为什么 Gson fromJson 会抛出 JsonSyntaxException: Expected BEGIN_OBJECT but was BEGIN_ARRAY?

2022-01-31 00:00:00 json java gson

(这篇文章旨在成为 规范问题，下面提供了示例答案.)

(This post is meant to be a canonical question with a sample answer provided below.)

我正在尝试使用 Gson#fromJson(String, Class).

I'm trying to deserialize some JSON content into a custom POJO type with Gson#fromJson(String, Class).

这段代码

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;
}

抛出跟随异常

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

为什么 Gson 不能正确地将我的 JSON 文本转换为我的 POJO 类型?

Why can't Gson properly convert my JSON text to my POJO type?

推荐答案

如异常信息所述

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

在反序列化时，Gson 期待一个 JSON 对象，但找到了一个 JSON 数组.由于它不能从一个转换到另一个，它抛出了这个异常.

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.

此处描述了 JSON 格式.简而言之，它定义了以下类型:对象、数组、字符串、数字、null，以及布尔值true和false.

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.

在 Gson(和大多数 JSON 解析器)中，存在以下映射: JSON 字符串映射到 Java String；JSON 数字映射到 Java Number 类型；JSON 数组映射到 Collection 类型或数组类型；JSON 对象映射到 Java Map 类型，或者通常是自定义 POJO 类型(不是前面提到过)；null 映射到 Java 的 null，布尔值映射到 Java 的 true 和 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 遍历您提供的 JSON 内容并尝试将其反序列化为您请求的相应类型.如果内容不匹配或无法转换为期望的类型，则会抛出相应的异常.

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.

在您的情况下，您提供了以下 JSON

In your case, you provided the following JSON

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

在根部，这是一个 JSON 对象，其中包含一个名为 nestedPojo 的成员，该成员是一个 JSON 数组.该 JSON 数组包含一个元素，即另一个具有两个成员的 JSON 对象.考虑到前面定义的映射，您希望这个 JSON 映射到一个 Java 对象，该对象具有一个名为 nestedPojo 的字段，该字段属于某个 Collection 或数组类型，其中该类型定义了两个分别命名为 name 和 value 的字段.

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.

但是，您已将 Pojo 类型定义为具有字段

However, you've defined your Pojo type as having a field

NestedPojo nestedPojo;

既不是数组类型，也不是 Collection 类型.Gson 无法反序列化该字段对应的 JSON.

that is neither an array type, nor a Collection type. Gson can't deserialize the corresponding JSON for this field.

相反，您有 3 个选项:

Instead, you have 3 options:

  • 更改您的 JSON 以匹配预期类型

  • Change your JSON to match the expected type

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

  • 更改您的 Pojo 类型以期望 Collection 或数组类型

  • Change your Pojo type to expect a Collection or array type

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

  • 使用您自己的解析规则为 NestedPojo 编写和注册自定义反序列化器.例如

  • 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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