This manual is for GNU WebSocket4J, version 1.3.
This chapter describes WebSocket protocol and it's GNU WebSocket4J implementation.
WebSocket is a protocol that provides bi-directional communication over a TCP socket, designed to be implemented in web browsers for use in HTML5 applications. While main use of this protocol is to allow a stable communication channel between a web server and JavaScript running in a web browser the protocol can be used by any application.
JavaScript WebSocket API is being standardised by W3C (latest draft can
be obtained from <http://dev.w3.org/html5/websockets/>).
WebSocket protocol is being standardised by the IETF. Latest standard
can be downloaded from
<http://www.whatwg.org/specs/web-socket-protocol/>.
GNU WebSocket4J is an implementation of WebSocket protocol in Java. GNU WebSocket4J tries to provide a simple API for both client and server sockets that would be as easy to use, as the WebSocket JavaScript API, or TCP sockets in Java standard library.
See Accepting connections, if you want to create a WebSocket server application.
See Connecting to server, if you want to create a WebSocket client application talking to a WebSocket server.
This chapter tells how to use GNU WebSocket4J in a server application.
Throughout this chapter WebServerSocket will refer to
websocket4j.server.WebServerSocket class and WebSocket
will refer to websocket4j.server.WebSocket class.
Accepting WebSocket connections is similar to using TCP sockets from the
standard library. You have to create an instance of
WebServerSocket (modelled after java.net.ServerSocket),
and then you can use accept() : WebSocket method to get incoming
connections. Accepted sockets have successfully completed an opening
handshake and are ready to use (see Using an established socket).
websocket4j.server.WebServerSocket has two constructors, first
without any parameters and second taking an Integer. Second one
creates a new socket listening on port specified as an argument, and the
first one chooses a random free port. If you create a socket listening
on a random port, you can use getLocalPort() : Integer method to get that
port number.
accept() : WebSocket works similar to method, of
java.net.ServerSocket class, of the same name. It waits until
someone connects and returns an established socket. You can limit the
time for which accept will block with setSoTimeout(Integer)
: void method (provided Integer is time in milliseconds) — if
no client connects in this much time a java.io.IOException will
be thrown.
After you've accepted a socket, you can use getRequestUri() :
String method of WebSocket to get URI requested by client, so
you can decide what to do next with this socket.
Both WebServerSocket and WebSocket have close() :
void method that closes the socket and ends connection in case of
WebSocket, or stops listening for new connections in case of
WebServerSocket. isClosed() : Boolean method can be used
to determine if socket is already closed.
This chapter tells how to use GNU WebSocket4J in a client application.
Throughout this chapter WebSocket will refer to
websocket4j.client.WebSocket class.
WebSocket class works similar to java.net.Socket — you
have to create an instance providing information where it should connect
to and you get back a socket that is ready to use (see Using an established socket).
WebSocket has two constructors, first one takes a String,
Integer and String, second one takes an additional
Integer. First String is the host name of the server you
want to connect to, first Integer is port on which the server
listens for incoming connections. Second String is URI that
should be requested when connecting — just as HTTP servers, WebSocket
servers can handle multiple kinds of connections over one port.
For example new WebSocket("localhost", 5432, "/echo") would
connect to localhost on port 5432 and request /echo URI. In JavaScript
API you'd use ws://localhost:5432/echo URI to connect to the same
server.
Fourth, optional, argument of the constructor is timeout in milliseconds, if the opening handshake takes more time than this, an IOException will be thrown.
Just like the server socket, client sockets have close() : void
method that ends the connection. Also you can use isClosed() :
Boolean method to check if the socket is already closed.
Once a connection is established and you have an instance of either
websocket4j.client.WebSocket, or
websocket4j.server.WebSocket you can use them in uniform way to
send and receive messages.
Throughout this chapter WebSocket will refer to both
websocket4j.client.WebSocket and
websocket4j.server.WebSocket.
To send messages you can use sendMessage(String) : void method of
WebSocket. This method immediately sends given string to the other
party.
To receive messages you can use synchronous getMessage(void) :
String method of WebSocket. This method waits until the other party
sends a message and then returns it.
This chapter shows how to build a simple threaded echo server. This example is included in the distributed package in 'src/websocket4j/examples/EchoServer.java'.
package websocket4j.examples;
/**
* This file is part of GNU WebSocket4J.
* Copyright (C) 2010 Marek Aaron Sapota
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
* As a special exception, the copyright holders of this library give you
* permission to link this library with independent modules to produce an
* executable, regardless of the license terms of these independent modules,
* and to copy and distribute the resulting executable under terms of your choice,
* provided that you also meet, for each linked independent module, the terms and
* conditions of the license of that module. An independent module is a module
* which is not derived from or based on this library. If you modify this library,
* you may extend this exception to your version of the library, but you are not
* obligated to do so. If you do not wish to do so, delete this exception
* statement from your version.
*/
import java.io.IOException;
import websocket4j.server.WebServerSocket;
import websocket4j.server.WebSocket;
/**
* Example presenting an echo server using WebSocket4j.
*/
public class EchoServer extends Thread {
private WebSocket ws;
public EchoServer(WebSocket ws) {
this.ws = ws;
}
private void handleConnection() {
try {
while (true) {
String message = ws.getMessage();
ws.sendMessage(message);
System.out.println("Received: " + message);
if (message.equals("exit"))
break;
}
} catch (IOException e) {
}finally {
try {
ws.close();
} catch (IOException e) {
}
}
}
public void run() {
handleConnection();
}
public static void main(String[] args) throws IOException {
WebServerSocket socket = new WebServerSocket(5555);
try {
while (true) {
WebSocket ws = socket.accept();
System.out.println("GET " + ws.getRequestUri());
if (ws.getRequestUri().equals("/echo"))
(new EchoServer(ws)).start();
else {
System.out.println("Unsupported Request-URI");
try {
ws.close();
} catch (IOException e) {
}
}
}
} finally {
socket.close();
}
}
}
This chapter shows how to build a simple echo client reading from stdin. This example is included in the distributed package in 'src/websocket4j/examples/EchoClient.java'.
package websocket4j.examples;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import websocket4j.client.WebSocket;
/**
* This file is part of GNU WebSocket4J.
* Copyright (C) 2010 Marek Aaron Sapota
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*
* As a special exception, the copyright holders of this library give you
* permission to link this library with independent modules to produce an
* executable, regardless of the license terms of these independent modules,
* and to copy and distribute the resulting executable under terms of your choice,
* provided that you also meet, for each linked independent module, the terms and
* conditions of the license of that module. An independent module is a module
* which is not derived from or based on this library. If you modify this library,
* you may extend this exception to your version of the library, but you are not
* obligated to do so. If you do not wish to do so, delete this exception
* statement from your version.
*/
/**
* Example presenting an echo client using WebSocket4J.
*/
public class EchoClient {
public static void main(String[] args) throws IOException {
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
WebSocket ws = new WebSocket("localhost", 5555, "/echo");
while (true) {
String message = in.readLine();
ws.sendMessage(message);
System.out.println("Response: " + ws.getMessage());
if (message.equals("exit"))
break;
}
ws.close();
}
}
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