SAX drivers bug ... or feature !

[david at megginson.com]

Toivo Lainevool writes:

 > Instead of clearing and reusing the the AttributeList object,
 > wouldn't it be better to create a new attribute list object?  If
 > the old Attribute list isn't being referenced, it will be garbage
 > collectible.  If the old Attribute list is being reference, it
 > won't be changed out from under the client.  This way of doing it
 > seems to offer the best of both worlds.

Depending on the virtual machine, this could be a killer.  Remember
that a medium-sized XML document (such as a book) might have 10,000
elements: that would mean an extra 10,000 attribute lists allocated
and then garbage collected in what should be only a few seconds of
parsing.

In fact, since AttributeList is an interface, drivers often implement
it themselves rather than allocating an object for it, as in

  import org.xml.sax.Parser;
  import org.xml.sax.AttributeList;

  public class MySAXDriver implements Parser, AttributeList
  {
    ...
  }

SAX was designed to be flexible so that people could write
highly-optimised implementations like this if they wanted.  It's also
designed to work with languages that don't support GC out-of-the-box,
like C++.

However, we also recognised that people might want to keep around
attribute lists sometimes, so we made it very easy by adding the
org.xml.sax.helpers.AttributeListImpl class with a copy constructor:

  AttributeList persistentAtts

  public void startElement (String name, AttributeList atts)
  {
    persistentAtts = new AttributeListImpl(atts);
  }

This approach does give you the best of both worlds -- it is trivially
easy to make a persistent copy when you need to, but you're not stuck
with the allocation/gc overhead when you don't need it.


All the best,


David

-- 
David Megginson                 david at megginson.com
           http://www.megginson.com/

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