Sachin Kulkarni Sachin Kulkarni - 6 months ago 27
JSON Question

Using Enums while parsing JSON with GSON

This is related to a previous question that I asked here earlier

JSON parsing using Gson

I am trying to parse the same JSON, but now I have changed my classes a little bit.

{
"lower": 20,
"upper": 40,
"delimiter": " ",
"scope": ["${title}"]
}


My class now looks like:

public class TruncateElement {

private int lower;
private int upper;
private String delimiter;
private List<AttributeScope> scope;

// getters and setters
}


public enum AttributeScope {

TITLE("${title}"),
DESCRIPTION("${description}"),

private String scope;

AttributeScope(String scope) {
this.scope = scope;
}

public String getScope() {
return this.scope;
}
}


This code throws an exception,

com.google.gson.JsonParseException: The JsonDeserializer EnumTypeAdapter failed to deserialized json object "${title}" given the type class com.amazon.seo.attribute.template.parse.data.AttributeScope
at


The exception is understandable, because as per the solution to my previous question, GSON is expecting the Enum objects to be actually be created as

${title}("${title}"),
${description}("${description}");


But since this is syntactically impossible, what are the recommended solutions, workarounds?

Answer

From the documentation for Gson:

Gson provides default serialization and deserialization for Enums... If you would prefer to change the default representation, you can do so by registering a type adapter through GsonBuilder.registerTypeAdapter(Type, Object).

Following is one such approach.

import java.io.FileReader;
import java.lang.reflect.Type;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;

public class GsonFoo
{
  public static void main(String[] args) throws Exception
  {
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(AttributeScope.class, new AttributeScopeDeserializer());
    Gson gson = gsonBuilder.create();

    TruncateElement element = gson.fromJson(new FileReader("input.json"), TruncateElement.class);

    System.out.println(element.lower);
    System.out.println(element.upper);
    System.out.println(element.delimiter);
    System.out.println(element.scope.get(0));
  }
}

class AttributeScopeDeserializer implements JsonDeserializer<AttributeScope>
{
  @Override
  public AttributeScope deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
      throws JsonParseException
  {
    AttributeScope[] scopes = AttributeScope.values();
    for (AttributeScope scope : scopes)
    {
      if (scope.scope.equals(json.getAsString()))
        return scope;
    }
    return null;
  }
}

class TruncateElement
{
  int lower;
  int upper;
  String delimiter;
  List<AttributeScope> scope;
}

enum AttributeScope
{
  TITLE("${title}"), DESCRIPTION("${description}");

  String scope;

  AttributeScope(String scope)
  {
    this.scope = scope;
  }
}