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Implement a custom request

This lesson describes how to implement your own custom request types, for types that don't have out-of-the-box Volley support.

Write a custom request

Most requests have ready-to-use implementations in the toolbox; if your response is a string, image, or JSON, you probably won't need to implement a custom Request.

For cases where you do need to implement a custom request, this is all you need to do:

  • Extend the Request<T> class, where <T> represents the type of parsed response the request expects. So if your parsed response is a string, for example, create your custom request by extending Request<String>. See the Volley toolbox classes StringRequest and ImageRequest for examples of extending Request<T>.
  • Implement the abstract methods parseNetworkResponse() and deliverResponse(), described in more detail below.

parseNetworkResponse

A Response encapsulates a parsed response for delivery, for a given type (such as string, image, or JSON). Here is a sample implementation of parseNetworkResponse():

Kotlin

override fun parseNetworkResponse(response: NetworkResponse?): Response<T> {
    return try {
        val json = String(
                response?.data ?: ByteArray(0),
                Charset.forName(HttpHeaderParser.parseCharset(response?.headers)))
        Response.success(
                gson.fromJson(json, clazz),
                HttpHeaderParser.parseCacheHeaders(response))
    }
    // handle errors
// ...
}

Java

@Override
protected Response<T> parseNetworkResponse(
        NetworkResponse response) {
    try {
        String json = new String(response.data,
        HttpHeaderParser.parseCharset(response.headers));
    return Response.success(gson.fromJson(json, clazz),
    HttpHeaderParser.parseCacheHeaders(response));
    }
    // handle errors
// ...
}

Note the following:

  • parseNetworkResponse() takes as its parameter a NetworkResponse, which contains the response payload as a byte[], HTTP status code, and response headers.
  • Your implementation must return a Response<T>, which contains your typed response object and cache metadata or an error, such as in the case of a parse failure.

If your protocol has non-standard cache semantics, you can build a Cache.Entry yourself, but most requests are fine with something like this:

Kotlin

return Response.success(myDecodedObject,
        HttpHeaderParser.parseCacheHeaders(response))

Java

return Response.success(myDecodedObject,
        HttpHeaderParser.parseCacheHeaders(response));

Volley calls parseNetworkResponse() from a worker thread. This ensures that expensive parsing operations, such as decoding a JPEG into a Bitmap, don't block the UI thread.

deliverResponse

Volley calls you back on the main thread with the object you returned in parseNetworkResponse(). Most requests invoke a callback interface here, for example:

Kotlin

override fun deliverResponse(response: T) = listener.onResponse(response)

Java

protected void deliverResponse(T response) {
        listener.onResponse(response);

Example: GsonRequest

Gson is a library for converting Java objects to and from JSON using reflection. You can define Java objects that have the same names as their corresponding JSON keys, pass Gson the class object, and Gson will fill in the fields for you. Here's a complete implementation of a Volley request that uses Gson for parsing:

Kotlin

/**
 * Make a GET request and return a parsed object from JSON.
 *
 * @param url URL of the request to make
 * @param clazz Relevant class object, for Gson's reflection
 * @param headers Map of request headers
 */
class GsonRequest<T>(
        url: String,
        private val clazz: Class<T>,
        private val headers: MutableMap<String, String>?,
        private val listener: Response.Listener<T>,
        errorListener: Response.ErrorListener
) : Request<T>(Method.GET, url, errorListener) {
    private val gson = Gson()


    override fun getHeaders(): MutableMap<String, String> = headers ?: super.getHeaders()

    override fun deliverResponse(response: T) = listener.onResponse(response)

    override fun parseNetworkResponse(response: NetworkResponse?): Response<T> {
        return try {
            val json = String(
                    response?.data ?: ByteArray(0),
                    Charset.forName(HttpHeaderParser.parseCharset(response?.headers)))
            Response.success(
                    gson.fromJson(json, clazz),
                    HttpHeaderParser.parseCacheHeaders(response))
        } catch (e: UnsupportedEncodingException) {
            Response.error(ParseError(e))
        } catch (e: JsonSyntaxException) {
            Response.error(ParseError(e))
        }
    }
}

Java

public class GsonRequest<T> extends Request<T> {
    private final Gson gson = new Gson();
    private final Class<T> clazz;
    private final Map<String, String> headers;
    private final Listener<T> listener;

    /**
     * Make a GET request and return a parsed object from JSON.
     *
     * @param url URL of the request to make
     * @param clazz Relevant class object, for Gson's reflection
     * @param headers Map of request headers
     */
    public GsonRequest(String url, Class<T> clazz, Map<String, String> headers,
            Listener<T> listener, ErrorListener errorListener) {
        super(Method.GET, url, errorListener);
        this.clazz = clazz;
        this.headers = headers;
        this.listener = listener;
    }

    @Override
    public Map<String, String> getHeaders() throws AuthFailureError {
        return headers != null ? headers : super.getHeaders();
    }

    @Override
    protected void deliverResponse(T response) {
        listener.onResponse(response);
    }

    @Override
    protected Response<T> parseNetworkResponse(NetworkResponse response) {
        try {
            String json = new String(
                    response.data,
                    HttpHeaderParser.parseCharset(response.headers));
            return Response.success(
                    gson.fromJson(json, clazz),
                    HttpHeaderParser.parseCacheHeaders(response));
        } catch (UnsupportedEncodingException e) {
            return Response.error(new ParseError(e));
        } catch (JsonSyntaxException e) {
            return Response.error(new ParseError(e));
        }
    }
}

Volley provides ready-to-use JsonArrayRequest and JsonArrayObject classes if you prefer to take that approach. See Make a standard request for more information.