如何使用 GSON 序列化和反序列化包含 HashMap 和 Pair 的对象

Question

我将此 Hashmap Map<Pair<String,String>,Map<String,String>> pathData = new HashMap<>(); 作为其他对象 (Tour_Object) 的属性，我正在尝试使用 GSON 库在 json 中对其进行序列化/反序列化。

public static String setTourToJson(Tour_Object tourObject)
{
    Gson gson = new Gson();
    return gson.toJson(tourObject);
}

public static Tour_Object getTourFromJson(String JsonString)
{
    Gson gson = new Gson();
    return gson.fromJson(JsonString, new TypeToken<Tour_Object>() {
    }.getType());
}

当我进行反序列化时抛出以下异常：

04-19 11:18:49.449 29076-29076/abff.fxguide E/AndroidRuntime: FATAL EXCEPTION: main
Process: abff.fxguide, PID: 29076
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 100 path $.pathData.
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
at com.google.gson.Gson.fromJson(Gson.java:879)
at com.google.gson.Gson.fromJson(Gson.java:844)
at com.google.gson.Gson.fromJson(Gson.java:793)
at com.google.gson.Gson.fromJson(Gson.java:765)
at abff.fxguide.Tour_Helper.getTourFromJson(Tour_Helper.java:311)
at abff.fxguide.Tour_All.showTourDetails(Tour_All.java:266)
at abff.fxguide.Tour_All$8.onClick(Tour_All.java:370)
at android.view.View.performClick(View.java:4856)
at android.view.View$PerformClick.run(View.java:19956)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:211)
at android.app.ActivityThread.main(ActivityThread.java:5389)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)
Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 100 path $.pathData.
at com.google.gson.stream.JsonReader.beginObject(JsonReader.java:388)
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:183)
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:40)
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:186)
at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:116)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:216)
at com.google.gson.Gson.fromJson(Gson.java:879) 
at com.google.gson.Gson.fromJson(Gson.java:844) 
at com.google.gson.Gson.fromJson(Gson.java:793) 
at com.google.gson.Gson.fromJson(Gson.java:765) 
at abff.fxguide.Tour_Helper.getTourFromJson(Tour_Helper.java:311) 
at abff.fxguide.Tour_All.showTourDetails(Tour_All.java:266) 
at abff.fxguide.Tour_All$8.onClick(Tour_All.java:370) 
at android.view.View.performClick(View.java:4856) 
at android.view.View$PerformClick.run(View.java:19956) 
at android.os.Handler.handleCallback(Handler.java:739) 
at android.os.Handler.dispatchMessage(Handler.java:95) 
at android.os.Looper.loop(Looper.java:211) 
at android.app.ActivityThread.main(ActivityThread.java:5389) 
at java.lang.reflect.Method.invoke(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:372) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)

我想知道在使用 pathData 等属性时，主对象 (Tour_Object) 是否可反序列化，如 getTourFromJson(String JsonString) 所示？

Answer 1

TL;DR 你需要一个适配器。

Gson 可以在没有显式适配器的情况下使用 Map，但它只希望键是 String，因为它将映射转换为 JSON 对象。如果不是String，它会将其转换为通常只调用Object#toString()。

另一种说法是：您的对象的 JSON 表示形式如何？不明显，因为您不能将复杂对象作为 JSON 对象的属性，它们始终是字符串。

因此，您需要确定您的表示并为其编写自定义适配器。

扩展的奖金答案

作为奖励，我建议您使用类似数组的方式来表示您的 pathData。您编写的对象数组不是映射，而是键/值对。像这样的：

{
    "pathData": [
        {
            "pair": {"a": "one", "b": "two"},
            "map": { ... }
        },
        {
            "pair": {"a": "one", "b": "two"},
            "map": { ... }
        }
    ]
}

pair 是你的Pair<String,String>，map 是你的Map<String,String>。

那么你可以这样写TypeAdapter。

package net.sargue.gson;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.TypeAdapter;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

import java.io.IOException;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class SO36716159 {
  public static void main(String[] args) {
    Tour_Object o = new Tour_Object();
    o.pathData.put(new Pair<>("one", "two"), Collections.emptyMap());
    Map<String, String> myMap = new HashMap<>();
    myMap.put("a", "b");
    myMap.put("c", "d");
    o.pathData.put(new Pair<>("three", "four"), myMap);

    Gson gson = new GsonBuilder()
            .registerTypeAdapter(Tour_Object.class,
                                 new TourObjectAdapter())
            .setPrettyPrinting()
            .create();
    String json = gson.toJson(o);
    System.out.println("json = " + json);
  }

  public static class Pair<A, B> {
    private A a;
    private B b;

    public Pair(A a, B b) {
      this.a = a;
      this.b = b;
    }
  }

  public static class Tour_Object {
    private Map<Pair<String, String>, Map<String, String>> pathData = new HashMap<>();

    public Map<Pair<String, String>, Map<String, String>> pathData() {
      return pathData;
    }
  }

  public static class TourObjectAdapter extends TypeAdapter<Tour_Object> {
    @Override
    public void write(JsonWriter out, Tour_Object value)
            throws IOException
    {
      Gson gson = new Gson();

      out.beginObject()
         .name("pathData")
         .beginArray();

      for (Map.Entry<Pair<String, String>, Map<String, String>> entry :
              value.pathData().entrySet()) {

        out.beginObject();
        out.name("pair");
        gson.getAdapter(new TypeToken<Pair<String,String>>() {})
            .write(out, entry.getKey());
        out.name("map");
        gson.getAdapter(new TypeToken<Map<String, String>>() {})
            .write(out, entry.getValue());
        out.endObject();
      }
      out.endArray()
         .endObject();
    }

    @Override
    public Tour_Object read(JsonReader in) throws IOException {
      throw new RuntimeException("Left as an exercise for the reader... ;-)");
    }
  }
}

注意：不是一个完整的例子，只是序列化，没有过多的检查等等。仅作为示例有效。

前面代码的输出是：

json = {
  "pathData": [
    {
      "pair": {
        "a": "one",
        "b": "two"
      },
      "map": {}
    },
    {
      "pair": {
        "a": "three",
        "b": "four"
      },
      "map": {
        "a": "b",
        "c": "d"
      }
    }
  ]
}