Question

I have a problem with a throw execution in GWT. So I want to show a simple String list in Client side located on “Server Side”. Ok, I have in my Main class this atributes:

private final GreetingServiceAsync greetingService = Util.getInstance(); // This is like > typing GWT.create( GreetingService.class );
public ArrayList songs = new ArrayList();

and in my onModuleLoad() method I have a call to another private method that make the Async call to the Server class:

songs.addAll(getSongsList());

So my getSongsList method is as follow:

public ArrayList<String> getSongsList() {
        final int defaultSize = 4;
        final ArrayList<String> temp = new ArrayList<String>();

        GWT.log("Enter in getSongsLists");
        greetingService.greetSongMostPopular(defaultSize,
                new AsyncCallback<ArrayList<String>>() {

                    public void onSuccess(ArrayList<String> result) {
                        GWT.log("Result is:" + result);
                        temp.addAll(result);
                        GWT.log("Case 1 TEMP= " + temp);
                    }

                    public void onFailure(Throwable caught) {
                        // throw new
                        // UnsupportedOperationException("Not supported yet.");
                        Window.alert("Error greeting data");

                    }

                });
        GWT.log("CASE 2 TEMP = " + temp);
        return temp;

    }

My problem is that in Case 1 I get

[INFO] [MainModule] – Case 1 TEMP= [Song 1, Song 2, Song 3, Song 4]

but in CASE 2 I get the ArrayList empty!!!

[INFO] [MainModule] – Case 1 TEMP= []

What am I doing wrong?

Thanks in advance!

Answer 1

The call to greetSongsMostPopular is asynchronous. This means that the call begins and the code continues directly to the next line of code, in your case GWT.log(“CASE 2 TEMP…”). When this line of code is executed, the results are not yet available and you end up with an empty list.

Later, in the background, onSuccess is called with the results. At this point you should call a function to process the results. You can use a pattern similar to the AsyncCallback class, with an onSuccess and onFailure so the caller of this code can handle both cases, or you can just have the caller pass in their own AsyncCallback instance which would make it a thin wrapper around your greetSongsMostPopular RPC function.