src/doc/tomcat-ejbclasses.xml - openejb - Git at Google

 <?xml version="1.0"?>
 <document>
   <properties>
     <title>Webapp libraries and EJB libraries</title>
   </properties>
   <body>

 <section title="The Rule">
 <p>
 Never ever put your EJBs in your WEB-INF directories.  The same rule applies
 to any libraries needed by your EJBs.
 </p>
 </section>

 <section title="The Consequences">
 <p>
 If you break the rule, you will be riddled with ClassCastExceptions like the following:
 </p>
 <PRE>
 java.lang.ClassCastException
     at org.apache.jsp.test$jsp._jspService(test$jsp.java:102)
     at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
     ...
 </PRE>
 <PRE>
 java.lang.ClassCastException
     at com.sun.corba.se.internal.javax.rmi.PortableRemoteObject.narrow(PortableRemoteObject.java:293)
     ...
 </PRE>
 </section>

 <section title="Java Classloading Rules">
 <p>
 By rule of the Java Language, two classes are only the same if they have the same type <B>AND</B> they were loaded by the same classloader!
 </p>
 </section>
 <section title="Webapp Classloader vs. Common Classloader">
 <p>
 In Tomcat, each webapp has it's very own classloader, called it's <A HREF="http://jakarta.apache.org/tomcat/tomcat-4.1-doc/class-loader-howto.html#Class%20Loader%20Definitions">Webapp Classloader</A>.
 </p>
 <p>
 When OpenEJB is loaded into Tomcat, it is loaded into Tomcat's <A HREF="http://jakarta.apache.org/tomcat/tomcat-4.1-doc/class-loader-howto.html#Class%20Loader%20Definitions">Common Classloader</A>.
 Classes in the Common Classloader are automatically available to all the Webapp classloaders.
 Having OpenEJB in the Common Classloader makes OpenEJB and your EJBs available to all webapps.
 </p>
 <p>
 There is an important thing to know about the Webapp Classloader; it will always load classes from the WEB-INF/classes or WEB-INF/lib directories even if the same class is available in the Common Classloader.
 </p>
 <p>
 Remember the Java classloading rules?  If your webapp loads a class file from the Webapp Classloader and OpenEJB loads the same class file from the Common Classloader, are they the same class? NO, definitly not!
 </p>
 </section>

 <section title="The Rule Revisited">
 <p>
 This brings us back to our rule: never ever put your EJBs in your WEB-INF directories.  The same rule applies
 to any libraries needed by your EJBs.
 </p>
 <p>
 OpenEJB creates instances and implementations of the EJBHome and EJBObject using classes it finds in
 it's classloader (the Common Classloader).  When your Servlet or JSP looks up an EJB, OpenEJB will
 generate an implementation of that bean's EJBHome interface and send it to the servlet.
 </p>
 <p>
 If your servlet's classloader (the Webapp Classloader) already loaded the class into
 it's classloader, you have a situation where the exact same class
 exists in two different classloaders.  When
 you try to cast one classloader's version to another classloader's
 version, everything blows up.
 </p>
 </section>
   </body>
 </document>
	<?xml version="1.0"?>
	<document>
	<properties>
	<title>Webapp libraries and EJB libraries</title>
	</properties>
	<body>

	<section title="The Rule">
	<p>
	Never ever put your EJBs in your WEB-INF directories. The same rule applies
	to any libraries needed by your EJBs.
	</p>
	</section>

	<section title="The Consequences">
	<p>
	If you break the rule, you will be riddled with ClassCastExceptions like the following:
	</p>
	<PRE>
	java.lang.ClassCastException
	at org.apache.jsp.test$jsp._jspService(test$jsp.java:102)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
	...
	</PRE>
	<PRE>
	java.lang.ClassCastException
	at com.sun.corba.se.internal.javax.rmi.PortableRemoteObject.narrow(PortableRemoteObject.java:293)
	...
	</PRE>
	</section>

	<section title="Java Classloading Rules">
	<p>
	By rule of the Java Language, two classes are only the same if they have the same type <B>AND</B> they were loaded by the same classloader!
	</p>
	</section>
	<section title="Webapp Classloader vs. Common Classloader">
	<p>
	In Tomcat, each webapp has it's very own classloader, called it's <A HREF="http://jakarta.apache.org/tomcat/tomcat-4.1-doc/class-loader-howto.html#Class%20Loader%20Definitions">Webapp Classloader</A>.
	</p>
	<p>
	When OpenEJB is loaded into Tomcat, it is loaded into Tomcat's <A HREF="http://jakarta.apache.org/tomcat/tomcat-4.1-doc/class-loader-howto.html#Class%20Loader%20Definitions">Common Classloader</A>.
	Classes in the Common Classloader are automatically available to all the Webapp classloaders.
	Having OpenEJB in the Common Classloader makes OpenEJB and your EJBs available to all webapps.
	</p>
	<p>
	There is an important thing to know about the Webapp Classloader; it will always load classes from the WEB-INF/classes or WEB-INF/lib directories even if the same class is available in the Common Classloader.
	</p>
	<p>
	Remember the Java classloading rules? If your webapp loads a class file from the Webapp Classloader and OpenEJB loads the same class file from the Common Classloader, are they the same class? NO, definitly not!
	</p>
	</section>

	<section title="The Rule Revisited">
	<p>
	This brings us back to our rule: never ever put your EJBs in your WEB-INF directories. The same rule applies
	to any libraries needed by your EJBs.
	</p>
	<p>
	OpenEJB creates instances and implementations of the EJBHome and EJBObject using classes it finds in
	it's classloader (the Common Classloader). When your Servlet or JSP looks up an EJB, OpenEJB will
	generate an implementation of that bean's EJBHome interface and send it to the servlet.
	</p>
	<p>
	If your servlet's classloader (the Webapp Classloader) already loaded the class into
	it's classloader, you have a situation where the exact same class
	exists in two different classloaders. When
	you try to cast one classloader's version to another classloader's
	version, everything blows up.
	</p>
	</section>
	</body>
	</document>