Deserialize JSONP with Jackson

Question

For some reason Jackson 2.3.0 is unable to parse a JSONP response.

com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'my_json_callback':

I've got the deserialization process to work without the callback.

I've tried with the Jackson JAX-RS package which includes a @JSONP annotation, but this seems to be used only when serializing.

Answer 1

Here's a trimmed-for-space version of the solution I came up with using a ReaderInterceptor. I am using Jersey 2.x in conjunction with Jackson to interact with a web service that only outputs JSONP.

public class CallbackStripInterceptor implements ReaderInterceptor {

    private final static byte[] callbackBytes = "callback(".getBytes();

    @Override
    public Object aroundReadFrom(ReaderInterceptorContext context) throws IOException, WebApplicationException {

    int howMany = callbackBytes.length;

    InputStream x = context.getInputStream();

    if( !(x.available() >= howMany) ) {
        return context.proceed();
    }

    x.mark( howMany );
    byte[] preamble = new byte[ howMany ];
    x.read( preamble );

    // In case the first part of our entity doesn't have the callback String, reset the stream so downstream exploiters get the full entity.
    if( !Arrays.equals( preamble, callbackBytes ) ) {
        x.reset();
    } 

    return context.proceed();
}

Use like so:

Client c = ClientBuilder.newBuilder()
    .register( new CallbackStripInterceptor() )
    .build();

Using this client, all responses with an entity will go through this interceptor (Jersey does not run interceptors on responses without entity bodies).

Answer 2

Finally I've been able to remove the callback part of the JSONP response.

First, Jackson is able to parse the JSON even when it ends with a parenthesis. So, by simply removing the my_json_callback( from the response is enough.

Since I'm using Apache's HTTP Client, this fixes the issue:

String callback = "my_json_callback(";
InputStreamReader r = new InputStreamReader(response.getEntity().getContent());
r.skip(callback.length());
return mapper.readValue(r, MyObject.class);

The idea is not having to convert the Reader to a String and then parsing that String after removing the callback part.

I was also able to accomplish the same results using JSONTokener from json.org library for a given JSONP String:

JSONTokener t = new JSONTokener(json);
t.nextValue(); // skip the callback
return mapper.readValue(t.nextValue().toString(), MyObject.class);