Converting JSON between string and byte[] with GSON

Question

I'm using hibernate to map objects to the database. A client (an iOS app) sends me particular objects in JSON format which I convert to their true representation using the following utility method:

/**
     * Convert any json string to a relevant object type
     * @param jsonString the string to convert
     * @param classType the class to convert it too
     * @return the Object created
     */
    public static <T> T getObjectFromJSONString(String jsonString, Class<T> classType) {
        
        if(stringEmptyOrNull(jsonString) || classType == null){
            throw new IllegalArgumentException("Cannot convert null or empty json to object");
        }

        try(Reader reader = new StringReader(jsonString)){
            Gson gson = new GsonBuilder().create();
            return gson.fromJson(reader, classType);
        } catch (IOException e) {
            Logger.error("Unable to close the reader when getting object as string", e);
        }
        return null;
    }

The issue however is, that in my pogo I store the value as a byte[] as can be seen below (as this is what is stored in the database - a blob):

@Entity
@Table(name = "PersonalCard")
public class PersonalCard implements Card{
    
    @Id @GeneratedValue
    @Column(name = "id")
    private int id;
    
    @OneToOne
    @JoinColumn(name="userid")
    private int userid;
    
    @Column(name = "homephonenumber")
    protected String homeContactNumber;
    
    @Column(name = "mobilephonenumber")
    protected String mobileContactNumber;
    
    @Column(name = "photo")
    private byte[] optionalImage;
    
    @Column(name = "address")
    private String address;

Now of course, the conversion fails because it can't convert between a byte[] and a String.

Is the best approach here to change the constructor to accept a String instead of a byte array and then do the conversion myself whilst setting the byte array value or is there a better approach to doing this.

The error thrown is as follows;

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 96 path $.optionalImage

Thanks.

Edit In fact even the approach I suggested will not work due to the way in which GSON generates the object.

Answer 1

You can use this adapter to serialize and deserialize byte arrays in base64. Here's the content.

   public static final Gson customGson = new GsonBuilder().registerTypeHierarchyAdapter(byte[].class,
            new ByteArrayToBase64TypeAdapter()).create();

    // Using Android's base64 libraries. This can be replaced with any base64 library.
    private static class ByteArrayToBase64TypeAdapter implements JsonSerializer<byte[]>, JsonDeserializer<byte[]> {
        public byte[] deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            return Base64.decode(json.getAsString(), Base64.NO_WRAP);
        }

        public JsonElement serialize(byte[] src, Type typeOfSrc, JsonSerializationContext context) {
            return new JsonPrimitive(Base64.encodeToString(src, Base64.NO_WRAP));
        }
    }

Credit to the author Ori Peleg.

Answer 2

From some blog for future reference, incase the link is not available, atleast users can refer here.

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;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

import java.lang.reflect.Type;
import java.util.Date;

public class GsonHelper {
    public static final Gson customGson = new GsonBuilder()
            .registerTypeAdapter(Date.class, new JsonDeserializer<Date>() {
                @Override
                public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
                return new Date(json.getAsLong());
                }
            })
            .registerTypeHierarchyAdapter(byte[].class,
                    new ByteArrayToBase64TypeAdapter()).create();

    // Using Android's base64 libraries. This can be replaced with any base64 library.
    private static class ByteArrayToBase64TypeAdapter implements JsonSerializer<byte[]>, JsonDeserializer<byte[]> {
        public byte[] deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            return Base64.decode(json.getAsString(), Base64.NO_WRAP);
        }

        public JsonElement serialize(byte[] src, Type typeOfSrc, JsonSerializationContext context) {
            return new JsonPrimitive(Base64.encodeToString(src, Base64.NO_WRAP));
        }
    }
}