server properly.
String translatedMessage = br.readLine();
br.close();
//Log.d("application_name",translatedMessage); Trying to check the contents begin returned from the server.
return translatedMessage;
}
Server Code:
public class ServerConnection {
public static void main(String[] args) throws Exception {
// Delete - Using while loop to keep connection open permanently.
boolean status = false;
while( !status){
ServerSocket serverSocket = null;
try {
serverSocket = new ServerSocket(4444);
} catch (IOException e) {
System.err.println("Could not listen on port: 4444.");
System.exit(1);
}
Socket clientSocket = null;
try {
clientSocket = serverSocket.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
System.exit(1);
}
// Delete - Working as of here, connection is established and program runs awaiting connection on 4444
BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String language = br.readLine();
String message = br.readLine();
// Test - Works
System.out.println(language);
// Test - Works
System.out.println(message);
// Delete - Working as of here, both messages are passed and applied. Messages are received as sent from client.
TranslateMessage tm = new TranslateMessage();
String translatedMessage = tm.translateMessage(language, message);
// NOTE: This seems to be where I am going wrong, either I am not sending the message correctly or I am not receiving it correctly..
// PrintWriter writer = new PrintWriter(new BufferedOutputStream(clientSocket.getOutputStream()));
PrintWriter pw = new PrintWriter(clientSocket.getOutputStream(),true);
// Send translation back
System.out.println(translatedMessage);
// pw.write(translatedMessage+"\n");
pw.write("Return test"); // Test message!
pw.flush();
// Send off the data
pw.close();
br.close();
clientSocket.close();
serverSocket.close();
}
}
}
The code is a bit of a mess and I can see a few duplicates, I have commented where I feel the problems occour.
Thanks for any help!
You are using BufferedReader.readLine() to read the response from the server, but in the test case you are sending a string that is not terminated with a \n or \r\n, so it will not get the line as far as I can tell from the docs...
public String readLine()
throws IOException
Read a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.
Returns:
A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached
An additional suggestion...
When writing request response protocols like this I would not rely on line endings to terminate the requests or responses. Typically I would use either a fully formatted JSON string, or my preference is for a binary protocol where all requests and response are prepended with a binary count (usually 4 bytes bigendian/network byte order). Then the client and server reads the 4 bytes then reads the number of bytes that follow. This handles the packet fragmentation that typically happens over network connections, also it helps avoid DOS attacks by malicious users sending long strings that never terminate.
In Java you can use ByteBuffer.order() to handle bigendian numbers.