5.3: Clarify handling of exceptions thrown by ClassLoaders

The ClassLoader.loadClass method allows arbitrary user-defined behavior to occur during loading of classes. Notably, the method can throw any exception type, including RuntimeExceptions and Errors.

5.3 claims that, if any error occurs, "an instance of a subclass of LinkageError" may be thrown. It does not account for the possibility of exceptions that are not LinkageErrors.

The following test confirms that a RuntimeException thrown by ClassLoader.loadClass is exposed directly by class resolution.

~~~

public class Test {
   public static void main(String... args) throws Exception {
       PreemptingClassLoader l = new PreemptingClassLoader("A", "B");
       Class<?> c = l.loadClass("A");
       Runnable r = (Runnable) c.getDeclaredConstructor().newInstance();
       r.run();
   }
}

~~~

public class A implements Runnable {
   public void run() {
       System.out.println(B.foo);
   }
}

~~~

public class B {
   public static String foo = "foo";
}

~~~

import java.util.*;
import java.io.*;

public class PreemptingClassLoader extends ClassLoader {

 private final Set<String> names = new HashSet<>();

 public PreemptingClassLoader(String... names) {
   for (String n : names) this.names.add(n);
 }

 protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
   if (!names.contains(name)) return super.loadClass(name, resolve);
   Class<?> result = findLoadedClass(name);
   if (result == null) {
     if (name.equals("B")) throw new RuntimeException("Hi, mom"); // INSERTED TO TRIGGER EXCEPTION
     String filename = name.replace('.', '/') + ".class";
     try (InputStream data = getResourceAsStream(filename)) {
       if (data == null) throw new ClassNotFoundException();
       try (ByteArrayOutputStream buffer = new ByteArrayOutputStream()) {
         int b;
         do {
           b = data.read();
           if (b >= 0) buffer.write(b);
         } while (b >= 0);
         byte[] bytes = buffer.toByteArray();
         result = defineClass(name, bytes, 0, bytes.length);
       }
     }
     catch (IOException e) {
       throw new ClassNotFoundException("Error reading class file", e);
     }
   }
   if (resolve) resolveClass(result);
   return result;
 }

}