Gson fails to parse simple map

[GoogleCodeExporter]

When parsing the following json with gson (using 2.2.2 from maven-central), 
gson (fromJson) throws an:

JsonSyntaxException: "duplicate key: null"

whereby the json is created by gson (toJson) itself.

Json:
{"appProperties":{"server.configuration.reset":"false","server.db.jndi":"testDB"
,"xml.basedata.list":"basedata.xml","server.configuration.cache":"false","xml.la
yout.list":"","server.rolemapper":"InsertionRoleMapper","xml.content.list":"data
.xml"}}

http://json.parser.online.fr/ validates the string as correct...

Original issue reported on code.google.com by phil...@meisen.net on 15 Feb 2013 at 12:04

[busylee999]

I faced the same trouble. But I figured out it was because of we have map<enum, T> and ones server started returns new enum type, we had not had yet, so it leaded to this error, two nulls as keys.

[perpetual-dork]

I'm serializing a map.. the json object only has one key in it.
"map": { "string_key": 0 }

Every time I attempt to deserialize this, it throws this exception.

com.google.gson.JsonSyntaxException: duplicate key: string_key

[TheDoctorOne]

Since issue is still open, i'm guessing its still here. Here is how i solved my case:
I had an issue with Map<enum, list> like everyone else. After seeing this issue and debugging realized it was because of the enum.
So I cast my Map<enum, list> to Map<Integer, list>. Then did my serialization / de-serialization.

Here is an example code, similar to mine. Changed variable names etc.

	public static <T extends Enum, V> Map<T, V> fromOrdinal(Map<Integer, V> ordMap, Class<T> enumClass) {
		Map<T, V> org = new HashMap<>();
		
		for(int ord : ordMap.keySet()) {
			org.put(enumClass.getEnumConstants()[ord], ordMap.get(ord));
		}
		return org;
	}
	
	public static <T extends Enum, V> Map<Integer, V> toOrdinal(Map<T, V> map) {
		Map<Integer, V> ord = new HashMap<>();
		for(T key : map.keySet()) {
			ord.put(key.ordinal(), map.get(key));
		}
		return ord;
	}

[eamonnmcmanus]

I don't think there's enough information in this issue to be actionable. There may be more than one underlying issue, or there may be none and people are just not using the API right. (So there might be a documentation issue.)

If you have a problem that seems like this one, please show an exception backtrace and the code you are using to deserialize.

[TheDoctorOne]

I don't think there's enough information in this issue to be actionable. There may be more than one underlying issue, or there may be none and people are just not using the API right. (So there might be a documentation issue.)

If you have a problem that seems like this one, please show an exception backtrace and the code you are using to deserialize.

Here is the exact code i am testing it right now with the latest version "2.8.8":

package net.mahmutkocas.dummy;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import java.util.*;

public class Main {

    public static void main(String[] args) {

        TestClass testClass = new TestClass();
        for(TestEnum s : TestEnum.values()) {
            testClass.typesMap.put(s, "test");
        }

        GsonBuilder builder = new GsonBuilder();
        Gson gson = builder.create();

        try {
            String json= gson.toJson(testClass);
            System.out.println(json); // Output is provided at below with backtrace.
            gson.fromJson(json, TestClass.class);
        } catch (Exception ex) {
            ex.printStackTrace();
        }

    }

    public enum TestEnum {
        ENUM1,
        ENUM2,
        ENUM3,
        ENUM4;

        @Override
        public String toString() {
            return super.toString().toLowerCase(Locale.ENGLISH);
        }
    }


    public static class TestClass {
        public Map<TestEnum, String> typesMap = new HashMap<>();
    }

}

And here is the backtrace:

Output of the system.out : {"typesMap":{"enum2":"test","enum4":"test","enum1":"test","enum3":"test"}}

com.google.gson.JsonSyntaxException: duplicate key: null
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:190)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$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:932)
	at com.google.gson.Gson.fromJson(Gson.java:897)
	at com.google.gson.Gson.fromJson(Gson.java:846)
	at com.google.gson.Gson.fromJson(Gson.java:817)
	at net.mahmutkocas.dummy.Main.main(Main.java:22)

Hope it helps.
Note: This only happens if and only if when Enum's toString() method gets overriden and returning value does not equal to original name.

[eamonnmcmanus]

Thanks @TheDoctorOne, that does look suspicious and we should look into it.

[TheDoctorOne]

Thanks @TheDoctorOne, that does look suspicious and we should look into it.

See #1950 , as far as i tested, it works. And should have backwards compatibility.

Edit: It checks on the highest level, where de-serialization done. It can be also done at serialization part which may be more convenient for future use. I don't know if that would be backwards compatible, if backwards compatibility matters to the project.