GWT - Restygwt - Response was NOT a valid JSON document

Question

I wantet to set up a simple demo Project on how to use Restygwt, for learning. I followed this Tutorial: https://ronanquillevere.github.io/2014/03/16/gwt-rest-app.html So far I got this on my client: In my SearchPresenter the method searchBook(String isbn) gets triggered when I click a button

    @Override
public void searchBook(String isbn) {
    clientContext.getBookStoreClient().getBook(isbn, new MethodCallback<List<Book>>() {
        @Override
        public void onFailure(Method method, Throwable exception) {
            Window.alert("Failure");
            GWT.log(exception.getMessage());
            GWT.log(stackTraceToString(exception));


        }
        @Override
        public void onSuccess(Method method, List<Book> response) {
            for (Book book : response) {
                Window.alert(book.getIsbn() + " " + book.getAuthor());
            }
        }

    });
}

private String stackTraceToString(Throwable e) {
    StringBuilder sb = new StringBuilder();
    for (StackTraceElement element : e.getStackTrace()) {
        sb.append(element.toString());
        sb.append("\n");
    }
    return sb.toString();
}

My BookstoreClient looks like this:

@Path("/api/bookstore/books")
public interface BookstoreClient extends RestService{

@GET
public void getBooks(MethodCallback<List<Book>> callback);

@GET
@Path("/{isbn}")
public void getBook(@PathParam("isbn") String isbn, MethodCallback<List<Book>> callback);
}

The BookResource on my Server:

@Path("books")
public class BookResource {

Map<String, Book> books;

public BookResource() {
    books = new HashMap<>();

    Book book1 = new Book("1", "Max Mustermann");
    books.put(book1.getIsbn(), book1);

    Book book2 = new Book("2", "Erika mustermann");
    books.put(book2.getIsbn(), book2);
}

@GET
@Produces("application/json")
public Collection<Book> getBooks() {
    return books.values();
}

@GET
@Path("/{isbn}")
@Produces("application/json")
public Book getBook(@PathParam("isbn") String isbn) {
    return books.get(isbn);
}
}

And the Book in my shared Package

public class Book {

private final String isbn;
private final String author;

@JsonCreator
public Book(@JsonProperty("isbn")String isbn, @JsonProperty("author")String author) {
    this.isbn = isbn;
    this.author = author;
}

public String getIsbn() {
    return isbn;
}

public String getAuthor() {
    return author;
}

}

But if I click the Button on my GUI I jump into the onFailure and get the Exception:

Response was NOT a valid JSON document

Unknown.Throwable_4_g$(GWT_Selenium-0.js@8:4312)
Unknown.Exception_4_g$(GWT_Selenium-0.js@18:4550)
Unknown.RuntimeException_4_g$(GWT_Selenium-0.js@18:4597)
Unknown.new ResponseFormatException_2_g$(GWT_Selenium-0.js@25:51000)
Unknown.parseResult_2_g$(GWT_Selenium-0.js@23:33367)
Unknown.parseResult_1_g$(GWT_Selenium-0.js@15:33355)
Unknown.onResponseReceived_0_g$(GWT_Selenium-0.js@27:33312)
Unknown.fireOnResponseReceived_0_g$(GWT_Selenium-0.js@17:14339)
Unknown.onReadyStateChange_0_g$(GWT_Selenium-0.js@28:14647)
Unknown.<anonymous>(GWT_Selenium-0.js@18:30998)
Unknown.apply_0_g$(GWT_Selenium-0.js@28:5498)
Unknown.entry0_0_g$(GWT_Selenium-0.js@16:5554)
Unknown.<anonymous>(GWT_Selenium-0.js@14:5534)

But if I go to url:

http://127.0.0.1:8888/api/bookstore/books

I get the following shown in My Browser

[{"isbn":"1","author":"Max Mustermann"},{"isbn":"2","author":"Erika mustermann"}]

Did I miss something or screwed up on the implementation of the RestService? Thanks in Advance for any comment, youre Help is apreciatet

Answer 1

Your question says that the error happens when you click a button, and the button does a search by isbn - it doesn't call GET /api/bookstore/books like you tested in the browser. This is what it actually does:

@GET
@Path("/{isbn}")
public void getBook(@PathParam("isbn") String isbn, MethodCallback<List<Book>> callback);

This client-side method expects a List<Book>, but on the server you defined

@GET
@Path("/{isbn}")
@Produces("application/json")
public Book getBook(@PathParam("isbn") String isbn) 

which returns a single book, not wrapped in a list. This is likely the source of your error.