I want to lay the groundwork on which we can start a discussion about dynamic and modular software systems. Class loaders may seem to be a dry topic, but I think it is one of the topics that separate the junior from the senior software engineer, so bear
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\"Why should I deal with multiple class loaders and their limitations and problems?\" a simple servlet or JSP program and deploy within a servlet container, your code is loaded by your very own class loader, preventing you from accessing other web applications\' classes. In addition, many \"container-type\" applications such as J2EE servers, web containers, NetBeans, and others are using custom class loaders in order to limit the impact of classes provided by a component, and thus will have an impact on the developer of such components.one class loaded in a particular JVM. Additional class loaders enable a developer to partition the JVM
First we need to explain some definitions:
CL: Class loader.
Initial CL: The CL that initiated the loading of the class.
Effective CL: The CL that actually loaded the class.
Class type: The fully qualified class name (package plus class name).
Class: A combination of the class type and effective class loader.
java.lang.Class: A class in the JDK that represents a class (name, fields, methods, etc.).
Symbolic Link: A class type used within the source code, such as superclasses, extended interfaces, variables, parameters, return values, instanceofs, and upcasts.
Class loaders and their usage follow a few simple rules:
* Class loaders are hierarchically organized, where each one has a parent class loader, except the bootstrap class loader (the root).
* Class loaders should (practically: must) delegate the loading of a class to the parent, but a custom class loader can define for itself when it should do so.
* A class is defined by its class type and the effective class loader.
* A class is only loaded once and then cached in the class loader to ensure that the byte code cannot change.
* Any symbolic links are loaded by the effective class loader (or one of its ancestors), if this is not already done. The JVM can defer this resolution until the class is actually used.
* An upcast of an instance to another class fails when the class of the instance and the class of the symbolic link do not match (meaning their class loaders do not match).
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