TIL: JSF + Guava not such a good idea

The other day I was working on a page that presents hierarchical data and I wanted to retrieve each new level with an Ajax request. So far so good, but JSF makes simple things harder than they should be.

I created a view-model for each level of data. It was very basic:

class EntityViewModel {
    private Entity entity;
    private List<EntityViewModel> children;
}

After querying the database for the entities, I had to transform and insert the retrieved data into the view-model object tree. The obvious solution is to wrap every entity in a view-model and set the parent’s children property to the new list.

My mistake was that I used Guava’s fancy Lists#transform method instead of a simple foreach. For the first level, everything worked fine, but when I wanted to retrieve the second level nothing came back in the response. Everything seemed to be fine, but somehow JSF did not pick up the changes in the object-tree and did not render anything. All the responses were empty.

After a few hours of staring at the screen helplessly and blaming JSF, I realized that the problem was with the List<> implementation that Guava returned. The #transform method only returns a view of the original list, so JSF did not have a chance to notice when the children property of an object changed, thus it never rendered anything below the first level.

The takeaway? Don’t shy away from using the good ol’ foreach. It will probably never let you down.