XOM API for Processing XML with Java

[Elliotte Harold <elharo AT metalab.unc.edu>]

luca wrote:


> mmmm, I don't get it. How hard could it be to have
>
>   Builder builder = new Builder();
>   builder.ignoreDtdNoMatterWhat();

But what do you do when you then encounter an entity reference to an 
entity whose replacement text is not available?

I really think it's safer (if not necessarily faster) to always read the 
external DTD subset. I wrote about this in Item 18 of Effective XML, 
which if somebody asks nicely I may put online, but here's the relevant 
part:

Consumers cannot assume that documents are self-contained. In the spirit 
of being liberal in what you accept but conservative in what you 
produce, when receiving a document from a third party you should do your 
utmost to read the external DTD subset, resolve all external entity 
references, and apply all default attribute values. You should only use 
parsers that can do this, especially for DTDs. While most APIs allow you 
to turn off resolution of external entity references and ignore the 
external DTD subset, you should almost never do so. Reading the external 
DTD subset will cost you little in the case where it isn’t necessary and 
may save your bacon in the case where it is. Programs that repeatedly 
read the same external DTD subsets can use the public identifiers, 
catalogs, and various caching strategies to cache the DTDs so repeated 
trips back to a server are not necessary. (See Item 47.)

This is especially useful with web browsers and XHTML, and indeed it’s 
sanctioned by the XHTML specification. Web browsers that support XHTML 
have, for practical intents and purposes, internally cached the XHTML 
DTD. Thus when they encounter a document whose public ID labels it as 
XHTML, they do not need to read the DTD. They already know how to 
resolve entity references like Θ and α. However, this option 
is not available to a general purpose XML parser that does not know 
which specific XML applications it will encounter. Such a processor must 
read the external DTD subset before it can confidently replace all entities.

--
Elliotte Rusty Harold
elharo AT metalab.unc.edu