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Use the Java EventSource API

Note: To use the Gateway, a KAAZING client library, or a KAAZING demo, fork the repository from kaazing.org.

This procedure describes how you can use the EventSource API--provided by the Kaazing Java client library--in Java. This API allows you to take advantage of the server-sent events standard as described in the HTML5 specification. For example, you can create a Java applet or stand-alone Java application that uses the Java HTML5 Communications client library to receive streaming data from a news feed or streaming financial data. The support for server-sent events is provided by the EventSource class and its supporting classes.

The following steps show you how to use the EventSource API in a Java applet or stand-alone Java application. This example highlights some of the most commonly used EventSource methods and is not meant to be an end-to-end tutorial. Refer to the EventSource API documentation for a complete description of all the available methods. View the out of the box Server Sent Events demo code in GATEWAY_HOME/demo/java/src/core/com/kaazing/net/sse/demo/ServerSentEventsApplet.java. The example code below is taken from this demo.

Before You Begin

This procedure is part of Build Java WebSocket Clients:

  1. Set Up Your Development Environment
  2. Use the Java WebSocket API
  3. Use the Java EventSource API
  4. Migrate WebSocket and ByteSocket Applications to KAAZING Gateway 5.0
  5. Secure Your Java and Android Clients
  6. Display Logs for the Java Client
  7. Troubleshoot Your Java Client

To Use the EventSource API in Java

  1. Add the necessary import statements:

    // Import java.net classes
import java.net.URI;
import java.net.URL;

// Import EventSource API classes
import org.kaazing.net.sse.SseEventReader;
import org.kaazing.net.sse.SseEventSource;
import org.kaazing.net.sse.SseEventSourceFactory;
import org.kaazing.net.sse.SseEventType;

  2. Create a new SseEventSource object and connection using the Server Sent Events factory class:

    SseEventSource eventSource = null; // Create a variable for the Event Source

// Create Event Source factory
SseEventSourceFactory factory = SseEventSourceFactory.createEventSourceFactory();

// Create a target location using the java.net.URI create() method
eventSource = factory.createEventSource(URI.create(url.getText()));

// Connect to the event source.                
eventSource.connect();

  3. Use the SseEventReader class to create a new reader object, and then use getEventReader() method to receive events:

    Thread sseEventReaderThread = new Thread() {
    public void run() {
      try {
          SseEventReader reader = eventSource.getEventReader(); // Receive event stream
  
          SseEventType type = null;
          while ((type = reader.next()) != SseEventType.EOS) { // Wait until type is DATA
              switch (type) {
                  case DATA:
                      // Return the payload of the last received event
                      logArea.setText("<html>" + reader.getData() + "</html>");
                      break;
                  case EMPTY:
                      logArea.setText("<html>" + "</html>");
                      break;
              }
          }
  
      }
      catch (Exception ex) {
          logArea.setText("Exception: " + ex.getMessage());
      }
    }
};

    Notes:

    • The SseEventReader class has a next() method that causes the thread to block until an event is received.
    • The getData() method returns the payload of the last received event. This is not a blocking call. This method should be invoked after next() only if the returned type is SseEventType.DATA. Otherwise, an IOException is thrown.
    • Receiving events is performed in a separate thread. In UI-based Java clients, receiving events should be done on a separate thread to avoid blocking the java.awt EventDispatchThread.
    • A switch block is used to manage different types of events. The SseEventType class is used to identify the type. It has three values: EOS for end of stream, EMPTY for empty events, and DATA for events.

  4. Later, you can call the close() method in case you want to stop listening to messages from a particular event source.

    eventSource.close();

Next Step

Migrate WebSocket and ByteSocket Applications to KAAZING Gateway 5.0