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Gson uses TypeAdapter or Json Deserializer to convert data from an error list to an empty list

Let's start from example:
If the data is correct, it should be ( the Beijing cities is empty )
{
"code":200,
"msg":"success",
"data":[
{
"id":1,
"name":"Beijing",
"cities":[]
},
{
"id":2,
"name":"Guangdong",
"cities":[
{
"id":1,
"name":"Guangzhou"
}
]
}
]
}
Now I got a wrong data. ( the Beijing cities is null )
{
"code":200,
"msg":"success",
"data":[
{
"id":1,
"name":"Beijing",
"cities":null
},
{
"id":2,
"name":"Guangdong",
"cities":[
{
"id":1,
"name":"Guangzhou"
}
]
}
]
}
I am using the Retrofit2 ResponseBodyConverter ,the entity class:
public class Result<T> {
private int code;
private String msg;
private T data;
// getters, setters
}
public class Province {
private int id;
private String name;
private List<City> cities;
}
public class City {
private int id;
private String name;
}
The data obtained after deserialization is like this:
but the data I need is like this:
In order to have better fault tolerance, when the data is list, I want to process it by myself.
First of all,I tried to use JsonDeserializer
Gson gson = new GsonBuilder()
.serializeNulls()
.registerTypeHierarchyAdapter(List.class, new GsonListAdapter())
.create();
static class GsonListAdapter implements JsonDeserializer<List<?>> {
#Override
public List<?> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
if (json.isJsonArray()) {
JsonArray array = json.getAsJsonArray();
Type itemType = ((ParameterizedType) typeOfT).getActualTypeArguments()[0];
List list = new ArrayList<>();
for (int i = 0; i < array.size(); i++) {
JsonElement element = array.get(i);
Object item = context.deserialize(element, itemType);
list.add(item);
}
return list;
} else {
return Collections.EMPTY_LIST;
}
}
}
JsonDeserializer is valid when the data is "", {}, and [],but data is null, it will not work.
Then I tried to use TypeAdapter
static class GsonListAdapter extends TypeAdapter<List<?>> {
#Override
public void write(JsonWriter out, List<?> value) throws IOException {
out.value(String.valueOf(value));
}
#Override
public List<?> read(JsonReader reader) throws IOException {
if (reader.peek() != JsonToken.BEGIN_ARRAY) {
reader.skipValue();
return Collections.EMPTY_LIST;
}
return new Gson().fromJson(reader, new TypeToken<List<?>>() {}.getType());
}
}
In this way, no matter what the data is, it can work properly.We know that using TypeToken<List<?>> will give us the LinkedHashMap，So although TypeAdapter can work properly, but I don't know how to convert JsonReader to the List <?>.
So I wonder if there are other ways that I can handle the wrong list data? Or convert JsonReader to the List <?> data I want.

I found the CollectionTypeAdapterFactory in Gson source code.I tried to modify it,it has been tested and it is useful.
public class CollectionTypeAdapterFactory implements TypeAdapterFactory {
private final ConstructorConstructor constructorConstructor;
public CollectionTypeAdapterFactory(ConstructorConstructor constructorConstructor) {
this.constructorConstructor = constructorConstructor;
}
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
Type type = typeToken.getType();
Class<? super T> rawType = typeToken.getRawType();
if (!Collection.class.isAssignableFrom(rawType)) {
return null;
}
Type elementType = $Gson$Types.getCollectionElementType(type, rawType);
TypeAdapter<?> elementTypeAdapter = gson.getAdapter(TypeToken.get(elementType));
ObjectConstructor<T> constructor = constructorConstructor.get(typeToken);
#SuppressWarnings({"unchecked", "rawtypes"}) // create() doesn't define a type parameter
TypeAdapter<T> result = new Adapter(gson, elementType, elementTypeAdapter, constructor);
return result;
}
private static final class Adapter<E> extends TypeAdapter<Collection<E>> {
private final TypeAdapter<E> elementTypeAdapter;
private final ObjectConstructor<? extends Collection<E>> constructor;
public Adapter(Gson context, Type elementType,
TypeAdapter<E> elementTypeAdapter,
ObjectConstructor<? extends Collection<E>> constructor) {
this.elementTypeAdapter =
new TypeAdapterRuntimeTypeWrapper<E>(context, elementTypeAdapter, elementType);
this.constructor = constructor;
}
public Collection<E> read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
//In the source code is return null, I changed to return an empty collection
return constructor.construct();
}
Collection<E> collection = constructor.construct();
in.beginArray();
while (in.hasNext()) {
E instance = elementTypeAdapter.read(in);
collection.add(instance);
}
in.endArray();
return collection;
}
public void write(JsonWriter out, Collection<E> collection) throws IOException {
if (collection == null) {
out.nullValue();
return;
}
out.beginArray();
for (E element : collection) {
elementTypeAdapter.write(out, element);
}
out.endArray();
}
}
}
In the source code the TypeAdapterRuntimeTypeWrapper is protected,We must make a copy.
public class TypeAdapterRuntimeTypeWrapper<T> extends TypeAdapter<T> {
private final Gson context;
private final TypeAdapter<T> delegate;
private final Type type;
TypeAdapterRuntimeTypeWrapper(Gson context, TypeAdapter<T> delegate, Type type) {
this.context = context;
this.delegate = delegate;
this.type = type;
}
#Override
public T read(JsonReader in) throws IOException {
return delegate.read(in);
}
#SuppressWarnings({"rawtypes", "unchecked"})
#Override
public void write(JsonWriter out, T value) throws IOException {
TypeAdapter chosen = delegate;
Type runtimeType = getRuntimeTypeIfMoreSpecific(type, value);
if (runtimeType != type) {
TypeAdapter runtimeTypeAdapter = context.getAdapter(TypeToken.get(runtimeType));
if (!(runtimeTypeAdapter instanceof ReflectiveTypeAdapterFactory.Adapter)) {
// The user registered a type adapter for the runtime type, so we will use that
chosen = runtimeTypeAdapter;
} else if (!(delegate instanceof ReflectiveTypeAdapterFactory.Adapter)) {
// The user registered a type adapter for Base class, so we prefer it over the
// reflective type adapter for the runtime type
chosen = delegate;
} else {
// Use the type adapter for runtime type
chosen = runtimeTypeAdapter;
}
}
chosen.write(out, value);
}
private Type getRuntimeTypeIfMoreSpecific(Type type, Object value) {
if (value != null
&& (type == Object.class || type instanceof TypeVariable<?> || type instanceof Class<?>)) {
type = value.getClass();
}
return type;
}
}
How to use
Gson gson = new GsonBuilder().serializeNulls()
.registerTypeAdapterFactory(
new CollectionTypeAdapterFactory(new ConstructorConstructor(new HashMap<>()))
)
.create();
Result<List<Province>> result = gson.fromJson(jsonStr, new TypeToken<Result<List<Province>>>() {}.getType());
prints:
Result{code=200, msg='success', data=[Province{id=1, name='Beijing', cities=[]}, Province{id=2, name='Guangdong', cities=[City{id=1, name='Guangzhou'}]}]}

Deserialize a value which can be an array or atomic

I'm using Gson and want to deserialize the following JSON alternatives into the same class Values. In the latter case I would like to receive a List of just one element. Is there a way to accomplish this in Gson without having to write a custom deserializer?
//Array:
{
"Value": [205.4, 13.5]
}
//Single value:
{
"Value": 205.4
}
Deserialize both into:
public static class Values {
private List<Double> Value;
}

There is no way to do this in Gson unless using custom TypeAdapter.
public class AwesomeType extends ArrayList<Double> {
...
}
public class AwesomeTypeAdapter extends TypeAdapter<AwesomeType> {
#Override
public void write(JsonWriter out, AwesomeType value) throws IOException {
...
}
#Override
public AwesomeType read(JsonReader in) throws IOException {
AwesomeType result = new AwesomeType();
if (in.peek() == JsonToken.BEGIN_ARRAY) {
in.beginArray();
while (in.peek() != JsonToken.END_ARRAY)
result.add(in.nextDouble());
in.endArray();
} else {
result.add(in.nextDouble());
}
return result;
}
}
Register this type adapter to your Gson and use this class in your model instead of List<Double>.

As #BornToCode mentioned the solution is to write a custom TypeAdapter and register it with the GsonBuilder:
public class ListTypeAdapter extends TypeAdapter<List<Double>> {
#Override
public void write(final JsonWriter writer, final List<Double> doubles) throws IOException {
writer.beginArray();
for (final double value : doubles) {
writer.value(value);
}
writer.endArray();
}
#Override
public List<Double> read(final JsonReader reader) throws IOException {
final List<Double> doubles = new ArrayList<>();
if (reader.peek() == JsonToken.BEGIN_ARRAY) {
reader.beginArray();
while (reader.hasNext()) {
doubles.add(reader.nextDouble());
}
reader.endArray();
} else {
doubles.add(reader.nextDouble());
}
return doubles;
}
}
Register it:
GSONBUILDER.registerTypeAdapter(new TypeToken<List<Double>>(){}.getType(),
new ListTypeAdapter().nullSafe());

GSON: serialize/deserialize object of class, that have registered type hierarchy adapter, using ReflectiveTypeAdapterFactory.Adapter

To be clear, let introduse some model:
interface A {
boolean isSomeCase();
}
class AAdapter implements JsonSerializer<A> {
public JsonElement serialize(A src, Type typeOfSrc, JsonSerializationContext context) {
if (src.isSomeCase()) {
/* some logic */
return result;
} else {
JsonObject json = new JsonObject();
JsonElement valueJson = <???>; // TODO serialize src like POJO
json.add(src.getClass().getSimpleName(), valueJson);
return json;
}
}
}
Gson gson = new GsonBuilder()
.registerTypeHierarchyAdapter(A.class. new AAdapter())
.create();
How it is possible to serealize some instance of A, which isSomeCase() = false, like any other object, that is serialized by ReflectiveTypeAdapterFactory.Adapter.

You can write a custom TypeAdapterFactory and handle incoming object's isSomeCase() result in its TypeAdapter's write() method and apply your logic there:
public class ATypeAdapterFactory implements TypeAdapterFactory {
public TypeAdapter<A> create(Gson gson, TypeToken type) {
if (!A.class.isAssignableFrom(type.getRawType())) {
// Check if incoming raw type is an instance of A interface
return null;
}
final TypeAdapter<A> delegate = gson.getDelegateAdapter(this, type);
return new TypeAdapter<A>() {
#Override
public void write(JsonWriter out, A value) throws IOException {
if(value.isSomeCase()) {
// your custom logic here
out.beginObject();
out.name("x").value(0);
out.endObject();
} else {
// default serialization here
delegate.write(out, value);
}
}
#Override
public A read(JsonReader in) throws IOException {
return delegate.read(in);
}
};
}
}
Test:
final GsonBuilder gsonBuilder = new GsonBuilder();
// Register custom type adapter factory
gsonBuilder.registerTypeAdapterFactory(new ATypeAdapterFactory());
final Gson gson = gsonBuilder.create();
A aSomeCaseTrue = new AImpl(true);
System.out.print("aSomeCaseTrue:" + gson.toJson(aSomeCaseTrue));
// writes; aSomeCaseTrue:{"x":0}
A aSomeCaseFalse = new AImpl(false);
System.out.print("aSomeCaseFalse:" + gson.toJson(aSomeCaseFalse););
// writes; aSomeCaseFalse:{"someCase":false}
Extras:
1) Your interface:
interface A {
boolean isSomeCase();
}
2) A sample class which implements your sample interface:
class AImpl implements A {
boolean someCase;
public AImpl(boolean value) {
this.someCase = value;
}
#Override
public boolean isSomeCase() {
return someCase;
}
}

Deserialize to type at runtime

I am trying to use TypeAdapterFactory to serialize and deserialize some customer objects. I would like to serialize all the objects to a particular type at runtime.
So given a String classpath and a JsonObject object I would like to deserialize the object to an instance of Class.forName(classpath).
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> tokenType)
{
final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, tokenType);
final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
return new TypeAdapter<T>()
{
#Override
public T read(JsonReader reader) throws IOException
{
Class<?> clazz = Class.forName(classpath);
JsonObject jsonObject = elementAdapter.read(reader).getAsJsonObject();
// Here I want to return an instance of clazz
}
#Override
public void write(JsonWriter writer, T value) throws IOException
{
}
};
}
How would I go about this?

You can try something like this (code wont compile, you need to catch exceptions). Maybe there is a better syntax for THIS too.
final class MyClass implements TypeAdapterFactory {
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> tokenType) {
final MyClass THIS = this;
return new TypeAdapter<T>() {
#Override
public T read(JsonReader reader) throws IOException {
Class<?> clazz = Class.forName(classpath);
TypeToken<T> token = (TypeToken<T>) TypeToken.get(clazz);
TypeAdapter<T> adapter = gson.getDelegateAdapter(THIS, token);
JsonElement tree = gson.getAdapter(JsonElement.class).read(reader);
T out = adapter.fromJsonTree(tree);
return out;
}
#Override
public void write(JsonWriter writer, T value) throws IOException {
}
};
}
}

Gson: can we get the serialized field name in a type adapter?

I've seen that the default TypeAdapter for Enum doesn't fit my need:
private static final class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
private final Map<String, T> nameToConstant = new HashMap<String, T>();
private final Map<T, String> constantToName = new HashMap<T, String>();
public EnumTypeAdapter(Class<T> classOfT) {
try {
for (T constant : classOfT.getEnumConstants()) {
String name = constant.name();
SerializedName annotation = classOfT.getField(name).getAnnotation(SerializedName.class);
if (annotation != null) {
name = annotation.value();
}
nameToConstant.put(name, constant);
constantToName.put(constant, name);
}
} catch (NoSuchFieldException e) {
throw new AssertionError();
}
}
public T read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
return nameToConstant.get(in.nextString());
}
public void write(JsonWriter out, T value) throws IOException {
out.value(value == null ? null : constantToName.get(value));
}
}
If the Enum has value ONE and TWO, when we try to parse THREE, then this value is unknown and Gson will map null instead of raising a parsing exception. I need something more fail-fast.
But I also need something which permits me to know the name of the field which is currently read and creates a parsing failure.
Is it possible with Gson?

Yes.
Gson is quite modular to allow you to use your own TypeAdapterFactory for the enum case. Your custom adapter will return your own EnumTypeAdapter and manage the wanted case. Let the code speak.
package stackoverflow.questions.q16715117;
import java.io.IOException;
import java.util.*;
import com.google.gson.*;
import com.google.gson.annotations.SerializedName;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.*;
public class Q16715117 {
public static void main(String[] args) {
GsonBuilder gb = new GsonBuilder();
gb.registerTypeAdapterFactory(CUSTOM_ENUM_FACTORY);
Container c1 = new Container();
Gson g = gb.create();
String s1 = "{\"colour\":\"RED\",\"number\":42}";
c1 = g.fromJson(s1, Container.class);
System.out.println("Result: "+ c1.toString());
}
public static final TypeAdapterFactory CUSTOM_ENUM_FACTORY = newEnumTypeHierarchyFactory();
public static TypeAdapterFactory newEnumTypeHierarchyFactory() {
return new TypeAdapterFactory() {
#SuppressWarnings({"rawtypes", "unchecked"})
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
Class<? super T> rawType = typeToken.getRawType();
if (!Enum.class.isAssignableFrom(rawType) || rawType == Enum.class) {
return null;
}
if (!rawType.isEnum()) {
rawType = rawType.getSuperclass(); // handle anonymous subclasses
}
return (TypeAdapter<T>) new CustomEnumTypeAdapter(rawType);
}
};
}
private static final class CustomEnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
private final Map<String, T> nameToConstant = new HashMap<String, T>();
private final Map<T, String> constantToName = new HashMap<T, String>();
private Class<T> classOfT;
public CustomEnumTypeAdapter(Class<T> classOfT) {
this.classOfT = classOfT;
try {
for (T constant : classOfT.getEnumConstants()) {
String name = constant.name();
SerializedName annotation = classOfT.getField(name).getAnnotation(SerializedName.class);
if (annotation != null) {
name = annotation.value();
}
nameToConstant.put(name, constant);
constantToName.put(constant, name);
}
} catch (NoSuchFieldException e) {
throw new AssertionError();
}
}
public T read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
String nextString = in.nextString();
T enumValue = nameToConstant.get(nextString);
if (enumValue == null)
throw new GsonEnumParsinException(nextString, classOfT.getName());
return enumValue;
}
public void write(JsonWriter out, T value) throws IOException {
out.value(value == null ? null : constantToName.get(value));
}
}
}
Plus I declared a custom runtime exception:
public class GsonEnumParsinException extends RuntimeException {
String notFoundEnumValue;
String enumName;
String fieldName;
public GsonEnumParsinException(String notFoundEnumValue, String enumName) {
this.notFoundEnumValue = notFoundEnumValue;
this.enumName = enumName;
}
#Override
public String toString() {
return "GsonEnumParsinException [notFoundEnumValue="
+ notFoundEnumValue + ", enumName=" + enumName + "]";
}
public String getNotFoundEnumValue() {
return notFoundEnumValue;
}
#Override
public String getMessage() {
return "Cannot found " + notFoundEnumValue + " for enum " + enumName;
}
}
These are the classes I used in the example:
public enum Colour {
WHITE, YELLOW, BLACK;
}
public class Container {
private Colour colour;
private int number;
public Colour getColour() {
return colour;
}
public void setColour(Colour colour) {
this.colour = colour;
}
public int getNumber() {
return number;
}
public void setNumber(int number) {
this.number = number;
}
#Override
public String toString() {
return "Container [colour=" + colour + ", number=" + number + "]";
}
}
This gives this stacktrace:
Exception in thread "main" GsonEnumParsinException [notFoundEnumValue=RED, enumName=stackoverflow.questions.q16715117.Colour]
at stackoverflow.questions.q16715117.Q16715117$CustomEnumTypeAdapter.read(Q16715117.java:77)
at stackoverflow.questions.q16715117.Q16715117$CustomEnumTypeAdapter.read(Q16715117.java:1)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:93)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:172)
at com.google.gson.Gson.fromJson(Gson.java:803)
at com.google.gson.Gson.fromJson(Gson.java:768)
at com.google.gson.Gson.fromJson(Gson.java:717)
at com.google.gson.Gson.fromJson(Gson.java:689)
at stackoverflow.questions.q16715117.Q16715117.main(Q16715117.java:22)
Unfortunately, the EnumTypeAdapter does not know anything about the context it's called, so this solution is not enough to catch the field name.
Edit
So you have to use also another TypeAdapter that I called CustomReflectiveTypeAdapterFactory and is almost a copy of CustomReflectiveTypeAdapterFactory and I changed a bit the exception, so:
public final class CustomReflectiveTypeAdapterFactory implements TypeAdapterFactory {
private final ConstructorConstructor constructorConstructor;
private final FieldNamingStrategy fieldNamingPolicy;
private final Excluder excluder;
public CustomReflectiveTypeAdapterFactory(ConstructorConstructor constructorConstructor,
FieldNamingStrategy fieldNamingPolicy, Excluder excluder) {
this.constructorConstructor = constructorConstructor;
this.fieldNamingPolicy = fieldNamingPolicy;
this.excluder = excluder;
}
public boolean excludeField(Field f, boolean serialize) {
return !excluder.excludeClass(f.getType(), serialize) && !excluder.excludeField(f, serialize);
}
private String getFieldName(Field f) {
SerializedName serializedName = f.getAnnotation(SerializedName.class);
return serializedName == null ? fieldNamingPolicy.translateName(f) : serializedName.value();
}
public <T> TypeAdapter<T> create(Gson gson, final TypeToken<T> type) {
Class<? super T> raw = type.getRawType();
if (!Object.class.isAssignableFrom(raw)) {
return null; // it's a primitive!
}
ObjectConstructor<T> constructor = constructorConstructor.get(type);
return new Adapter<T>(constructor, getBoundFields(gson, type, raw));
}
private CustomReflectiveTypeAdapterFactory.BoundField createBoundField(
final Gson context, final Field field, final String name,
final TypeToken<?> fieldType, boolean serialize, boolean deserialize) {
final boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());
// special casing primitives here saves ~5% on Android...
return new CustomReflectiveTypeAdapterFactory.BoundField(name, serialize, deserialize) {
final TypeAdapter<?> typeAdapter = context.getAdapter(fieldType);
#SuppressWarnings({"unchecked", "rawtypes"}) // the type adapter and field type always agree
#Override void write(JsonWriter writer, Object value)
throws IOException, IllegalAccessException {
Object fieldValue = field.get(value);
TypeAdapter t =
new CustomTypeAdapterRuntimeTypeWrapper(context, this.typeAdapter, fieldType.getType());
t.write(writer, fieldValue);
}
#Override void read(JsonReader reader, Object value)
throws IOException, IllegalAccessException {
Object fieldValue = null;
try {
fieldValue = typeAdapter.read(reader);
} catch (GsonEnumParsinException e){
e.setFieldName(field.getName());
throw e;
}
if (fieldValue != null || !isPrimitive) {
field.set(value, fieldValue);
}
}
};
}
// more copy&paste code follows
The most important part is read method where I catch the exception and add the field name and throw again exception. Note that class CustomTypeAdapterRuntimeTypeWrapper is simply a renamed copy of TypeAdapterRuntimeTypeWrapper in library internals since class is private.
So, main method changes as follows:
Map<Type, InstanceCreator<?>> instanceCreators
= new HashMap<Type, InstanceCreator<?>>();
Excluder excluder = Excluder.DEFAULT;
FieldNamingStrategy fieldNamingPolicy = FieldNamingPolicy.IDENTITY;
GsonBuilder gb = new GsonBuilder();
gb.registerTypeAdapterFactory(new CustomReflectiveTypeAdapterFactory(new ConstructorConstructor(instanceCreators), fieldNamingPolicy, excluder));
gb.registerTypeAdapterFactory(CUSTOM_ENUM_FACTORY);
Gson g = gb.create();
and now you have this stacktrace (changes to exception are so simple that I omitted them):
Exception in thread "main" GsonEnumParsinException [notFoundEnumValue=RED, enumName=stackoverflow.questions.q16715117.Colour, fieldName=colour]
at stackoverflow.questions.q16715117.Q16715117$CustomEnumTypeAdapter.read(Q16715117.java:90)
at stackoverflow.questions.q16715117.Q16715117$CustomEnumTypeAdapter.read(Q16715117.java:1)
at stackoverflow.questions.q16715117.CustomReflectiveTypeAdapterFactory$1.read(CustomReflectiveTypeAdapterFactory.java:79)
at stackoverflow.questions.q16715117.CustomReflectiveTypeAdapterFactory$Adapter.read(CustomReflectiveTypeAdapterFactory.java:162)
at com.google.gson.Gson.fromJson(Gson.java:803)
at com.google.gson.Gson.fromJson(Gson.java:768)
at com.google.gson.Gson.fromJson(Gson.java:717)
at com.google.gson.Gson.fromJson(Gson.java:689)
at stackoverflow.questions.q16715117.Q16715117.main(Q16715117.java:35)
Of course this solution comes at some costs.
First off all, you have to copy some private/final classes and do your changes. If library get updated, you have to check again your code (a fork of source code would be the same, but at least you do not have to copy all that code).
If you customize field exclusion strategy, constructors or field naming policies you have to replicate them into the CustomReflectiveTypeAdapterFactory since I do not find any possibility to pass them from the builder.