JSF 1.2: ValueExpression and null Strings

[Adam Winer <aw...@gmail.com>]

In JSF 1.2, our TLD now includes:

  <deferred-value><type>String.class</type></deferred-value>

... for any String-typed property.  This is all seems well
and good, since it means that our runtime is getting
String objects instead of needing to manually cast
to string.

However, section 1.18.2 of the EL spec has some lousy
language:

1.18.2 CoerceAtoString
■ If A is String: return A
■ Otherwise, if A is null: return""
■ Otherwise, i fA is Enum, returnA.name()
■ Otherwise, if A.toString() throws an exception, error
■ Otherwise, return A.toString()

Specifically, it's that second bullet.  Now, if you have an
EL binding to a String, and return null, we're getting the
empty string, never null.  That's fine if you don't care
about the difference, but we almost always do.  For
example, ResponseWriter.writeAttribute() will actually
write out the attribute, so you'd get:
  <span class="">...</span>
if you had an EL binding for styleClass that conditionally
returned null.

We have two choices:
(1) In many, many locations, start checking for the
    empty string.
(2) Don't use String.class as a deferred type, and
   convert to string in the code (again, at many,
   many locations).

I strongly prefer #2, especially because our code is already
good about converting to strings automatically.  #1
means we're throwing away information - the empty
string is not always the same thing as null - which is
never a good idea.

-- Adam