如何使用GSON

Question

我有这个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中序列化/反序列化它。

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());
}

当我继续进行反序列化时，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)

我想知道主要对象（Tour_Object）是否可以反序列化，如getTourFromJson(String JsonString)中所示，当使用pathData这样的属性时？

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，但它只是期望密钥为String，因为它将地图转换为JSON对象。如果它不是String，则会将其转换为通常只是调用Object#toString()。

另一种表达方式是：对象的JSON表示如何？不明显，因为你不能将复杂的对象作为JSON对象的属性，它们总是字符串。

因此，您需要确定您的代表和为其编写自定义适配器。

延长奖金anwser

作为奖励我可以建议你在类似数组的东西中思考来表示你的pathData。您可以编写一个对象数组来代替映射，这些对象是键/值对。像这样：

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

pair Pair<String,String>是map，Map<String,String>是TypeAdapter。

然后你可以像这样写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... ;-)"); } } } 。

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

注意：不是一个完整的例子，只是序列化而没有太多检查等等。仅作为示例有效。

上一代码的输出是：

img