How to Obtain the Bytes of a Class Loaded by the Bootstrap Class Loader in Java

Описание: Discover how to retrieve the bytecode of classes loaded by the bootstrap class loader in Java, bypassing common limitations and enhancing your coding toolkit.
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This video is based on the question https://stackoverflow.com/q/65852376/ asked by the user 'Moiré' ( https://stackoverflow.com/u/10638556/ ) and on the answer https://stackoverflow.com/a/67644676/ provided by the user 'Moiré' ( https://stackoverflow.com/u/10638556/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Class Loading in Java

Java's class loading mechanism is a powerful feature that is crucial for running Java applications. When you load classes in Java, different class loaders handle this task. One significant class loader is the bootstrap class loader. It is primarily responsible for loading core Java classes, such as java.lang.Object. However, when it comes to obtaining the bytecode of classes loaded by this loader, developers encounter a common problem: the bootstrap class loader returns null for ClassLoader.

The Problem: Accessing Class Bytes

While many resources recommend using the following code to retrieve class bytes, it becomes problematic with classes loaded by the bootstrap class loader:

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When attempting to get the class loader of a core class, such as java.lang.Object, you will notice:

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This means that standard approaches do not work for classes loaded by the bootstrap class loader. So how can we access their bytecode?

The Solution: Utilizing the System Class Loader

Fortunately, there is a solution to this problem! By leveraging the system class loader, we can successfully access the bytecode of the class. Below is a step-by-step breakdown of how to implement this:

Step 1: Retrieve the Class Loader

You first attempt to get the regular class loader. If it is null, you fall back to the system class loader.

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Step 2: Access the Resource

Next, you need to obtain the input stream of the class file. This is done by using the resource path:

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Step 3: Read Class Bytes

With the InputStream now available, you can read the class bytes using your preferred method. Here’s a simple way to convert the input stream into a byte array:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Obtaining the bytes of a class loaded by the bootstrap class loader in Java is not as straightforward as other classes. By using the system class loader to access the class's bytecode, you can seamlessly retrieve the necessary data. This approach is effective and avoids more complicated methods, such as manual resource locators.

With this knowledge, you can enhance your Java programming and better understand the inner workings of Java’s class loading system. Happy coding!