Support exists for downloading, parsing, and loading the English
version of wikipedia (enwiki).

The build file can automatically try to download the most current
enwiki dataset (pages-articles.xml.bz2) from the "latest" directory,
http://download.wikimedia.org/enwiki/latest/. However, this file
doesn't always exist, depending on where wikipedia is in the dump
process and whether prior dumps have succeeded. If this file doesn't
exist, you can sometimes find an older or in progress version by
looking in the dated directories under
http://download.wikimedia.org/enwiki/. For example, as of this
writing, there is a page file in
http://download.wikimedia.org/enwiki/20070402/. You can download this
file manually and put it in temp. Note that the file you download will
probably have the date in the name, e.g.,
http://download.wikimedia.org/enwiki/20070402/enwiki-20070402-pages-articles.xml.bz2. When
you put it in temp, rename it to enwiki-latest-pages-articles.xml.bz2.

After that, ant enwiki should process the data set and run a load
test. Ant targets get-enwiki, expand-enwiki, and extract-enwiki can
also be used to download, decompress, and extract (to individual files
in work/enwiki) the dataset, respectively.

NOTE: This bug in Xerces:

  https://issues.apache.org/jira/browse/XERCESJ-1257

which is still present as of 2.9.1, causes an exception like this when
processing Wikipedia's XML:

Caused by: org.apache.xerces.impl.io.MalformedByteSequenceException: Invalid byte 2 of 4-byte UTF-8 sequence.
	at org.apache.xerces.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
	at org.apache.xerces.util.ErrorHandlerWrapper.fatalError(Unknown Source)
	at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
	at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.lucene.benchmark.byTask.feeds.EnwikiDocMaker$Parser.run(EnwikiDocMaker.java:77)
	... 1 more

The original poster in the Xerces bug provided this patch:

--- UTF8Reader.java	2006-11-23 00:36:53.000000000 +0100
+++ /home/rainman/lucene/xerces-2_9_0/src/org/apache/xerces/impl/io/UTF8Reader.java	2008-04-04 00:40:58.000000000 +0200
@@ -534,6 +534,16 @@
                     invalidByte(4, 4, b2);
                 }
 
+                // check if output buffer is large enough to hold 2 surrogate chars
+                if( out + 1 >= offset + length ){
+                    fBuffer[0] = (byte)b0;
+                    fBuffer[1] = (byte)b1;
+                    fBuffer[2] = (byte)b2;
+                    fBuffer[3] = (byte)b3;
+                    fOffset = 4;
+                    return out - offset;
+		}
+
                 // decode bytes into surrogate characters
                 int uuuuu = ((b0 << 2) & 0x001C) | ((b1 >> 4) & 0x0003);
                 if (uuuuu > 0x10) {

which I've applied to Xerces 2.9.1 sources, and committed under
lib/xerces-2.9.1-patched-XERCESJ-1257.jar.  Once XERCESJ-1257 is fixed
we can upgrade to a standard Xerces release.