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In a JVM a class is identified by its |
fully-qualified class name AND its classloader. A class with the same name but different package name is different, and a class with the same fully-qualified name but which has been loaded with a different classloader is also different. According to \[1\] §2.8.1, "Two classes are the _same class_ (and therefore the _same type_) if they are loaded by the same class loader and they have the same fully qualified name". |
You may frequently want to know whether a given object has a specific class, or whether 2 objects have the same class, for example, in implementing the equals() method. If the comparison is performed incorrectly, your code might assume that 2 objects are of the same class when they're not.
Depending on the function that this code performs, this might introduce a vulnerability to a mix-and-match attack.An attacker can supply code that is the same name as your class; if you rely on comparing the classname as a string, you might end up believing it is privileged code and granting it undue privileges.
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Conversely, a source of error is assuming that the same codebase will result in the same class in a JVM. While it is usually true in common user applications, it is not the case with J2EE servers like servlet containers, which could use different instances of classloaders to be able to deploy and undeploy applications at runtime without restarting the JVM. In this situation, 2 objects whose classes come from the same codebase could be different classes to the JVM and it could be confusing to get false from the equals() method on their respective classes.
References
- http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html
Wiki Markup [http://www.javaworld.com/javaworld/jw-12-1998/jw-12-securityrules.html?page=4
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] \[Mcgraw and Felten\]
- http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/java.html
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