Significant performance improvement for freemarker TagModel

The class org.apache.struts2.views.freemarker.tags.TagModel creates a new freemarker.template.DefaultObjectWrapper for each call to unwrapParameters(Map). The problem is that that class is quite expensive to construct as the super-class does a quite bit of introspection to fill up its caches. Not only is this a lot of work, it tends to hit the Introspector class pretty hard, which internally does a lot of sychronising. In our soak performance tests this was a significant cause of blocking in our application.

The DefaultObjectWrapper (or more properly the freemarker.ext.beans.BeansWrapper class as that is actually all the TagModel is using) is actually meant to be a cache that prevents exactly this kind of slowness, so constructing a new one every time is counter-productive.

I don't have the latest source handy so I can't provide a patch, but the change is to simply replace the line:

DefaultObjectWrapper objectWrapper = new DefaultObjectWrapper();

with:

BeansWrapper objectWrapper = BeansWrapper.getDefaultInstance();

in the second line of TagModel.unwrapParameters(Map params)