2009/5/27 Jon Pearson <jon.pear...@sixnet.com>: >> -----Original Message----- >> From: Konstantin Kolinko [mailto:knst.koli...@gmail.com] >> Sent: Tuesday, May 26, 2009 4:44 PM >> To: Tomcat Users List >> Subject: Re: Classloaders >> >> 1. What tomcat version? > > Tomcat v. 6.0.18, JVM 1.6.0_13, Ubuntu 9.04 on kernel 2.6.28-11 >
OK >> 2. The documentation on classloaders is here: >> http://tomcat.apache.org/tomcat-6.0-doc/class-loader-howto.html > > Thanks, but I've read through that a few times. It describes the > existing classloader layout but does not describe how to avoid problems > when using your own within Tomcat. > Well, at least you should cooperate with this layout. ;) Some notes: The class loader of a web application can be retrieved as Thread#getContextClassLoader() Also, if you run in development mode (see Jasper configuration), each JSP is executed with its own class loader, so that they can be reloaded. http://tomcat.apache.org/tomcat-6.0-doc/jasper-howto.html >> 3. Some JDBC drivers use native code. Those cannot be loaded from >> different class loaders in the same time. Thus, they cannot be loaded >> from /WEB-INF/lib, as there will be conflicts between applications, >> or between several instances of the same application if it is >> restarted. > > This one does not include any native code, it is self-contained in a > single JAR file. > >> 4. Put the jar file into WEB-INF/lib, and then restart your >> web application. >> (E.g., using Tomcat Manager). Your jar library will be loaded. >> http://tomcat.apache.org/tomcat-6.0-doc/html-manager-howto.html >> http://tomcat.apache.org/tomcat-6.0-doc/manager-howto.html > > It is more of a plugin than a library. The location is user-specified, a > directory to watch for plugin JARs. Each JAR contains metadata in the > manifest describing the main class of the plugin. That main class is > loaded using Class.forName(), causing a static{} block in the class to > run, performing any registrations necessary to integrate with the > application. > > When the class is needed (i.e. when I try to connect to a database), a > new instance is created. That involves calling Class.forName() Do you pass a ClassLoader reference to that forName() call? What is classloader hierarchy for the ClassLoader used by that call? See http://java.sun.com/javase/6/docs/api/java/lang/Class.html#forName(java.lang.String) http://java.sun.com/javase/6/docs/api/java/lang/Class.html#forName(java.lang.String,%20boolean,%20java.lang.ClassLoader) > to load > the PostgreSQL database driver (a class called 'org.postgresql.Driver'), > contained in postgresql-8.3-604.jdbc4.jar (present in WEB-INF/lib). But > that call to Class.forName() to load org.postgresql.Driver fails because > the class cannot be found. > > This is confounding because an ancestor classloader of my URLClassLoader > that made the classes in my plugin JAR available should have access to > org.postgresql.Driver. In fact, one of them *must* because when the > classes in my plugin JAR are placed into WEB-INF/classes in their raw > .class state (not packaged into a jar), everything works. > It might be. What happens, step-by-step, in this case? Additional classes will be picked up by web-app classloader instantly. Additional JARs - only upon restart of the web application, because it requires reconfiguration of the class loader (adds additional URLs to scan for classes). Best regards, Konstantin Kolinko --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.org
