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javaclassloader

Obtain the bytes of a class loaded by the boostrap class loader?

I would like to know if it is possible to get the bytes of a class loaded by the bootstrap class loader. Everywhere I look, people suggest to get the bytes of an arbitrary class, one must simply do

Class.forName(className).getClassLoader().getResourceAsStream(className.replace('.', '/') + ".class");

And then read the InputStream with a method of your choice, however this is not possible on classes loaded by the bootstrap class loader (such as java.lang.Object), as for those classes Class#getClassLoader() returns null.

> Class.forName("java.lang.Object").getClassLoader()

null

Is this even possible? I've heard it can be done with instrumentation but I'm hoping there's a "better" way to do it (outside of locating rt.jar in the Java installation and reading the class data from there or something)

Solution

  • I've figured it out. You just have to get the system class loader from ClassLoader#getSystemClassLoader().

    ClassLoader loader = Class.forName(className).getClassLoader();
if (loader == null) loader = ClassLoader.getSystemClassLoader();
final URL url = loader.getResourceAsStream(className.replace('.', '/') + ".class");
// read...