gson - How to include class name property when serializing object of any type

Question

Came to realize I need to include the class name as a property when serializing an object in my application. It would probably be best if I added the class name property for any non-primitive object that is serialized.

I saw that this is a built-in feature in Genson with the useClassMetadata method. But I'm already using gson in my project, so it would be beneficial if I could stick with it.

This is my current attempt:

package com.mycompany.javatest;

import com.google.gson.*;
import java.lang.reflect.*;

public class JavaTest {

    public static class GenericSerializer implements JsonSerializer<Object>, JsonDeserializer<Object> {

        private static final String CLASS_PROPERTY_NAME = "class";

        @Override
        public JsonElement serialize(Object src, Type typeOfSrc,
                                     JsonSerializationContext context) {

            JsonElement retValue = context.serialize(src);
            if (retValue.isJsonObject()) {
                retValue.getAsJsonObject().addProperty(CLASS_PROPERTY_NAME, src.getClass().getName());
            }
            return retValue;
        }

        @Override
        public Object deserialize(JsonElement json, Type typeOfT,
                                  JsonDeserializationContext context) throws JsonParseException {

            Class actualClass;
            if (json.isJsonObject()) {
                JsonObject jsonObject = json.getAsJsonObject();
                String className = jsonObject.get(CLASS_PROPERTY_NAME).getAsString();

                try {
                    actualClass = Class.forName(className);
                }
                catch (ClassNotFoundException e) {
                    e.printStackTrace();
                    throw new JsonParseException(e.getMessage());
                }
            }
            else {
                actualClass = typeOfT.getClass();
            }
            return context.deserialize(json, actualClass);
        }
    }

    public static class MyClass {

        private final String name = "SpongePants SquareBob";

    }

    public static void main(String[] args) {

        MyClass obj = new MyClass();

        GsonBuilder gb = new GsonBuilder();
        gb.registerTypeAdapter(Object.class, new GenericSerializer());
        Gson gson = gb.create();

        System.out.println(gson.toJson(obj, Object.class));

    }
}

Prints

{"name":"SpongePants SquareBob"}

I want it to print

{"name":"SpongePants SquareBob","class":"com.mycompany.javatest$MyClass"}

EDIT: Another attempt (this time using GsonFire)

package com.mycompany.javatest;

import com.google.gson.*;
import io.gsonfire.*;

public class JavaTest {

    public static class DummyData {

        private final String someData = "1337";

    }

    private static final String CLASS_PROPERTY_NAME = "class";

    public static void main(String[] args) {

        GsonFireBuilder gfb = new GsonFireBuilder();
        gfb.registerPostProcessor(Object.class, new PostProcessor<Object>() {

                              @Override
                              public void postDeserialize(Object t, JsonElement je, Gson gson) {
                                  // Ignore
                              }

                              @Override
                              public void postSerialize(JsonElement je, Object t, Gson gson) {
                                  if (je.isJsonObject()) {
                                      je.getAsJsonObject().add(CLASS_PROPERTY_NAME, new JsonPrimitive(t.getClass().getTypeName()));
                                  }
                              }

                          });

        gfb.registerTypeSelector(Object.class, (JsonElement je) -> {
            System.out.println(je);
                             if (je.isJsonObject()) {
                                 try {
                                     return Class.forName(je.getAsJsonObject().get(CLASS_PROPERTY_NAME).getAsString());
                                 }
                                 catch (ClassNotFoundException ex) {
                                     ex.printStackTrace();
                                 }
                             }

                             return null;
                         });

        Gson gson = gfb.createGson();

        DummyData dd = new DummyData();
        String json = gson.toJson(dd);
        System.out.println(json);

        DummyData dd2 = (DummyData) gson.fromJson(json, Object.class); // <-- gives me a ClassCastException

    }

}

Answer 1

Accepted @pandaadb's answer but just wanted to paste the code I'm using. It takes care of serializing with type and de-serializing into the proper subtybe:

package com.mycompany.javatest;

import com.google.gson.*;
import java.lang.reflect.*;
import org.junit.*;

public class JavaTest {

    public static class GenericSerializer implements JsonSerializer<Object>, JsonDeserializer<Object> {

        private static final String CLASS_PROPERTY_NAME = "class";
        private final Gson gson;

        public GenericSerializer() {
            gson = new Gson();
        }

        public GenericSerializer(Gson gson) {
            this.gson = gson;
        }

        @Override
        public Object deserialize(JsonElement json, Type typeOfT,
                                  JsonDeserializationContext context) throws JsonParseException {

            Class actualClass;
            if (json.isJsonObject()) {
                JsonObject jsonObject = json.getAsJsonObject();
                String className = jsonObject.get(CLASS_PROPERTY_NAME).getAsString();
                try {
                    actualClass = Class.forName(className);
                }
                catch (ClassNotFoundException e) {
                    e.printStackTrace();
                    throw new JsonParseException(e.getMessage());
                }
            }
            else {
                actualClass = typeOfT.getClass();
            }

            return gson.fromJson(json, actualClass);
        }

        @Override
        public JsonElement serialize(Object src, Type typeOfSrc,
                                     JsonSerializationContext context) {
            JsonElement retValue = gson.toJsonTree(src);
            if (retValue.isJsonObject()) {
                retValue.getAsJsonObject().addProperty(CLASS_PROPERTY_NAME, src.getClass().getName());
            }
            return retValue;
        }

    }

    public static void main(String[] args) {

        GsonBuilder builder = new GsonBuilder();
        builder.registerTypeHierarchyAdapter(Object.class, new GenericSerializer());
        Gson gson = builder.create();

        SomeSuperClass x = new SomeSubClass();
        String json = gson.toJson(x);
        
        SomeSuperClass y = gson.fromJson(json, SomeSuperClass.class); // Usually, y would now be of type SomeSuperClass
        Assert.assertEquals(x.getClass(), y.getClass()); // y is actually of type SomeSubClass (!)
        
        System.out.println("y.getClass()= " + y.getClass());
    }

    public static class SomeSuperClass {
    }

    public static class SomeSubClass extends SomeSuperClass {

        private final String someMember = "12345";
    }
}

Answer 2

pandaadb's amazing answer wasn't completely working for me since it does not handle arrays/lists and there was a problem with deserialization, so I made a couple of changes:

package org.ctbto.osi.fieldapp.util.gson;

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.internal.Streams;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

import java.io.IOException;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * <p>
*  Disclaimer: taken from here https://stackoverflow.com/a/40133286/285091 with some modifications
 * </p>
 *
 * Adapts values whose runtime type may differ from their declaration type. This
 * is necessary when a field's type is not the same type that GSON should create
 * when deserializing that field. For example, consider these types:
 * <pre>   {@code
 *   abstract class Shape {
 *     int x;
 *     int y;
 *   }
 *   class Circle extends Shape {
 *     int radius;
 *   }
 *   class Rectangle extends Shape {
 *     int width;
 *     int height;
 *   }
 *   class Diamond extends Shape {
 *     int width;
 *     int height;
 *   }
 *   class Drawing {
 *     Shape bottomShape;
 *     Shape topShape;
 *   }
 * }</pre>
 * <p>Without additional type information, the serialized JSON is ambiguous. Is
 * the bottom shape in this drawing a rectangle or a diamond? <pre>   {@code
 *   {
 *     "bottomShape": {
 *       "width": 10,
 *       "height": 5,
 *       "x": 0,
 *       "y": 0
 *     },
 *     "topShape": {
 *       "radius": 2,
 *       "x": 4,
 *       "y": 1
 *     }
 *   }}</pre>
 * This class addresses this problem by adding type information to the
 * serialized JSON and honoring that type information when the JSON is
 * deserialized: <pre>   {@code
 *   {
 *     "bottomShape": {
 *       "type": "Diamond",
 *       "width": 10,
 *       "height": 5,
 *       "x": 0,
 *       "y": 0
 *     },
 *     "topShape": {
 *       "type": "Circle",
 *       "radius": 2,
 *       "x": 4,
 *       "y": 1
 *     }
 *   }}</pre>
 * Both the type field name ({@code "type"}) and the type labels ({@code
 * "Rectangle"}) are configurable.
 * <p>
 * <h3>Registering Types</h3>
 * Create a {@code RuntimeTypeAdapterFactory} by passing the base type and type field
 * name to the {@link #of} factory method. If you don't supply an explicit type
 * field name, {@code "type"} will be used. <pre>   {@code
 *   RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory
 *       = RuntimeTypeAdapterFactory.of(Shape.class, "type");
 * }</pre>
 * Next register all of your subtypes. Every subtype must be explicitly
 * registered. This protects your application from injection attacks. If you
 * don't supply an explicit type label, the type's simple name will be used.
 * <pre>   {@code
 *   shapeAdapter.registerSubtype(Rectangle.class, "Rectangle");
 *   shapeAdapter.registerSubtype(Circle.class, "Circle");
 *   shapeAdapter.registerSubtype(Diamond.class, "Diamond");
 * }</pre>
 * Finally, register the type adapter factory in your application's GSON builder:
 * <pre>   {@code
 *   Gson gson = new GsonBuilder()
 *       .registerTypeAdapterFactory(shapeAdapterFactory)
 *       .create();
 * }</pre>
 * Like {@code GsonBuilder}, this API supports chaining: <pre>   {@code
 *   RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class)
 *       .registerSubtype(Rectangle.class)
 *       .registerSubtype(Circle.class)
 *       .registerSubtype(Diamond.class);
 * }</pre>
 */
public final class RuntimeClassNameTypeAdapterFactory<T> implements TypeAdapterFactory {
    private final Class<?> baseType;
    private final String typeFieldName;
    private final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<String, Class<?>>();
    private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<Class<?>, String>();

    private RuntimeClassNameTypeAdapterFactory(Class<?> baseType, String typeFieldName) {
        if (typeFieldName == null || baseType == null) {
            throw new NullPointerException();
        }
        this.baseType = baseType;
        this.typeFieldName = typeFieldName;
    }

    /**
     * Creates a new runtime type adapter using for {@code baseType} using {@code
     * typeFieldName} as the type field name. Type field names are case sensitive.
     */
    public static <T> RuntimeClassNameTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName) {
        return new RuntimeClassNameTypeAdapterFactory<T>(baseType, typeFieldName);
    }

    /**
     * Creates a new runtime type adapter for {@code baseType} using {@code "type"} as
     * the type field name.
     */
    public static <T> RuntimeClassNameTypeAdapterFactory<T> of(Class<T> baseType) {
        return new RuntimeClassNameTypeAdapterFactory<T>(baseType, "class");
    }

    /**
     * Registers {@code type} identified by {@code label}. Labels are case
     * sensitive.
     *
     * @throws IllegalArgumentException if either {@code type} or {@code label}
     *                                  have already been registered on this type adapter.
     */
    public RuntimeClassNameTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label) {
        if (type == null || label == null) {
            throw new NullPointerException();
        }
        if (subtypeToLabel.containsKey(type) || labelToSubtype.containsKey(label)) {
            throw new IllegalArgumentException("types and labels must be unique");
        }
        labelToSubtype.put(label, type);
        subtypeToLabel.put(type, label);
        return this;
    }

    /**
     * Registers {@code type} identified by its {@link Class#getSimpleName simple
     * name}. Labels are case sensitive.
     *
     * @throws IllegalArgumentException if either {@code type} or its simple name
     *                                  have already been registered on this type adapter.
     */
    public RuntimeClassNameTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
        return registerSubtype(type, type.getSimpleName());
    }

    public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {

        final Map<String, TypeAdapter<?>> labelToDelegate
                = new LinkedHashMap<String, TypeAdapter<?>>();
        final Map<Class<?>, TypeAdapter<?>> subtypeToDelegate
                = new LinkedHashMap<Class<?>, TypeAdapter<?>>();

//    && !String.class.isAssignableFrom(type.getRawType())

        if (Object.class.isAssignableFrom(type.getRawType())) {
            TypeAdapter<?> delegate = gson.getDelegateAdapter(this, type);
            labelToDelegate.put(type.getRawType().getName(), delegate);
            subtypeToDelegate.put(type.getRawType(), delegate);
        }

//    for (Map.Entry<String, Class<?>> entry : labelToSubtype.entrySet()) {
//      TypeAdapter<?> delegate = gson.getDelegateAdapter(this, TypeToken.get(entry.getValue()));
//      labelToDelegate.put(entry.getKey(), delegate);
//      subtypeToDelegate.put(entry.getValue(), delegate);
//    }

        return new TypeAdapter<R>() {
            @SuppressWarnings("unchecked")
            @Override
            public R read(JsonReader in) throws IOException {
                JsonElement jsonElement = Streams.parse(in);
                if (jsonElement.isJsonObject()) {
                    JsonElement labelJsonElement = jsonElement.getAsJsonObject().remove(typeFieldName);
                    if (labelJsonElement == null) {
                        throw new JsonParseException("cannot deserialize " + baseType
                                + " because it does not define a field named " + typeFieldName);
                    }
                    String label = labelJsonElement.getAsString();
                    TypeAdapter<R> delegate = (TypeAdapter<R>) labelToDelegate.get(label);
                    if (delegate == null) {
                        Class<R> aClass;
                        try {
                            aClass = (Class<R>) Class.forName(label);
                        } catch (ClassNotFoundException e) {
                            throw new JsonParseException("Cannot find class " + label, e);
                        }

                        TypeToken<R> subClass = TypeToken.get(aClass);
                        delegate = gson.getDelegateAdapter(RuntimeClassNameTypeAdapterFactory.this, subClass);
                        if (delegate == null) {
                            throw new JsonParseException("cannot deserialize " + baseType + " subtype named "
                                    + label + "; did you forget to register a subtype?");
                        }
                    }
                    return delegate.fromJsonTree(jsonElement);
                } else if (jsonElement.isJsonNull()) {
                    return null;
                } else {
                    TypeAdapter<R> delegate = gson.getDelegateAdapter(RuntimeClassNameTypeAdapterFactory.this, type);
                    if (delegate == null) {
                        throw new JsonParseException("cannot deserialize " + baseType + "; did you forget to register a subtype?");
                    }
                    return delegate.fromJsonTree(jsonElement);
                }
            }

            @Override
            public void write(JsonWriter out, R value) throws IOException {
                Class<?> srcType = value.getClass();
                String label = srcType.getName();
                TypeAdapter<R> delegate = getDelegate(srcType);
                if (delegate == null) {
                    throw new JsonParseException("cannot serialize " + srcType.getName()
                            + "; did you forget to register a subtype?");
                }
                JsonElement jsonTree = delegate.toJsonTree(value);
                if (!jsonTree.isJsonObject()) {
                    Streams.write(jsonTree, out);
                } else {
                    JsonObject jsonObject = jsonTree.getAsJsonObject();
                    if (jsonObject.has(typeFieldName)) {
                        throw new JsonParseException("cannot serialize " + srcType.getName()
                                + " because it already defines a field named " + typeFieldName);
                    }
                    JsonObject clone = new JsonObject();
                    clone.add(typeFieldName, new JsonPrimitive(label));
                    for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
                        clone.add(e.getKey(), e.getValue());
                    }
                    Streams.write(clone, out);
                }
            }

            @SuppressWarnings("unchecked")
            private TypeAdapter<R> getDelegate(Class<?> srcType) {
                TypeAdapter<?> typeAdapter = subtypeToDelegate.get(srcType);
                if (typeAdapter != null) {
                    return (TypeAdapter<R>) typeAdapter;
                }

                for (Map.Entry<Class<?>, TypeAdapter<?>> classTypeAdapterEntry : subtypeToDelegate.entrySet()) {
                    if (classTypeAdapterEntry.getKey().isAssignableFrom(srcType)) {
                        return (TypeAdapter<R>) classTypeAdapterEntry.getValue();
                    }
                }
                return null;
            }
        }.nullSafe();
    }
}

All credit still goes to him/her, though. As s/he says, please test this code before using it!

Answer 3

Just tried this myself and this seems to work:

public class GsonClassNameTest {


    public static void main(String[] args) {

        Gson create = new GsonBuilder().registerTypeHierarchyAdapter(Object.class, new ODeserialiser()).create();
        String json = create.toJson(new X());
        System.out.println(json);

    }

    public static class ODeserialiser implements JsonSerializer<Object> {

        @Override
        public JsonElement serialize(Object src, Type typeOfSrc, JsonSerializationContext context) {
            Gson gson = new Gson();
            JsonElement serialize = gson.toJsonTree(src);
            JsonObject o = (JsonObject) serialize;
            o.addProperty("class", src.getClass().getName());
            return serialize;
        }
    }

    public static class X {
        public String test = "asd";
    }
}

This prints:

{"test":"asd","class":"google.GsonClassNameTest$X"}

details:

You have to register a Hierarchy adapter, so that if you register it with the Object class, it will be called for any type you pass into it.

You also have to use a different Gson instance within the custom Serializer, otherwise you just keep running in circles and get a Stackoverflow.

Other than that, pretty straight forward :)

Note: I have rather little experience with gson, so there may be a cooler solution to this.

Regards,

Artur