Using Custom Type Adapter

In the earlier tutorials we have seen how gson can serialize and deserialize java classes with or without hierarchies. By default, it introspects the classes and comes with with a strategy for serializing and deserializing it. However, in some cases, you want to specify your own conversion strategy. That is, you want to control how the java object is converted to json string and the other way round. Gson provides a capability to specify a custom type adapter. You tell Gson that for a particular class, use the conversion strategy specified by your custom adapter. Lets look at how to write the type adapter :
To write a custom adapter extend the com.google.gson.TypeAdapter abstract class. Implement the public abstract T read(JsonReader in) throws IOException; and public abstract void write(JsonWriter out, T value) throws IOException; methods. The adapter should also handle nulls. Create the Type adapter
instance and then register it with the GsonBuilder. Create the Gson object from the GsonBuilder and then use that to serialize and deserialize. Lets look at an example

package com.studytrails.json.gson;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

import org.apache.commons.io.IOUtils;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class DatasetTypeAdapterExample8 {

	public static void main(String[] args) throws MalformedURLException, IOException {
		String url = "http://freemusicarchive.org/api/get/albums.json?api_key=60BLHNQCAOUFPIBZ&limit=5";
		String json = IOUtils.toString(new URL(url));
		// Create the custom type adapter and register it with the GsonBuilder
		// class.
		Gson gson = new GsonBuilder().registerTypeAdapter(Dataset.class, new DatasetTypeAdapter()).create();
		// deserialize the json to Albums class. The Dataset objects are part of
		// the Albums class. Whenever Gson encounters an object of type DataSet
		// it calls the DatasetTypeAdapter to read and write json.
		Albums albums = gson.fromJson(json, Albums.class);
		System.out.println(albums.getDataset()[1].getAlbum_title());
		// prints
		// http://freemusicarchive.org/music/The_Yes_Sirs/Through_The_Cracks_Mix_Vol_1/
	}
}
	

The Adapter

package com.studytrails.json.gson;

import java.io.IOException;

import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

/**
 * The Dataset class contains the information about a particular Album.
 * album_title and album_url are two distinct fields in the json. The Dataset
 * object contains the field album_title. Normally Gson would map the
 * album_title property in the json the the album_title field in the Dataset
 * object. However, we dont want that. We want to use the album_url property
 * from the json object to populate the album_title field in the Dataset object.
 * we build a custom TypeAdapter to do that. This is just a trivial case, you
 * could also combine album_url and album_title properties and set it to the
 * album_title field of the Dataset Object.
 * 
 */
public class DatasetTypeAdapter extends TypeAdapter<Dataset> {
	@Override
	public Dataset read(JsonReader reader) throws IOException {
		// the first token is the start object
		JsonToken token = reader.peek();
		Dataset dataset = new Dataset();
		if (token.equals(JsonToken.BEGIN_OBJECT)) {
			reader.beginObject();
			while (!reader.peek().equals(JsonToken.END_OBJECT)) {
				if (reader.peek().equals(JsonToken.NAME)) {
					if (reader.nextName().equals("album_url"))
						dataset.setAlbum_title(reader.nextString());
					else
						reader.skipValue();

				}
			}
			reader.endObject();

		}
		return dataset;
	}

	@Override
	public void write(JsonWriter out, Dataset value) throws IOException {

	}

}

The Albums class

package com.studytrails.json.gson;


public class Albums {

	private String title;
	private Dataset[] dataset;

	public void setTitle(String title) {
		this.title = title;
	}

	public void setDataset(Dataset[] dataset) {
		this.dataset = dataset;
	}

	public String getTitle() {
		return title;
	}

	public Dataset[] getDataset() {
		return dataset;
	}
}

The Dataset class

package com.studytrails.json.gson;

import java.util.HashMap;
import java.util.Map;

public class Dataset {
	private String album_id;
	private String album_title;
	private Map<String , Object> otherProperties = new HashMap<String , Object>();

	public String getAlbum_id() {
		return album_id;
	}

	public void setAlbum_id(String album_id) {
		this.album_id = album_id;
	}

	public String getAlbum_title() {
		return album_title;
	}

	public void setAlbum_title(String album_title) {
		this.album_title = album_title;
	}

	public Object get(String name) {
		return otherProperties.get(name);
	}

}