(This post is meant to be a canonical question with a sample answer provided below.)
I'm trying to deserialize some JSON content into a custom POJO type with Gson#fromJson(String, Class)
.
This piece of code
import com.google.gson.Gson;
public class Sample {
public static void main(String[] args) {
String json = "{\"nestedPojo\":[{\"name\":null, \"value\":42}]}";
Gson gson = new Gson();
gson.fromJson(json, Pojo.class);
}
}
class Pojo {
NestedPojo nestedPojo;
}
class NestedPojo {
String name;
int value;
}
throws the follow exception
Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 16 path $.nestedPojo
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:200)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:103)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:196)
at com.google.gson.Gson.fromJson(Gson.java:810)
at com.google.gson.Gson.fromJson(Gson.java:775)
at com.google.gson.Gson.fromJson(Gson.java:724)
at com.google.gson.Gson.fromJson(Gson.java:696)
at com.example.Sample.main(Sample.java:23)
Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 16 path $.nestedPojo
at com.google.gson.stream.JsonReader.beginObject(JsonReader.java:387)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:189)
... 7 more
Why can't Gson properly convert my JSON text to my POJO type?
begin_object means the json response is an object which will look something like this {....} gson is one cool library that will provide us with cool tips in the form of errors while handling json responses.
3. Deserialize JSON With Extra Unknown Fields to Object. As you can see, Gson will ignore the unknown fields and simply match the fields that it's able to.
Introduction. Gson is the main actor class of Google Gson library. It provides functionalities to convert Java objects to matching JSON constructs and vice versa. Gson is first constructed using GsonBuilder and then toJson(Object) or fromJson(String, Class) methods are used to read/write JSON constructs.
As the exception message states
Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 16 path $.nestedPojo
while deserializing, Gson was expecting a JSON object, but found a JSON array. Since it couldn't convert from one to the other, it threw this exception.
The JSON format is described here. In short, it defines the following types: objects, arrays, strings, numbers, null
, and the boolean values true
and false
.
In Gson (and most JSON parsers), the following mappings exist: a JSON string maps to a Java String
; a JSON number maps to a Java Number
type; a JSON array maps to a Collection
type or an array type; a JSON object maps to a Java Map
type or, typically, a custom POJO type (not mentioned previously); null
maps to Java's null
, and the boolean values map to Java's true
and false
.
Gson iterates through the JSON content that you provide and tries to deserialize it to the corresponding type you've requested. If the content doesn't match or can't be converted to the expected type, it'll throw a corresponding exception.
In your case, you provided the following JSON
{
"nestedPojo": [
{
"name": null,
"value": 42
}
]
}
At the root, this is a JSON object which contains a member named nestedPojo
which is a JSON array. That JSON array contains a single element, another JSON object with two members. Considering the mappings defined earlier, you'd expect this JSON to map to a Java object which has a field named nestedPojo
of some Collection
or array type, where that types defines two fields named name
and value
, respectively.
However, you've defined your Pojo
type as having a field
NestedPojo nestedPojo;
that is neither an array type, nor a Collection
type. Gson can't deserialize the corresponding JSON for this field.
Instead, you have 3 options:
Change your JSON to match the expected type
{
"nestedPojo": {
"name": null,
"value": 42
}
}
Change your Pojo
type to expect a Collection
or array type
List<NestedPojo> nestedPojo; // consider changing the name and using @SerializedName
NestedPojo[] nestedPojo;
Write and register a custom deserializer for NestedPojo
with your own parsing rules. For example
class Custom implements JsonDeserializer<NestedPojo> {
@Override
public NestedPojo deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
NestedPojo nestedPojo = new NestedPojo();
JsonArray jsonArray = json.getAsJsonArray();
if (jsonArray.size() != 1) {
throw new IllegalStateException("unexpected json");
}
JsonObject jsonObject = jsonArray.get(0).getAsJsonObject(); // get only element
JsonElement jsonElement = jsonObject.get("name");
if (!jsonElement.isJsonNull()) {
nestedPojo.name = jsonElement.getAsString();
}
nestedPojo.value = jsonObject.get("value").getAsInt();
return nestedPojo;
}
}
Gson gson = new GsonBuilder().registerTypeAdapter(NestedPojo.class, new Custom()).create();
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