Server Sent Events, Async Servlet, Web Sockets and JSON; born to work together!

Server Sent Events, Async Servlet,
WebSockets and JSON; born to
work
Masoud Kalali: Principal Software Bhakti Mehta: Principal Member of Technical
Engineer at ORACLE Staff at ORACLE
Blog: Http://kalali.me Blog: http://www.java.net/blogs/bhaktimehta
Twitter: @MasoudKalali Twitter: @bhakti_mehta

1Copyright © 2012, Oracle and/or its affiliates. All rights reserved. Insert Information Protection Policy Classification from Slide 13

Program Agenda
§ Introduction
§ Polling
§ Server Sent Events (SSE)
§ WebSockets
§ JSON-P
§ JAXRS 2.0
§ AsyncServlet
§ Demo
§ Q&A

Copyright © 2012, Oracle and/or its affiliates. All rights reserved.

Polling
§ Used by vast majority of AJAX applications
§ Poll the server for data
§ Client --->request--> Server
§ If no data empty response is returned


Polling Drawbacks
§ Http overhead
§ Reducing the interval will consume more
bandwidth and processing resources.


Long Polling
§ Uses persistent or long-lasting HTTP connection between the server
and the client
§ If server does not have data holds request open
§ COMET


Long Polling Drawbacks
§ Missing error handling
§ Involves hacks by adding script tags to an infinite iframe
§ Hard to check the state of request


Server Sent Events
§ Unidirectional channel between server and client
§ Server pushes data to your app when it wants
§ No need to make initial request
§ Updates can be streamed froms server to client as they happen


Polling vs Long Polling vs Server Sent Events
§ Polling: GET. If data, process data. GET...
1 GET = 1 HTTP response header and maybe a chunk of data.

§ Long-polling: GET. Wait. Process data. GET...
1 GET = 1 HTTP response header and chunks of data.

§ SSE: Subscribe to event stream. Wait. Process data. Wait. Process
data. Wait..
1 GET = 1 HTTP response header, many chunks of data


Server Sent Events and EventSource
§ Subscribing to event stream
To subscribe to an event stream, create an EventSource object and
pass it the URL of your stream:
Example in javascript
eventSource = new EventSource(url);
eventSource.onmessage = function (event) {
}

§ Setting up handlers for events
You can optionally listen for onopen and onerror:


Server Sent Events and Message format
§ Sending an event stream
§ Construct a plaintext response, served with a text/event-stream
Content-Type, that follows the SSE format.
§ The response should contain a "data:" line, followed by your
message, followed by two "n" characters to end the stream:
§ Sample message format

data: My messagenn


Server Sent Events and JSON
§ Sending JSON
You can send multiple lines without breaking JSON format by
sending messages like this
data: {n
data: "name": "John Doe",n
data: "id": 12345n
data: }nn

§ On client side
source.addEventListener('message', function(e) {
var data = JSON.parse(e.data);


Server Sent Events and Reconnection
§ If the connection drops, the EventSource fires an error event and
automatically tries to reconnect.
§ The server can also control the timeout before the client tries to
reconnect.


Server Sent Events and Jersey 2.0 apis

§ Client apis in Jersey 2.0
Client client = ClientFactory.newClient();
WebTarget webTarget= client.target(new URI(TARGET_URI)) ;
§ EventSource apis in Jersey 2.0
EventSource eventSource = new EventSource(webTarget, executorService) {
@Override
public void onEvent(InboundEvent inboundEvent) {
// get the data from the InboundEvent

}


Best practices for ServerSentEvents

§ Check if eventSource's origin attribute is the expected
domain to get the messages from
if (e.origin != 'http://foo.com') {
alert('Origin was not http://foo.com');
return;



§ Check that the data in question is of the expected format..
§ Only accept a certain number of messages per minute to avoid DOS
§ This will avoid cases where attackers can send high volume of
messages and receiving page does expensive computations



§ Associating an ID with an event
Setting an ID lets the browser keep track of the last event fired
●
Incase connection is dropped a special Last-Event-ID is set
with new request
●
This lets the browser determine which event is appropriate to
fire. The message event contains a e.lastEventId property.


WebSockets
§ Full duplex communication in either direction
§ A component of HTML5
§ API under w3c, protocol under IETF(RFC 6455)
§ Good support by different browsers
§ Use existing infrastructure


WebSockets Client Server Handshake
§ Client and Server upgrade from Http protocol to WebSocket protocol
during initial handshake
GET /text HTTP/1.1rn
Upgrade: WebSocketrn
Connection: Upgradern
Host: www.websocket.orgrn …rn
§ Handshake from server looks like
HTTP/1.1 101 WebSocket Protocol Handshakern
Upgrade: WebSocketrn
Connection: Upgradern …rn


WebSockets Code Snippet
§ After the upgrade HTTP is completely out of the picture at this point.
§ Using the lightweight WebSocket wire protocol, messages can now
be sent or received by either endpoint at any time.

§ Creating a Websocket
ws = new WebSocket("ws://localhost:8080/../WebSocketChat");

§ You can set handlers for events onopen or onerror


WebSockets API JSR 356
§ @WebSocketEndpoint
signiﬁes that the Java class it decorates is to be deployed as
a WebSocket endpoint.
§ Additionally the following components can be annotated with
@WebServiceEndpoint
●
a stateless session EJB
●
a singleton EJB
●
a CDI managed bean


WebSockets Annotations
§ Decorate methods on @WebSocketEndpoint annotated Java class
with
●
@WebSocketOpen to specify when the resulting endpoint
receives a new connection
●
@WebSocketClose to specify when the connection is closed.


WebSockets and security
§ WebSockets URI starts with ws/wss
§ Conform to the same security that already exists at container level


Java API for Processing JSON (JSON-P)
JSR 353
§ Streaming API to produce/consume JSON
§ Similar to StAX API in XML world
§ Object model API to represent JSON
§ Similar to DOM API in XML world


JsonParser
§ JsonParser – Parses JSON in a streaming way from input sources
§ Similar to StAX’s XMLStreamReader, a pull parser
§ Parser state events :
START_ARRAY, START_OBJECT, KEY_NAME, VALUE_STRING, VALUE_NUMBER,
VALUE_TRUE, VALUE_FALSE, VALUE_NULL, END_OBJECT, END_ARRAY

§ Created using :
Json.createParser(…),

Json.createParserFactory(...).createParser(…)


JSON sample data
{

"firstName": "John", "lastName": "Smith", "age": 25,

"phoneNumber": [

{ "type": "home", "number": "212 555-1234" },

{ "type": "fax", "number": "646 555-4567" }]}

]

}


JSonParser
Iterator<Event> it = parser.iterator();

Event event = it.next(); // START_OBJECT

event = it.next(); // KEY_NAME

event = it.next(); // VALUE_STRING

String name = parser.getString(); // "John”


JsonGenerator
§ JsonGenerator – Generates JSON in a streaming way to output
sources
§ Similar to StAX’s XMLStreamWriter
§ Created using :
Json.createGenerator(…),

Json.createGeneratorFactory(...).createGenerator(…)


JsonGenerator
JsonArray address= Json.createGenerator().beginArray() [

.beginObject() {

.add("type", "home”).add("number", "212 555-1234") "type": "home", "number": "212 555-1234"

.endObject() }

.beginObject() ,{

.add("type", "fax”).add("number", "646 555-4567") "type": "fax", "number": "646 555-4567"

.endObject() }

.endArray(); ]


Object Model API
§ JsonObject/JsonArray – JSON object and array structure
§ JsonString and JsonNumber for string and number value
§ JsonBuilder – Builds JsonObject and JsonArray Programmatically
§ JsonReader – Reads JsonObject and JsonArray from input source
§ JsonWriter – Writes JsonObject and JsonArray to output source


JsonReader
§ Reads JsonObject and JsonArray from input source
§ Uses pluggable JsonParser

try (JsonReader reader = new JsonReader(io)) {

JsonObject obj = reader.readObject();

}


JsonWriter
§ Writes JsonObject and JsonArray to output source
§ Uses pluggable JsonGenerator

// Writes a JSON object

try (JsonWriter writer = new JsonWriter(io)) {

writer.writeObject(obj);

}


JAX-RS 2.0
§ New in JAX-RS 2.0
§ Client API
§ Filters and Interceptors
§ Client-side and Server-side Asynchronous
§ Improved Connection Negotiation
§ Validation Alignment with JSR 330


JAXRS 2.0 Client API
§ Code snippet


WebTarget webTarget= client.target(new URI(TARGET_URI)) ;

webTarget.request().post(Entity.text(message));


JAXRS 2.0 and Asynchronous support
@Path("/async/longRunning")

public class MyResource {

@Context private ExecutionContext ctx;

@GET @Produces("text/plain")

public void longRunningOp() {

Executors.newSingleThreadExecutor().submit( new Runnable() {

public void run() {

Thread.sleep(10000); // Sleep 10 secs

ctx.resume("Hello async world!");

} }); ctx.suspend(); // Suspend connection and return

}…}


JAXRS 2.0 and @Suspend annotation
@Path("/async/longRunning")

public class MyResource {

@Context private ExecutionContext ctx;

@GET @Produces("text/plain") @Suspend

public void longRunningOp() {

Executors.newSingleThreadExecutor().submit( new Runnable() {

public void run() {

Thread.sleep(10000); // Sleep 10 secs

ctx.resume("Hello async world!");

} });

//ctx.suspend(); // Suspend connection and return

}…}

Async Servlet components
§ Thread Pool and Queue
§ AsyncContext
§ Runnable instance
§ Servlet/Filter chain


DEMO


Demo class diagram

Create EventSource
ParseJson data
Display in servlet

Writes the message
Gets data from on the EventChannel
twitter search
apis


AsyncServlet code sample
protected void service(final HttpServletRequest request, final HttpServletResponse response)
throws ServletException, IOException {
final AsyncContext asyncContext = request.startAsync();
asyncContext.setTimeout(TIMEOUT);
asyncContext.addListener(new AsyncListener() {
// Override the methods for onComplete, onError, onAsyncStartup
}

Thread t = new Thread(new AsyncRequestProcessor(asyncContext));
t.start();
}


AsyncServlet code sample
class AsyncRequestProcessor implements Runnable {
@Override
public void run() {
webTarget = client.target(new URI(TARGET_URI));
EventSource eventSource = new EventSource(webTarget, executorService) {
public void onEvent(InboundEvent inboundEvent) {
try {
//get the JSON data and parse it
}


Trying the sample

Clone the sample from the following repo:
https://github.com/kalali/jersey-sse-twitter-sample/
Follow the readme which involves:
● Do a mvn clean install

● Get latest GlassFish build

● Deploy the application

● Hit the http://localhost:8080/jersey-sse-twitter-sample/TestClient


Related sessions at JavaOne
●
HOL4461: Developing JAX-RS Web Applications Utilizing Server-
Sent Events and WebSocket Tuesday, Oct 2, 4:30 PM - 6:30 PM
●
CON4435: JAX-RS 2.0: New and Noteworthy in the RESTful Web
Services API Tuesday, Oct 2, 1:00 PM - 2:00 PM
●
CON3566: JSR 353: Java API for JSON Processing Wednesday,
Oct 3, 10:00 AM - 11:00 AM
●
CON7001: HTML5 WebSocket and Java Wednesday, Oct 3, 4:30
PM - 5:30 PM


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