How to handle a NumberFormatException with Gson in deserialization a JSON response

Question

I\'m reading a JSON response with Gson, which returns somtimes a NumberFormatException because an expected int value is set to an empty string. Now

Answer 1

At first, I tried to write a general custom type adaptor for Integer values, to catch the NumberFormatException and return 0, but Gson doesn't allow TypeAdaptors for primitive Types:

java.lang.IllegalArgumentException: Cannot register type adapters for class java.lang.Integer

After that I introduced a new Type FooRuntime for the runtime field, so the Foo class now looks like this:

public class Foo
{
    private String name;
    private FooRuntime runtime;

    public int getRuntime()
    {
        return runtime.getValue();
    }
}

public class FooRuntime
{
    private int value;

    public FooRuntime(int runtime)
    {
        this.value = runtime;
    }

    public int getValue()
    {
        return value;
    }
}

A type adaptor handles the custom deserialization process:

public class FooRuntimeTypeAdapter implements JsonDeserializer<FooRuntime>, JsonSerializer<FooRuntime>
{
    public FooRuntime deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException
    {
        int runtime;
        try
        {
            runtime = json.getAsInt();
        }
        catch (NumberFormatException e)
        {
            runtime = 0;
        }
        return new FooRuntime(runtime);
    }

    public JsonElement serialize(FooRuntime src, Type typeOfSrc, JsonSerializationContext context)
    {
        return new JsonPrimitive(src.getValue());
    }
}

Now it's necessary to use GsonBuilder to register the type adapter, so an empty string is interpreted as 0 instead of throwing a NumberFormatException.

String input = "{\n" +
               "   \"name\" : \"Test\",\n" +
               "   \"runtime\" : \"\"\n" +
               "}";

GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(FooRuntime.class, new FooRuntimeTypeAdapter());
Gson gson = builder.create();
Foo foo = gson.fromJson(input, Foo.class);

Answer 2

Here is example I did for Long type.This is better option:

    public class LongTypeAdapter extends TypeAdapter<Long>{
    @Override
    public Long read(JsonReader reader) throws IOException {
        if(reader.peek() == JsonToken.NULL){
            reader.nextNull();
            return null;
        }
        String stringValue = reader.nextString();
        try{
            Long value = Long.valueOf(stringValue);
            return value;
        }catch(NumberFormatException e){
            return null;
        }
    }
    @Override
    public void write(JsonWriter writer, Long value) throws IOException {
        if (value == null) {
            writer.nullValue();
            return;
        }
        writer.value(value);
    }
}

Register the adapter upon creation of the gson util:

Gson gson = new GsonBuilder().registerTypeAdapter(Long.class, new LongTypeAdapter()).create();

You can refer to this link for more.

Answer 3

Quick and easy workaround - Just change your member type field of runtime to String and access it via getter that returns runtime as an int:

public class Foo
{
    private String name;
    private String runtime;

    public int getRuntime(){
        if(runtime == null || runtime.equals("")){
            return 0;
        }
        return Integer.valueOf(trackId);
    }
}

=> no json deserialization neccessary

Answer 4

This solution works for Double types. This will only work for non-primitive types:

public class DoubleGsonTypeAdapter implements JsonDeserializer<Double> {

    @Override
    public Double deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        Double result = null;
        try {
            result = jsonElement.getAsDouble();
        } catch (NumberFormatException e) {
            return result;
        }
        return result;
    }
}

Model:

@SerializedName("rateOfInterest")
public Double rateOfInterest;
@SerializedName("repaymentTenure")
public Double repaymentTenure;
@SerializedName("emiAmount")
public Double emiAmount;

Retrofit client:

Gson gson = new GsonBuilder().registerTypeAdapter(Double.class, new DoubleGsonTypeAdapter()) .create();

Retrofit retrofit = new Retrofit.Builder()
                .client(okHttpClient)
                .baseUrl(API_BASE_URL)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();

Answer 5

I've made this TypeAdapter which check for empty strings and return 0

public class IntegerTypeAdapter extends TypeAdapter<Number> {
@Override
public void write(JsonWriter jsonWriter, Number number) throws IOException {
    if (number == null) {
        jsonWriter.nullValue();
        return;
    }
    jsonWriter.value(number);
}

@Override
public Number read(JsonReader jsonReader) throws IOException {
    if (jsonReader.peek() == JsonToken.NULL) {
        jsonReader.nextNull();
        return null;
    }

    try {
        String value = jsonReader.nextString();
        if ("".equals(value)) {
            return 0;
        }
        return Integer.parseInt(value);
    } catch (NumberFormatException e) {
        throw new JsonSyntaxException(e);
    }
}

}

Answer 6

It might help you to always assume a default value of 0 for the field runtime in case of a NumberFormatException, since it can be the only source of error.