I have the following class
private static class ClassWithGenericType<T> {
Set<T> values;
}
If I initialize now the class with a Set of Enum-values, serialize and deserialize the object by using gson, the Set of the deserialized object does not contain the Enum-values, but the values as String.
I think this is because the generic type is thrown away through the serialization. I saw, that I could use new TypeToken<...>(){}.getType();, but the problem is, that the class above is part of a bigger object, so I cannot call gson.fromJson(classWithGenericType, typeToken) directly.
Is there a smart way of solving this problem? I thought of a TypeAdapter, which does not serialize only the values of the Set, but also it's type.
I found now a solution and created a TypeAdapter.
public class SetTypeAdapterFactory implements TypeAdapterFactory {
#Override
public <T> TypeAdapter<T> create(Gson gson, #NonNull TypeToken<T> type) {
if (!Set.class.isAssignableFrom(type.getRawType())) {
return null;
}
return (TypeAdapter<T>) new SetTypeAdapter(gson);
}
}
public class SetTypeAdapter extends TypeAdapter<Set<?>> {
public static final String TYPE = "#type";
public static final String DATA = "#data";
private final Gson gson;
public SetTypeAdapter(#NonNull Gson gson) {
this.gson = gson;
}
#Override
public void write(final JsonWriter out, final Set<?> set
) throws IOException {
out.beginArray();
for (Object item : set) {
out.beginObject();
out.name(TYPE).value(item.getClass().getName());
out.name(DATA).jsonValue(gson.toJson(item));
out.endObject();
}
out.endArray();
}
#Override
public Set<?> read(final JsonReader in) throws IOException {
final Set<Object> set = Sets.newHashSet();
in.beginArray();
while (in.hasNext()) {
in.beginObject();
set.add(readNextObject(in));
in.endObject();
}
in.endArray();
return set;
}
private Object readNextObject(JsonReader in) throws IOException {
try {
checkNextName(in, TYPE);
Class<?> cls = Class.forName(in.nextString());
checkNextName(in, DATA);
return gson.fromJson(in, cls);
} catch (ClassNotFoundException exception) {
throw new IOException(exception);
}
}
private void checkNextName(JsonReader in, String name) throws IOException {
if (!in.nextName().equals(name)) {
throw new IOException("Name was not: " + name);
}
}
}
We can add the factory to the GsonBuilder and afterwards we are capable of serializing a Set with generic types.
var gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapterFactory(new SetTypeAdapterFactory());
var gson = gsonBuilder.create();
The serialized Set has then the following structure:
[
{
"#type":<class_name_first_element>,
"#data":<first_element_as_json>
},
...
]
A client has decided to start sending empty JSON objects at the end of arrays (to help their caching facepalm) but this has caused a whole bunch of unexpected behaviour in my app.
For example this is the data I am being sent...
[{object}, {object}, {}, {object}]
I currently use GSON to deserialize data sent from the server. I have been looking into type adapters to filter out these empty objects, however I am not fully sure how to implement the filtering. Any ideas on how to skip empty objects using GSON?
You can try this solution from here
class CollectionAdapter implements JsonSerializer<Collection<?>> {
#Override
public JsonElement serialize(Collection<?> src, Type typeOfSrc, JsonSerializationContext context) {
if (src == null || src.isEmpty()) // exclusion is made here
return null;
JsonArray array = new JsonArray();
for (Object child : src) {
JsonElement element = context.serialize(child);
array.add(element);
}
return array;
}
}
Then register it
Gson gson = new GsonBuilder().registerTypeHierarchyAdapter(Collection.class, new CollectionAdapter()).create();
I solved this issue ... I had to make a TypeAdapterFactory that set empty objects to null then filtered out the nulls from the resulting list.
Here is my TypeAdapterFactory
private static class EmptyCheckTypeAdapterFactory implements TypeAdapterFactory {
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> type) {
// We filter out the EmptyCheckTypeAdapter as we need to check this for emptiness!
if (Story.class.isAssignableFrom(type.getRawType())) {
final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
return new EmptyCheckTypeAdapter<>(delegate, elementAdapter).nullSafe();
}
return null;
}
public class EmptyCheckTypeAdapter<T> extends TypeAdapter<T> {
private final TypeAdapter<T> delegate;
private final TypeAdapter<JsonElement> elementAdapter;
public EmptyCheckTypeAdapter(final TypeAdapter<T> delegate,
final TypeAdapter<JsonElement> elementAdapter) {
this.delegate = delegate;
this.elementAdapter = elementAdapter;
}
#Override
public void write(final JsonWriter out, final T value) throws IOException {
this.delegate.write(out, value);
}
#Override
public T read(final JsonReader in) throws IOException {
final JsonObject asJsonObject = elementAdapter.read(in).getAsJsonObject();
if (asJsonObject.entrySet().isEmpty()) {
return null;
}
return this.delegate.fromJsonTree(asJsonObject);
}
}
}
Finally, filtered out the nulls using the following code
myDto.stories.removeAll(Collections.singleton(null));
I'm working on a nice solution to internationalize Enums by Gson deserialize (.toJson).
For now I have it:
private static final class GenericEnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
private ResourceBundle bundle = ResourceBundle.getBundle("Messages");
private Class<T> classOfT;
public GenericEnumTypeAdapter(Class<T> classOfT) {
this.classOfT = classOfT;
}
public T read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
return Enum.valueOf(classOfT, in.nextString());
}
public void write(JsonWriter out, T value) throws IOException {
out.value(value == null ? null : bundle.getString("enum." + value.getClass().getSimpleName() + "."
+ value.name()));
}
}
The problem of this solution is: For each enum you should register a new Adapter:
gsonBuilder.registerTypeAdapter(EventSensorState.class,
new GenericEnumTypeAdapter<>(FirstEnum.class)
Do someone has an idea to do it better?
Use a TypeAdapterFactory to generate ALL the Adapters. See How do I implement TypeAdapterFactory in Gson?
To convert your TypeAdapter into a TypeAdapterFactory, the key is detecting the class properly, and then using the create method. Warning: this solution will register every type of Enum in your system; you may have to tweak it to only work with Enums that implement a particular interface, or register Enum classes with the subclass, etc. I created an EnumGenerator class to do most of the work of the reading conversion, which you should be able to figure out on your own.
public class EnumAdapterFactory implements TypeAdapterFactory {
private final ResourceBundle bundle;
private final EnumGenerator generator;
public EnumAdapterFactory(ResourceBundle bundle, EnumGenerator generator) {
this.bundle = bundle;
this.generator = generator;
}
#SuppressWarnings("unchecked")
#Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
if (!Enum.class.isAssignableFrom(type.getRawType())) return null;
return (TypeAdapter<T>) new GenericEnumTypeAdapter();
}
private final class GenericEnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
public T read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
return generator.create(in.nextString());
}
public void write(JsonWriter out, T value) throws IOException {
if(value == null) {
out.nullValue();
return;
}
out.value(bundle.getString("enum."
+ value.getClass().getSimpleName() + "."
+ value.name()));
}
}
}
And EnumGenerator's interface:
public interface EnumGenerator {
<T extends Enum<T>> T create(String nextString);
}
I need to output enum values using Gson which due to client limitations need to be in lower case.
For example CLOSE_FILE would be close_file.
Is there a simple way of doing this? I have looked at making a class which implements JsonSerializer but it looks like I would have to manually serialize the whole class (which is quite complex) is this the case?
If you have control over the enum type, annotate its members with #SerializedName and give it the appropriate serialized value. For example,
enum Action {
#SerializedName("close_file")
CLOSE_FILE;
}
If you don't have control over the enum, provide a custom TypeAdapter when creating a Gson instance. For example,
Gson gson = new GsonBuilder().registerTypeAdapter(Action.class, new TypeAdapter<Action>() {
#Override
public void write(JsonWriter out, Action value) throws IOException {
out.value(value.name().toLowerCase());
}
#Override
public Action read(JsonReader in) throws IOException {
return Action.valueOf(in.nextString().toUpperCase());
}
}).create();
If you want to serialize all enum to lowercase, you can use this code
Gson gson = new GsonBuilder().registerTypeHierarchyAdapter(Enum.class, new EnumToLowerCaseJsonConverter()).create();
public final class EnumToLowerCaseJsonConverter implements JsonSerializer<Enum<?>>, JsonDeserializer<Enum<?>> {
private static Map<String, Class<? extends Enum<?>>> typesToClass = new HashMap<>();
#Override
public JsonElement serialize(final Enum<?> src, final Type typeOfSrc,
final JsonSerializationContext context) {
if (src == null) {
return JsonNull.INSTANCE;
}
return new JsonPrimitive(src.name().toLowerCase());
}
#SuppressWarnings("unchecked")
#Override
public Enum<?> deserialize(final JsonElement json, final Type typeOfT,
final JsonDeserializationContext context) throws JsonParseException {
if (json == null || json.isJsonNull()) {
return null;
}
if (!json.isJsonPrimitive() || !json.getAsJsonPrimitive().isString()) {
throw new JsonParseException(
"Expecting a String JsonPrimitive, getting " + json.toString());
}
try {
final String enumClassName = typeOfT.getTypeName();
Class<? extends Enum<?>> clazz = typesToClass.get(enumClassName);
if (clazz == null) {
clazz = (Class<? extends Enum<?>>) Class.forName(enumClassName);
typesToClass.put(enumClassName, clazz);
}
return Enum.valueOf((Class) clazz, json.getAsString().toUpperCase());
} catch (final ClassNotFoundException e) {
throw new JsonParseException(e);
}
}
}
An easy to use specific type adapter:
Gson gson = new GsonBuilder().registerTypeAdapter(Action.class, new EnumToLowerCaseTypeAdapter<>(Action.class)).create();
public final class EnumToLowerCaseTypeAdapter<T extends Enum<?>> extends TypeAdapter<T> {
private final Class<T> clazz;
public EnumToLowerCaseTypeAdapter(final Class<T> clazz) {
this.clazz = clazz;
}
#Override
public void write(final JsonWriter out, final T value) throws IOException {
if (value == null) {
out.nullValue();
} else {
out.value(value.name().toLowerCase());
}
}
#SuppressWarnings("unchecked")
#Override
public T read(final JsonReader in) throws IOException {
switch (in.peek()) {
case NULL:
in.nextNull();
return null;
default:
final String value = in.nextString();
return (T) Enum.valueOf((Class) clazz, value.toUpperCase());
}
}
}
I have the following code:
public static class A
{
public A() {}
private List<B> bs = new ArrayList<B>();
public List<B> getBs() {
return bs;
}
public void setBs(List<B> bs) {
this.bs = bs;
}
}
public static class B
{
B(String foo){this.foo=foo;}
private String foo;
public String getFoo() {
return foo;
}
public void setFoo(String foo) {
this.foo = foo;
}
}
public static void main(String[] args) throws Exception {
Gson gson = new Gson();
A a = new A();
a.getBs().add(new B("bar"));
System.out.println(gson.toJson(a));
}
and as expected the output is:
{"bs":[{"foo":"bar"}]}
However, if I make A a subclass of HashMap:
public static class A extends HashMap
I get an empty set returned: {}
I have even tried:
System.out.println(gson.toJson(a, new TypeToken<A>(){}.getType()));
and:
System.out.println(gson.toJson(a, new TypeToken<HashMap>(){}.getType()));
Can someone tell me whether/how I can serialise this HashMap subclass using GSON?
Gson works with (default and custom) TypeAdapterFactory instances and the TypeAdapter objects they create to serialize/deserialize your objects.
It goes through the list of registered TypeAdapterFactory objects and picks the first one that can create an appropriate TypeAdapter for the type of the object your are providing. One of these TypeAdapterFactory objects, is one of type MapTypeAdapterFactory which creates a TypeAdapter (of type MapTypeAdapterFactory$Adapter) that serializes/deserializes based on the java.util.Map interface (keys/values). It does nothing about your custom sub type's fields.
If you want Gson to serialize your type as both a Map and a custom type, you will need to register either a custom TypeAdapter directly or a custom TypeAdapterFactory that creates TypeAdapter objects.
Here is the custom TypeAdapterFactory.
Test:
public static void main(String[] args) throws Exception{
Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(new RetainFieldMapFactory())
.create();
Foo f = gson.fromJson("{'key1':'value1','key2':'value2'}", Foo.class);
System.out.println("in map:\t" + f.toString());
System.out.println("f.key1:\t"+f.key1);
System.out.println("toJson:\t"+gson.toJson(f));
}
public static class Foo extends HashMap<String, String> {
private String key1;
}
Output:
in map: {key2=value2}
f.key1: value1
toJson: {"key2":"value2","key1":"value1"}
RetainFieldMapFactory.java:
/**
* Created by linfaxin on 2015/4/9 009.
* Email: linlinfaxin#163.com
*/
public class RetainFieldMapFactory implements TypeAdapterFactory {
FieldNamingPolicy fieldNamingPolicy = FieldNamingPolicy.IDENTITY;
ConstructorConstructor constructorConstructor = new ConstructorConstructor(Collections.<Type, InstanceCreator<?>>emptyMap());
MapTypeAdapterFactory defaultMapFactory = new MapTypeAdapterFactory(constructorConstructor, false);
ReflectiveFilterMapFieldFactory defaultObjectFactory = new ReflectiveFilterMapFieldFactory(constructorConstructor,
fieldNamingPolicy, Excluder.DEFAULT);
#Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
final TypeAdapter<T> mapAdapter = defaultMapFactory.create(gson, type);
if(mapAdapter!=null){
return (TypeAdapter<T>) new RetainFieldMapAdapter(mapAdapter, defaultObjectFactory.create(gson, type));
}
return mapAdapter;
}
class RetainFieldMapAdapter extends TypeAdapter<Map<String, Object>>{
TypeAdapter<Map<String, Object>> mapAdapter;
ReflectiveTypeAdapterFactory.Adapter<Map<String, Object>> objectAdapter;
RetainFieldMapAdapter(TypeAdapter mapAdapter, ReflectiveTypeAdapterFactory.Adapter objectAdapter) {
this.mapAdapter = mapAdapter;
this.objectAdapter = objectAdapter;
}
#Override
public void write(final JsonWriter out, Map<String, Object> value) throws IOException {
//1.write object
StringWriter sw = new StringWriter();
objectAdapter.write(new JsonWriter(sw), value);
//2.convert object to a map
Map<String, Object> objectMap = mapAdapter.fromJson(sw.toString());
//3.overwrite fields in object to a copy map
value = new LinkedHashMap<String, Object>(value);
value.putAll(objectMap);
//4.write the copy map
mapAdapter.write(out, value);
}
#Override
public Map<String, Object> read(JsonReader in) throws IOException {
//1.create map, all key-value retain in map
Map<String, Object> map = mapAdapter.read(in);
//2.create object from created map
Map<String, Object> object = objectAdapter.fromJsonTree(mapAdapter.toJsonTree(map));
//3.remove fields in object from map
for(String field : objectAdapter.boundFields.keySet()){
map.remove(field);
}
//4.put map to object
object.putAll(map);
return object;
}
}
/**
* If class is extends from some custom map,
* class should implement this to avoid serialize custom map's fields
*/
public interface RetainFieldFlag {}
static class ReflectiveFilterMapFieldFactory extends ReflectiveTypeAdapterFactory{
public ReflectiveFilterMapFieldFactory(ConstructorConstructor constructorConstructor, FieldNamingStrategy fieldNamingPolicy, Excluder excluder) {
super(constructorConstructor, fieldNamingPolicy, excluder);
}
#Override
protected boolean shouldFindFieldInClass(Class willFindClass, Class<?> originalRaw) {
if(RetainFieldFlag.class.isAssignableFrom(originalRaw)){
return RetainFieldFlag.class.isAssignableFrom(willFindClass);
}else{
Class[] endClasses = new Class[]{Object.class, HashMap.class, LinkedHashMap.class,
LinkedTreeMap.class, Hashtable.class, TreeMap.class, ConcurrentHashMap.class,
IdentityHashMap.class, WeakHashMap.class, EnumMap.class};
for(Class c : endClasses){
if(willFindClass == c) return false;
}
}
return super.shouldFindFieldInClass(willFindClass, originalRaw);
}
}
/**
* below code copy from {#link com.google.gson.internal.bind.ReflectiveTypeAdapterFactory}
* (little modify, in source this class is final)
* Type adapter that reflects over the fields and methods of a class.
*/
static class ReflectiveTypeAdapterFactory implements TypeAdapterFactory {
private final ConstructorConstructor constructorConstructor;
private final FieldNamingStrategy fieldNamingPolicy;
private final Excluder excluder;
public ReflectiveTypeAdapterFactory(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> Adapter<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 ReflectiveTypeAdapterFactory.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 ReflectiveTypeAdapterFactory.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 TypeAdapterRuntimeTypeWrapper(context, this.typeAdapter, fieldType.getType());
t.write(writer, fieldValue);
}
#Override void read(JsonReader reader, Object value)
throws IOException, IllegalAccessException {
Object fieldValue = typeAdapter.read(reader);
if (fieldValue != null || !isPrimitive) {
field.set(value, fieldValue);
}
}
};
}
private Map<String, BoundField> getBoundFields(Gson context, TypeToken<?> type, Class<?> raw) {
Map<String, BoundField> result = new LinkedHashMap<String, BoundField>();
if (raw.isInterface()) {
return result;
}
Type declaredType = type.getType();
Class<?> originalRaw = type.getRawType();
while (shouldFindFieldInClass(raw, originalRaw)) {
Field[] fields = raw.getDeclaredFields();
for (Field field : fields) {
boolean serialize = excludeField(field, true);
boolean deserialize = excludeField(field, false);
if (!serialize && !deserialize) {
continue;
}
field.setAccessible(true);
Type fieldType = $Gson$Types.resolve(type.getType(), raw, field.getGenericType());
BoundField boundField = createBoundField(context, field, getFieldName(field),
TypeToken.get(fieldType), serialize, deserialize);
BoundField previous = result.put(boundField.name, boundField);
if (previous != null) {
throw new IllegalArgumentException(declaredType
+ " declares multiple JSON fields named " + previous.name);
}
}
type = TypeToken.get($Gson$Types.resolve(type.getType(), raw, raw.getGenericSuperclass()));
raw = type.getRawType();
}
return result;
}
protected boolean shouldFindFieldInClass(Class willFindClass, Class<?> originalRaw){
return willFindClass != Object.class;
}
static abstract class BoundField {
final String name;
final boolean serialized;
final boolean deserialized;
protected BoundField(String name, boolean serialized, boolean deserialized) {
this.name = name;
this.serialized = serialized;
this.deserialized = deserialized;
}
abstract void write(JsonWriter writer, Object value) throws IOException, IllegalAccessException;
abstract void read(JsonReader reader, Object value) throws IOException, IllegalAccessException;
}
public static final class Adapter<T> extends TypeAdapter<T> {
private final ObjectConstructor<T> constructor;
private final Map<String, BoundField> boundFields;
private Adapter(ObjectConstructor<T> constructor, Map<String, BoundField> boundFields) {
this.constructor = constructor;
this.boundFields = boundFields;
}
#Override public T read(JsonReader in) throws IOException {
if (in.peek() == JsonToken.NULL) {
in.nextNull();
return null;
}
T instance = constructor.construct();
try {
in.beginObject();
while (in.hasNext()) {
String name = in.nextName();
BoundField field = boundFields.get(name);
if (field == null || !field.deserialized) {
in.skipValue();
} else {
field.read(in, instance);
}
}
} catch (IllegalStateException e) {
throw new JsonSyntaxException(e);
} catch (IllegalAccessException e) {
throw new AssertionError(e);
}
in.endObject();
return instance;
}
#Override public void write(JsonWriter out, T value) throws IOException {
if (value == null) {
out.nullValue();
return;
}
out.beginObject();
try {
for (BoundField boundField : boundFields.values()) {
if (boundField.serialized) {
out.name(boundField.name);
boundField.write(out, value);
}
}
} catch (IllegalAccessException e) {
throw new AssertionError();
}
out.endObject();
}
}
}
static 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 {
// Order of preference for choosing type adapters
// First preference: a type adapter registered for the runtime type
// Second preference: a type adapter registered for the declared type
// Third preference: reflective type adapter for the runtime type (if it is a sub class of the declared type)
// Fourth preference: reflective type adapter for the declared type
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);
}
/**
* Finds a compatible runtime type if it is more specific
*/
private Type getRuntimeTypeIfMoreSpecific(Type type, Object value) {
if (value != null
&& (type == Object.class || type instanceof TypeVariable<?> || type instanceof Class<?>)) {
type = value.getClass();
}
return type;
}
}
}