Deserialize a List<T> object with Gson?

Question

I want to transfer a list object via Google Gson, but I don't know how to deserialize generic types.

What I tried after looking at this (BalusC's answer):

MyClass mc = new Gson().fromJson(result, new List<MyClass>() {}.getClass());

but then I get an error in Eclipse saying "The type new List<MyClass>() {} must implement the inherited abstract method..." and if I use a quick fix I get a monster of over 20 method stubs.

I am pretty sure that there is an easier solution, but I seem unable to find it!

Now I have this:

Type listType = new TypeToken<List<MyClass>>() {}.getType();

MyClass mc = new Gson().fromJson(result, listType);

However, I do get the following exception at the fromJson line:

java.lang.NullPointerException
at org.apache.harmony.luni.lang.reflect.ListOfTypes.length(ListOfTypes.java:47)
at org.apache.harmony.luni.lang.reflect.ImplForType.toString(ImplForType.java:83)
at java.lang.StringBuilder.append(StringBuilder.java:203)
at com.google.gson.JsonDeserializerExceptionWrapper.deserialize(JsonDeserializerExceptionWrapper.java:56)
at com.google.gson.JsonDeserializationVisitor.invokeCustomDeserializer(JsonDeserializationVisitor.java:88)
at com.google.gson.JsonDeserializationVisitor.visitUsingCustomHandler(JsonDeserializationVisitor.java:76)
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:106)
at com.google.gson.JsonDeserializationContextDefault.fromJsonArray(JsonDeserializationContextDefault.java:64)
at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:49)
at com.google.gson.Gson.fromJson(Gson.java:568)
at com.google.gson.Gson.fromJson(Gson.java:515)
at com.google.gson.Gson.fromJson(Gson.java:484)
at com.google.gson.Gson.fromJson(Gson.java:434)

I do catch JsonParseExceptions and result is not null.

I checked listType with the debugger and got the following:

• list Type
  • args = ListOfTypes
    • list = null
    • resolvedTypes = Type[ 1 ]
  • loader = PathClassLoader
  • ownerType0 = null
  • ownerTypeRes = null
  • rawType = Class (java.util.ArrayList)
  • rawTypeName = "java.util.ArrayList"

So it seems the getClass invocation didn't work properly. Any suggestions...?

I've checked on the Gson User Guide. It mentions a runtime exception that should happen during parsing a generic type to Json. I did it "wrong" (not shown above), just as in the example, but didn't get that exception at all. So I changed the serialization as in the user guide suggested. Didn't help, though.

Edit:

Solved, see my answer below.

Solution 1

Method to deserialize generic collection:

import java.lang.reflect.Type;
import com.google.gson.reflect.TypeToken;

...

Type listType = new TypeToken<ArrayList<YourClass>>(){}.getType();
List<YourClass> yourClassList = new Gson().fromJson(jsonArray, listType);

Since several people in the comments have mentioned it, here's an explanation of how the TypeToken class is being used. The construction new TypeToken<...>() {}.getType() captures a compile-time type (between the < and >) into a runtime java.lang.reflect.Type object. Unlike a Class object, which can only represent a raw (erased) type, the Type object can represent any type in the Java language, including a parameterized instantiation of a generic type.

The TypeToken class itself does not have a public constructor, because you're not supposed to construct it directly. Instead, you always construct an anonymous subclass (hence the {}, which is a necessary part of this expression).

Due to type erasure, the TypeToken class is only able to capture types that are fully known at compile time. (That is, you can't do new TypeToken<List<T>>() {}.getType() for a type parameter T.)

For more information, see the documentation for the TypeToken class.

Solution 2

Another way is to use an array as a type, e.g.:

MyClass[] mcArray = gson.fromJson(jsonString, MyClass[].class);

This way you avoid all the hassle with the Type object, and if you really need a list you can always convert the array to a list by:

List<MyClass> mcList = Arrays.asList(mcArray);

IMHO this is much more readable.

And to make it be an actual list (that can be modified, see limitations of Arrays.asList()) then just do the following:

List<MyClass> mcList = new ArrayList<>(Arrays.asList(mcArray));

Solution 3

Since Gson 2.8, we can create util function like this:

public <T> List<T> getList(String jsonArray, Class<T> clazz) {
    Type typeOfT = TypeToken.getParameterized(List.class, clazz).getType();
    return new Gson().fromJson(jsonArray, typeOfT);
}

Example usage:

String jsonArray = ...
List<User> user = getList(jsonArray, User.class);

Solution 4

Refer to this post. Java Type Generic as Argument for GSON

I have better solution for this. Here's the wrapper class for list so the wrapper can store the exactly type of list.

public class ListOfJson<T> implements ParameterizedType
{
  private Class<?> wrapped;

  public ListOfJson(Class<T> wrapper)
  {
    this.wrapped = wrapper;
  }

  @Override
  public Type[] getActualTypeArguments()
  {
      return new Type[] { wrapped };
  }

  @Override
  public Type getRawType()
  {
    return List.class;
  }

  @Override
  public Type getOwnerType()
  {
    return null;
  }
}

And then, the code can be simple:

public static <T> List<T> toList(String json, Class<T> typeClass)
{
    return sGson.fromJson(json, new ListOfJson<T>(typeClass));
}

Solution 5

Wep, another way to achieve the same result. We use it for its readability.

Instead of doing this hard-to-read sentence:

Type listType = new TypeToken<ArrayList<YourClass>>(){}.getType();
List<YourClass> list = new Gson().fromJson(jsonArray, listType);

Create a empty class that extends a List of your object:

public class YourClassList extends ArrayList<YourClass> {}

And use it when parsing the JSON:

List<YourClass> list = new Gson().fromJson(jsonArray, YourClassList.class);

View more solutions

Comments

[Drew]

If you have a list of abstract classes you'll get the same error. I guess this is GSON's general error message for "Unable to instantiate class".

[Pablo]

In new versions of GSON the TypeToken contructor is not public, hence here you get constructor not visible error. What do you have to do in this case?

[Admin]

Using actual version of GSON (2.2.4) it works again. You can access the constructor here.

[Amin Sh]

this is great! How can I use it with reflection? I dont know the MyClass value and it will be defined dynamically!

[CQM]

my json object starts with an object, then contains an array of the object I want { "myObjectArray":[ {....} , {....} , {....} ] }, I have made the model file for {....} , how do I get this generic collection code to not assume my root element is an array without making a new nested object file

[njzk2]

nota: with this, be careful that mcList is not a full-fledged list. many things will not work.

[Happier]

Check the link to have another way to write a better util class. stackoverflow.com/questions/14139437/…

[Pawel Cioch]

How to use it with generics? T[] yourClassList = gson.fromJson(message, T[].class); //cannot select from type variable

[Jose Ospina]

which package is neede for the type.getType() instruction? I am getting "The method getType() is undefined for the type new ArrayList<XXXX>(){}"

[Al Lelopath]

What is mEntity.rulePattern?

[Happier]

It's just a sample object for test. You don't need to care about it. Use toList method and everything goes well.

[Pratik Butani]

It gives me error if i use proguard only otherwise its work. any suggestion

[Umair Saleem]

Following Imports required --- import java.lang.reflect.Type; import com.google.gson.reflect.TypeToken

[jasxir]

This is good if YourClass is fixed in code. What if the class comes at runtime?

[Jan Rabe]

then you will most likely run into a class cast exception (java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap) when you try to use it

[Rune]

The tip about adding a constructor helped me realize why I had all null-values. I had field names like "To" and "From" in my JSON-string, but the corresponding fields in my object were "to" and "from" in lower case, so they were skipped

[Fermin Silva]

Those {} before .getType look so non-Java to me. I'm not even able to tell what they are syntacically. Is it an empty anonymous class or what?

[Mateus Viccari]

@njzk2 What exactly is a full-fledged list?

[njzk2]

@MateusViccari at the time of that comment, mcList in this answer was only the result of the call to Arrays.asList. This method returns a list on which most if not all optional methods are left unimplemented and throw exceptions. For instance, you cannot add any element to that list. As the later edit suggests, Arrays.asList has limitations, and wrapping it into an actual ArrayList allows you to get a list that is more useful in many cases.

[JHH]

Surely this cannot work since you're trying to use T in compile-time. This will effectively deserialize to a List of StringMap, no?

[anshulkatta]

dont forget before Gson gson = new Gson(); for begineers

[Lambart]

Good one. But this duplicates DevNGs above answer, written 2 years earlier: stackoverflow.com/a/17300003/1339923 (and read that answer for caveats to this approach)

[coffeemakr]

I've used your code to create this function using reified types: inline fun <reified T> buildType() = object : TypeToken<T>() {}.type!! and call it with the List type: buildType<List<YourMagicObject>>()

[Chad Bingham]

@coffeemakr You don't need reified types here.

[coffeemakr]

Oh. But why do you create the type token of a ArrayList in buildType and also call the function with the generic type? Is this a typo? - This would create ArrayList<ArrayList<YourMagicObject>>

[Chad Bingham]

@coffeemakr ah, yeah. Typo

[Daniel Pryden]

If you need to construct an array type at runtime for an arbitrary element type, you can use Array.newInstance(clazz, 0).getClass() as described in David Wood's answer.

[Nikolay Kulachenko]

TypeToken#getParameterized looks a way better then the hack with an anonymous subclass

[Steve Moretz]

@CQM You have two ways.1-Just remove the first and the last character from the string and you're good to go.2-Make a Class put your ArrayList<Whatever> in that class,Now just use Whatever.class,For the second argument you're good to go.

[illusionJJ]

for kotlin val listType = object : TypeToken<ArrayList<YourClass>>() {}.type

[leguminator]

I copied your method "as is" and it does not work : compiler says "The method getParameterized(Class<List>, Class<T>) is undefined for the type TypeToken". I checked both my Gson version (2.8.0) and documentation and everything is fine on this side... I ended up using @Happier solution which works fine

[Linh]

@leguminator did you import TypeToken correct? and you are using java or kotlin. I will try to test again

[leguminator]

@PhanVanLinh Absolutely : I am using Java and imported "com.google.gson.reflect.TypeToken" and "java.lang.reflect.Type". I doubled check method implementation : it is declared as "public static TypeToken<?> getParameterized(Type rawType, Type... typeArguments)"

[4gus71n]

This should be the accepted solution, simple, it uses the Gson API, and there are no hacks around it. +1