HibernateProxy. Forgot to register a type adapter?

使用Gson转换Hibernate对象遇到一个问题，当对象的Lazy加载的，就会出现上面的错误。处理方式摘抄自网上，留存一份以后自己看。

/**
 * 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);
    }
}

实现上面的类，然后就是使用：

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

解释，只是看不懂。

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.