A parameterized AutoBean type containing a typed member

Question

Question

Is there any way to deserialize JSON using the AutoBean framework such that the resulting bean has a type parameter that affects the type of one or more of its members?

Background

RPC with JSON results

I'm using GWT (RequestBuilder) to perform RPC requests. The JSON payload returned is of the following form:

{
  "resultSet": [{...}, {...}, ...], // items requested; say, items 150-160
  "totalCount": 15330               // total matching items in DB
}

The objects in resultSet vary in type depending on the specific RPC I'm calling.

AutoBean interface

I'd like to deserialize this JSON using AutoBean. I'm trying to represent this object as follows:

interface RpcResults<T> {

  List<T> getResultSet();
  void setResultSet(List<T> resultSet);

  int getTotalCount();
  void setTotalCount(int totalCount);

}

I've also created appropriate interfaces representing each type of object that could exist within resultSet. Finally, I set up the appropriate call to AutoBeanCodex.decode.

Running the code

Attempting to run this code in development mode causes the following stack trace to appear in the console:

19:44:23.791 [ERROR] [xcbackend] Uncaught exception escaped
java.lang.IllegalArgumentException: The AutoBeanFactory cannot create a java.lang.Object
    at com.google.gwt.autobean.shared.AutoBeanCodex$Decoder.push(AutoBeanCodex.java:240)
    at com.google.gwt.autobean.shared.AutoBeanCodex$Decoder.decode(AutoBeanCodex.java:50)
    at com.google.gwt.autobean.shared.AutoBeanCodex$Decoder.visitCollectionProperty(AutoBeanCodex.java:83)
    at com.citrix.xenclient.backend.client.json.RpcResultsAutoBean.traverseProperties(RpcResultsAutoBean.java:100)
    at com.google.gwt.autobean.shared.impl.AbstractAutoBean.traverse(AbstractAutoBean.java:153)
    at com.google.gwt.autobean.shared.impl.AbstractAutoBean.accept(AbstractAutoBean.java:112)
    at com.google.gwt.autobean.shared.AutoBeanCodex$Decoder.decode(AutoBeanCodex.java:51)
    at com.google.gwt.autobean.shared.AutoBeanCodex.decode(AutoBeanCodex.java:505)
    at com.google.gwt.autobean.shared.AutoBeanCodex.decode(AutoBeanCodex.java:521)
    at com.citrix.xenclient.backend.client.services.JSONResponseResultSetHandler.onResponseReceived(JSONResponseResultSetHandler.java:51)
    at com.google.gwt.http.client.Request.fireOnResponseReceived(Request.java:287)
    at com.google.gwt.http.client.RequestBuilder$1.onReadyStateChange(RequestBuilder.java:395)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)
    at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
    at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
    at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)
    at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:326)
    at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:207)
    at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:126)
    at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:561)
    at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:269)
    at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
    at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
    at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:214)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)
    at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
    at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
    at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:157)
    at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:281)
    at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:531)
    at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:352)
    at java.lang.Thread.run(Thread.java:636)

Based on this stack trace, my hunch is the following:

1. Type erasure makes it seem that RpcResults.getResultSet() is returning a raw List.
2. The AutoBean deserialiser attempts to create Object instances for each item in resultSet.
3. Failure

Question again

Am I missing something in the AutoBean API that will allow me to do this easily? If not, is there an obvious point of attack I should look into? Is there a more sensible alternative for what I'm doing (other than JSONParser and JavaScriptObject, which I'm already using)?

Answer 1

This is not simple, due to Java type erasure. The type T does not exist at runtime, having been erased to Object in lieu of any other lower bound. The AutoBeanCodex requires type information in order to reify the elements of the incoming json payload. This type information is usually provided by the AutoBean implementation, but due to the T erasure, all it knows is that it contains a List<Object>.

If you can provide a class literal at runtime, the getter could be declared as Splittable getResultSet() and the individual elements of the list reified by calling AutoBeanCodex.decode(autoBeanFactory, SomeInterfaceType.class, getResultSet().get(index)). By using a Category, you could add a <T> T getResultAs(Class<T> clazz, int index) method to the AutoBean interface. This would look something like:

@Category(MyCategory.class)
interface MyFactory extends AutoBeanFactory {
  AutoBean<ResultContainer> resultContainer();
}
interface ResultContainer<T> {
  Splittable getResultSet();
  // It's the class literal that makes it work
  T getResultAs(Class<T> clazz, int index);
}
class MyCategory {
  public static <T> T getResultAs(Autobean<ResultContainer> bean,
      Class<T> clazz, int index) {
    return AutoBeanCodex.decode(bean.getFactory(), clazz,
      bean.as().getResultSet().get(index)).as();
  }
}

Answer 2

Try overriding the .getResultSet() and .setResultSet() methods in your object-specific interfaces:

interface FooRpcResults extends RpcResults<Foo> {
    @Override
    List<Foo> getResultSet();
    @Override
    void setResultSet(List<Foo> value);
}

The following test works for me (GWT 2.3.0):

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;

import com.google.web.bindery.autobean.shared.AutoBean;
import com.google.web.bindery.autobean.shared.AutoBeanCodex;
import com.google.web.bindery.autobean.shared.AutoBeanFactory;
import com.google.web.bindery.autobean.shared.AutoBeanUtils;
import com.google.web.bindery.autobean.vm.AutoBeanFactorySource;

public class AutoBeanTest {
 public static interface Page<T> {
  int getDataSize();

  List<T> getPage();

  int getStartIndex();

  void setDataSize(int value);

  void setPage(List<T> value);

  void setStartIndex(int value);
 }

 public static interface Thing {
  String getName();

  void setName(String value);
 }

 public static interface ThingFactory extends AutoBeanFactory {
  AutoBean<Thing> createThing();

  AutoBean<ThingPage> createThingPage();
 }

 public static interface ThingPage extends Page<Thing> {
  @Override
  List<Thing> getPage();

  @Override
  void setPage(List<Thing> value);
 }

 @Test
 public void testAutoBean() {
  final ThingFactory factory = AutoBeanFactorySource
    .create(ThingFactory.class);

  final Thing thing1 = factory.createThing().as();
  thing1.setName("One");

  final Thing thing2 = factory.createThing().as();
  thing2.setName("Two");

  final List<Thing> things = new ArrayList<Thing>();
  things.add(thing1);
  things.add(thing2);

  final Page<Thing> page = factory.createThingPage().as();
  page.setStartIndex(50);
  page.setDataSize(1000);
  page.setPage(things);

  final String json = AutoBeanCodex.encode(
    AutoBeanUtils.getAutoBean(page)).getPayload();

  final Page<Thing> receivedPage = AutoBeanCodex.decode(factory,
    ThingPage.class, json).as();

  assertEquals(receivedPage.getStartIndex(), page.getStartIndex());
  assertEquals(receivedPage.getDataSize(), page.getDataSize());
  assertNotNull(receivedPage.getPage());
  assertEquals(receivedPage.getPage().size(), page.getPage().size());
  for (int i = 0; i < receivedPage.getPage().size(); i++) {
   assertNotNull(receivedPage.getPage().get(i));
   assertEquals(receivedPage.getPage().get(i).getName(), page
     .getPage().get(i).getName());
  }
 }
}

Removing the overrides in the ThingPage interface will break it.