Summary

Specify MethodHandles::lookup to throw IllegalCallerException if invoked from JNI attached thread with no caller frame

Problem

The MethodHandles::lookup method is a caller sensitive method but does not specify what should happen when there are no stack frames to determine the caller. A NullPointerException results which doesn't provide much diagnostic information. If a call is made from JNI attached thread, the thread will have no call frame to determine the caller.

Solution

When there is no caller frame on the stack, simply throw an IllegalCallerException to provide better diagnostics.

Specification

The spec of java.lang.invoke.MethodHandles::lookup is updated as follows:

@@ -107,18 +107,29 @@ * This lookup object is a capability which may be delegated to trusted agents. * Do not store it in place where untrusted code can access it. *

* This method is caller sensitive, which means that it may return different * values to different callers. + * In cases where {@code MethodHandles.lookup} is called from a context where + * there is no caller frame on the stack (e.g. when called directly + * from a JNI attached thread), {@code IllegalCallerException} is thrown. + * To obtain a {@link Lookup lookup object} in such a context, use an auxiliary class that will + * implicitly be identified as the caller, or use {@link MethodHandles#publicLookup()} + * to obtain a low-privileged lookup instead. * @return a lookup object for the caller of this method, with * {@linkplain Lookup#ORIGINAL original} and * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}. + * @throws IllegalCallerException if there is no caller frame on the stack. */