Saturday, January 26, 2013

Marshaling POJOs with Jersey

When a web service has multiple consumers, you obviously don't want to duplicate a lot of code.  So what if you want to push data to/from a Java API and a web site like jQuery?  Jersey can help you do both with JSON.  A lot of what I'll cover is in the Jersey docs.
  • Turn JSON support on in your servlet's web.xml:
  • This example also uses compression so add the gzip filters too.
  • Annotate your POJO with JAXB.  Make sure you have an empty constructor.  I like to make the member variables private and force use of public getters.  I also elect to ignore unknown properties.
@XmlRootElement(name = "foo")
@JsonIgnoreProperties(ignoreUnknown = true)
public class Foo {
    private String blah;

    Foo() {}
    public String getBlah() { return blah; }
  • Set up your web service.  Since you're returning JSON, jQuery will be happy with basically no further consideration.  Note I didn't define a setBlah() above but I hope you get the picture.  
@GET @Path("/")
public Foo getFoo() {
  Foo foo = new Foo();
  return foo;
  • Connect your WebResource properly.  Note the addition of the JacksonJsonProvider class.  The gzip filter is not required but you should probably use it.

ClientConfig config = new DefaultClientConfig();
Client client = Client.create(config);
client.addFilter(new GZIPContentEncodingFilter(true)); 
WebResource webResource = client.resource(hostname);
  • Now request your POJO response (note the use of gzip here):

Foo foo = webResource.path("/")
               .header("Accept-Encoding", "gzip")
               .get(new GenericType<Foo>() {});
  • You can also pass POJOs to the web service using POSTs.  This snippet assumes you annotated class Bar is a similar manner to Foo and the "/bar" POST endpoint accepts a Bar object.

Bar bar = new Bar("stuff");

Foo foo = webResource.path("/bar").post(new GenericType<Foo>() {}, bar);

Edited on 2/10/13 to include compression in example.

Sunday, January 6, 2013

"Multiple dex files" error with Eclipse + ADT

I haven't done much Android development in the last year but this week wanted to update my app a bit.  So I installed Eclipse Juno (4.2), grabbed the latest Android Developer Tools (r21), and the latest AWS SDK for Android (1.4.4) since my app persists data on AWS.

These are all pretty big jumps from what I used to have - Eclipse Indigo, probably a single digit ADT, and AWS SDK 0.2.X IIRC.  I've only been using my System76 laptop for grad school work of late, not "real" coding obviously.  Almost everything worked fine.  Thanks go out to these teams for honoring backward compatibility!  The one problem I had was "Unable to execute dex: Multiple dex files define X" (X being something in the AWS package).

So off to StackOverflow.  I tried all the suggestions:
  • Open/close Eclipse a few times, doing project cleans
  • Reinstalling ADT
  • Deleting my /bin and /gen directories, then cleaning
  • Made sure my build path was legit - several people traced it back to having /bin in the build path
The AWS JARs I imported were core, debug, and sdb (my app uses SimpleDB - don't ask).  Since my testing only consisted of moving the .apk to dropbox and making sure it worked on my Galaxy S3, I didn't need the debug JAR.  Once I removed it, everything worked okay.

Kinda perturbed this is still a problem - many SO posts and blogs I've seen are by people who appear to know what they're talking about so I don't think it's always a silly oversight by junior developers.  I'm thinking it's a dependency problem - maybe AWS SDK 1.4.4 wasn't developed with ADT r21?  If that was the case, Eclipse and SDK providers make it really hard to grab older versions like you would with a POM file.  If I needed to debug in this instance, I'd be in real trouble.