Develop Reference

Use writeUTF and readUTF for http requests in Java

This is a a Java method that tries to crawl a designated web page. I am using writeUTF and readUTF for socket communications to a server.
static void get_html(String host, String page, int port) throws IOException {
Socket sock = new Socket(host, port);
String msg = MessageFormat.format("GET {0} HTTP/1.1\r\nHost: {1}\r\n\r\n", page, host);
DataOutputStream outToServer = new DataOutputStream(sock.getOutputStream());
DataInputStream inFromServer = new DataInputStream(sock.getInputStream());
InputStream stream = new ByteArrayInputStream(msg.getBytes(StandardCharsets.UTF_8));
BufferedReader buf = new BufferedReader(new InputStreamReader(stream));
String outMsg;
while ((outMsg = buf.readLine()) != null) {
System.out.println("Sending message: " + outMsg);
outToServer.writeUTF(outMsg);
String inMsg;
try {
inMsg = inFromServer.readUTF();
} catch (EOFException eof) {
break;
}
System.out.println(inMsg);
}
sock.close();
}
The reason I am writing it this way was to mimic the c code, where you have a while loop of send() making all deliveries from a buffer, and another while loop of recv() from a buffer untill it hits 'null'. When execute my code, it just hangs there, I suspect that is due to a call of readUTF before I finished sending all my messages. If this is the case, is there any way to fix it?

You can't do this. HTTP is defined as text lines. writeUTF() does not write text, it writes a special format starting with a 16-bit binary length word. Similarly the HTTP server won't reply with that format into your readUTF() call. See the Javadoc.
You have to use binary streams and the write() method, with \r\n as the line terminator. Depending on the output format you may or may not be able to use readLine(). Best not, then you don't have to write two pieces of code: use binary streams again.
In fact you should throw it all away and use HttpURLConnection. Implementing HTTP is not as simple as may hastily be supposed.

Java communication with TCP socket and PIC stuck in read()

I try to communicate with a java application to a µController in wifi (Flyport).
I have a problem with the java application :
It first create a socket to communicate with the Flyport server, then send a message and receive the Flyport answer.
Everything work fine until the read part. I'm polling the read() function of the BufferedReader until it return -1, but it doesn't. The first read works fine, all the answer are red, but the application stay stuck when it tries to read again.
My code is very simple :
Java application :
try (
Socket clientSocket = new Socket(hostName, portNumber);
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in =
new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()));
)
{
...//Connection and sending message works fine
}
char[] buffer = new char[500];
while ((in.read(buffer)) != -1) { // first read() works fine, second read() stay stuck...
System.out.println(buffer); // display all answer sent by flyport
}
The code in the flyport :
while(isClientConnected){
//check if client is still connected
...
//read client message
while((RxLen=TCPRxLen(sock))>0)
{
TCPRead(sock,bff,RxLen);
strcat(msg,bff);
}
//write back to the client that the order is received
TCPWrite(sock, msg, strlen(msg));
//process the client order
...
//Write to the client that the process is done
TCPWrite(sock, msg2, strlen(msg2));
}
The java application read msg and msg2 with the first read(). msg and msg2 have "\r\n" at the end.
Doesn't somebody can tell me where I am wrong ?
Is there a function from BufferedReading that tells how much data there is left to read ?
Thanks and regards.
NB : I try with a small buffer in the java application, the problem is the same, read() is stuck when there is nothing left to read...

You're reading from the socket until end of stream, and you're never causing end of stream, as you are never closing the socket at the sender. Either close the socket or don't read until end of stream.

How to handle sending multiple messages over a socket connection?

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() )

Confused About Reading and Writing From Input Streams (Sockets)

I have a question about reading and writing from input streams related to a socket. For instance, I have
ServerSocket testie = new ServerSocket(0);
Socket putClient = new Socket("localhost",testie.getLocalPort());
InputStream is = putClient.getInputStream();
OutputStream os = putClient.getOutputStream();
os.write("testt".getBytes());
System.out.println(convertStreamToString(is));
but "testt" is oddly never printed. The method convertStreamToString is from
Read/convert an InputStream to a String
public static String convertStreamToString(java.io.InputStream is) {
java.util.Scanner s = new java.util.Scanner(is).useDelimiter("\\A");
return s.hasNext() ? s.next() : "";
}
Is there some flaw in my understanding of streams and sockets. I thought that if you wrote to a socket's output stream you can retrieve it from its input stream? The method just hangs indefinitely.

If you write to a socket's output stream you can read from the remote socket's input stream. In your code you are writing to one side of the socket and then trying to read from that same side.
Unfortunately, you haven't even created the remote socket yet. The next thing you will want to do is call accept() on the ServerSocket. This will receive the connection from putClient and create a Socket. This new socket will be the server side socket. Essentially you will have the following:
//Server Thread
ServerSocket testie = new ServerSocket(0);
Socket remoteSocket = testie.accept();
remoteIs = remoteSocket.getInputStream();
remoteOs = remoteSocket.getOutputStream();
System.out.println(new Scanner(remoteIs).next());
//Client Thread
Socket putClient = new Socket("localhost",testie.getLocalPort());
InputStream is = putClient.getInputStream();
OutputStream os = putClient.getOutputStream();
os.write("testt\n".getBytes());
os.flush();
Notice that I said two different threads. The problem is that the call testie.accept() will block until it gets a connection. But also, new Socket("localhost",testie.getLocalPort()) will block until the connection is established. These calls can't be made on the same thread.
UPDATE:
It's probably worth pointing out that I'm not very confident in that convertStringToStream method. You might want to start with something simple like what I've fixed the above to (note the newline in the write call).

How do I read a server reply without it blocking me?

I'm writing a proxy and have the following code:
Socket conUser;
Socket conDest;
try{
ServerSocket ss = new ServerSocket(Integer.parseInt(p.getProperty("proxy.port")));
while(true){
//Connect to user
conUser = ss.accept();
BufferedReader inputFromUser = new BufferedReader(new InputStreamReader(conUser.getInputStream()));
BufferedWriter outputToUser = new BufferedWriter(new OutputStreamWriter(conUser.getOutputStream(), "UTF8"));
//Get user request
StringBuffer req = new StringBuffer();
getUserRequest(inputFromUser, req);
System.out.println("User requested the following:");
System.out.println(req);
//Connect to server
InetAddress a = InetAddress.getByName(determineHost(req));
conDest = new Socket(a,80);
//Send request to server
BufferedWriter outputToServer = new BufferedWriter(new OutputStreamWriter(conDest.getOutputStream(), "UTF8"));
InputStreamReader inputFromServer = new InputStreamReader(conDest.getInputStream(), "UTF8");
outputToServer.write(req.toString());
outputToServer.flush();
System.out.println("==============================");
System.out.println("Server replied with the following:");
//Read reply from the server
//=========================================
int chars;
while ((chars = inputFromServer.read()) != -1) {
System.out.print((char)chars);
outputToUser.write(chars);
outputToUser.flush();
//serverReply.append(chars);
}
//Relay reply to user
//outputToUser.write(serverReply.toString());
//System.out.println(serverReply);
//outputToUser.flush();
conUser.close();
conDest.close();
}
}
catch (Exception e) {
System.err.println(e);
}
What happens is: I make a connection and it succeeds. I also send the request, and that succeeds too. I also get a reply, and am able to load the entire page's HTML, except that the read doesn't seem to terminate when it reaches the end of the content.
Specifically, I was attempting to load Google's homepage and the chunked transfer reached 0 (that is- end of chanked transfer), and thus there should've been no more input to read, but this did not cause the loop to stop reading. What's also strange to me is that pretty much all code examples of proxies do use this loop, and assuming they work, I don't see much differences between their code and mine.
How do I make the loop terminate correctly?
EDIT: for the record, yes- I know that the TCP connection should be kept open to handle further connections. This is not relevant to the problem I'm having. I need to get this loop to terminate per response.

In general the connection is not closed at the end of each response. Creating TCP connections is relatively time-consuming so the connection is left open, ready for you to send your next request.
Here are a couple of explanatory links:
http://en.wikipedia.org/wiki/HTTP_persistent_connection
http://en.wikipedia.org/wiki/HTTP_pipelining

If you want to terminate connection correctly after receiving HTTP response, your simple loop is not enough. You have to determine the end of message as described in section 4.4 Message Length of RFC 2616 and then close the connection.
However, it would be better to use existing libraries, such as built-in URLConnection.