Java: Gson - hashmap to JSON in custom JsonSerializer

Question

I'm trying to create custom JsonSerializer for a class (and it's children). So I'm iterating through the Fields of the class and doing stuff. I faced with problem: If I have something like this in my class:

public Map<String, Map<String, Integer>> nestedHashMap = new HashMap<String, Map<String, Integer>>() {{
    put("levelOneKey", new HashMap<String, Integer>() {{
        put("levelTwoKey", 25);
    }});
}};

It turns into {}. I checked, it has elements on creation, GSON just can't handle it properly. If I'll try to use GSON without JsonSerializer, it works. Clearly, I'm doing something wrong. I don't have much experience with Java Reflection.

Part of my code:

@Override
public JsonElement serialize(JsonConfig src, Type typeOfSrc, JsonSerializationContext context) {
    JsonObject o = new JsonObject();
    //...
    Field[] fields = src.getClass().getDeclaredFields();
    for (Field field : fields) {
        Object fieldValue = null;
        try {
            fieldValue = field.get(src);
        } catch (IllegalAccessException e) {
            System.out.print("IllegalAccessException for field " + field.getName() + ": " + e.toString());
            continue;
        }
        o.add(field.getName(), context.serialize(fieldValue, field.getType()));
    }
    //...
}

I tried to create another instance of Gson and replace o.add with:

o.add(field.getName(), gson.toJson(fieldValue, field.getType()));

I checked, fieldValue has elements.

tima · Accepted Answer · 2017-08-06 03:00:29Z

1

The issue is field.getType(), you should change it to field.getGenericType().

o.add(field.getName(), context.serialize(fieldValue, field.getGenericType()));

If you print both values to the console, the .getType() returns just a java.util.Map, but the .getGenericType() returns the actual type of your map Map<String, Map<String, Integer>>

answered Aug 6, 2017 at 3:00

tima

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