RE: Classes in tomcat\server\lib folder sometimes are visible to web application
Hi Chuck, Thank you very much for your feedback. Your comments about reflection were right on the money and have helped me resolve the underlying issue. It turns out that my initial assertion about our server/lib classes being visible to our web app was incorrect -- it was just that our custom classloader was SOMETIMES loading these classes from a different location, and therefore no error was reported. This situation was the result of other problems we had, but your comments have helped me develop a much cleaner solution. In order to help others, here is our resolution: Background: We have a simple custom ClassLoader that uses a number of helper classes to determine which jars on our system should be loaded from some well-known location (instead of just using jars in the WEB-INF/lib folder). We bundle the custom ClassLoader and the helper classes in a jar together. The custom ClassLoader obviously needs to be loaded by Tomcat, and the helper classes are used not only by our custom ClassLoader, but by our web app as well. First attempt: Since the helper classes are also used by the web-app itself, our initial plan was to place this jar in common/lib. However, this results in Tomcat errors because the custom class loader can't exist in common/lib because it extends WebappClassLoader, which exists in server/lib and is not visible to common/lib. Second attempt: So we moved the combined jar to server/lib. Tomcat now uses the custom ClassLoader fine, but our web app can't see the helper classes, because they are in the Tomcat-private shared/lib folder. Third attempt: We place a duplicate of the combined jar in the well-known location for our custom ClassLoader to find. For the most part, this works okay, but it is very klugey, sometimes leads to errors, and is hard to debug. This is because we now have the same helper classes being loaded twice by different ClassLoaders. Final solution: Break the custom ClassLoader and helper classes into two jars. The custom ClassLoader goes in server/lib, and the helper classes go in common/lib. We now no longer need to duplicate the jars -- all classes are loaded by only one ClassLoader. And the one part of our system where the wep-abb needs to call a custom ClassLoader-specific method is handled by reflection rather than by casting the ClassLoader and making a direct call. Once again, thanks for spending the time to respond so thoroughly and accurately. Regards, Gary -Original Message- From: Caldarale, Charles R [mailto:[EMAIL PROTECTED] Sent: Friday, October 17, 2008 11:27 AM To: Tomcat Users List Subject: RE: Classes in tomcat\server\lib folder sometimes are visible to web application From: Gary Hirschhorn [mailto:[EMAIL PROTECTED] Subject: RE: Classes in tomcat\server\lib folder sometimes are visible to web application The Catalina ClassLoader in server/lib creates Webapp1 ClassLoader specifically for webapp1. It's actually org.apache.catalina.loader.WebappLoader that creates each webapp's classloader, but the result is the same as you surmised. the only way for this to work in practice is that these specific implementations are referenced in Webapp1 as Interfaces which are defined NOT in server/lib, but rather by jars higher up in hierarchy, such as common/lib. Essentially correct; in addition to Interface classes, there are also abstract classes in the top-level jars. a) if my webapp code has any specific reference (e.g. the Class name is in my code as a variable or parameter type) to a class in server/lib, I should get a ClassDefNotFoundError. Correct. b) however, if my webapp has a reference to an Interface defined in common/lib and at runtime is handed a Class that implements this class, I may not get a ClassDefNotFoundError. Not just may not, but should not. Be aware that Tomcat uses a fair amount of reflection to avoid having direct references at compile and load time; depending on what your code needs, you may have to do something similar. - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers. - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
RE: Classes in tomcat\server\lib folder sometimes are visible to web application
Thank you for your quick reply. I think I understand the distinction you make between a class being available and a class being loaded, although it is a bit confusing. I suspect that if I understand this clearly, it will help me track down the cause of my problem. Given the Tomcat ClassLoader hierarchy diagram in the documentation and your clarification below, is the following correct? The Catalina ClassLoader in server/lib creates Webapp1 ClassLoader specifically for webapp1. The parent classloader of Webapp1 ClassLoader is NOT set to Catalina ClassLoader but rather to Shared ClassLoader, so requests from webapp1 to load classes are never delegated to Catalina ClassLoader. Thus a reference to a Class in server/lib in Webapp1 will result in a ClassDefNotFound error. However, the classes in sever/lib ARE aware of WebApp1, and may call methods in Webapp1 and pass in references to themselves. When this happens, Webapp1 is now working with an implementation of a class in server/lib. I assume, however, that the only way for this to work in practice is that these specific implementations are referenced in Webapp1 as Interfaces which are defined NOT in server/lib, but rather by jars higher up in hierarchy, such as common/lib. If this understanding is true, then a) if my webapp code has any specific reference (e.g. the Class name is in my code as a variable or parameter type) to a class in server/lib, I should get a ClassDefNotFoundError. b) however, if my webapp has a reference to an Interface defined in common/lib and at runtime is handed a Class that implements this class, I may not get a ClassDefNotFoundError. Regards, Gary -Original Message- From: Caldarale, Charles R [mailto:[EMAIL PROTECTED] Sent: Thursday, October 16, 2008 6:55 PM To: Tomcat Users List Subject: RE: Classes in tomcat\server\lib folder sometimes are visible to web application From: Gary Hirschhorn [mailto:[EMAIL PROTECTED] Subject: Classes in tomcat\server\lib folder sometimes are visible to web application According to this documentation, classes in this jar should be TOTALLY invisible to web applications. That's not quite the case. Classes outside of a given classloader's branch can not be *loaded* by that classloader. However, if references to instances of such outside classes are made available to code from the classloader branch of interest, that code can use those objects. Nearly all of Tomcat's code, including the classloaders, request/response wrappers, servlet API handlers, etc., are in server/lib, yet are obviously used by application servlet code. So I understand why it does not work (class should not be visible), but why does it ever work? Without seeing the code, it's difficult to even speculate. - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers. - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
RE: Classes in tomcat\server\lib folder sometimes are visible to web application
From: Gary Hirschhorn [mailto:[EMAIL PROTECTED] Subject: RE: Classes in tomcat\server\lib folder sometimes are visible to web application The Catalina ClassLoader in server/lib creates Webapp1 ClassLoader specifically for webapp1. It's actually org.apache.catalina.loader.WebappLoader that creates each webapp's classloader, but the result is the same as you surmised. the only way for this to work in practice is that these specific implementations are referenced in Webapp1 as Interfaces which are defined NOT in server/lib, but rather by jars higher up in hierarchy, such as common/lib. Essentially correct; in addition to Interface classes, there are also abstract classes in the top-level jars. a) if my webapp code has any specific reference (e.g. the Class name is in my code as a variable or parameter type) to a class in server/lib, I should get a ClassDefNotFoundError. Correct. b) however, if my webapp has a reference to an Interface defined in common/lib and at runtime is handed a Class that implements this class, I may not get a ClassDefNotFoundError. Not just may not, but should not. Be aware that Tomcat uses a fair amount of reflection to avoid having direct references at compile and load time; depending on what your code needs, you may have to do something similar. - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers. - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
RE: Classes in tomcat\server\lib folder sometimes are visible to web application
From: Gary Hirschhorn [mailto:[EMAIL PROTECTED] Subject: Classes in tomcat\server\lib folder sometimes are visible to web application According to this documentation, classes in this jar should be TOTALLY invisible to web applications. That's not quite the case. Classes outside of a given classloader's branch can not be *loaded* by that classloader. However, if references to instances of such outside classes are made available to code from the classloader branch of interest, that code can use those objects. Nearly all of Tomcat's code, including the classloaders, request/response wrappers, servlet API handlers, etc., are in server/lib, yet are obviously used by application servlet code. So I understand why it does not work (class should not be visible), but why does it ever work? Without seeing the code, it's difficult to even speculate. - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers. - To start a new topic, e-mail: users@tomcat.apache.org To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
