GWT: How to avoiding calls to dynamicCast and canCastUnsafe in generated JavaScript code?

Question

I\'m writing some special purpose data structures in Java, intended for use in the browser, (compiled to JavaScript with GWT).

I\'m trying to match the performance o

Answer 1

Unfortunately I'm still not exactly clear on the cause, but from my experience, it seems from explicit casts, like:

((Comparable) obj).compareTo(other)

The Javascript generated looks like:

dynamicCast(obj, 1).compareTo(other);

Where 1 is a generated typeId representing the target of the cast. dynamicCast in turn calls canCastUnsafe and if false, it throws a ClassCastException. The value of this has been debated, since this would already be caught in hosted mode.

It can be sidestepped with JSNI:

public static native int compare(Object a, Object b) /*-{
    return a.@java.lang.Comparable::compareTo(Ljava/lang/Object;)(b); 
}-*/;

Answer 2

Dunno if you've seen this thread in the GWT Contributor's forum...

Basically, it starts with the same problem you've identified, proposes some new compiler flags, and goes on to show how to use some JSNI to get around the casts.

Edit In the GWT trunk there's a new compiler flag. See the wiki...

Answer 3

An updated answer for GWT version 2.1 and later:

Since GWT 2.1 (at least that's the first mention), the GWT compiler has a new compiler argument called -XdisableCastChecking that disables all runtime checking of casts. Note, this option is marked as experimental (probably because this would make class cast exceptions very hard to debug).

In my app dynamicCast was called thousands of times in a short profile run, and were the the 3rd most time consuming method in the Firebug profiler. Using this compiler argument significantly reduced the number of "Long Duration Events" messages in the Chrome Speed Tracer.

See GWT Compiler Options for this and other Compiler arguments.

Answer 4

Does the use of java 1.5 generics and wildcards could avoid this ?

Answer 5

It's definitely a compiler problem: I have the problem on the following line:

final DefaultIconedSuggestBox<SuggestValueProxy, IconedValueHolderItem<SuggestValueProxy>> fieldValueWidget = getCategoryWidget().getFieldValueWidget();

I don't really know how I can workaround it: this line happens in a moment I'm changing from a module to another (it is maybe related to the code splitter issue: even though I'm not using code split: I'm just loading another page with another module)