Why does Gson from Json throw JsonSyntaxException: BEGIN_OBJECT expected, but there was BEGIN_ARRAY?

As stated in the error message

Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 16 path $.nestedPojo

      

        
        
        
      

    

when deserializing, Gson was expecting a JSON object but found a JSON array. Since he could not convert from one to the other, he threw this exception.

The JSON format is described here . In short, it defines the following types: objects, arrays, strings, numbers, null

and booleans true

and false

.

Gson (and most JSON parsers) have the following mappings: JSON string maps to Java String

; JSON number maps to Java type Number

; a JSON array is mapped to an array type Collection

or type; the JSON object maps to a Java type, Map

or generally a custom POJO type (not previously mentioned); null

maps to Java null

, and booleans maps to Java true

and false

.

Gson iterates through the JSON content you provide and tries to deserialize it to the appropriate type you requested. If the content does not match or cannot be converted to the expected type, it will throw a corresponding exception.

In your case, you provided the following JSON

{
    "nestedPojo": [
        {
            "name": null,
            "value": 42
        }
    ]
}

      

        
        
        
      

    

At the root, it is a JSON object that contains a named member nestedPojo

, which is a JSON array. This JSON array contains one element, another JSON object with two members. Given the mappings defined earlier, you would expect this JSON to represent a Java object that has a field named nestedPojo

some Collection

or array type, where those types define two fields named name

and value

, respectively.

However, you have defined the type Pojo

as having a field

NestedPojo nestedPojo;

      

        
        
        
      

    

which is not an array type, not a type Collection

. Gson cannot deserialize the corresponding JSON for this field.

Instead, you have 3 options:

• Modify your JSON to match the expected type

  {
      "nestedPojo": {
          "name": null,
          "value": 42
      }
  }
  
        
  
          
          
          
        
  
      

• Change type Pojo
  
  to expect Collection
  
  or array type

  List<NestedPojo> nestedPojo; // consider changing the name and using @SerializedName
  NestedPojo[] nestedPojo;
  
        
  
          
          
          
        
  
      

• Write and register your own deserializer for nestedPojo
  
  using 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();