Use of VMSecurityManager.getClassContext()

[Andrew Haley]

David Holmes writes:
 > I filed a documentation bug against this method because it implied that it
 > was only called by SecurityManager.getClassContext() when in fact it is also
 > calld by Class and ClassLoader directly. This makes it awkward for
 > VMSecurityManager.getClassContext() to know how many stack frames it needs
 > to pop to return the "true" caller.
 > 
 > But looking at how Class and ClassLoader use this method we have code of the
 > form:
 > 
 >      Class c = VMSecurityManager.getClassContext()[1];
 >      ClassLoader cl = c.getClassLoader();
 > 
 > getClassContext() is only being used to find the class of the immediate
 > caller. This is horrendously inefficient in terms of time and space. Might I
 > suggest that a method be added to Class/VMClass or VMClassLoader or
 > VMSecurityManager to simply return a single Class from the current calling
 > context:
 > 
 >    /**
 >     * Return the Class of the method at the given position on the call
 > stack. This is
 >     * logically equivalent to
 > <tt>VMSecurityManager.getClassContext()[pos]</tt> but does
 >     * not need to traverse the whole call stack nor construct an array.
 >     * @param pos the position in the callstack: 0 is the caller of this
 > method, 1 the
 >     * caller of that method, etc.
 >     * @return the Class of the method at the given position in the callstack
 >     * @throws IllegalArgumentException if <tt>pos</tt> is negative or
 > greater than the
 >     * depth of the call stack.
 >     */
 >    static native Class getClassFromContext(int pos);
 > 
 > That also simplifies VMSecurityManager.getClassContext() because it now
 > knows that its caller is always SecurityManager.getClassContext().

This seems like a reasonable idea.  In several cases we need to say
"who is our caller?" and some VM assistance is useful here.

Andrew.