Deserisation fails with when using null numeric primitives as map keys

What did you do:

Ran:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.util.HashMap;
import java.util.Map;

public class Test {
    static class MyType {
        public Map<Integer, Integer> map = new HashMap<>();
    }

    public static void main(String[] args) {
        MyType instance = new MyType();
        instance.map.put(null, 1);

        Gson serialiser = new GsonBuilder()
                .enableComplexMapKeySerialization()
                .serializeNulls()
                .setPrettyPrinting()
                .create();

        String data = serialiser.toJson(instance);
        System.out.println(data);

        try {
            MyType newInstance = serialiser.fromJson(data, MyType.class);
            System.out.println("Success");
        } catch (Exception e) {
            e.printStackTrace(System.out);
        }
    }
}

What happened

Deserialisation fails:

{
  "map": {
    "null": 1
  }
}
com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: For input string: "null"
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:228)
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:218)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:186)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222)
	at com.google.gson.Gson.fromJson(Gson.java:927)
	at com.google.gson.Gson.fromJson(Gson.java:892)
	at com.google.gson.Gson.fromJson(Gson.java:841)
	at com.google.gson.Gson.fromJson(Gson.java:813)
	at Test.main(Test.java:27)
Caused by: java.lang.NumberFormatException: For input string: "null"
	at java.base/jdk.internal.math.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2054)
	at java.base/jdk.internal.math.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.base/java.lang.Double.parseDouble(Double.java:543)
	at com.google.gson.stream.JsonReader.nextInt(JsonReader.java:1201)
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:226)
	... 11 more

What did you expect to happen:

Either both serialisation and deserilisation to either succeed or fail (making the handling consistent between the two modes).

It seems that only one mode does not handle this case. I know that null is an invalid json map key, but perhaps the library should handle a special “null” key when the type is a nullable primitive.

It seems that this is also broken for strings, as you can serialise a null, and get "null" back after deserilisation.

Note

Strangely it works okay if you serialise a simple map (not embedded in an object).

[AudriusButkevicius]

What exactly would you be breaking? Preventing null key serialisation? What about deserialising “null” strings into nulls for boxed primitives (other than string I guess)

[yanxinyuan]

I also think it is an issue, here is the code:

Scenario 1:

Gson gson = new GsonBuilder().serializeNulls().create();
Map<String, Integer> mapData = new HashMap<>();
mapData.put("abc", null);
String data = gson.toJson(mapData);
System.out.println(data);
Type type = TypeToken.getParameterized(HashMap.class, String.class, Integer.class).getType();
Object obj = gson.fromJson(data, type);
System.out.println(obj);

The code runs well and got the result

{"abc":null}
{abc=null}

Scenario 2:

Gson gson = new GsonBuilder().serializeNulls().create();
Map<Integer, String> mapData = new HashMap<>();
mapData.put(null, "abc");
String data = gson.toJson(mapData);
System.out.println(data);
Type type = TypeToken.getParameterized(HashMap.class, Integer.class, String.class).getType();
Object obj = gson.fromJson(data, type);
System.out.println(obj);

The code runs failed, and get an exception:

Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: For input string: "null"
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:227)
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:217)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:186)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
	at com.google.gson.Gson.fromJson(Gson.java:888)
	at com.google.gson.Gson.fromJson(Gson.java:853)
	at com.google.gson.Gson.fromJson(Gson.java:802)
	at com.delta.cornerstone.common.serialization.gson.user.UserSerializationByGson.main(UserSerializationByGson.java:41)
Caused by: java.lang.NumberFormatException: For input string: "null"
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.lang.Double.parseDouble(Double.java:538)
	at com.google.gson.stream.JsonReader.nextInt(JsonReader.java:1201)
	at com.google.gson.internal.bind.TypeAdapters$7.read(TypeAdapters.java:225)
	... 8 more

So it seems Gson can only handle the “NULL” when it is the value of the map.