View source for FAQ Why doesn't Eclipse play well with Xerces?

Many plug-ins in the Eclipse Platform require an XML parser for reading and storing
various data in XML format.  In particular, the platform uses an XML parser to
read each plug-in&#146;s <tt>plugin.xml</tt> file during start-up.
Because the JDK before 1.4 did not provide an XML 
parser, Eclipse used to ship with a plug-in containing a Xerces parser.  Xerces is one of the
two XML parser implementations maintained by the Apache Project.


The first problems began to appear when users tried to start Eclipse using
a JDK that also contained an implementation of Xerces. Prior to 
JDK 1.4, it was common practice to throw a copy of Xerces or Xalan 
into the JDK&#146;s 
<tt>ext</tt> directory so that it could be used by all applications.
Thus, two copies of Xerces were available when Eclipse was starting up,
one in the <tt>ext</tt> directory and one in the <tt>org.apache.xerces</tt>
plug-in.  Because libraries provided by the JDK always appear at the beginning of
the runtime classpath, the one in the JDK is always found first.
If this copy of Xerces was slightly different from the one the platform expected,
various linkage errors or <tt>ClassCastException</tt>s occurred on start-up,
often preventing Eclipse from starting up at all.  The workaround in this case
was pretty straightforward: Omit the <tt>ext</tt> directory from
the classpath when starting Eclipse:
<pre>
   eclipse -vmargs -Djava.ext.dirs=
</pre>


The situation became worse with JDK 1.4, which added a specification
for XML called Java API for XML Processing (JAXP).  The exact parser 
implementation was left unspecified, so each JDK was free to choose 
its own, as long as it was compliant with the interfaces defined by JAXP.
Sun decided to use the Apache Crimson parser, and IBM went with the 
Apache Xerces parser.  Worst of all, because these packages were now
part of the standard JDK libraries, there was no easy workaround as there
has been for the <tt>ext</tt> problem.  The JDK people, realizing that 
they had messed up by bundling such widely
used packages in their JDKs, thereby breaking any application that used slightly
different versions of those packages, plan to prefix
the implementation package names with a unique prefix to prevent
these collisions in the future.  However, it was too late to fix this problem for JDK 1.4.  The bottom
line: You cannot use Eclipse 2.1 or earlier with an IBM 1.4 VM.


For Eclipse 3.0, the problem was solved by tossing out the Xerces plug-in
and simply using JAXP for any XML processing.  Now at most one
Xerces is on the classpath, so there cannot be any collisions.  Of course, this
means that you need at least JDK 1.4 to run Eclipse 3.0.

Update March 29/2006:  WTP ships its own '''org.apache.xerces''' plugin because it needs to use the schema validation capability from Xerces. The Sun JRE is one of the JREs that WTP supports. It comes with the Crimson XML parser, which does not offer the schema validation capability that WTP is looking for. As a result, WTP needs to package Xerces explicitly.

<hr><font size=-2>This FAQ was originally published in [http://www.eclipsefaq.org Official Eclipse 3.0 FAQs]. Copyright 2004, Pearson Education, Inc. All rights reserved. This text is made available here under the terms of the [http://www.eclipse.org/legal/epl-v10.html Eclipse Public License v1.0].</font>