Gson将map转换成JsonObject出现null值

异常信息：

Exception in thread “main” java.lang.IllegalStateException: Not a JSON Object: null
at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:84)

Gson doesn’t serialize anonymous classes. Use Map.of to create maps, not subclasses.
当拿到的map是通过初始化赋值得到的时，你可以通过添加type参数来进行Json格式转换。

code:

import com.google.common.collect.Maps;

import com.google.gson.Gson;
import com.google.gson.JsonParser;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test3 {
    private static Gson gson = new Gson();
    private static JsonParser jsonParser = new JsonParser();
    public static final Map<String, String> map = new HashMap<String, String>(){
        {
            put("111", "22");
            put("11", "11");
        }
    };
    public static void main(String[] args) {
        List<Map<String, String>> list = new ArrayList<>();
        Map<String, String> map1 = Maps.newHashMap();
        map1.put("11","11");
        Map<String, String> map2 = Maps.newHashMap();
        map2.put("22","22");
        list.add(map1);
        list.add(map2);
        String s = gson.toJson(list);
        System.out.println(jsonParser.parse(s).getAsJsonArray());
        String s1 = gson.toJson(map1);
        System.out.println(jsonParser.parse(s1).getAsJsonObject());

        Type type = new TypeToken<HashMap<String, String>>() {}.getType();
        String s2 = gson.toJson(map, type); // 通过reflect告知Gson Map键值对的类型

        System.out.println(jsonParser.parse(s2).getAsJsonObject());

        String s3 = gson.toJson(map); //为null
        System.out.println("s3 = " + s3);
        System.out.println(jsonParser.parse(s3).getAsJsonObject());
    }
}

result:

Exception in thread "main" java.lang.IllegalStateException: Not a JSON Object: null
	at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:84)
	at Test3.main(Test3.java:42)
[{"11":"11"},{"22":"22"}]
{"11":"11"}
{"11":"11","111":"22"}
s3 = null

对于ImmutableMap却可以转换json格式

  Map<String, String> of = ImmutableMap.of("11", "22");
        String s4 = gson.toJson(of);
        JsonObject asJsonObject = jsonParser.parse(s4).getAsJsonObject();

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