With Google Web Toolkit (GWT), you write your AJAX front-end in the Java programming language which GWT then cross-compiles into optimized JavaScript that automatically works across all major browsers.

…claims Google. And I must say, I’m pretty impressed by the ease of development GWT offers. I’ve used it at work on a project that I probably couldn’t have done without it, given my poor JavaScript skills. Especially the fact that you don’t have to worry about whether your code works in all browsers is great.

There are a couple of caveats, however:

• The set of supported Java classes is limited, which sometimes causes confusion. For instance, there is a Character class, but the isWhitespace() method is not supported. And neither is List.subList().
• Serialization works differently from the Java standard.
• You can’t work with regular W3C DOM objects on the client. Instead, GWT provides it’s own DOM hierarchy.
• Even though Google claims that GWT “allows developers to quickly build and maintain complex yet highly performant JavaScript front-end applications in the Java programming language”, performance can be a problem.

The purpose of this post is to elaborate on that last point.

Java isn’t JavaScript

Most developers evolve a sense of intuition about what type of code can be a performance problem, and what not. The problem with GWT development, however, is that even though you write Java code, the browser executes JavaScript code. So any intuitions about Java performance are misleading.

Therefore, your usual rules of thumb won’t work in GWT development. Here’s a couple that may.

Serialization is slow

Our application created a domain model on the server, serialized that to the client, and had the client render it. The problem was that the model could become quite large, consisting of thousands of objects. This is not something that GWT currently handles well. Serialization performance appears to be proportional to the number of objects, not just to their total size. Therefore, we translated the model to XML, and serialized that as a single string. This was way faster.

However, this meant that we needed to parse the XML on the client side to reconstruct our model. GWT provides an XMLParser class to handle just that. This class is very efficient in parsing XML, but it turned out that traversing the resulting DOM document was still too slow.

So I wrote a dedicated XML parser, one that can only parse the subset of XML documents that represent our domain model. This parser builds the domain model directly, without intermediate representation. This proved to be faster than the generic approach, but only after being very careful with handling strings.

Some string handling is slow, particularly in Internet Explorer

Java developers know that string handling can be a performance bottleneck. This is also true for GWT development, but not in quite the same way. For instance, using StringBuffer or StringBuilder is usually sufficient for improving String handling performance in Java code. But not so in JavaScript. StringBuffer.append() can be very slow on Internet Explorer, for instance.

GWT 1.6 will contain its own version of StringBuffer that will alleviate some of these problems. Since 1.6 isn’t released yet, we just copied this class into our project.

But even with the new StringBuffer class, you still need to be careful when dealing with strings. Some of the StringBuffer methods are implemented by calling toString() and doing something on the result. That can be a real performance killer. So anything you can do to stay away from substring(), charAt(), etc. will help you. This can mean that it’s sometimes better to work with plain Strings instead of StringBuffers!

Tables

For displaying data in a table-like format, you can use the Grid widget. This is not terribly fast, however, so you may want to consider the bulk table renderers.

The downside of these is that they translate any widgets you insert into the table to HTML, and you loose all the functionality to attached to them, like click handling. Instead, you can add a TableListener, that has a onCellClicked() method.

Varargs are slow

Variable argument lists are sometimes quite handy. However, we’ve found that they come at a severe performance penalty, because a JavaScript array needs to be created to wrap the arguments. So if at all possible, use fixed argument lists, even though this may be less convenient from a code writing/maintaining perspective.

Don’t trust rules of thumb, use a profiler

The problem with rules of thumb is that they are just that: principles with broad application that are not intended to be strictly accurate or reliable for every situation. You shouldn’t put all your trust in them, but measure where the pain really is. This means using a profiler.

You could, of course, run your favorite Java profiler against hosted mode to get a sense of performance of your code. But that would be besides the point. Your Java code is compiled to JavaScript and it is the JavaScript code that gets executed by the browser. So you should use a JavaScript profiler.

We used Firebug to profile the compiled JavaScript code and this helped us enormously. As usual with profiling, we found performance bottlenecks that we didn’t anticipate. As a result, we were able to make our application load over 60 times faster! (on Internet Explorer 7)

Performance in different browsers

The only problem with Firebug is that it’s a FireFox plugin, and therefore not available for Internet Explorer. [Firebug Lite doesn’t contain a profiler.] Not that I personally would want to use IE, but our customers do, unfortunately.

The irony is that you need a JavaScript profiler in Internet Explorer the most: FireFox is unbelievably much faster than Internet Explorer when it comes to processing JavaScript, especially with string handling. For instance, for one data set, FireFox 3 loaded the page in 51 seconds, while Internet Explorer 7 took 12 minutes and 4 seconds!

Internet Explorer 8 will supposedly be better:

We have made huge improvements to widely-used JScript functionality including faster string, array, and lookup operations.

We’ll have to see how that works out when IE8 is released…

BTW, if you’re interested in browser performance, check out this comparison.