Java Tutorial

The easiest way to understand how Wave extensions work is to build a Wave robot. Robots are applications that interact with a Wave through the Wave protocol (HTTP interface). Currently, we only support robots hosted with Google App Engine. In the future, we will support any client architecture that implements the Wave protocol.

In this tutorial, we'll use the Java client library to develop a sample robot. (The concepts apply here equally well to users of the Python client library, though implementation details will be different.) In this brief tutorial, you will create a simple robot, upload it to App Engine, and see it working with Wave.

Before you get started, make sure you have the Java 6 development kit installed. You can determine if you have Java installed (and what version is installed) by executing the following command from the command line:

hostname$ java -version
java version "1.6.0_07"
Java(TM) SE Runtime Environment (build 1.6.0_07-b06-153)
Java HotSpot(TM) 64-Bit Server VM (build 1.6.0_07-b06-57, mixed mode)

Note: to enable Java 6 on Mac OS X, you may need to set Java 6 as the default using the /Applications/Utilties/Java/JavaPreferences application.

The Java Client Library

Development of Google Wave robots requires compiling with a client library. This tutorial uses the Java client library and some other auxiliary libraries for communication and authentication. You can download these libraries from the Wave Robot Java Client Library home page on Google Project Hosting: 

A Python client library is also available.

Setting Up App Engine

You can develop Wave robots and other Java web applications for App Engine using your favorite Java development tools. The Google Plugin for Eclipse makes it especially easy to develop App Engine applications with the Eclipse IDE. The Plugin includes the App Engine Java SDK, and adds several features to Eclipse for creating, testing and uploading projects.

Registering Your Application with App Engine

You will also want to register your robot using an Application ID at You are allowed to register up to 10 application IDs, and application registrations cannot be undone or deleted, nor can an application ID be changed after it is registered. If you wish to conserve your allotted application registrations, you may want to choose an application ID you know you will use for a future project. As well, you may wish to reserve one ID for testing purposes of your robots.

Go to the App Engine Administrator Console in your web browser. Sign in using your Google account, creating one if necessary. If you haven't used this account with App Engine before, you may also be prompted to verify your account using SMS and a mobile phone.

Under My Applications click the Create an Application button. Choose an application ID, and follow the prompts to complete the registration. The new application appears in the list. You can click its name to visit the Administrator Console for this application.

Setting Up Eclipse

To keep things brief, this tutorial assumes you are using Eclipse and the Google Plugin. All of the Plugin's features are also provided by the App Engine SDK as Apache Ant build tasks, and as command-line tools and Java classes. If you'd prefer not to use Eclipse or the Plugin, see the App Engine Java documentation for alternatives.

Download and install Eclipse from the Eclipse website. Be sure to get the Java EE bundle, which includes several useful features for web application development.

You can install the Google Plugin and the App Engine SDK using the Software Update feature of Eclipse.

If you are using Eclipse 3.5 (Galileo), use the following Software Update location:

If you are using Eclipse 3.4 (Ganymede), use the following Software Update location:

Select the Plugin and the App Engine SDK from the list of available software, then install. For more information on installing the Google Plugin for Eclipse, see the Google Plugin for Eclipse documentation and the App Engine docs for the Plugin.

Create a new App Engine project, as follows:

  1. Select File > New > Web Application Project. Alternatively, click the New Web Application Project button in the toolbar: The New Web Application Project button.
  2. The "Create a Web Application Project" wizard opens. For "Project name," enter a name for your project, such as Parroty. For "Package," enter an appropriate package name, such as parroty.
  3. Since we're not going to use Google Web Toolkit for this project, uncheck "Use Google Web Toolkit." Verify that "Use Google App Engine" is checked.
  4. Click Finish to create the project.

The wizard creates a directory structure for the project, including a src/ directory for Java source files, and a war/ directory for compiled classes and other files for the application, libraries, configuration files, static files and other data files. The wizard also creates a servlet source file and two configuration files. The complete directory structure looks like this:

        ...App Engine JARs... 

For more information on getting started with App Engine, see the App Engine Getting Started Guide for Java.

Hello, Robot!

To make this app into a robot, we need the Java robot application library from the Wave API SDK. We also need several addiitional libraries, also included with the SDK. These libraries are provided by the following JARs:

  • wave-robot-api.jar: The core robot API.
  • wave-model.jar: Models the components of a wave as classes.
  • gson.jar: Converts JSON into Java objects, and vice versa.
  • oauth.jar: Helps register the robots with OAuth.
  • commons-codec.jar: Computes OAuth-related information.

Copy these files from the SDK to the following directory in your project:


Refresh the project by selecting the File menu > Refresh. Select the Project menu > Properties, then in the Properties window, select the "Java Build Path" category. Click the "Libraries" tab, then click the Add JARs... button. Navigate to and select the new JARs, then click OK. Click OK to close the Properties window.

Event Handing

To implement the main event handler, create a servlet that extends the AbstractRobot class (from the package). This abstract class requires that you override and implement a few methods. Additionally, you'll want to implement method handlers for events which the robot should handle. These event handlers get fired when particular events occur while the Robot is listening on a Wave, and contain data (usually the wavelet) that gives the event context.

As your code manipulates the data, the API generates a list of operations to be performed by Wave. The servlet sends these operations back to Wave when the event handler exits. The AbstractRobot class takes care of processing the HTTP request, parsing event data, calling the event handlers, and communicating the operations back to Wave.

Let's create a simple robot that adds a greeting to a wave when it is added to the wave. Additionally, we will add a separate greeting whenever a new participant is added to the Wave. Edit src/parroty/, and give it the following contents:

package parroty;
public class ParrotyServlet extends AbstractRobot {
  protected String getRobotName() {
    return "Parroty";
  protected String getRobotAvatarUrl() {
    return "";
  protected String getRobotProfilePageUrl() {
    return "";
  public void onWaveletSelfAdded(WaveletSelfAddedEvent event) {
    Blip blip = event.getWavelet().reply("\nHi everybody!");
  public void onWaveletParticipantsChanged(WaveletParticipantsChangedEvent event) {
    for (String newParticipant: event.getParticipantsAdded()) {
      Blip blip = event.getWavelet().reply("\nHi : " + newParticipant);

Note that we override several methods:

  • getRobotName(), getRobotAvatarUrl() and getRobotProfilePageUrl() are fired when the Wave client requests identification of the robot. You must provide a handler for at least the getRobotName() method.
  • onWaveletSelfAdded() is fired when the robot is first added to a wave. Since robots (and humans) only act within the context of wavelets, note that this event handler acts on the context of Wavelet.
  • onWaveletParticipantsChanged() is fired whenever a participant is added or removed from the wave.

The latter two methods are event handlers. By defining an event handler, you indicate interest in an event occuring on the Wave server. When the Wave server notices that event occuring, it will dispatch the event and associated event data to the robot. Only events in which you've directly expressed interest in receiving will be sent to your robot.

Each event handler is typically passed a typed event containing the context for that event. For example the onWaveletSelfAdded() event handler will passed an event of type WaveletSelfAddedEvent. As this event is a WaveletXXX event, we can access the wavelet data through a getWavelet() method on that event. We then use the reply() method on the wavelet, which creates a new Blip and adds some text. We use this method to announce that the robot has been added, as well as welcome any new participants.

Note that each time we write a blip to the wavelet, we begin it with a newline character ("\n"). The current wave model requires all blips to start with this character.

Servlet Mapping

To map this servlet to the URL path /_wave/*, edit the file war/WEB-INF/web.xml so that it looks like this:

<?xml version="1.0" encoding="utf-8"?>
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
<web-app xmlns="" version="2.5">

This mapping ensures that anytime the Wave server communicates with the robot using the _wave/ path, it will invoke the Parroty servlet code we've written.

Robot Profiles

A Robot's Profile contains information about the robot, which a Wave client may use to display information identifying the robot (such as its avatar). Robots built using the Java client library create profiles by overriding methods in the AbstractRobot class, which responds to profile information requests.

To set up a Robot's profile, simply implement the following methods of the AbstractRobot class:

  • getRobotName() returns a Robot's name.
  • getRobotAvatarUrl() returns a Robot's imageUrl.
  • getRobotProfilePageUrl() returns a Robot's profileUrl, which should contain more information about this robot.
public class MyRobot extends AbstractRobot {
  public String getRobotName() {
    return "My Robot Name";
  public String getRobotAvatarUrl() {
    return "http://My Robot Image URL";
  public String getRobotProfilePageUrl() {
    return "http://My Robot Profile URL";

Robot Capabilities

The set of events which the robot handles implicitly define its capabilities. The Java client library automatically creates a document served at /_wave/capabilities.xml which indicates what events for which the robot has implemented event handlers. The following capabilities.xml file is similar to that which will be created by the above code:

<w:robot xmlns:w="">
    <w:capability name="WAVELET_SELF_ADDED" context="PARENT,CHILDREN"/>
    <w:capability name="WAVELET_PARTICIPANTS_CHANGED" context="PARENT,CHILDREN"/>

This capabilities file indicates that the robot handles Wave events named WAVELET_SELF_ADDED and WAVELET_PARTICIPANTS_CHANGED. These events are used internally by the Wave Robot protocol for communication between the Wave server and the robot. When the robot receives such events, it encapsulates any associated data (such as the current wavelet) and passes them off to your event handlers (onWaveletSelfAdded() and onWaveletParticipantsChanged() in this case).

Robots within the Wave API are versioned. This versioning allows the Wave system to detect when robots have changed and/or their capabilities have been altered. If you modify a robot's capabilities (by adding or removing monitored events, for example), or modifying the events to pass different data, the Wave system will check if the robot version is different than what it has cached. (The robot version is simply a hash string.) If so, Wave will refresh the capabilities.xml file and alter the system to generate any new events you've indicated interest in.

Deploying the Robot

You can test your new robot by deploying it to App Engine, then adding it to a wave.

Note: No mechanism currently exists to test Wave robots on your local machine with the App Engine development server. A future release of the Wave SDK will include tools to test robots locally before deploying them to App Engine.

Within Eclipse, edit the file war/WEB-INF/appengine-web.xml. Inside the <application> element, enter the application ID you registered. For example, if your registered application ID is parrotybot, your appengine-web.xml file might look like this:

<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="">

Note: the version value within this AppEngine configuration file does not reflect the robot version value used by Google Wave. We recommend you leave this value stable during development.

To deploy the application to App Engine, click the "Deploy to App Engine" button in the Eclipse toolbar: . Enter your Google account email address and password when prompted. Eclipse builds your project, then uploads it to App Engine.

You can check that your application is available by loading the following URL:

You should receive an XML capabilities file like the one shown below:

You can also check your application within App Engine by logging into

Adding the Robot to the Wave

You add a robot to a wave by adding it as a participant in the wave with which you want it to interact. To do so, you must first add the Robot's address to your existing contacts. (You must do this outside of the current wave.)

Within Wave, now create a new wave. Add your robot to the wave using its Wave ID, which is the App Engine application ID followed by (for example, The robot joins the wave, and adds its greeting.

Congratulations! You've built your first Wave Robot!

If you run into an issue when running your robot check out this article on Debugging Wave Robots.