Getting Started with WebSocket and Server-Sent Events using Java by Arun Gupta

Getting Started with
WebSocket and Server-Sent
Event in Java
Arun Gupta
blogs.oracle.com/arungupta, @arungupta

Copyright © 2013, Oracle and/or its affiliates. All rights reserved.3 Copyright © 2012, Oracle and/or its affiliates. All rights reserved. Insert Information Protection Policy Classification from Slide 16
Program
Agenda
§  WebSocket Primer
§  Getting Started with WebSocket
§  Server-Sent Event Primer
§  Getting Started with Server-Sent Event
§  Resources

Interactive Web Sites
§  HTTP is half-duplex
§  HTTP is verbose
§  Hacks for Server Push
–  Polling
–  Long Polling
–  Comet/Ajax
§  Complex, Inefficient, Wasteful

WebSocket to the Rescue
§  TCP based, bi-directional, full-duplex messaging
§  Originally proposed as part of HTML5
§  IETF-defined Protocol: RFC 6455
–  Handshake
–  Data Transfer
§  W3C defined JavaScript API
–  Candidate Recommendation

Establish a connection
Client
Handshake Request
Handshake Response
Server

Handshake Request
GET /chat HTTP/1.1 
Host: server.example.com 
Upgrade: websocket 
Connection: Upgrade 
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== 
Origin: http://example.com 
Sec-WebSocket-Protocol: chat, superchat 
Sec-WebSocket-Version: 13 "

Handshake Response
HTTP/1.1 101 Switching Protocols 
Upgrade: websocket 
Connection: Upgrade 
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo= 
Sec-WebSocket-Protocol: chat "

ServerClient
Handshake Request
Handshake Response
Connected !
Establishing a Connection

Peer
(server)
Peer
(client)
Connected !
open open
close
message
error
message
message
message
message
Disconnected
WebSocket Lifecycle

WebSocket API
www.w3.org/TR/websockets/

Copyright © 2013, Oracle and/or its affiliates. All rights reserved.12 http://caniuse.com/websockets
Browser Support

Java API for WebSocket Features
§  API for WebSocket Endpoints/Client
–  Annotation-driven (@ServerEndpoint)
–  Interface-driven (Endpoint)
§  WebSocket opening handshake negotiation
–  Client (@ClientEndpoint)
§  Integration with Java EE Web container

Hello World
import javax.websocket.*; 
 
@ServerEndpoint("/hello") 
public class HelloBean { 
 
@OnMessage 
public String sayHello(String name) { 
return “Hello “ + name; 
} 
}"

Annotations
Annotation Level Purpose
@ServerEndpoint" class Turns a POJO into a WebSocket Endpoint
@ClientEndpoint" class POJO wants to act as client
@OnMessage" method Intercepts WebSocket Message events
@PathParam"
method
parameter
Flags a matched path segment of a URI-template
@OnOpen" method Intercepts WebSocket Open events
@OnClose" method Intercepts WebSocket Close events
@OnError" method Intercepts errors during a conversation

@ServerEndpoint Attributes
value"
Relative URI or URI template
e.g. /hello or /chat/{subscriber-level}
decoders" list of message decoder classnames
encoders" list of message encoder classnames
subprotocols" list of the names of the supported subprotocols

Custom Payloads
@ServerEndpoint( 
value="/hello", 
encoders={MyMessage.class}, 
decoders={MyMessage.class} 
) 
public class MyEndpoint { 
. . . 
}"
"
"

Custom Payloads – Text
public class MyMessage implements Decoder.Text<MyMessage>, Encoder.Text<MyMessage> { 
private JsonObject jsonObject; 
 
public MyMessage decode(String s) { 
jsonObject = Json.createReader(new StringReader(s)).readObject(); 
return this;"
}"
public boolean willDecode(String string) { 
return true; // Only if can process the payload 
}"
"
public String encode(MyMessage myMessage) { 
return myMessage.jsonObject.toString(); 
} 
}"

Custom Payloads – Binary
public class MyMessage implements Decoder.Binary<MyMessage>, Encoder.Binary<MyMessage> { 
 
public MyMessage decode(byte[] bytes) { 
. . . 
return this;"
}"
public boolean willDecode(byte[] bytes) { 
. . . 
return true; // Only if can process the payload 
}"
"
public byte[] encode(MyMessage myMessage) { 
. . . 
} 
}"

Which methods can be @OnMessage ?
§  Exactly one of the following
–  Text: String, Java primitive or equivalent class, String and boolean,
Reader, any type for which there is a decoder
–  Binary: byte[], ByteBuffer, byte[] and boolean, ByteBuffer and
boolean, InptuStream, any type for which there is a decoder
–  Pong messages: PongMessage"
§  An optional Session parameter
§  0..n String parameters annotated with @PathParam"
§  Return type: String, byte[], ByteBuffer, Java primitive or class
equivalent or any type for which there is a encoder

Sample Messages
§  void m(String s);"
§  void m(Float f, @PathParam(“id”)int id);"
§  Product m(Reader reader, Session s);"
§  void m(byte[] b); or void m(ByteBuffer b);"
§  Book m(int i, Session s, @PathParam(“isbn”)String
isbn, @PathParam(“store”)String store);"

Chat Server
@ServerEndpoint("/chat")"
public class ChatBean {"
static Set<Session> peers = Collections.synchronizedSet(…); 
 
@OnOpen 
public void onOpen(Session peer) { 
peers.add(peer); 
} 
 
@OnClose 
public void onClose(Session peer) { 
peers.remove(peer); 
} 
 
. . ."

Chat Server
. . . 
 
@OnMessage"
public void message(String message, Session client) {"
for (Session peer : peers) { 
peer.getBasicRemote().sendObject(message); 
} 
} 
}"

WebSocket Client
@ClientEndpoint 
public class HelloClient { 
@OnMessage 
public void message(String message, Session session) { 
// process message from server 
} 
} 
"
WebSocketContainer c = ContainerProvider.getWebSocketContainer(); 
c.connectToServer(HelloClient.class, “hello”);"
"

Interface-driven Endpoint
public class MyEndpoint extends Endpoint { 
 
@Override 
public void onOpen(Session session) { 
session.addMessageHandler(new MessageHandler.Text() { 
public void onMessage(String name) { 
try { 
session.getBasicRemote().sendText(“Hello “ + name); 
} catch (IOException ex) { 
} 
}  
}); 
} 
}"

How to view WebSocket messages ?
Capture traffic on loopback

How to view WebSocket messages ?
chrome://net-internals -> Sockets -> View live sockets

Server-Sent Events
§  Part of HTML5 Specification
§  Server-push notifications
§  Cross-browser JavaScript API: EventSource"
§  Message callbacks
§  MIME type: text/eventstream"

EventSource API
dev.w3.org/html5/eventsource/

Server-Sent Events Example
var url = ‘webresources/items/events’; 
var source = new EventSource(url);"
source.onmessage = function (event) { 
console.log(event.data); 
} 
source.addEventListener(“size”, function(event) {"
console.log(event.name + ‘ ‘ + event.data); 
}"
Client-side

private final SseBroadcaster BROADCASTER = new SseBroadcaster(); 
 
@GET 
@Path("events”) 
@Produces(SseFeature.SERVER_SENT_EVENTS) 
public EventOutput fruitEvents() { 
final EventOutput eventOutput = new EventOutput(); 
BROADCASTER.add(eventOutput); 
return eventOutput; 
}

@POST 
@Consumes(MediaType.APPLICATION_FORM_URLENCODED) 
public void addFruit(@FormParam("fruit")String fruit) { 
FRUITS.add(fruit); 
 
// Broadcasting an un-named event with the name of the newly added item in data 
BROADCASTER.broadcast(new OutboundEvent.Builder().data(String.class, fruit).build()); 
 
// Broadcasting a named "add" event with the current size of the items collection in
data 
BROADCASTER.broadcast(new OutboundEvent.Builder().name("size").data(Integer.class,
FRUITS.size()).build()); 
}

WebSocket and Server-Sent Event
Competing technologies ?
WebSocket Server-Sent Event
Over a custom protocol Over simple HTTP
Full Duplex, Bi-directional Server-Push Only, Client->Server
is out-of-band (higher latency)
Native support in most browsers Can be poly-filled to backport
Not straight forward protocol Simpler protocol

WebSocket and Server-Sent Event
Competing technologies ?
WebSocket Server-Sent Event
Pre-defined message handlers Arbitrary events
Application-specific Built-in support for re-connection
and event id
Require server and/or proxy
configurations
No server or proxy changes
required
ArrayBuffer and Blob No support for binary types

Resources
§  Java API for WebSocket
–  Specification: jcp.org/en/jsr/detail?id=356
–  Reference Implementation: java.net/projects/tyrus
–  Part of Java EE 7
–  Integrated in GlassFish Server 4.0
§  Server-Sent Event
–  Integrated in Jersey and GlassFish Server 4.0
–  Not part of Java EE 7

Getting Started with WebSocket and Server-Sent Events using Java by Arun Gupta

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