FrontEnd: jsp with AngularJS
BackEnd: Spring MVC/Java

I am uploading a file using ng-flow, angularJS. Source: <https://github.com/flowjs/ng-flow>

File upload is successful. I need to return a json from my Spring Controller. Any clues how to go about it? 
P.S. can't find where to put in .success() function, if at all that is applicable.

Spring Controller:

```
@RequestMapping(value = "/upload", method = RequestMethod.POST)
        public String uploadFileHandler(@RequestParam("file") MultipartFile file, Model model) {
       //Upload file and process

       JsonObject jo = Json.createObjectBuilder().add(path, folderPath.toString())
                                .add(aContentsAttrib, aContents)
                                .add(bContentsAttrib, bContents).build();
}

```

app.js code:

```
(function() {
    var app = angular.module('app', ['flow'])
    .config(['flowFactoryProvider', function (flowFactoryProvider) {
      flowFactoryProvider.defaults = {
        target: 'upload',
        permanentErrors: [404, 500, 501],
        maxChunkRetries: 4,
        chunkRetryInterval: 500,
        simultaneousUploads: 4
      };
      flowFactoryProvider.on('catchAll', function (event) {
        console.log('catchAll', arguments);
      });
      // Can be used with different implementations of Flow.js
      // flowFactoryProvider.factory = fustyFlowFactory;
    }]);

  app.controller('PageController', function() {
    //this.products = gems;
  });

  app.controller("TabController", function() {
    this.tab = 1;
    this.showOutput = false;
    this.viewEvents = false;

    this.isSet = function(checkTab) {
      return this.tab === checkTab;
    };

    this.changeVal = function() {
        this.viewEvents = true;
    };

    this.setTab = function(setTab) {
      this.tab = setTab;
    };
  });

})();

```

What exactly should be returned from the spring controller? (`String/@ResponseBody` String etc)
How to collect that `json` in `angular`?