Draconian Overlord

Oppressing software entropy

gwt-mpv-apt 1.1

Update 2011/12/29: This post was originally about the gwt-mpv-apt project, which has since been merged into/rechristened Tessell, see tessell.org.

gwt-mpv-apt is a annotation processor (natively supported by Eclipse and javac) that generates gwt-dispatch and GwtEvents for GWT projects.

The benefit is a 10x decrease in boilerplate code, which is something every GWT project needs.

The github page is pretty self-explanatory, but the basic idea is that gwt-mpv-apt can generate both your Action and Result command-pattern DTOs and then also your GWT event DTOs from “spec” classes that contain only the minimal amount of information needed to derive the resulting DTOs.

Manually Creating Action/Result DTOs

Let’s start out without gwt-mpv-apt and see how things typically work.

For example, if you were writing a gwt-dispatch Action/Result by hand for creating a new User, you’d have to:

  1. Make a NewUserAction class
  2. Have it implement the Action<NewUserResult> interface
  3. Add a field/getter/setter for String username
  4. Add a protected NewUserAction constructor for serialization
  5. Add a public constructor for NewUserAction(String username) for real usage
  6. Add a default serialVersionUID to avoid the Eclipse warning (assuming the warning is enabled)
  7. Make a NewUserResult class
  8. Have it implement the Result interface
  9. Add a field/getter/setter for boolean success, and Integer newUserId
  10. Add a protected NewUserResult constructor for serialization
  11. Add a public constructor for NewUserResult(boolean success, Integer newUserId) for real usage
  12. Add a default serialVersionUID to avoid the Eclipse warning (assuming the warning is enabled)
  13. If you want caching add hashCode/equals to NewUserAction
  14. If you want nice debugging add toString to NewUserAction
  15. If you want nice debugging add toString to NewUserResult

In the end, you’ll end up with something like:

public class NewUserAction implements Action<NewUserResult> {
  private static final long serialVersionUID = 1L;
  private String username;

  public NewUserAction(String username) {
    this.username = username;

  protected NewUserAction() {

  public String getUsername() {
    return this.username;

  public int hashCode() {
      int hashCode = 23;
      hashCode = (hashCode * 37) + getClass().hashCode();
      hashCode = (hashCode * 37) + (username == null ? 1 : username.hashCode());
      return hashCode;

  public boolean equals(Object other) {
      if (other != null && other.getClass().equals(this.getClass())) {
          NewUserAction o = (NewUserAction) other;
          return (o.username == null && this.username == null)
                 || (o.username != null && o.username.equals(this.username));
      return false;

  public String toString() {
      return "NewUserAction[" + username + "]";

public class NewUserResult implements Result {
  private static final long serialVersionUID = 1L;
  private boolean success;
  private Integer newUserId;

  public NewUserResult(boolean success, Integer newUserId) {
    this.success = success;
    this.newUserId = newUserId;

  protected NewUserResult() {

  public boolean getSuccess() {
    return this.success;

  public Integer getNewUserId() {
    return this.newUserId;

  public String toString() {
      return "NewUserResult[" + success + "," + newUserId + "]";

So, roughly 15 steps leading to 60 LOC.

Which isn’t necessarily a big deal, until you have to repeat it for every Action/Result pair in your project.

Generating Action/Result DTOs

So, let’s try this again. If we think about it, what are we really trying to do?

We have 1 operation: creating a new user. We know it should take 1 input parameter (username) and return two output parameters (success and newUserId).

With gwt-mpv-apt, we can specify this directly by:

  1. Make a new NewUserSpec class
  2. Add a String in1username field (in1 == first input parameter)
  3. Add a boolean out1success field (out1 == first output parameter)
  4. Add a Integer out2newUserId field (out2 == second output parameter)
  5. Add a GenDispatch annotation

So, it while look like:

public class NewUserSpec {
  String in1username;
  boolean out1success;
  Integer out2newUserId;

And that’s it.

gwt-mpv-apt will step in (if you’re in Eclipse as soon as you hit Save), and generate the NewUserAction and NewUserResult classes that look exactly like the ~60 lines of boilerplate above from this 6 lines of spec.

That’s an order of magnitude decrease in LOC (60 -> 6) and a 3x decrease in manual steps (15 -> 5).

Adding Up The Savings

Saving ~50 LOC for one example is good. But, even better, this savings is applied to every command Action/Result in your project.

I’m currently writing what I consider a small project, and it has 20 command patterns. So, 50 LOC x 20 commands = 1,000 LOC I didn’t have to write.

So, pretty quickly, we’re talking about real LOC that gwt-mpv-apt should save you.

Events Too

I’ll do another post sometime with the details of gwt-mpv-apt’s GWT event support, but for now I’ll leave you with a short of example:

public class FooChangedEventSpec {
  Foo p1foo;

That will generate a FooChangedEvent, FooChangedHandler, and all sorts of other boilerplate that, if you’ve written GWT events before, you’ll know is a PITA to type out each time.

Getting gwt-mpv-apt

The github page is the best place to go.