I have an assignment where I have to build a Client-Server communication using Java. So what I did was building a Client-Server connect using Sockets, the following way:
//Client code
Socket socket = new Socket("127.0.0.1", 4999);
//Server code
ServerSocket ss = new ServerSocket(4999);
Socket socket = ss.accept();
System.out.println("Client is connected");
For now, the communication between the client and the server is successful, I am able to send messages from client to server and vice versa. but the main problem is storing data on the server host, we were asked to store relevant data on the server host so the server could send it to the client.
Can someone please explain to me how to store data on the server and how to retrieve it ??
Thanks
To read from the server socket:
ServerSocket server = new ServerSocket(port);
Socket socket = server.accept();
DataInputStream in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
//Do whatever you want with the DataInputStream
Source: https://www.baeldung.com/java-inputstream-server-socket
Then we can move to the input stream processing, here is a sample:
...
int i;
char c;
...
try{
// reads till the end of the stream
while((i = in .read())!=-1) {
// converts integer to character
c = (char)i;
// prints character
System.out.print(c);
}
} catch(Exception e){
//Handle....
} finally {
// Close stream...
}
Source: https://www.tutorialspoint.com/java/io/inputstream_read.htm#:~:text=io.-,InputStream.,the%20returned%20value%20is%20%2D1.
For writing to the socket (the response to the client), just follow the following tutorial, no need to repeat it here:
http://www.avajava.com/tutorials/lessons/how-do-i-write-a-server-socket-that-can-handle-input-and-output.html
I hope it helps
Related
I've just started with both java and networking with servers and clients. Although i understand the basics of whats going on, i was struggling to put it all together and do what i wanted to do in the title.
I was able to make this to send a message to the server, however i was wondering how i'd turn the message into a input string from the user, and also how id send multiple messages between the client and server
thanks
SERVER
import java.io.*;
import java.net.*;
public class Server {
//Main Method:- called when running the class file.
public static void main(String[] args){
//Portnumber:- number of the port we wish to connect on.
int portNumber = 15882;
try{
//Setup the socket for communication and accept incoming communication
ServerSocket serverSoc = new ServerSocket(portNumber);
Socket soc = serverSoc.accept();
//Catch the incoming data in a data stream, read a line and output it to the console
DataInputStream dataIn = new DataInputStream(soc.getInputStream());
System.out.println("--> " + dataIn.readUTF());
//Remember to close the socket once we have finished with it.
soc.close();
}
catch (Exception except){
//Exception thrown (except) when something went wrong, pushing message to the console
System.out.println("Error --> " + except.getMessage());
}
}}
CLIENT
import java.io.*;
import java.net.*;
public class Client {
//Main Method:- called when running the class file.
public static void main(String[] args){
//Portnumber:- number of the port we wish to connect on.
int portNumber = 15882;
//ServerIP:- IP address of the server.
String serverIP = "localhost";
try{
//Create a new socket for communication
Socket soc = new Socket(serverIP,portNumber);
//Create the outputstream to send data through
DataOutputStream dataOut = new DataOutputStream(soc.getOutputStream());
//Write message to output stream and send through socket
dataOut.writeUTF("Hello other world!");
dataOut.flush();
//close the data stream and socket
dataOut.close();
soc.close();
}
catch (Exception except){
//Exception thrown (except) when something went wrong, pushing message to the console
System.out.println("Error --> " + except.getMessage());
}
}}
There are some "problems" with your code.
You should only close the ServerSocket if you are done.
You should handle the newly connected client inside a thread to allow multiple clients to simultaniously "send messages".
1.
you could easily wrap your code inside an while loop.
boolean someCondition = true;
try{
//Setup the socket for communication and accept incoming communication
ServerSocket serverSoc = new ServerSocket(portNumber);
// repeat the whole process over and over again.
while(someCondition) {
Socket soc = serverSoc.accept();
//Catch the incoming data in a data stream, read a line and output it to the console
DataInputStream dataIn = new DataInputStream(soc.getInputStream());
System.out.println("--> " + dataIn.readUTF());
}
//Remember to close the socket once we have finished with it.
soc.close();
}
Now your programm should continue to accept clients. But only one at a time. You can now terminate the Server by stopping the programm or by changing the someCondition to false and accepting the next client.
A bit more advanced would be, to shutdown the ServerSocket to stop the programm and catching the exception inside the while loop.
2.
To allow multiple clients to be handled simultaniously, you should pack the handle part into another Thread.
private ExecutorService threadPool = Executors.newCachedThreadPool();
boolean someCondition = true;
try{
//Setup the socket for communication and accept incoming communication
ServerSocket serverSoc = new ServerSocket(portNumber);
// repeat the whole process over and over again.
while(someCondition) {
Socket soc = serverSoc.accept();
//Catch the incoming data in a data stream, read a line and output it to the console in a new Thread.
threadPool.submit(() -> {
DataInputStream dataIn = new
DataInputStream(soc.getInputStream());
System.out.println("--> " + dataIn.readUTF());
}
}
//Remember to close the socket once we have finished with it.
soc.close();
}
The part inside the threadPool.submit block could be specified as an custom instance of the Runnable interface of as an method, to call it using method reference.
I assumed you are knowing about ThreadPools. They have multiple advantages over Threads
This should get you going for any number of clients.
Note: This is not good designed, but it is for demonstrational porpurses only.
I am creating a program where an android device requests a file from a Web Server(running python).The server can receive over sockets with no problem the path of the requested file but i dont know how i can make my android device to wait for a responce.
Here is the android code(as a client requesting a file from web server):
try {
Socket socket = null;
socket = new Socket("192.168.1.9", 4000);
DataInputStream input = new DataInputStream(socket.getInputStream());
DataOutputStream output = new DataOutputStream(socket.getOutputStream());
String str = getURL();
output.writeBytes(str);
output.close();
input.close();
socket.close();
{
}
} catch (IOException e) {
}
Log.d("communicationService", "URL transferred with success");
And the python script running on Web Server(It can receive thefile path but i have problem sending the file)
import socket
import sys
HOST, PORT = '192.168.1.9', 4000
serverSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serverSocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serverSocket.bind((HOST,PORT))
serverSocket.listen(10)
print 'Server is on and listening to %s ... ' % PORT
while True:
clientSocket, clientAddress = serverSocket.accept()
print 'A client was connected.....'
incomingURL = clientSocket.recv(1024)
print incomingURL
clientSocket.close()
Any advice and tip would be really helpful...
I imagine you should be able to get away with SimpleHTTPServer
If you need to get fancier with a full blown webservice, WSGI is very popular.
On the client side Requests library is by far the easiest way that I've found to make http requests in python. (just had to plug that one because it's that good)
Well i managed to transfer the files in the end(For those that are interested in apps of this kind).What i did was to create another socket and sent a stream back to client.
file = open("path_of_file", "rb")
s = socket.socket()
s = connect((addr,port))
l = file.read(1024)
while (l):
s.send(l)
l.f.read(1024)
file.close()
s.close()
I have TCP server-client application. It works but sometime something happens. Client connects to server but server says he doesn't accepted him.
Server side code:
while(!stopped){
try {
AcceptClient();
} catch(SocketTimeoutException ex){
continue;
} catch (IOException ex) {
System.err.println("AppServer: Client cannot be accepted.\n"+ex.getMessage()+"\n");
break;
}
...
private void AcceptClient() throws IOException {
clientSocket = serverSocket.accept();
clientSocket.setSoTimeout(200);
out = new ObjectOutputStream(clientSocket.getOutputStream());
in = new ObjectInputStream(clientSocket.getInputStream());
System.out.println("Accepted connection from "+clientSocket.getInetAddress());
}
Client side code:
try {
socket = new Socket(IPAddress, serverPort);
socket.setSoTimeout(5000);
out = new ObjectOutputStream(socket.getOutputStream());
in = new ObjectInputStream(socket.getInputStream());
} catch (IOException e1) {
sendSystemMessage("DISCONNECTED");
sendSystemMessage(e1.getMessage());
return;
}
sendSystemMessage("CONNECTED");
If client connects the message:
Accepted connection from ... appears. But sometimes it doesn't appear
even if client sends message "CONNECTED"
Server is still runing the loop trying to get client and it is catching socketTimeoutException. Client is connected, sends message and waits for response.
I suspect a missing 'flush' inside your client's 'sendSystemMessage()'.
Unfortunately the constructor of ObjectInputStream attempts to read a header from the underlying stream (which is not very intuitive IMHO). So if the client fails to flush the data - the server may remain stuck on the line "in = new ObjectInputStream(socket.getInputStream())"...
As a side note it's usually better for a server to launch a thread per incoming client, but that's just a side remark (plus it obviously depends on requirements).
I found the problem. The communication on my net is too slow so it timeouts in getting inputstream. The solution has two parts. Flushing outputstream before getting inputstream. And set socket timout after streams are initialized.
serverside:
clientSocket = serverSocket.accept();
out = new ObjectOutputStream(clientSocket.getOutputStream());
out.flush()
in = new ObjectInputStream(clientSocket.getInputStream());
clientSocket.setSoTimeout(200);
So I've been trying to figure out how to send messages between my arduino and my java servlet (tomcat) and have been bumping into some problems. I'm using this code in my java servlet:
ServerSocket server;
//socket server port on which it will listen
int port = 9876;
String message = "";
//create the socket server object
server = new ServerSocket(port);
System.out.println("Server socket created");
//keep listens indefinitely until receives 'exit' call or program terminates
while(true){
System.out.println("Waiting for client request...");
//creating socket and waiting for client connection
Socket socket = server.accept();
BufferedReader buff = new BufferedReader(new InputStreamReader (socket.getInputStream()));
System.out.println("Input stream established");
message = buff.readLine();
System.out.println("Message Received: " + message);
buff.close();
socket.close();
//terminate the server if client sends exit request
if(message.equalsIgnoreCase("exit")) break;
}
System.out.println("Shutting down Socket server!!");
//close the ServerSocket object
server.close();
But it's hanging on the
Socket socket = server.accept();
line. I assume this is because the messages i'm sending from the arduino aren't arriving. On the arduino side of things, this is my client:
if (client.connect(ip, 9876)) {
Serial.println("connected");
client.println("12345678");
Serial.println("Message sent");
} else {
Serial.println("connection failed");
}
Not much to it. Now, I've tried sending this as a HTTP POST request, but without success (surely i'm doing it incorrectly, i just don't know how to do it, i've tried for a while). I know this is probably horrifically written, but i'm very open to learning from you guys today.
The weird thing is that this works exactly how i want it to when i put the java server code in a new regular java project. Can anyone help me figure this out? Why is it hanging in the servlet but not in a regular java project?
I created a java chat application (client and server)
Everything works fine when I'm on my LAN (using LAN IP address of the server into my client).
But when I'm using the Internet address of my server in my client, the strings are sent only when I close the output Data stream of my client (and all the strings are sent at once).
Here's a quick snap of my code (I have port forward from 6791 to 6790 in the example below),
My server (thread):
// this line is actually on my global server class, used below with theServer
ServerSocket svrSocket= new ServerSocket(6790);
//wait for incoming connection
connectionSocket = svrSocket.accept();
connectionSocket.setSoTimeout(10000);
// free the accepting port
svrSocket.close();
//create a new thread to accept future connections (creates a new svrSocket)
theServer.openNewConnection();
//create input stream
BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
boolean threadRunning = true);
while (threadRunning) {
//System.out.println("thread: in the while");
try {
Thread.sleep(100);
clientSentence = inFromClient.readLine();
System.out.println(clientSentence);
}
catch...
}
My client:
InetAddress dnsName;
Socket clientSocket;
PrintWriter out;
dnsName = InetAddress.getByName("myAddress.me");
clientSocket = new Socket(dnsName.getHostAddress(), 6791);
Thread.sleep(10);
out = new PrintWriter(clientSocket.getOutputStream(), true );
int i=140;
while (i>130){
try {
out.println(Integer.toString(i));
out.flush();
Thread.sleep(200);
}
catch(Exception e) {
e.printStackTrace();
}
i--;
}
out.flush();
out.close();
clientSocket.close();
I've tried with DataOutStreams, there's nothing to do.
My server will only receive the strings when out.close() is called on client side.
Is there a reason why, over the Internet, the data stream has to be closed for data to be sent? Is there a way around this? Am I doing something wrong?