I am trying to implement a client-server application by myself. Its working so far, except the fact, that when I am accessing the port via another application (e.g. Web-Browser) the server application is blocking.
Is there any easy way/good practice to check, if the connecting application is the "client application"?
Here is an short example for a socket listening to port 8080. The socket awaits 2 Strings. If you now connect with the browser (localhost:8080) the connection gets established, but is waiting for the first UTF8.
public class MainSocket {
public static void main(String[] args) {
ServerSocket serverSocket;
try {
serverSocket = new ServerSocket(8080);
while (true) {
try {
Socket clientSocket = serverSocket.accept();
System.out.println("Connection established");
DataInputStream in = new DataInputStream(clientSocket.getInputStream());
System.out.println(in.readUTF());
System.out.println(in.readUTF());
in.close();
clientSocket.close();
} catch (Exception e) {
e.printStackTrace();
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
You're calling DataInputStream.readUTF8. That expects data like this:
First, two bytes are read and used to construct an unsigned 16-bit integer in exactly the manner of the readUnsignedShort method . This integer value is called the UTF length and specifies the number of additional bytes to be read. These bytes are then converted to characters by considering them in groups. The length of each group is computed from the value of the first byte of the group. The byte following a group, if any, is the first byte of the next group.
(etc)
In other words, this isn't just UTF-8. It's a slight variation on UTF-8 with a length prefix. That's not what a browser is going to write to the connection. Basically, DataInput and DataOutput are symmetric but not entirely general-purpose - they're usually used together, with one side reading via DataInput what the other side has written with DataOutput.
If you just want to read lines of UTF-8, you can use:
try (Reader inputReader = new InputStreamReader(clientSocket.getInputStream(), StandardCharsets.UTF_8);
BufferedReader reader = new BufferedReader(inputReader)) {
System.out.println(read.readLine());
System.out.println(read.readLine());
}
(Now it's not at all clear that you should expect a browser to write UTF-8 to the socket either, but the above is more likely to get you to the next step, I'd say...)
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 have created a server by using ServerSocket. After that, I have created Client using Socket, and connect to this server.
After that, I do "some stuff" with InputStream and OutputStream is taken from Socket Object. But, I don't really understand inputStream and outputStream so much. Here is my simple code :
private Socket sock = null;
private InputStream sockInput = null;
private OutputStream sockOutput = null;
...
String msg = "Hello World";
byte[] buffer = null;
try {
sockOutput.write(msg.getBytes(), 0, test.length());
sockOutput.write("Hello StackOverFlow".getBytes(), 0, test.length());
buffer = new byte[test.length()];
sockInput.read(buffer, 0, test.length());
System.out.println(new String(buffer));
sockInput.read(buffer, 0, test.length());
System.out.println(new String(buffer));
} catch (IOException e1) {
e1.printStackTrace();
}
The result will be : "Hello World" and "Hello StackOverFlow".
Here is server side code :
private int serverPort = 0;
private ServerSocket serverSock = null;
public VerySimpleServer(int serverPort) {
this.serverPort = serverPort;
try {
serverSock = new ServerSocket(this.serverPort);
}
catch (IOException e){
e.printStackTrace(System.err);
}
}
// All this method does is wait for some bytes from the
// connection, read them, then write them back again, until the
// socket is closed from the other side.
public void handleConnection(InputStream sockInput, OutputStream sockOutput) {
while(true) {
byte[] buf=new byte[1024];
int bytes_read = 0;
try {
// This call to read() will wait forever, until the
// program on the other side either sends some data,
// or closes the socket.
bytes_read = sockInput.read(buf, 0, buf.length);
// If the socket is closed, sockInput.read() will return -1.
if(bytes_read < 0) {
System.err.println("Server: Tried to read from socket, read() returned < 0, Closing socket.");
return;
}
System.err.println("Server: Received "+bytes_read
+" bytes, sending them back to client, data="
+(new String(buf, 0, bytes_read)));
sockOutput.write(buf, 0, bytes_read);
// This call to flush() is optional - we're saying go
// ahead and send the data now instead of buffering
// it.
sockOutput.flush();
}
catch (Exception e){
System.err.println("Exception reading from/writing to socket, e="+e);
e.printStackTrace(System.err);
return;
}
}
}
public void waitForConnections() {
Socket sock = null;
InputStream sockInput = null;
OutputStream sockOutput = null;
while (true) {
try {
// This method call, accept(), blocks and waits
// (forever if necessary) until some other program
// opens a socket connection to our server. When some
// other program opens a connection to our server,
// accept() creates a new socket to represent that
// connection and returns.
sock = serverSock.accept();
System.err.println("Server : Have accepted new socket.");
// From this point on, no new socket connections can
// be made to our server until we call accept() again.
sockInput = sock.getInputStream();
sockOutput = sock.getOutputStream();
}
catch (IOException e){
e.printStackTrace(System.err);
}
// Do something with the socket - read bytes from the
// socket and write them back to the socket until the
// other side closes the connection.
handleConnection(sockInput, sockOutput);
// Now we close the socket.
try {
System.err.println("Closing socket.");
sock.close();
}
catch (Exception e){
System.err.println("Exception while closing socket.");
e.printStackTrace(System.err);
}
System.err.println("Finished with socket, waiting for next connection.");
}
}
public static void main(String argv[]) {
int port = 54321;
VerySimpleServer server = new VerySimpleServer(port);
server.waitForConnections();
}
My question is :
When I use sockOutput.write and I can get back those message back by sockInput.read. So, those message has been saved, right? If this true, does it saved on Server I have created or just saved in some other thing such as Socket Object.
If I have written to socket String A1, A2,... An so I will receive A1, A2, ... An String respectively, right?
A socket is an abstraction that you use to talk to something across the network. See diagram below...
In Java, to send data via the socket, you get an OutputStream (1) from it, and write to the OutputStream (you output some data).
To read data from the socket, you get its InputStream, and read input from this second stream.
You can think of the streams as a pair of one-way pipes connected to a socket on the wall. What happens on the other side of the wall is not your problem!
In your case, the server has another socket (the other end of the connection) and another pair of streams. It uses its InputStream (2) to read from the network, and its OutputStream (3) to write the same data back across the network to your client, which reads it again via its InputStream (4) completing the round trip.
Client Server
1. OutputStream -->\ /--> 2. InputStream -->
Socket <--> network <--> ServerSocket |
4. InputStream <--/ \<--3. OutputStream <--
Updated: in reply to comment:
Note that the streams and sockets just send raw bytes; they have no notion of a "message" at this level of abstraction. So if you send X bytes and another X bytes, then read X bytes and read another X bytes, then your system behaves as if there are two messages, because that's how you've divided up the bytes.
If you send X bytes, and another X bytes, then read a reply of length 2X, then you might be able to read a single combined "message", but as you've noticed, the underlying implementation of the streams can choose when to deliver chunks of bytes, so it might return X bytes, then X bytes, later, or 2X at once, or 0.5X four times...
InputStream and OutputStream are two completely separate streams. What you write into one has no a priori relation to what you read from the other. The InputStream gives you whatever data the server decides to send to you. I would also like to comment on this piece of your code:
sockOutput.write(msg.getBytes(), 0, test.length());
sockOutput.write("Hello StackOverFlow".getBytes(), 0, test.length());
You use the length of a string test (not shown in your code), which has nothing to do with the byte array you are passing as the first argument. This can cause an ArrayIndexOutOfBoundsException or truncation of your intended message.
Additional comments to your updated question
Reviewing your server-side code, it is not quite correctly written. You need to have try { handleConnection(...); } finally { socket.close(); } to ensure proper cleanup after an error, as well as when completing normally. Your code never closes anything on the server side.
Finally, and most critically, your entire code is written in a way that can result in a deadlock. Normally you need a separate thread to read and to write; otherwise the following may happen:
You attempt to write some data to the output;
The server reads it and tries to respond with data in your input;
But, since the buffers are too small, you don't manage to send everything because the server wants to first send something to you, then receive the rest; but you don't get to the receiving part before you have sent everything you've got.
I am trying to send data from a C#.NET (Windows Application) program to a Java (Android App) program and vice versa, via TCP connection through Wifi. Till now I am success to send data from Java to C#, but unable to do so from C# to Java.
Following is the Java code, I used to create a connection and receive data:
ServerSocket serverSocket = null;
DataInputStream socketInputStream;
while (true) {
try {
String localIPAddr = getLocalIPAddress();
InetSocketAddress ipEndPoint = new InetSocketAddress(
InetAddress.getByName(localIPAddr), 8222);
serverSocket = new ServerSocket();
serverSocket.bind(ipEndPoint, 4);
workerSocket = serverSocket.accept();
socketInputStream = new DataInputStream(
workerSocket.getInputStream());
inputText.setText(socketInputStream.readUTF());
} catch (Exception ex) {
throw ex;
}
}
Here getLocalIPAddress() method returns the IP Address of the Android Device.
Following is the C# code in Windows Application to connect to the Android's IP Address (192.168.1.6) and send data to it:
Socket clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
void button1_Click(object sender, EventArgs e)
{
try
{
if (!clientSocket.Connected)
clientSocket.Connect(IPAddress.Parse("192.168.1.6"), 8222);
clientSocket.Send(Encoding.UTF8.GetBytes(txtInput.Text));
}
catch (Exception ex)
{
throw ex;
}
}
Well, client (C#) is failing to connect to the server (Java) That means data is not leaving from client. But it will, if it get connected. Please tell me what am I missing and where am I mistaken. :)
After you have launched your android app and it is connected to the wifi, did you try to do a ping to the ip where the application is launched.
ping 192.168.1.6
If the IP is accessible from the workstation where C# app is running, try to perform a telnet on the IP and Port of the android ip, to see whether it works or not.
telnet 192.168.1.6 8222
If either of the two steps fail then could be a problem in the wifi network. As i have noticed many times the firewall of the routers filters out all the ports except 8080 and 80. So you would need to open the ports on the router.
Did you try to do this?
Runnable showmessage = new Runnable() {
public void run() {
myTextView.setText(membervariabletext);
}
};
and from your thread, after the readUTF(), call
runOnUiThread(showmessage);
Found this here
Well, I have solved this myself, but of course Dilberted has helped me a little bit. I thank him for what he has provided. :)
Check out the solved Java code below:
ServerSocket serverSocket = null;
Socket workerSocket;
DataInputStream socketInputStream;
try {
if (serverSocket == null) {
// No need to get local IP address and to bind InetSocketAddress.
// Following single line make it very simple.
serverSocket = new ServerSocket(8222, 4);
workerSocket = serverSocket.accept();
}
// When data are accepted socketInputStream will be invoked.
socketInputStream = new DataInputStream(
workerSocket.getInputStream());
/* Since data are accepted as byte, all of them will be collected in the
following byte array which initialised with accepted data length. */
byte[] rvdMsgByte = new byte[socketInputStream.available()];
// Collecting data into byte array
for (int i = 0; i < rvdMsgByte.length; i++)
rvdMsgByte[i] = socketInputStream.readByte();
// Converting collected data in byte array into String.
String rvdMsgTxt = new String(rvdMsgByte);
// Setting String to the text view.
receivedMsg.setText(rvdMsgTxt);
} catch (Exception ex) {
throw ex;
}
Note that a separate thread is to be used to run this code in background.
I am writing a client application that will receive a continuous flow of data through tcp/ip. The problem I'm having is that the buffered reader object isn't receiving any data and is hanging at the readline method.
The way the server works is that you connect to it, and then send authentication information in order to receive data. The gist of my code is below
socket = new Socket(strHost, port);
authenticate();
inStream = new BufferedReader(new InputStreamReader(socket.getInputStream()));
process(inStream);
authenticate()
{
PrintWriter pwriter = new PrintWriter(socket.getOutputStream(), true);
pwriter.println(authString);
}
process(BufferedReader bufferedReader)
{
while((line = bufferedReader.readLine()) != null)
dostuff
}
I created a sample server application that sends data the way (I think) the server is sending data and it connects, and receives and processes the data fine. I can connect to the server fine in my application. I can also telnet to the server and write the authentication string and receive a flood of data using telnet. However my application just hangs at readLine with the server and I'm out of idea's why.
The data coming in (through telnet atleast) looks like a continuous stream of the following:
data;data;data;data;data
data;data;data;data;data
Why is my app hanging at readline, am I not outputting the authentication line correctly? I'm not receiving any errors...
EDIT
My sample server code (which is working correctly)...again this is only mimicking the way I think the real server is running but I can connect to both in my application just not receive data from the real server.
public static void main(String[] args) throws IOException
{
ServerSocket serverSocket = null;
try
{
serverSocket = new ServerSocket(1987);
}
catch (IOException e)
{
System.out.println("Couldn't listen on port: 1987");
System.exit(-1);
}
Socket clientSocket = null;
try
{
clientSocket = serverSocket.accept();
}
catch (IOException e) {
System.out.println("Accept failed: 1987");
System.exit(-1);
}
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String something;
while ((something = in.readLine()) != null)
{
while(true)
{
out.println(message);
}
}
out.close();
in.close();
clientSocket.close();
serverSocket.close();
}
Firstly you should call BufferedReader.ready() before calling readLine(), as the ready() will tell you if it's ok to read.
PrintWriter doesn't throw I/O Exception so the write may have failed without your knowledge which is why there is nothing to read. Use PrintWriter.checkError() to see if anything as gone wrong during the write.
You ought to set up the input and output streams on the Socket at the same time before you write anything down the pipe. If your reader is not ready when the other end tries to write you will get a broken pipe in the server and it won't send any more data. Telnet sets up read and write before you have written or read anything.
You can make use of Wireshark to tell if the server is actually sending data.
BufferdReader.readLine() reads lines, i.e. sequences of characters ended with \r or \r\n. I guess that your server writes its output into one single line. Your telnet output proves this assumption. Just use PrintWriter.println() at server side.
this work with me
with socket without flush
void start_listen()
{
String result1="";
char[] incoming = new char[1024];
while (!s.isClosed())
{
try {
int lenght = input.read(incoming);
result1 = String.copyValueOf(incoming,0,lenght);
}
catch (IOException e)
{
e.printStackTrace();
}
Log.d("ddddddddddd",result1);
}
I want to send multiple byte array from the client and server ?
I was able to send/Receive one byte array from client and send / Receive one byte array from server :
My code is like this :
server :
Socket sock=null;
ByteArrayOutputStream input=null;
OutputStream out=null;
InputStream in=null;
try{
ServerSocket server_sock=new ServerSocket(2972);
sock=server_sock.accept();
in =
sock.getInputStream();
out=sock.getOutputStream();
}catch(IOException e){
System.out.println(e.getMessage());
}
String word="";
//1-Receive
try{
ByteArrayOutputStream serverinput=new ByteArrayOutputStream();
int len=0;
byte[] buf=new byte[1000];
while ((len = in.read(buf))>=0) {
serverinput.write(buf, 0, len);
}
sock.shutdownInput();
word=new String(serverinput.toByteArray());
System.out.println("Client send 1"+word);
}catch(Exception e){
System.out.println(e.getMessage());
}
String st="Server is a king";
try{
out.write(st.getBytes());
out.flush();
}catch(Exception e){
System.out.println(e.getMessage());
}
client :
Socket sock=null;
OutputStream out=null;
InputStream in=null;
try{
sock=new Socket("127.0.0.1",2972);
}catch(IOException e){
System.out.println(e.getMessage());
}
String word="Hellow World" ;
try{
in =
sock.getInputStream();
out=sock.getOutputStream();
}catch(IOException e){
System.out.println(e.getMessage());
}
//1- send
try{
System.out.println("Your string is"+word+"converted to byte"+word.getBytes());
out.write(word.getBytes());
out.flush();
sock.shutdownOutput();
}catch(Exception e){
System.out.println(e.getMessage());
}
try{ ByteArrayOutputStream serverinput=new ByteArrayOutputStream();
int len=0;
byte[] buf=new byte[1000];
while ((len = in.read(buf))>=0) {
serverinput.write(buf, 0, len);
}
System.out.println("server send 1 "+new String(serverinput.toByteArray()));
System.out.println("Your string is"+word+"converted to byte"+word.getBytes());
}catch(Exception e){
System.out.println(e.getMessage());
}
This code is working fine for one submitting from client and server but it does not work when I want to send / receive more byte array ?
It is working only when I use shutdown because both client and server reading and writing to the data.
Therefore, I can not use the socket channel again ... is there is alternative solution? ...that does not lead to deadlock.
The problem you have is that you don't currently have any way to say when one byte array ends and the next one starts. (In your "one array" solution, the end of the byte array corresponds to the end of stream. And of course, once the stream has been ended / closed, it cannot be reopened without creating a new Socket, etcetera.)
The simple way to solve this is as follows, using DataOutputStream and DataInputStream pairs wrapped around the respective socket streams:
To send a byte array:
Convert data to bytes.
Send the byte array size using the DataOutputStream.writeInt(int) method.
Send the byte array using DataOutputStream.write(byte[]) method.
To receive a byte array:
Receive the byte array size using the DataInputStream.readInt() method.
Allocate a byte array of the required size.
Receive the bytes into the byte array using the DataInputStream.read(byte[], int, int) method ... repeatedly until you've gotten all of the bytes.
By sending the size of the byte array at the front, you tell the receiver how many bytes to read. You can repeat this process as many times as you need. The sender can indicate to the receiver that there are no more byte arrays to send by simply closing the socket stream.
Note - this is pseudo-code. I assume that you are capable of turning it into working Java.
Don't forget to insert BufferedInputStreams and BufferedOutputStreams into the respective stream chains ... to reduce system call overheads.
Try wrapping your socket streams in DataInputStream and DataOutputStream. That should allow you to do what you want.
You should really have a look at this tutorial : Reading from and Writing to a Socket
It seems to outline how to read and write to a socket. The reading should be as easy as creating a server socket and listening on the port you expect and then waiting for data.
You could also create an object that would hold your byte array and send it out with ObjectOutputStream, then use the writeObject(Object) method to send the info.