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How to use Jackson ObjectMapper.readValue with generic class

how to use Jackson ObjectMapper.readValue with generic class, someone says that need JavaType, but JavaType is also splicing other class， is Jackson can use like gson TypeToken?
my code is like this
public static void main(String[] args) throws IOException {
String json = "{\"code\":200,\"msg\":\"success\",\"reqId\":\"d1ef3b76e73b40379f895a3a7f1389e2\",\"cost\":819,\"result\":{\"taskId\":1103,\"taskName\":\"ei_custom_config\",\"jobId\":233455,\"status\":2,\"interrupt\":false,\"pass\":true}}";
RestResponse<TaskResult> result = get(json);
System.out.println(result);
System.out.println(result.getResult().getJobId());
}
public static <T> RestResponse<T> get(String json) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
return objectMapper.readValue(json, new TypeReference<RestResponse<T>>() {});
}
and error is
org.example.zk.RestResponse#6fd02e5
Exception in thread "main" java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to org.example.zk.TaskResult
at org.example.zk.JacksonTest.main(JacksonTest.java:15)

You need to provide jackson with concrete type information for T. I would suggest using readValue() overload with parameter - JavaType.
Add the class of T as parameter of get() and construct parametric type using it.
public static <T> RestResponse<T> get(String json, Class<T> classOfT) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
JavaType type = TypeFactory.defaultInstance().constructParametricType(RestResponse.class, classOfT);
return objectMapper.readValue(json, type);
}
Usage:
RestResponse<TaskResult> result = get(json, TaskResult.class);

We can make T with upper-bound to help infering object type.
public static <T extends TaskResult> RestResponse<T> get(String json) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
return objectMapper.readValue(json, new TypeReference<RestResponse<T>>() {});
}
Without type bounduary, RestResponse<T> equals to RestResponse<Object>
We can not new a generic class with T.

How to get use GSON.fromJson with type provided in a variable?

I wanted to use GSON.fromJSON in the following way:
Class<?> type = Abc.class;
Class<?> parametersObject = GSON.fromJson(parameters, type);
But I am getting compilation error on 2nd line. How do I infer the return type of the following line? I know it would be of Abc type in the above case. But I want to make this dynamic based on what is specified in type variable. How can I do that?

Try it with the below code. There is a question about the difference between ? and T in class and method signatures, link.
public class Tests<T> {
#Test
public void test01() {
Tests<String> stringTest= new Tests<>();
System.out.println(stringTest.parseObject("testtestt", String.class));
}
public T parseObject(String parameters, Class<T> clazz) {
Gson gson = new Gson();
T obj = (T) gson.fromJson(parameters, clazz);
return obj;
}
}
``
UPD: declare <T> T returning type of method.
#Test
public void test02() {
String stringstring = parseObject02("stringstring", String.class);
System.out.println(stringstring);
List list = parseObject02("[1,2,3]", List.class);
System.out.println(list);
// output in console
// stringstring
// [1.0, 2.0, 3.0]
}
public <T> T parseObject02(String parameters, Class<T> clazz) {
Gson gson = new Gson();
T obj = gson.fromJson(parameters, clazz);
return obj;
}

Make GSON accept single objects where it expects arrays

I have bunch of model classes which have fields of type List<X> where X is one of many things (e.g. String, Integer, but also some of my own types). I'm using GSON to parse JSON representations of these models.
My problem is that the server I'm dealing with (which is beyond my control) somehow removed singleton arrays and replaces them by the contained object.
For example, instead of returning:
{
"foo": [ "bar"],
"bleh": [ { "some": "object" } ]
}
It returns:
{
"foo": "bar",
"bleh": { "some": "object" }
}
Now assume that the Java model class look something like this:
public class Model {
private List<String> foo;
private List<SomeObject> bleh;
}
Currently this causes GSON to throw an exception because it finds BEGIN_STRING or BEGIN_OBJECT where it expects BEGIN_ARRAY.
For arrays or lists of Strings this is easily solved using a TypeAdapter<List<String>>. But the problem is I have Lists with many different element types and I don't want to write a separate TypeAdapter for each case. Nor have I been able to a generic TypeAdapter<List<?>>, because at some point you need to know the type.
So is there another way to configure GSON to be smart enough to turn single objects or values into arrays/lists? Or in other words, just "pretend" that the [ and ] are there where it expects to find them although they aren't there?

But the problem is I have Lists with many different element types and I don't want to write a separate TypeAdapter for each case. Nor have I been able to a generic TypeAdapter>, because at some point you need to know the type.
This is what type adapter factories are designed for: you can control every type in Gson instance configuration.
final class AlwaysListTypeAdapterFactory<E>
implements TypeAdapterFactory {
// Gson can instantiate it itself
private AlwaysListTypeAdapterFactory() {
}
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// If it's not a List -- just delegate the job to Gson and let it pick the best type adapter itself
if ( !List.class.isAssignableFrom(typeToken.getRawType()) ) {
return null;
}
// Resolving the list parameter type
final Type elementType = resolveTypeArgument(typeToken.getType());
#SuppressWarnings("unchecked")
final TypeAdapter<E> elementTypeAdapter = (TypeAdapter<E>) gson.getAdapter(TypeToken.get(elementType));
// Note that the always-list type adapter is made null-safe, so we don't have to check nulls ourselves
#SuppressWarnings("unchecked")
final TypeAdapter<T> alwaysListTypeAdapter = (TypeAdapter<T>) new AlwaysListTypeAdapter<>(elementTypeAdapter).nullSafe();
return alwaysListTypeAdapter;
}
private static Type resolveTypeArgument(final Type type) {
// The given type is not parameterized?
if ( !(type instanceof ParameterizedType) ) {
// No, raw
return Object.class;
}
final ParameterizedType parameterizedType = (ParameterizedType) type;
return parameterizedType.getActualTypeArguments()[0];
}
private static final class AlwaysListTypeAdapter<E>
extends TypeAdapter<List<E>> {
private final TypeAdapter<E> elementTypeAdapter;
private AlwaysListTypeAdapter(final TypeAdapter<E> elementTypeAdapter) {
this.elementTypeAdapter = elementTypeAdapter;
}
#Override
public void write(final JsonWriter out, final List<E> list) {
throw new UnsupportedOperationException();
}
#Override
public List<E> read(final JsonReader in)
throws IOException {
// This is where we detect the list "type"
final List<E> list = new ArrayList<>();
final JsonToken token = in.peek();
switch ( token ) {
case BEGIN_ARRAY:
// If it's a regular list, just consume [, <all elements>, and ]
in.beginArray();
while ( in.hasNext() ) {
list.add(elementTypeAdapter.read(in));
}
in.endArray();
break;
case BEGIN_OBJECT:
case STRING:
case NUMBER:
case BOOLEAN:
// An object or a primitive? Just add the current value to the result list
list.add(elementTypeAdapter.read(in));
break;
case NULL:
throw new AssertionError("Must never happen: check if the type adapter configured with .nullSafe()");
case NAME:
case END_ARRAY:
case END_OBJECT:
case END_DOCUMENT:
throw new MalformedJsonException("Unexpected token: " + token);
default:
throw new AssertionError("Must never happen: " + token);
}
return list;
}
}
}
Now you just have to tell Gson which fields are not well-formed.
Of course, you might configure the whole Gson instance to accept such lists, but let it be more precise using the #JsonAdapter annotation:
final class Model {
#JsonAdapter(AlwaysListTypeAdapterFactory.class)
final List<String> foo = null;
#JsonAdapter(AlwaysListTypeAdapterFactory.class)
final List<SomeObject> bleh = null;
#Override
public String toString() {
return "Model{" + "foo=" + foo + ", bleh=" + bleh + '}';
}
}
final class SomeObject {
final String some = null;
#Override
public String toString() {
return "SomeObject{" + "some='" + some + '\'' + '}';
}
}
Test data:
single.json
{
"foo": "bar",
"bleh": {"some": "object"}
}
list.json
{
"foo": ["bar"],
"bleh": [{"some": "object"}]
}
Example:
private static final Gson gson = new Gson();
public static void main(final String... args)
throws IOException {
for ( final String resource : ImmutableList.of("single.json", "list.json") ) {
try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q43412261.class, resource) ) {
final Model model = gson.fromJson(jsonReader, Model.class);
System.out.println(model);
}
}
}
And the output:
Model{foo=[bar], bleh=[SomeObject{some='object'}]}
Model{foo=[bar], bleh=[SomeObject{some='object'}]}

You can simply write your own JsonDeserializer where you check whether your bleh or foo are JsonObjects or JsonArrays.
To check if a JsonElement is an array or an object:
JsonElement element = ...;
if (element.isJsonObject()) {
//element is a JsonObject
} else if (element.isJsonArray()) {
//element is a JsonArray
}

One solution to this would be to write a custom TypeAdapterFactory which creates an adapter which peeks at the JSON data. If it encounters something other than a JSON array (or JSON null) it wraps it inside a JSON array before deserializing it:
// Only intended for usage with #JsonAdapter on fields
class SingleValueOrListAdapterFactory implements TypeAdapterFactory {
#Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
// Note: Cannot use getDelegateAdapter due to https://github.com/google/gson/issues/1028
TypeAdapter<T> listAdapterDelegate = gson.getAdapter(type);
TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
return new TypeAdapter<T>() {
#Override
public void write(JsonWriter out, T value) throws IOException {
listAdapterDelegate.write(out, value);
}
#Override
public T read(JsonReader in) throws IOException {
JsonToken peeked = in.peek();
if (peeked == JsonToken.NULL || peeked == JsonToken.BEGIN_ARRAY) {
return listAdapterDelegate.read(in);
} else {
// Wrap JSON element in a new JSON array before deserializing it
JsonElement jsonElement = jsonElementAdapter.read(in);
JsonArray jsonArray = new JsonArray();
jsonArray.add(jsonElement);
return listAdapterDelegate.fromJsonTree(jsonArray);
}
}
};
}
}
The above implementation is designed only for usage with #JsonAdapter on fields, for example:
#JsonAdapter(SingleValueOrListAdapterFactory.class)
private List<MyClass> myField;
Compared to the currently accepted answer this provides the following advantages because it simply delegates the actual deserialization to listAdapterDelegate:
Custom List (or Collection) subclasses are supported because creation of them is delegated to Gson
Gson's default type resolution logic is used to determine the element type and to deserialize it
But it also has the following disadvantage:
Decreased performance because if the data is not already in a JSON array it is first deserialized to a JsonElement before the actual deserialization is performed

When using the GSON library, you could just check whether or not the following token is an object or an array. This of course requires you to go more fine grained while parsing the XML, but it allows you full control of what do you want to get from it. Sometimes we are not under control of the XML, and it could come handy.
This is an example to check if the next token is an object or an array, using the JsonReader class to parse the file:
if (jsonReader.peek() == JsonToken.BEGIN_ARRAY) {
jsonReader.beginArray()
} else if (jsonReader.peek() == JsonToken.BEGIN_OBJECT) {
jsonReader.beginObject()
}
And at the end of the array / object, you could do the same, but for the end tokens:
if (jsonReader.peek() == JsonToken.END_ARRAY) {
jsonReader.endArray()
} else if (jsonReader.peek() == JsonToken.END_OBJECT) {
jsonReader.endObject()
}
This way, you could have identical code (adding an extra check, to verify if you are on an array or on an object) to parse your array of objects, or a single object.

I had this same problem consuming xml / json from a vendor - they certainly weren't going to change their code for me :) There were several resources on the web that I used before changing adapting them to my own version This SO answer was very helpful. I spent some time looking at the gson code and finding a lot of private variable that I wanted access to. So, essentially what my custom collection adapter does is peek to see if the next element is an object. If not, we just delegate the read to the previous adapter (that we have overridden).
If the next element is an object, we use gson to process that. We then convert that to an array of one object. Use gson to write that to a string, then pass that string as a JsonReader to the underlying adapter. This can then create an instance of the underlying list and add the one element we have.
Here's the AdapterTypeFactory:
public enum ListSingleObjectAdapterFactory implements TypeAdapterFactory {
INSTANCE; // Josh Bloch's Enum singleton pattern
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
Class<? super T> rawType = typeToken.getRawType();
if (!Collection.class.isAssignableFrom(rawType)) {
return null;
}
TypeAdapter collectionAdapter = gson.getDelegateAdapter(this, typeToken);
Class genericClass = (Class) ((ParameterizedType) typeToken.getType())
.getActualTypeArguments()[0];
return new SingleObjectOrCollectionAdapter(
gson, collectionAdapter, genericClass);
}
}
Then the type adapter I have is:
public class SingleObjectOrCollectionAdapter<T> extends TypeAdapter<Collection<T>> {
private Class<T> adapterclass;
private Gson gson;
private TypeAdapter arrayTypeAdapter;
public SingleObjectOrCollectionAdapter(Gson gson, TypeAdapter<T> collectionTypeAdapter, Class<T> componentType) {
arrayTypeAdapter = collectionTypeAdapter;
this.gson = gson;
adapterclass = componentType;
}
#Override
public Collection<T> read(JsonReader reader) throws IOException {
Collection<T> collection;
JsonReader myReader = reader;
if (reader.peek() == JsonToken.BEGIN_OBJECT) {
T inning = gson.fromJson(reader, adapterclass);
String s = gson.toJson(new Object[]{inning});
myReader = new JsonReader(new StringReader(s));
}
collection = (Collection)arrayTypeAdapter.read( myReader );
return collection;
}
#Override
public void write(JsonWriter writer, Collection<T> value) throws IOException {
arrayTypeAdapter.write(writer, value);
}
}
Finally, we need to register the adapter factory:
GsonBuilder gb = new GsonBuilder().registerTypeAdapterFactory(ListSingleObjectAdapterFactory.INSTANCE);
So far, it seems to be working well handling both single and multiple objects - although I wouldn't be surprised if it needs some tweaking down the road.

Java Generics and Super Type Tokens

I am trying to make the three following methods into one generic solution, I tried some ideas which compile but don't do well at runtime.
public static List<User> parseToUsers(HttpResponse response) {
ObjectMapper mapper = new ObjectMapper();
String results = parseToString(response);
return mapper.readValue(results, new TypeReference<List<User>>() {});
}
public static List<Record> parseToRecords(HttpResponse response) {
ObjectMapper mapper = new ObjectMapper();
String results = parseToString(response);
return mapper.readValue(results, new TypeReference<List<Record>>() {});
}
public static Record parseToRecord(HttpResponse response) {
ObjectMapper mapper = new ObjectMapper();
String results = parseToString(response);
return mapper.readValue(results, new TypeReference<Record>() {});;
}
I have also tried to understand this blog post about Super Type Tokens.
EDIT:
This is what I came up with so far:
public static <T> T parseJsonResponse(TypeReference<T> type, HttpResponse response) throws DroidException {
ObjectMapper mapper = new ObjectMapper();
String results = parseResponseToString(response);
return readValue = mapper.readValue(results, type);
}
Then I call it like this.
parseJsonResponseToList(new TypeReference<List<Record>>() {}, response)
Not really satisfieng.Is there a better solution?

So what exactly is the problem? In what way do you not like it?
Jackson has other ways for constructing generic types; so perhaps what are looking for is along lines of:
public List<T> listOf(String json, Class<T> elementType) {
ObjectMapper mapper = new ObjectMapper(); // should (re)use static instance for perf!
JavaType listType = mapper.getTypeFactory().constructCollectionType(ArrayList.class, elementType);
return mapper.readValue(json, listType);
}
TypeFactory can be used to programmatically construct types that use generics -- return type is JavaType, because basic Java Class is type-erased.
TypeFactory is actually used to convert TypeReference to JavaType internally as well.
EDIT
As to regular, non-Collection/Map types, it's really quite simple:
public T parseSingle(Class<T> cls, InputStream src) throws IOException {
return mapper.readValue(src, cls);
}
(you also do NOT want to read contents as String -- not only is it slow, but it's easy to mess up character encodings, so if possible, feed InputStream or byte[] instead)

I don't really know what your ObjectMapper and TypeReference classes do, so maybe this answer doesn't fit you all that well, but here's how I'd probably do it if I understand your situation at all:
public interface Parser<T> {
public T parse(String results);
public static class MapperParser<T> implements Parser<T> {
private final TypeReference<T> type;
public MapperParser(TypeReference<T> type) {this.type = type;}
public T parse(String results) {
return(new ObjectMapper().readValue(results, type));
}
}
public static final Parser<List<User>> users = new MapperParser(new TypeReference<List<User>>());
public static final Parser<List<Record>> records = new MapperParser(new TypeReference<List<Record>>());
public static final Parser<Record> record = new MapperParser(new TypeReference<Record>());
}
/* And then, in the class you were in your question: */
public static <T> T parseJsonResponse(Parser<T> parser, HttpResponse response) {
return(parser.parse(parseResponseToString(response)));
}
Then, you may call it as such:
parseJsonResponse(Parser.users, response)
Is that more to your liking?

Ok this is my favorite solution, inspired by Dolda2000, i keep as is in my initial post and add an enum.
public enum TypeRef {
RECORDS(new TypeReference<List<Record>>() {}), USERS(new TypeReference<List<User>>() {}), USER(new TypeReference<User>() {});
private TypeReference<?> type;
private TypeRef(TypeReference<?> type) {
this.type = type;
}
public TypeReference<?> getType() {
return this.type;
}
}
and then instead of writing:
readJsonResponse(new TypeReference<List<Record>>() {}, response)
i can write:
readJsonResponse(TypeRef.RECORDS, response);
no magic going on but i like it more than wrapping it in another interface

Ok after running into a compile error:
type parameters of T cannot be determined; no unique maximal instance exists for type variable T with upper bounds T,java.lang.Object
i quit the over engeneering session and keep it simple
private static TypeReference<List<Record>> RECORDS = new TypeReference<List<Record>>() {};
public static <T> T readJson(TypeReference<T> type, String text) {
ObjectMapper mapper = new ObjectMapper();
return readValue = mapper.readValue(text, type);
}
use it like this
readJson(RECORDS, text);
no enums, i just use static fields for the TypeReference and everyone can read the code easily without understanding TypeReference
thank you guys i learned something about over engeneering today :P

GSON and Generic types

I've come across a problem of using Gson library and generic types(my types and collections). However they have an answer how to solve this problem, I don't think it's appropriate to write a specific message converter for the every type I've already implemented and I'll implement.
What I did is:
Implemented my own message converter:
public class SuperHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
private final Charset charset;
private final Gson gson;
public CostomHttpMC_1(MediaType mediaType, String charset) {
super(mediaType);
this.charset = Charset.forName(charset);
gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create();
}
#Override
protected Object readInternal(Class clazz, HttpInputMessage inputMessage) throws IOException {
String jsonString = FileCopyUtils.copyToString(new InputStreamReader(inputMessage.getBody(), charset));
return gson.fromJson(jsonString, clazz);
}
#Override
protected Long getContentLength(Object obj, MediaType contentType) {
try {
String jsonString = gson.toJson(obj);
return (long) jsonString.getBytes(charset.name()).length;
} catch (UnsupportedEncodingException ex) {
throw new InternalError(ex.getMessage());
}
}
#Override
protected void writeInternal(Object obj, HttpOutputMessage outputMessage) throws IOException {
String jsonString = gson.toJson(obj);
FileCopyUtils.copy(jsonString, new OutputStreamWriter(outputMessage.getBody(), charset));
}
#Override
public boolean supports(Class<?> clazz) {
return true;
}
}
It works well until I try to send a collection like List<String> or some Type<T>.
Gson has the solutions here: http://sites.google.com/site/gson/gson-user-guide
Also I tried the json-lib library yesterday. What I don't like about it is in-depth scanning of all objects which I have in the hierarchy. I tried to change the cycle detection strategy from CycleDetectionStrategy.STRICT to CycleDetectionStrategy.LENIENT, it didn't help at all!
#Override
protected void writeInternal(Object obj, HttpOutputMessage outputMessage) throws IOException {
JsonConfig jsonConfig = new JsonConfig();
jsonConfig.setCycleDetectionStrategy(CycleDetectionStrategy.LENIENT);
String jsonString = JSONObject.fromObject( obj ).toString();
FileCopyUtils.copy(jsonString, new OutputStreamWriter(outputMessage.getBody(), charset));
}
Finally, a work-around for the generic collection's problem was found out: changing from ArrayList to simple array helps to do serialization and deserialization. To be more specific you have to do it in a web-service, which you use in an application.
#RequestMapping(value = "/country/info/{code}")
public void info(#PathVariable("code") String code, Model model) {
//list
StuffImpl[] stuffList= new StuffImpl[0]; <-- this is the array I used!
stuffList= restTemplate.getForObject("http://localhost:8084/yourApp/restService/stuff", stuffList.getClass());
model.addAttribute("stuffList", stuffList);
}
So this approach is working good.
I failed to found out what a solution for generic type is. I really do hate an idea to write a new converter every time I implement a new generic type.
If you know any possible solution I'd appreciate your help a lot!
I'd be on the cloud nine if anyone could help me :)
L.

There are some methods where you can pass java.lang.reflect.Type. These methods are useful if the specified object is a generic type, e.g.:
Gson gson = new GsonBuilder().create();
List<String> names = new ArrayList<String>();
names.add("Foo");
names.add("Bar");
// marshal
String jsonLiteral = gson.toJson(names);
System.out.println(jsonLiteral);
// unmarshal
List<String> names2;
Type type = new TypeToken<List<String>>() {
}.getType();
names2 = gson.fromJson(jsonLiteral, type);
System.out.println(names2.get(0));
System.out.println(names2.get(1));
This will output:
["Foo","Bar"]
Foo
Bar