Summary

Specify ClassLoader::registerAsParallelCapable to throw IllegalCallerException if invoked with no caller frame or a class that is not a ClassLoader.

Problem

The ClassLoader::registerAsParallelCapable method is a caller sensitive method but does not specify what should happen when there are no stack frames to determine the caller or if the caller is not a ClassLoader. A NullPointerException results from the case where there is no caller frame. A ClassCastException results from the case where the caller is not a ClassLoader. These don't provide much diagnostic information.

Solution

When there is no caller frame on the stack or the caller class is not a ClassLoader, simply throw an IllegalCallerException to provide better diagnostics.

Specification

The spec of java.lang.ClassLoader.registerAsParallelCapable is updated as follows:

 @@ -1606,9 +1606,16 @@ public abstract class ClassLoader {
      * </ol>
      * <p>Note that once a class loader is registered as parallel capable, there
      * is no way to change it back.</p>
+     * <p>
+     * In cases where this method is called from a context where the caller is
+     * not a subclass of {@code ClassLoader} or there is no caller frame on the
+     * stack (e.g. when called directly from a JNI attached thread),
+     * {@code IllegalCallerException} is thrown.
+     * </p>
      *
      * @return  {@code true} if the caller is successfully registered as
      *          parallel capable and {@code false} if otherwise.
+     * @throws IllegalCallerException if the caller is not a subclass of {@code ClassLoader}
      *
      * @see #isRegisteredAsParallelCapable()
      *