java server (simple example) displaying only the first string sent - sockets

Hi i want to test if my client app is working by testing it on a very simple java server. My client will send a string almost every second. My idea is that the server will display the string in the eclipse console once it receives the string. But the problem is that the simple java server only prints out the first string. Below is the simple java server i got from the site
package serverjava;
/* ChatServer.java */
import java.net.ServerSocket;
import java.net.Socket;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class serverjava {
private static int port = 1234; /* port the server listens on */
public static void main (String[] args) throws IOException {
ServerSocket server = null;
try {
server = new ServerSocket(port); /* start listening on the port */
} catch (IOException e) {
System.err.println("Could not listen on port: " + port);
System.err.println(e);
System.exit(1);
}
Socket client = null;
try {
client = server.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
System.err.println(e);
System.exit(1);
}
/* obtain an input stream to the client */
BufferedReader in = new BufferedReader(new InputStreamReader(
client.getInputStream()));
String msg;
/* loop reading lines from the client and display them */
while ((msg = in.readLine()) != null) {
System.out.println("Client says: " + msg);
}
}
}
Is the java server capable of displaying only an instance of a string per launch? If yes, how can I make it to print or show in the console all the strings that are being sent every second?

The problem you have is that you read the buffer only one time. The server waits for client lines and when it reads them, then it finish the execution because 'msg' is null. You must have a process that reads every second or minute, or hour... That means using THREADS.
If you want to see more examples, go here:
http://www.isr.umd.edu/~austin/ence489c.d/threads.html.
And here you have some information:
http://www.javaworld.com/javaworld/jw-04-1996/jw-04-threads.html
Be careful! Don't use a while(true) structure of something like that, because the machine will be overloaded.

Related

should I use the same port for tcp-based server-client-connection

I am trying to understand the topic of networking by writing java-programs (learning by doing). I wrote a client and a server in java using java.net.Socket. The client is a class and the server is a class. Then I instantiated the two classes in one Main-class. The java-Application run on my computer. When I assign the same port to the client and the server, then I get no error (ex: port 80). But when I assign different ports (ex: port 80 to server and port 9090 to client) then I get an error. Should I use the same port for tcp-connection?
I want to understand why I get an error.
here is my whole code put in one java-package:
package client_server;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.*;
import java.util.Date;
public class Server {
ServerSocket server;
Server(){
System.out.println("server starts");
try{
//server runs on port 9090
server=new ServerSocket(9090);
}catch(IOException e){
e.printStackTrace();
}
}
void listen(){
Socket socket = null;
try {
System.out.println("server is listining");
socket=server.accept();
PrintWriter out =new PrintWriter(socket.getOutputStream(), true);
out.println(new Date().toString());
} catch (IOException e) {
e.printStackTrace();
}finally{
try {
socket.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
package client_server;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.net.*;
public class Client {
Socket clientSocket;
Client(){
System.out.println("Client starts");
final String server_ip_adress="127.0.0.1";
try {
clientSocket = new Socket(server_ip_adress,80);
} catch (IOException e) {
e.printStackTrace();
}
}
void connectToServer(){
try{
System.out.println("client connects to server");
InputStream clientInput=clientSocket.getInputStream();
BufferedReader input = new BufferedReader(new InputStreamReader(clientInput));
String answer = input.readLine();
System.out.println(answer);
}catch(IOException e){
e.printStackTrace();
}
}
}
package client_server;
public class Main {
public static void main(String [] args){
Server server = new Server();
Client client = new Client();
server.listen();
client. connectToServer();
}
}
Theoretically it should work, because each process (server-process and client process) will be assigned a port and it does not matter if the interprocess-communication occurs on different ports.
But in my case I have one process. So when I assign different ports it throw an error. Is my analysis correct?
I getthis error: Address already in use: JVM_Bind
The server listens at port 9090, so the client has to connect to port 9090. But it isn't the case that the client is 'assigned' port 9090. Actually when you run netstat after connecting the client, you will see that the client is using another port, allocated by the system.
NB by the time you print 'client connects to server' the client is already connected, when new Socket(...) returns.

Socket implementation with ObjectInputStream - can't read object

For a Java class I am taking, I need to use sockets to pass data back and forth between client and server. While I can get examples to work passing string data, I need to be able to pass custom class objects (i.e. a product) and lists of these objects back and forth. I cannot get the server piece to successfully read the input. I tried to create a simple example of my code to see if anyone can pinpoint the issue. I do understand that I don't have the code complete, but I cannot even get the server to read the object the the class is writing to the stream (in this case, I am writing a string just in an attempt to get it to work, but need to read/write objects). Here is my code. I have spent hours and hours trying this and researching other people's questions and answere, but still can't get this to work.
Here the sample code:
simple server:
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
public class simpleServer {
public static final int PORT_NO = 8888;
static ObjectInputStream serverReader = null;
public static void main(String[] args) throws IOException, InterruptedException {
ServerSocket serverSocket = new ServerSocket(PORT_NO);
System.out.println("... server is accepting request");
Object myObject = null;
while (true) {
Socket socket = serverSocket.accept();
System.out.println("creating reader");
ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
serverReader = new ObjectInputStream(socket.getInputStream());
System.out.println("created reader");
try {
System.out.println("try to read");
myObject = serverReader.readObject();
System.out.println("read it");
System.out.println(myObject);
if (myObject != null) objOut.writeUTF("Got something");
else objOut.writeUTF("got nothing");
if ("quit".equals(myObject.toString())) serverSocket.close();
} catch (ClassNotFoundException e1) {
// TODO Auto-generated catch block
System.out.println("cath for readobject");
}
catch (Exception e) {
System.out.println("other error");
System.out.println(e.getMessage());
}
}
}
}
simple client:
public static void main(String[] args) {
Socket socket;
try {
socket = new Socket("localhost", ProductDBServer.PORT_NO);
ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream objIn = new ObjectInputStream(socket.getInputStream());
objOut.writeUTF("loadProductsFromDisk");
objOut.flush();
String myString = objIn.toString();
//System.out.println(myString);
if (!"quit".equals(objIn.toString().trim())) {
//System.out.println("reading line 1");
String line;
try {
line = (String)objIn.readObject();
//System.out.println("line is " + line);
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
objIn.close();
//System.out.println("result: " + line);
}
System.out.println("closing socket");
socket.close();
} catch (UnknownHostException e) {
// TODO Auto-generated catch block
System.out.println("Unknownhostexception");
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
System.out.println("ioexception");
e.printStackTrace();
}
}
The code appears to run to the point on the server side where it trys to read the object I sent, and then dies. Can someone see what I am doing wrong? This seems to be such a simple thing to do, and yet I can't seem to get it to work. Thanks for any help!
To write objects to an ObjectOutputStream you need to call writeObject().
Not writeUTF().
To read objects from an ObjectInputStream you need to call readObject().
Not toString().
See in your code:
// Simple Client
objOut.writeUTF("loadProductsFromDisk"); // Line 8
You are sending the String "loadProductsFromDisk" in the UTF-8 format towards the server side.
So in order to receive it and read it over the server side, you will need something like this:
String clientReq = serverReader.readUTF();
Where, serverReader is your ObjectInputStream object.
Otherwise, if you wish to send and receive objects you must use the
writeObject() & readObject() methods respectively.

How to terminate a java program that listens for client's requests to create a socket?

How do I make this program exit when I run it in terminal using the Java JDK in Ubuntu?
I want to type in "end" when this program is running, but I can't get it to work.
It listens for clients and calls a thread thingy to create a socket for the new client.
I can't figure out how to make this program end. I tried everything.
I started learning java yesterday.
Please help me....
import java.net.*; //jave lib
import java.io.*; //io lib
public class MultiServerConnections { //initiate class
public static void main(String[] args) throws IOException {
int portNum = 5342; //set server port number
boolean listen = true;
System.out.println("Listening for Connections"); //print message
ServerSocket server_Socket = null; //set server_Socket to null
try {
server_Socket = new ServerSocket(portNum); //set server port
} catch (IOException e) {
System.err.println("Port " + portNum + " is unavailable"); //port is taken error
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in)); //set reader
String input;
//while (TCPglobals.checkRequests){
while (listen == true){
new MultiServer(server_Socket.accept()).start();
input = stdIn.readLine();
//if(TCPglobals.checkRequests == false) //|| (input = stdIn.readLine()) == "end")
if(input == "end") {
System.out.println("end connection?");
System.exit(1);
}
}//while
server_Socket.close(); //close server socket
}
}

Java 7 doesn't throw BindException when binding an already used port using ServerSocket

I'm experimenting on ServerSocket in Java on Windows 7 x64.
I wrote a little program that host a HTTP server on port 8080 and only returns a static HTML response that contains the toString() of the class loader.
What I did in the program mainly:
Create a ServerSocket
call setReuseAddress(false) on the serverSocket
Bind port 8080 to this socket
Use a forever loop to accept socket and give response
First I tried with JRE 1.6.0_23 and everything is great: first instance launched and responds normally, second instance cannot be launched since exception is thrown:
Exception in thread "main" java.net.BindException: Address already in use: JVM_Bind
Unexpected thing happens when I tried with JRE 1.7.0_5: both instance can be launched successfully but only the first instance gives responses. After the first instance is kill, the second instance then starts to responds.
Am I doing anything wrong or is this a bug of JRE 7?
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
public class TestServerSocket {
private static final String HEADER = "HTTP/1.1 200 OK\r\n" + "Content-type: text/html\r\n"
+ "Connection: close\r\n" + "\r\n";
private static final int PORT = 8080;
private static void handle(Socket socket) {
System.out.println(socket.getInetAddress() + ":" + socket.getPort());
StringBuilder buffer = new StringBuilder();
buffer.append(HEADER);
buffer.append(TestServerSocket.class.getClassLoader());
try {
socket.getOutputStream().write(buffer.toString().getBytes());
} catch (IOException e) {
} finally {
try {
socket.close();
} catch (IOException e) {
}
}
}
public static void main(String[] args) throws IOException {
int port;
try {
port = Integer.parseInt(args[0]);
} catch (Exception e) {
port = PORT;
}
final ServerSocket server = new ServerSocket();
server.setReuseAddress(false);
server.bind(new InetSocketAddress(port));
// Terminator thread, stop when Ctrl-D is entered
new Thread() {
public void run() {
try {
while (System.in.read() != 4);
} catch (IOException e) {
e.printStackTrace();
}
try {
server.close();
} catch (IOException e) {
}
System.exit(0);
}
}.start();
System.out.println("Listening on: " + port);
Socket client = null;
while (true) {
try {
client = server.accept();
handle(client);
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
To Isolate the problem, I would recommend that you run the following test code.
Apache HttpCore basic server. It's standard API and uses ServerSocket in this particular example, so there is a very small chance that it would fail on your environment ( java 7).
In case it fails you will know for sure problem is not with your code. Meanwhile I will try your code on JDK 7 on my work-machine and will update.

Java socket programming problem

Hey,
I am trying to run this socket programming code.
This is the code on the server side -
package sockettest;
import java.net.*;
import java.io.*;
public class Server {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = null;
try
{
serverSocket = new ServerSocket(139);
}
catch (IOException e)
{
System.err.println("not able to listen on port");
System.exit(1);
}
Socket clientSocket = null;
try
{
clientSocket = serverSocket.accept();
}
catch (IOException e)
{
System.err.println("Accept failed.");
System.exit(1);
}
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true); // Out is Outputstream is used to write to the Client .
BufferedReader in = new BufferedReader( new InputStreamReader(clientSocket.getInputStream())); // in is used to read the Client's input.
String inputLine, outputLine;
out.println("Hey! . Who are you?"); // Writes to client as "Hey! . Who are you?"
while ((inputLine = in.readLine()) != null)
{
// Reads the input from the Client. if it is "bye" the program ends.
if (inputLine.equalsIgnoreCase("Bye"))
{
out.println("Bye");
break;
}
else
{
out.println("Hello Mr. " + inputLine);
}
}
out.close();
in.close();
clientSocket.close();
serverSocket.close();
}
}
This is the code running on the client side -
import java.io.*;
import java.net.*;
public class Client
{
public static void main(String[] args) throws IOException {
Socket kkSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try
{
kkSocket = new Socket("192.168.2.3", 139);
out = new PrintWriter(kkSocket.getOutputStream(), true); // Out may be used to write to server from the client
in = new BufferedReader(new InputStreamReader(kkSocket.getInputStream())); // in will be used to read the lines sent by the Server.
}
catch (UnknownHostException e)
{
System.err.println("Unidentified host.");
System.exit(1);
}
catch (IOException e)
{
System.err.println("Couldn't get I/O for the connection to.");
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String fromServer;
String fromClient;
while ((fromServer = in.readLine()) != null) {
System.out.println("Server: " + fromServer);
if (fromServer.equals("Bye"))
break;
fromClient = stdIn.readLine();
if (fromClient != null) {
System.out.println("Client: " + fromClient);
out.println(fromClient);
}
}
out.close();
in.close();
stdIn.close();
kkSocket.close();
}
}
I'm running the codes on eclipse in both the client and the server side. Using netstat -an command in cmd prompt, i can see that a connection has been established between the client and the server but i cannot communicate and eclipse is not showing any output too. What seems to be wrong??
You haven't told us what the problem is. However, from a cursory glance at your code, I would advise against listening on port 139 as this is already used by NetBios under Windows and may cause a conflict.
also your Server code is missing
the initialization of inputLine,
e.g
String inputline = "";
before the while loop
keep in mind that Socket's are blocked if you read or write...
your client is reading all the time because it waits for every information on the server
until it is null
and your server also reads all the time and is waiting for any input..
so as long as server and client are waiting for input, no one will receive any data.
try to think of a protocol to communicate between the server and the client.
e.g
Sever to Client: Hello Who are you?
Client receives Data and replies: Client
Server receives Information: You Are now authorized, what ya gonna do?
and so on ^^
also out.flush() is needed to send a message