Boilerplate Generation

GWTP provides annotation processors that reduce the effort of creating classes that primarily consist of boilerplate code. Annotation processors enable compiler-/IDE-driven generated-as-you-type code generation based on Java files in your project.

Using annotations to generate classes

To generate the boilerplate code, configure your build environment and then create simple classes with specific annotations that specify the metadata.

For full information refer to the javadoc of your @GenEvent, @GenDispatch, @GenDTO or @GenProxy.

Generate Event and Event Handler

The @GenEvent will generate an Event class and an EventHandler interface.

public class FooChanged {
    @Order(1) Foo foo;
    @Order(2) boolean originator;

The above example will generate FooChangedEvent and FooChangedEventHandler.

Generate Action and Result

The @GenDispatch will generate Action and Result classes.

public class RetrieveFoo {
    @In(1) Key<Foo> fooKey;
    @Out(1) Foo foo;
    @Out(2) boolean bar;

The above example will generate RetrieveFooAction and RetrieveFooResult.

Generate Data transfer objects

The @GenDto annotation will generate simple Data Transfer Object (DTO) classes to be used solely for transferring data between the client and the server.

public class LineItem {
    @Order(1) Key<Product> productKey;
    @Order(2) int quantity;

The above example will generate a LineItemDto class that could be used on the following action when creating an invoice.

public class CreateInvoice {
    @In Key<Customer> customerKey;
    @In LineItemDto[] lineItems;
    @Out Invoice invoice;

The alternative to using DTO classes would be to construct complete LineItem entites on the client and send them as part of the gwt-rpc call. (i.e. each element of lineItems would contain an id field, and maybe a price field, etc.) With DTO, you can just pass the minimal information required to create the object.

Using DTO classes is a better choice because:
* The client cannot be trusted. The LineItem entity probably has fields that you want to populate on the server (i.e. id or price).
* Sending the minimal amount of information lowers the bandwidth required for your operation.

Generate RequestFactory proxies

The @GenProxy will generate EntityProxy or ValueProxy interfaces. The generated interfaces equals the given examples on the offical Getting Started with RequestFactory guide.

Note: You can embed other generated proxy interfaces (or even the same proxy) using @UseProxyName.

public class Person {
    String id;
    Address address;
    Detail detail;

The above example will generate PersonProxy.

Declare optional fields

You can use the @Optional annotation to specify optional fields in all classes that are involved in the generation process (except @GenProxy).

Normally two constructors are created with either all or only non-optionals. In the second variant the optional annotated fields are not initialized and will contain their default value (i.e. objects will be initialized to null).

Note: There is no way to declare optional fields while using the @GenProxy annotation. Instead, you can use the 'filterGetter' and/or 'filterSetter' property.

public class FooChanged {
    @Order(1) @Optional Foo foo;
    @Order(2) int bar;
    @Order(3) @Optional boolean originator;

The following constructor and fire methods will be generated:

public class FooChangedEvent {
    protected FooChangedEvent() { ...
    public FooChangedEvent(int bar){ ...
    public FooChangedEvent(Foo foo, int bar, boolean originator) { ...
    public static void fire(HasEventBus source, int bar) { ...
    public static void fire(HasEventBus source, Foo foo, int bar, boolean originator) { ...

Modifiers and Constants

Since GWTP 0.5, the generation process will also consider modifiers and constant fields. If you want to use constants with @GenDispatch, do not forget to annotate them as well with @In or @Out.

Complex example

The following code snippet shows a more complex example using the @GenEvent annotation.

public class FireAnswerOfLife {
    public static final String ANSWER = "42";
    public final boolean REALLY = true;

    private final long TIME_TO_ANSWER = 99999999L;

    @Order(1) @Optional public String myName;
    @Order(2) String myQuestion;
    @Order(3) @Optional public int myAge;
    @Order(4) @Optional HairColor myHairColor;
    @Order(5) protected Boolean doIReallyNeedAnotherField;
    @Order(6) private HashMap<Project, Role> yep;
    @Order(7) private @Optional Boolean questionHasASense;

Configuring your build environment

Annotation processing is an accepted Java standard – the Pluggable Annotation Processing API (JSR 269) – and is part the Java Development Kit.


Maven can easily generate the sources for the project.

  • Maven Configuration - see maven configuration options on how to configure maven.
  • Then run mvn generate-sources to generate the sources.

Eclipse Annotation Processing

In Eclipse, the annotation processor kicks in as soon as you save the file you're working on and incrementally
changes only the required files. Another method exists by using the Eclipse lifecycle mapping. If the lifecycle
mapping is used, the annotation processor doesn't have to be setup. Find out more in the Maven Configuration on
how to setup the annotation processing with out touching eclipse properties.

To enable GWTP annotation processing in eclipse:
* Open the properties for your project
* Ticking all the boxes on the Annotation Processing page.
Ticking all the boxes on the Annotation Processing page.
* Add the GWTP jar to the factory path.
Add the GWTP jar to the factory path.


The latest version of ant automatically recognizes annotation processing. A good practice is to put the generated source in an alternate directory. To do this you will have to instruct ant to create that directory:

<mkdir dir=".apt_generated" />

And then, within your javac task, you specify that this directory should be used for generated source via the -s flag:

<compilerarg value="-s" />
<compilerarg path=".apt_generated" />

Don't forget to delete this directory in your clean target!