Starting with Java 1.4 a copy of JAXP APIs and an XML parser are packed inside the JRE. This has impacts on applications that wish to use their own XML parser.
In old versions of Tomcat, you could simply replace the XML parser in the Tomcat libraries directory to change the parser used by all web applications. However, this technique will not be effective when you are running modern versions of Java, because the usual class loader delegation process will always choose the implementation inside the JDK in preference to this one.
Java supports a mechanism called the “Endorsed Standards Override Mechanism” to allow replacement of APIs created outside of the JCP (i.e. DOM and SAX from W3C). It can also be used to update the XML parser implementation. For more information, see: http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html.
Tomcat utilizes this mechanism by including the system property setting -Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS in the command line that starts the container. The default value of this option is $CATALINA_HOME/endorsed. This endorsed directory is not created by default.
