Why does Gson fromJson throw a JsonSyntaxException: Expected BEGIN_OBJECT but was BEGIN_ARRAY?

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();
  

class Pojo {
  NestedPojo nestedPojo;
}

in your json you have an array of nestedPojo so either you change the code

  NestedPojo[] nestedPojo;

or you change the json string

String json = "{\"nestedPojo\":{\"name\":null, \"value\":42}}";