jdk.internal.loader.ClassLoaders$AppClassLoader incompatible with java.net.URLClassLoader (j8 - j11)

Question

I created a custom class to add urls into a class loader dynamically.

   void addURL(URL url) {
         try {
             Class<?>[] parameters = new Class[]{URL.class};
             Class<?> sysclass = URLClassLoader.class;
             Method method = sysclass.getDeclaredMethod("addURL", parameters);
             method.setAccessible(true);

             URLClassLoader sysloader = (URLClassLoader) (getClass().getClassLoader());
             while (sysloader != null) {
                 method.invoke(sysloader, url);
                 sysloader = (URLClassLoader) sysloader.getParent();
             }
         }catch (Exception e) {
             e.printStackTrace();
         }
          }

And then I'm trying to add the url with the following method

String filePath = "jar path";

File file = new File(filePath);
URL url = file.toURI().toURL();
    CustomClassLoader customURLClassLoader = new CustomClassLoader();
    customURLClassLoader.addURL(url);

I'm getting the following exception

java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')

Does this answer your question? Java Class cast Exception - Spring boot — juanlumn
– juanlumn, Commented May 30, 2023 at 14:39
You're doing something which is not valid anymore in Java 9 and higher. The type of the system classloader changed, so you can't just cast to URLClassLoader and expect it to work. — Mark Rotteveel
– Mark Rotteveel, Commented May 30, 2023 at 16:24
@MarkRotteveel to be more precise, this has never been valid at all. It just happened to work in a specific implementation. — Holger
– Holger, Commented May 31, 2023 at 9:49
Since this question has been tagged with “agent” and “instrumentation”, an explanation is needed why you don’t use the Instrumentation API but resort to this hack. — Holger
– Holger, Commented May 31, 2023 at 9:50
If you use an agent, then just call Instrumentation.appendToSystemClassLoaderSearch. — Johannes Kuhn
– Johannes Kuhn, Commented May 31, 2023 at 11:39

Johannes Kuhn · Accepted Answer · 2023-05-31 16:31:01Z

1

You you have tagged this question with both agent and instrumentation - so you may actually be interested in adding a jar to the system class loader from a javaagent.

The Instrumentation interface already has a supported method to do exactly that: Instumentation.appendToSystemClassLoaderSearch.

By using this supported method you avoid all the problems that the reflective solution has - as this method is part of Java, it will be updated to account for any internal changes - preserving its functionality.

answered May 31, 2023 at 16:31

Johannes Kuhn

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