如何使反射在 JDK 16 及更高版本上工作？

Question

I have the following legacy code that I migrated to Java 16 but, due to the strong encapsulation introduced by this new version, it doesn't work:

try {
    Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
    method.setAccessible(true);
    method.invoke(new URLClassLoader(
        new URL[] {}),
        new File("C:/external-folder/my.jar").toURI().toURL()
    );
} catch (Exception exc) {
    exc.printStackTrace();
}

Is there a way to make it work?

Answer 1

The problem is not that reflection "doesn't work"; it is that reflection is finally enforcing more of the accessibility model that the compiler and runtime have always enforced. URLClassLoader::addUrl is intended only for subclasses; it is not intended to be accessed from outside the implementation, which is what you're doing. Over time, starting with Java 9 and continuing in later versions (including 17), access restrictions are increasingly recognized by reflection, with warnings, to give broken code a chance to migrate to something supportable.

The code in question only really ever worked accidentally; it depended on being able to break into an unsupported interface. Using setAccessible should be a clue. Sure, you can get into locked houses by breaking windows, but if you have to break a window (and it's not your house), you should be aware of where the problem lies.

Look at it as glass-half-full; this accidentally-working code worked for a long time. But the bill has come due; it is time to fix your code.

Answer 2

The code is very weird. At first glance, I thought it used reflection to access URLClassLoader internals to add myJar to an existing class loader , but instead it uses it to add it to a new class loader . There's no reason to do that - you can just use the URLClassLoader constructor for that.

It should look something like this (untested because I am away from an IDE):

URL jar = new File("C:/external-folder/my.jar").toURI().toURL();
URL[] urls = { jar };
new URLClassLoader(urls);