Understanding Why Java ClassLoader Can Load the Same Class Twice Using URLClassLoader скачать в хорошем качестве

Understanding Why Java ClassLoader Can Load the Same Class Twice Using URLClassLoader

Discover the intriguing behavior of Java's `URLClassLoader`—how and why it allows the same class to be loaded multiple times by different class loaders. --- This video is based on the question https://stackoverflow.com/q/75206840/ asked by the user 'Machi' ( https://stackoverflow.com/u/13160706/ ) and on the answer https://stackoverflow.com/a/75207018/ provided by the user 'queeg' ( https://stackoverflow.com/u/4222206/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why this class can be loaded twice by Java classloader? Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding Java ClassLoader's Behavior with Duplicate Classes Java is a powerful, versatile programming language that allows developers to create efficient applications. However, it also presents unique challenges, particularly when it comes to class loading. One such challenge is understanding why a class can be loaded twice by different class loaders, particularly using the URLClassLoader. In this guide, we will delve into this phenomenon, clarify why it occurs, and explore its implications in the world of Java development. The Problem: Duplicate Class Loading You might find yourself puzzled when, upon using a URLClassLoader, you can load the same Java class multiple times. Consider the following Java code snippet: [[See Video to Reveal this Text or Code Snippet]] With this code, we create two distinct instances of URLClassLoader, each loading the same class, example.ToBeLoaded. Surprisingly, checking child1 == child2 returns false, as does child2.equals(child1). The ClassLoader Hierarchy and Parent Delegation Principle At first glance, it might seem illogical for the Java ClassLoader to violate the parent delegation principle, where a class should ideally be loaded once by its parent class loader. However, this behavior is not only intentional but also essential in certain contexts, particularly in Java EE (JEE) applications. The Reason: Contextual Class Loading 1. Java EE Applications In JEE containers, it is common for different applications to access the same class but potentially require different versions of that class. For example, one application might depend on mypackage.MyClass version 1, while another might rely on version 2. Having each application load its own version allows for this flexibility and maintains compatibility without conflicts. 2. Memory and Class Distinction Each class loader is responsible for its own namespace. Since two instances of URLClassLoader can be created for the same jar file, they will load their respective classes into memory as distinct instances. Even if the class names are identical, they are from different loaders, thereby making them distinct types. Example Scenario To visualize this better, think of how you’d handle separate product versions in diverse applications: Application A: Uses mypackage.MyClass, version 1.0 Application B: Uses mypackage.MyClass, version 2.0 By maintaining different class loaders for these applications, Java ensures they do not interfere with each other, preventing errors and maintaining application integrity. Conclusion: Comparing Classes Understanding this behavior of Java's class loading system emphasizes the importance of considering not only class names but also their class loaders when checking if two classes are equivalent. The JVM does not typically check class content at runtime for equality, which is crucial to understanding why the same class can exist in memory multiple times. Key Takeaways: ClassLoaders Are Isolated: Each class loader functions independently, allowing the same class to be loaded multiple times under different versions. Parent Delegation is Contextual: While the principle exists, certain environments deliberately create conditions where classes can be loaded multiple times. Class Comparison: Always take the class loader into account when deciding if two classes are the same. Understanding these concepts will enhance your Java development skills and help you embrace the complexities of Java's class loading mechanism.