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Rest API Error Handling

A major rework of the Rest API to support better/simpler error handling
Rest API Error Handling

Rest API Error Handling

We added a lot of features and fixed bugs over the past couple of months and I’ve been a bit lax on blogging. I’ll try to fix that as we approach the revised 5.0 release date. One of the big changes we added over the weekend (it will be in the builds on Friday), is a huge rework of the Rest API.

If you are unfamiliar with the Rest API check out this older post. The Rest API had a few problems, the most pressing of these was the inconsistent error handling behavior. A failure of the Rest call didn’t always trigger error handling correctly and behaved inconsistently on reading the response of the error.

Another inherent problem which we ran into was processing errors differently. E.g. I have a JSON webservice but an error might return a String instead of JSON. With the existing API there was no way of handling this.

This mostly impacted the asynchronous calls but was a problem with the synchronous calls too. Another problem was the SuccessCallback interface used through the API. In itself it’s fine but the callback method is named: onSucess which somehow slipped through…​
Unfortunately fixing interfaces is nearly impossible so we had to add a new interface OnComplete. Like the SuccessCallback the interface is "lambda friendly" by featuring only one method.

A Word About Deprecation

We use deprecation a lot to indicate the better API for usage. As a result we had to deprecate all the existing asynchronous calls in the Rest class and replace them with new API’s.

We won’t remove these deprecated API’s in the foreseeable future. The deprecation is there to show the better/newer API’s.

Better Properties Integration

As part of this work we also squeezed in some work on property business object support. So if I want to make a JSON request to the server with a User PropertyBusinessObject I can do something like:

User userPropertyObject = ...;
ServerResultObject result =

Notice that the result can also be parsed from JSON into a property business object seamlessly!

Fetch API’s

The existing synchronous get API’s such as getAsString() didn’t change. We left the signature as is and they should work the same way.

However, the asynchronous API’s relied on the old SuccessCallback interface so we had to change the signature. If this was old Java 5 we could have just overloaded the method but this would have created a conflict for lambda calls so we decided to change the method name to a fetch prefix.

So the existing code:

   getAsString(c -> callback(c));

Would become:

   fetchAsString(c -> callback(c));

Since I used a lambda the fact that we replaced SuccessCallback with OnComplete becomes irrelevant.

All the versions of the methods that received FailureCallback instances are now deprecated as there were serious problems with error handling.

Error Handling

The new error handling code is far more consistent and would work both for the synchronous and asynchronous calls. Notice that if you don’t define an error handler the behavior in the case of an error might be slightly inconsistent but should generally just return the error value in the standard response.


   onErrorCodeString(r -> error(r)).
   fetchAsJsonMap(c -> callback(c));

There are five new onError style methods:

public RequestBuilder onErrorCodeBytes(ErrorCodeHandler<byte[]> err);
public RequestBuilder onErrorCodeJSON(ErrorCodeHandler<Map> err);
public RequestBuilder onErrorCode(ErrorCodeHandler<PropertyBusinessObject> err, Class errorClass);
public RequestBuilder onErrorCodeString(ErrorCodeHandler<String> err);
public RequestBuilder onError(ActionListener<NetworkEvent> error);

The first four methods are invoked for an error code from the server (value that isn’t 2xx or 3xx from the HTTP response). Notice that each type parses the result and can use a different parsing system from the default parsing logic. We even support parsing errors into a PropertyBusinessObject which can be of a type different from the one used in the main result. So you can define a ServerError business object and parse error result JSON directly into a type-safe object.

The ErrorCodeHandler interface is a simple lambda friendly interface that returns the Result object.

The last method onError is invoked in case of an exception during the connection. We separate a failure such as exceptions which occur due to connectivity or physical issues from a server error code failure. Normally, I would recommend ignoring onError and focusing on a global error handler using NetworkManager as exists in the boilerplate code in a new project.

Moving Forward

Let us know what you think of these changes and how we can further improve the syntax/utility of this API. I think there is a lot we can do to take it forward.

Two omissions in the API that I’ve run into over the years are:

  • Support for XML — this should be easy to add but there wasn’t much demand for it. If there is we can probably add XML support too

  • Image handling — we already have great Image download/caching API’s so we didn’t add them to this class. I’m not sure if this would be the right API to do image requests but it might

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Posted by Shai Almog

Shai is the co-founder of Codename One. He's been a professional programmer for over 25 years. During that time he has worked with dozens of companies including Sun Microsystems.
For more follow Shai on Twitter & github.