XOM API for Processing XML with Java

[Elliotte Rusty Harold <elharo AT ibiblio.org>]

My initial efforts to mitigate the billion laughs attack succeeded. However
this document snuck by:

<?xml version="1.0"?>
<!DOCTYPE root [
  <!ENTITY ha1 "ha ha ha ha ha ha ha ha ha ha ha ha ha ha ha">
  <!ENTITY ha1 "ha ha ha ha ha ha ha ha ha ha ha ha ha ha ha">
  <!ENTITY ha2 "&ha1; &ha1; &ha1; &ha1; &ha1; &ha1; &ha1; &ha1; &ha1; &ha1;
&ha1; &ha1; &ha1; &ha1; &ha1;">
  <!ENTITY ha3 "&ha2; &ha2; &ha2; &ha2; &ha2; &ha2; &ha2; &ha2; &ha2; &ha2;
&ha2; &ha2; &ha2; &ha2; &ha2;">
  <!ENTITY ha4 "&ha3; &ha3; &ha3; &ha3; &ha3; &ha3; &ha3; &ha3; &ha3; &ha3;
&ha3; &ha3; &ha3; &ha3; &ha3;">
  <!ENTITY ha5 "&ha4; &ha4; &ha4; &ha4; &ha4; &ha4; &ha4; &ha4; &ha4; &ha4;
&ha4; &ha4; &ha4; &ha4; &ha4;">
  <!ENTITY ha6 "&ha5; &ha5; &ha5; &ha5; &ha5; &ha5; &ha5; &ha5; &ha5; &ha5;
&ha5; &ha5; &ha5; &ha5; &ha5;">
  <!ENTITY ha7 "&ha6; &ha6; &ha6; &ha6; &ha6; &ha6; &ha6; &ha6; &ha6; &ha6;
&ha6; &ha6; &ha6; &ha6; &ha6;">
  <!ENTITY ha8 "&ha7; &ha7; &ha7; &ha7; &ha7; &ha7; &ha7; &ha7; &ha7; &ha7;
&ha7; &ha7; &ha7; &ha7; &ha7;">
  <!ENTITY ha9 "&ha8; &ha8; &ha8; &ha8; &ha8; &ha8; &ha8; &ha8; &ha8; &ha8;
&ha8; &ha8; &ha8; &ha8; &ha8;">
  <!ENTITY ha "&ha9; &ha9; &ha9; &ha9; &ha9; &ha9; &ha9; &ha9; &ha9; &ha9;
&ha9; &ha9; &ha9; &ha9; &ha9;">
]>
<root name='&ha;'>Ha!</root>


In particular, rather than causing an out of memory error, it seems to have
hung Xerces, possibly pushing it into an infinite loop.

So here's my thought. In addition to setting a maximum memory size on
documents, I will set a maximum execution *time*. I.e. I'll move the parse
into a separate thread and abort it if it exceeds a configurable amount of
time. The code is something like this:

        Document result = null;
        try {
          ParseJob job = new ParseJob(in);
            Thread t = new Thread(job);
            t.start();
            t.join(60000);
            if (job.getProblem() != null) {
              throw job.getProblem();
            }
            result = handler.getDocument();
        }




    private class ParseJob implements Runnable {

      private InputSource in;
      Exception problem = null;

      ParseJob(InputSource in) {
        this.in = in;
      }

      public void run() {
        try {
          parser.parse(in);
        } catch (Exception throwable) {
          this.problem = throwable;
        }
      }

      Exception getProblem() {
        return problem;
      }
    }

Yes, this would be easier if I weren't trying to maintain compatibility
with Java 1.4.

What do folks think? Am I making any obvious mistakes?


-- 
Elliotte Rusty Harold
elharo AT ibiblio.org