Question

I am getting the following error response from the server.

HTTP Status 500 –

type Exception report

message

descriptionThe server encountered an internal error () that prevented
it from fulfilling this request.

exception

javax.servlet.ServletException:
java.lang.UnsupportedOperationException: Attempted to serialize
java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to
register a type adapter?

root cause

java.lang.UnsupportedOperationException: Attempted to serialize
java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to
register a type adapter?

From Java debugger:

org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer@7632012e

I am using Gson to convert my Java objects to JSON. Below I have pasted some of my code.

This is my resource:

@Stateless
@LocalBean
@Path("/autos")
@Produces(MediaType.APPLICATION_JSON)
public class AutoResource {

    @EJB
    private CarAssembler warehouse;
    @Context
    private UriInfo uriInfo;

    @GET
    public Response allAutos() {
        // Building a context, lots of code...
        // Creating a Gson instance and configures it...

        final Auto auto = warehouse.list(context);
        final String autoJson = gson.toJson(auto);

        return Response.ok(autoJson).build();
    }
}

The CarAssembler is just an service which invokes a repository. I have not pasted the code of the service here.

Repository:

@Override
public Question findById(final int id, final FetchType fetchType) {

    final Auto question = getEntityManager().find(Auto.class, id);

    if (fetchType == FetchType.LAZY) {
        return auto;
    }

    Hibernate.initialize(auto.getManufacturer());
    Hibernate.initialize(auto.getAssemblyHouse());

    return auto;
}

As you see, I provide both lazy and eager loading of objects. I use Hibernate.initialize to eager fetch JPA associations. However, the question is how I can fix the proxy error I am getting. Why come there is only AssemblyHouse that is still attached to JavaAssist, while Manufacturer is not (I have seen the type in Java Debugger). How do I know when to unproxy objects? Should I unproxy all associations this auto may have? And in which layer of my code? Does it affect the performance of my application when I unproxy? Are there other solutions? I see from the error message that I can make a type adapter. Yes, I could but then I have to do that for all domain objects to be sure that conversion is done correctly. Maybe other objects in my domain starts failing when I try to convert it into JSON representation too, but I don’t know when or why. Is it just luck that the other objects are fine?

This is the way I am unproxing objects, but I have not implemented it yet, because I don’t know if this is good or bad, and in what layer to do this, and when I should do it. Should I unproxy objects all the time?

public class HibernateUtilities {

    public static <T> T unproxy(T proxy) {
        if (proxy == null) {
            return null;
        }

        if (proxy instanceof HibernateProxy) {
            Hibernate.initialize(proxy);

            HibernateProxy hibernateProxy = (HibernateProxy) proxy;
            T unproxiedObject = (T) hibernateProxy.getHibernateLazyInitializer().getImplementation();

            return unproxiedObject;
        }

        return proxy;
    }
}

---

Stacktrace as requested:

[#|2012-11-22T17:17:13.285+0100|WARNING|glassfish3.1.2|javax.enterprise.system.container.web.com.sun.enterprise.web|_ThreadID=71;_ThreadName=Thread-8;|StandardWrapperValve[javax.ws.rs.core.Application]:
PWC1406: Servlet.service() for servlet javax.ws.rs.core.Application
threw exception java.lang.UnsupportedOperationException: Attempted to
serialize java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot
to register a type adapter?
    at com.google.gson.internal.bind.TypeAdapters$1.write(TypeAdapters.java:64)
    at com.google.gson.internal.bind.TypeAdapters$1.write(TypeAdapters.java:61)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ArrayTypeAdapter.write(ArrayTypeAdapter.java:93)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)
    at com.google.gson.Gson.toJson(Gson.java:586)
    at com.google.gson.Gson.toJson(Gson.java:565)
    at com.google.gson.Gson.toJson(Gson.java:520)
    at com.myapp.AutoResource.produceAuto(AutoResource.java:48)
    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:601)
    at org.glassfish.ejb.security.application.EJBSecurityManager.runMethod(EJBSecurityManager.java:1052)
    at org.glassfish.ejb.security.application.EJBSecurityManager.invoke(EJBSecurityManager.java:1124)
    at com.sun.ejb.containers.BaseContainer.invokeBeanMethod(BaseContainer.java:5388)
    at com.sun.ejb.EjbInvocation.invokeBeanMethod(EjbInvocation.java:619)
    at com.sun.ejb.containers.interceptors.AroundInvokeChainImpl.invokeNext(InterceptorManager.java:800)
    at com.sun.ejb.EjbInvocation.proceed(EjbInvocation.java:571)
    at com.sun.ejb.containers.interceptors.SystemInterceptorProxy.doAround(SystemInterceptorProxy.java:162)
    at com.sun.ejb.containers.interceptors.SystemInterceptorProxy.aroundInvoke(SystemInterceptorProxy.java:144)
    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:601)
    at com.sun.ejb.containers.interceptors.AroundInvokeInterceptor.intercept(InterceptorManager.java:861)
    at com.sun.ejb.containers.interceptors.AroundInvokeChainImpl.invokeNext(InterceptorManager.java:800)
    at com.sun.ejb.containers.interceptors.InterceptorManager.intercept(InterceptorManager.java:370)
    at com.sun.ejb.containers.BaseContainer.__intercept(BaseContainer.java:5360)
    at com.sun.ejb.containers.BaseContainer.intercept(BaseContainer.java:5348)
    at com.sun.ejb.containers.EJBLocalObjectInvocationHandler.invoke(EJBLocalObjectInvocationHandler.java:214)
    at com.sun.ejb.containers.EJBLocalObjectInvocationHandlerDelegate.invoke(EJBLocalObjectInvocationHandlerDelegate.java:89)
    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:601)
    at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)
    at com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$ResponseOutInvoker._dispatch(AbstractResourceMethodDispatchProvider.java:205)
    at com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75)
    at com.sun.jersey.server.impl.uri.rules.HttpMethodRule.accept(HttpMethodRule.java:288)
    at com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:108)
    at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
    at com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1469)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1400)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1349)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1339)
    at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:708)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:770)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1550)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
    at org.apache.catalina.core.StandardContextValve.__invoke(StandardContextValve.java:175)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
    at org.apache.catalina.core.StandardHostValve.__invoke(StandardHostValve.java:161)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:331)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
    at com.sun.enterprise.v3.services.impl.ContainerMapper$AdapterCallable.call(ContainerMapper.java:317)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:195)
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:860)
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:757)
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1056)
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:229)
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
    at java.lang.Thread.run(Thread.java:722) |#]

Answer 1

You can do without manually unproxying everything by using a custom TypeAdapter.
Something along these lines:

/**
 * This TypeAdapter unproxies Hibernate proxied objects, and serializes them
 * through the registered (or default) TypeAdapter of the base class.
 */
public class HibernateProxyTypeAdapter extends TypeAdapter<HibernateProxy> {

    public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
        @Override
        @SuppressWarnings("unchecked")
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            return (HibernateProxy.class.isAssignableFrom(type.getRawType()) ? (TypeAdapter<T>) new HibernateProxyTypeAdapter(gson) : null);
        }
    };
    private final Gson context;

    private HibernateProxyTypeAdapter(Gson context) {
        this.context = context;
    }

    @Override
    public HibernateProxy read(JsonReader in) throws IOException {
        throw new UnsupportedOperationException("Not supported");
    }

    @SuppressWarnings({"rawtypes", "unchecked"})
    @Override
    public void write(JsonWriter out, HibernateProxy value) throws IOException {
        if (value == null) {
            out.nullValue();
            return;
        }
        // Retrieve the original (not proxy) class
        Class<?> baseType = Hibernate.getClass(value);
        // Get the TypeAdapter of the original class, to delegate the serialization
        TypeAdapter delegate = context.getAdapter(TypeToken.get(baseType));
        // Get a filled instance of the original class
        Object unproxiedValue = ((HibernateProxy) value).getHibernateLazyInitializer()
                .getImplementation();
        // Serialize the value
        delegate.write(out, unproxiedValue);
    }
}

To use it you must first register it:

GsonBuilder b = new GsonBuilder();
...
b.registerTypeAdapterFactory(HibernateProxyTypeAdapter.FACTORY);
...
Gson gson = b.create();

Notice that this will recursively initialize every proxy you have in the object hierarchy; since however you have to serialize the whole data, you should have done that anyway.

How does this work?

GSON contains a number of TypeAdapterFactory implementations, for various types (primitive types, common types like String or Date, lists, arrays…). Each factory is asked if it is able to serialize a certain Java type (the parameter to create is a TypeToken instead of a Class in order to capture possible information about generic types, which Class does not have). If the factory is able to serialize/deserialize a type, it responds with a TypeAdapter instance; otherwise it responds with null.

HibernateProxyTypeAdapter.FACTORY verifies whether type implements HibernateProxy; in that case, it returns an instance of HibernateProxyTypeAdapter for serialization.
The write method is called when an actual object has to be serialized; the adapter extracts the original type of the underlying object, and asks GSON for the standard TypeAdapter for the original type, which generally is a ReflectiveTypeAdapter.

Then it retrieves an instance of the original class, instead of directly using the proxy. This is necessary because ReflectiveTypeAdapter accesses directly to fields, instead of using getters; accessing to the fields of a proxied object does not work, and is a classical Hibernate pitfall.

As a possible performance improvement, the delegate TypeAdapter should be acquired in the create method. I found out that calling getSuperclass() on the proxy Class appears to yield the original base class. The code can then become:

public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
    @Override
    @SuppressWarnings("unchecked")
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        return (HibernateProxy.class.isAssignableFrom(type.getRawType())
                ? (TypeAdapter<T>) new HibernateProxyTypeAdapter((TypeAdapter)gson.getAdapter(TypeToken.get(type.getRawType().getSuperclass()))) 
     : null);
    }
};
private final TypeAdapter<Object> delegate;

private HibernateProxyTypeAdapter(TypeAdapter<Object> delegate) {
    this.delegate = delegate;
}

@SuppressWarnings({"rawtypes", "unchecked"})
@Override
public void write(JsonWriter out, HibernateProxy value) throws IOException {
    if (value == null) {
        out.nullValue();
        return;
    }
    delegate.write(out, ((HibernateProxy) value).getHibernateLazyInitializer()
            .getImplementation());
}