1 /*
   2  * Copyright (c) 2008, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.invoke;
  27 
  28 import jdk.internal.ref.CleanerFactory;
  29 import sun.invoke.util.Wrapper;
  30 
  31 import java.lang.invoke.MethodHandles.Lookup;
  32 import java.lang.reflect.Field;
  33 
  34 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  35 import static java.lang.invoke.MethodHandleStatics.TRACE_METHOD_LINKAGE;
  36 import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
  37 
  38 /**
  39  * The JVM interface for the method handles package is all here.
  40  * This is an interface internal and private to an implementation of JSR 292.
  41  * <em>This class is not part of the JSR 292 standard.</em>
  42  * @author jrose
  43  */
  44 class MethodHandleNatives {
  45 
  46     private MethodHandleNatives() { } // static only
  47 
  48     /// MemberName support
  49 
  50     static native void init(MemberName self, Object ref);
  51     static native void expand(MemberName self);
  52     static native MemberName resolve(MemberName self, Class<?> caller) throws LinkageError, ClassNotFoundException;
  53     static native int getMembers(Class<?> defc, String matchName, String matchSig,
  54             int matchFlags, Class<?> caller, int skip, MemberName[] results);
  55 
  56     /// Field layout queries parallel to jdk.internal.misc.Unsafe:
  57     static native long objectFieldOffset(MemberName self);  // e.g., returns vmindex
  58     static native long staticFieldOffset(MemberName self);  // e.g., returns vmindex
  59     static native Object staticFieldBase(MemberName self);  // e.g., returns clazz
  60     static native Object getMemberVMInfo(MemberName self);  // returns {vmindex,vmtarget}
  61 
  62     /// CallSite support
  63 
  64     /** Tell the JVM that we need to change the target of a CallSite. */
  65     static native void setCallSiteTargetNormal(CallSite site, MethodHandle target);
  66     static native void setCallSiteTargetVolatile(CallSite site, MethodHandle target);
  67 
  68     /** Represents a context to track nmethod dependencies on CallSite instance target. */
  69     static class CallSiteContext implements Runnable {
  70         //@Injected JVM_nmethodBucket* vmdependencies;
  71 
  72         static CallSiteContext make(CallSite cs) {
  73             final CallSiteContext newContext = new CallSiteContext();
  74             // CallSite instance is tracked by a Cleanable which clears native
  75             // structures allocated for CallSite context. Though the CallSite can
  76             // become unreachable, its Context is retained by the Cleanable instance
  77             // (which is referenced from Cleaner instance which is referenced from
  78             // CleanerFactory class) until cleanup is performed.
  79             CleanerFactory.cleaner().register(cs, newContext);
  80             return newContext;
  81         }
  82 
  83         @Override
  84         public void run() {
  85             MethodHandleNatives.clearCallSiteContext(this);
  86         }
  87     }
  88 
  89     /** Invalidate all recorded nmethods. */
  90     private static native void clearCallSiteContext(CallSiteContext context);
  91 
  92     private static native void registerNatives();
  93     static {
  94         registerNatives();
  95     }
  96 
  97     /**
  98      * Compile-time constants go here. This collection exists not only for
  99      * reference from clients, but also for ensuring the VM and JDK agree on the
 100      * values of these constants (see {@link #verifyConstants()}).
 101      */
 102     static class Constants {
 103         Constants() { } // static only
 104 
 105         static final int
 106             MN_IS_METHOD           = 0x00010000, // method (not constructor)
 107             MN_IS_CONSTRUCTOR      = 0x00020000, // constructor
 108             MN_IS_FIELD            = 0x00040000, // field
 109             MN_IS_TYPE             = 0x00080000, // nested type
 110             MN_CALLER_SENSITIVE    = 0x00100000, // @CallerSensitive annotation detected
 111             MN_REFERENCE_KIND_SHIFT = 24, // refKind
 112             MN_REFERENCE_KIND_MASK = 0x0F000000 >> MN_REFERENCE_KIND_SHIFT,
 113             // The SEARCH_* bits are not for MN.flags but for the matchFlags argument of MHN.getMembers:
 114             MN_SEARCH_SUPERCLASSES = 0x00100000,
 115             MN_SEARCH_INTERFACES   = 0x00200000;
 116 
 117         /**
 118          * Constant pool reference-kind codes, as used by CONSTANT_MethodHandle CP entries.
 119          */
 120         static final byte
 121             REF_NONE                    = 0,  // null value
 122             REF_getField                = 1,
 123             REF_getStatic               = 2,
 124             REF_putField                = 3,
 125             REF_putStatic               = 4,
 126             REF_invokeVirtual           = 5,
 127             REF_invokeStatic            = 6,
 128             REF_invokeSpecial           = 7,
 129             REF_newInvokeSpecial        = 8,
 130             REF_invokeInterface         = 9,
 131             REF_LIMIT                  = 10;
 132     }
 133 
 134     static boolean refKindIsValid(int refKind) {
 135         return (refKind > REF_NONE && refKind < REF_LIMIT);
 136     }
 137     static boolean refKindIsField(byte refKind) {
 138         assert(refKindIsValid(refKind));
 139         return (refKind <= REF_putStatic);
 140     }
 141     static boolean refKindIsGetter(byte refKind) {
 142         assert(refKindIsValid(refKind));
 143         return (refKind <= REF_getStatic);
 144     }
 145     static boolean refKindIsSetter(byte refKind) {
 146         return refKindIsField(refKind) && !refKindIsGetter(refKind);
 147     }
 148     static boolean refKindIsMethod(byte refKind) {
 149         return !refKindIsField(refKind) && (refKind != REF_newInvokeSpecial);
 150     }
 151     static boolean refKindIsConstructor(byte refKind) {
 152         return (refKind == REF_newInvokeSpecial);
 153     }
 154     static boolean refKindHasReceiver(byte refKind) {
 155         assert(refKindIsValid(refKind));
 156         return (refKind & 1) != 0;
 157     }
 158     static boolean refKindIsStatic(byte refKind) {
 159         return !refKindHasReceiver(refKind) && (refKind != REF_newInvokeSpecial);
 160     }
 161     static boolean refKindDoesDispatch(byte refKind) {
 162         assert(refKindIsValid(refKind));
 163         return (refKind == REF_invokeVirtual ||
 164                 refKind == REF_invokeInterface);
 165     }
 166     static {
 167         final int HR_MASK = ((1 << REF_getField) |
 168                              (1 << REF_putField) |
 169                              (1 << REF_invokeVirtual) |
 170                              (1 << REF_invokeSpecial) |
 171                              (1 << REF_invokeInterface)
 172                             );
 173         for (byte refKind = REF_NONE+1; refKind < REF_LIMIT; refKind++) {
 174             assert(refKindHasReceiver(refKind) == (((1<<refKind) & HR_MASK) != 0)) : refKind;
 175         }
 176     }
 177     static String refKindName(byte refKind) {
 178         assert(refKindIsValid(refKind));
 179         switch (refKind) {
 180         case REF_getField:          return "getField";
 181         case REF_getStatic:         return "getStatic";
 182         case REF_putField:          return "putField";
 183         case REF_putStatic:         return "putStatic";
 184         case REF_invokeVirtual:     return "invokeVirtual";
 185         case REF_invokeStatic:      return "invokeStatic";
 186         case REF_invokeSpecial:     return "invokeSpecial";
 187         case REF_newInvokeSpecial:  return "newInvokeSpecial";
 188         case REF_invokeInterface:   return "invokeInterface";
 189         default:                    return "REF_???";
 190         }
 191     }
 192 
 193     private static native int getNamedCon(int which, Object[] name);
 194     static boolean verifyConstants() {
 195         Object[] box = { null };
 196         for (int i = 0; ; i++) {
 197             box[0] = null;
 198             int vmval = getNamedCon(i, box);
 199             if (box[0] == null)  break;
 200             String name = (String) box[0];
 201             try {
 202                 Field con = Constants.class.getDeclaredField(name);
 203                 int jval = con.getInt(null);
 204                 if (jval == vmval)  continue;
 205                 String err = (name+": JVM has "+vmval+" while Java has "+jval);
 206                 if (name.equals("CONV_OP_LIMIT")) {
 207                     System.err.println("warning: "+err);
 208                     continue;
 209                 }
 210                 throw new InternalError(err);
 211             } catch (NoSuchFieldException | IllegalAccessException ex) {
 212                 String err = (name+": JVM has "+vmval+" which Java does not define");
 213                 // ignore exotic ops the JVM cares about; we just wont issue them
 214                 //System.err.println("warning: "+err);
 215                 continue;
 216             }
 217         }
 218         return true;
 219     }
 220     static {
 221         assert(verifyConstants());
 222     }
 223 
 224     // Up-calls from the JVM.
 225     // These must NOT be public.
 226 
 227     /**
 228      * The JVM is linking an invokedynamic instruction.  Create a reified call site for it.
 229      */
 230     static MemberName linkCallSite(Object callerObj,
 231                                    Object bootstrapMethodObj,
 232                                    Object nameObj, Object typeObj,
 233                                    Object staticArguments,
 234                                    Object[] appendixResult) {
 235         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
 236         Class<?> caller = (Class<?>)callerObj;
 237         String name = nameObj.toString().intern();
 238         MethodType type = (MethodType)typeObj;
 239         if (!TRACE_METHOD_LINKAGE)
 240             return linkCallSiteImpl(caller, bootstrapMethod, name, type,
 241                                     staticArguments, appendixResult);
 242         return linkCallSiteTracing(caller, bootstrapMethod, name, type,
 243                                    staticArguments, appendixResult);
 244     }
 245     static MemberName linkCallSiteImpl(Class<?> caller,
 246                                        MethodHandle bootstrapMethod,
 247                                        String name, MethodType type,
 248                                        Object staticArguments,
 249                                        Object[] appendixResult) {
 250         CallSite callSite = CallSite.makeSite(bootstrapMethod,
 251                                               name,
 252                                               type,
 253                                               staticArguments,
 254                                               caller);
 255         if (callSite instanceof ConstantCallSite) {
 256             appendixResult[0] = callSite.dynamicInvoker();
 257             return Invokers.linkToTargetMethod(type);
 258         } else {
 259             appendixResult[0] = callSite;
 260             return Invokers.linkToCallSiteMethod(type);
 261         }
 262     }
 263     // Tracing logic:
 264     static MemberName linkCallSiteTracing(Class<?> caller,
 265                                           MethodHandle bootstrapMethod,
 266                                           String name, MethodType type,
 267                                           Object staticArguments,
 268                                           Object[] appendixResult) {
 269         Object bsmReference = bootstrapMethod.internalMemberName();
 270         if (bsmReference == null)  bsmReference = bootstrapMethod;
 271         Object staticArglist = (staticArguments instanceof Object[] ?
 272                                 java.util.Arrays.asList((Object[]) staticArguments) :
 273                                 staticArguments);
 274         System.out.println("linkCallSite "+caller.getName()+" "+
 275                            bsmReference+" "+
 276                            name+type+"/"+staticArglist);
 277         try {
 278             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
 279                                               staticArguments, appendixResult);
 280             System.out.println("linkCallSite => "+res+" + "+appendixResult[0]);
 281             return res;
 282         } catch (Throwable ex) {
 283             System.out.println("linkCallSite => throw "+ex);
 284             throw ex;
 285         }
 286     }
 287 
 288     /**
 289      * The JVM wants a pointer to a MethodType.  Oblige it by finding or creating one.
 290      */
 291     static MethodType findMethodHandleType(Class<?> rtype, Class<?>[] ptypes) {
 292         return MethodType.makeImpl(rtype, ptypes, true);
 293     }
 294 
 295     /**
 296      * The JVM wants to link a call site that requires a dynamic type check.
 297      * Name is a type-checking invoker, invokeExact or invoke.
 298      * Return a JVM method (MemberName) to handle the invoking.
 299      * The method assumes the following arguments on the stack:
 300      * 0: the method handle being invoked
 301      * 1-N: the arguments to the method handle invocation
 302      * N+1: an optional, implicitly added argument (typically the given MethodType)
 303      * <p>
 304      * The nominal method at such a call site is an instance of
 305      * a signature-polymorphic method (see @PolymorphicSignature).
 306      * Such method instances are user-visible entities which are
 307      * "split" from the generic placeholder method in {@code MethodHandle}.
 308      * (Note that the placeholder method is not identical with any of
 309      * its instances.  If invoked reflectively, is guaranteed to throw an
 310      * {@code UnsupportedOperationException}.)
 311      * If the signature-polymorphic method instance is ever reified,
 312      * it appears as a "copy" of the original placeholder
 313      * (a native final member of {@code MethodHandle}) except
 314      * that its type descriptor has shape required by the instance,
 315      * and the method instance is <em>not</em> varargs.
 316      * The method instance is also marked synthetic, since the
 317      * method (by definition) does not appear in Java source code.
 318      * <p>
 319      * The JVM is allowed to reify this method as instance metadata.
 320      * For example, {@code invokeBasic} is always reified.
 321      * But the JVM may instead call {@code linkMethod}.
 322      * If the result is an * ordered pair of a {@code (method, appendix)},
 323      * the method gets all the arguments (0..N inclusive)
 324      * plus the appendix (N+1), and uses the appendix to complete the call.
 325      * In this way, one reusable method (called a "linker method")
 326      * can perform the function of any number of polymorphic instance
 327      * methods.
 328      * <p>
 329      * Linker methods are allowed to be weakly typed, with any or
 330      * all references rewritten to {@code Object} and any primitives
 331      * (except {@code long}/{@code float}/{@code double})
 332      * rewritten to {@code int}.
 333      * A linker method is trusted to return a strongly typed result,
 334      * according to the specific method type descriptor of the
 335      * signature-polymorphic instance it is emulating.
 336      * This can involve (as necessary) a dynamic check using
 337      * data extracted from the appendix argument.
 338      * <p>
 339      * The JVM does not inspect the appendix, other than to pass
 340      * it verbatim to the linker method at every call.
 341      * This means that the JDK runtime has wide latitude
 342      * for choosing the shape of each linker method and its
 343      * corresponding appendix.
 344      * Linker methods should be generated from {@code LambdaForm}s
 345      * so that they do not become visible on stack traces.
 346      * <p>
 347      * The {@code linkMethod} call is free to omit the appendix
 348      * (returning null) and instead emulate the required function
 349      * completely in the linker method.
 350      * As a corner case, if N==255, no appendix is possible.
 351      * In this case, the method returned must be custom-generated to
 352      * to perform any needed type checking.
 353      * <p>
 354      * If the JVM does not reify a method at a call site, but instead
 355      * calls {@code linkMethod}, the corresponding call represented
 356      * in the bytecodes may mention a valid method which is not
 357      * representable with a {@code MemberName}.
 358      * Therefore, use cases for {@code linkMethod} tend to correspond to
 359      * special cases in reflective code such as {@code findVirtual}
 360      * or {@code revealDirect}.
 361      */
 362     static MemberName linkMethod(Class<?> callerClass, int refKind,
 363                                  Class<?> defc, String name, Object type,
 364                                  Object[] appendixResult) {
 365         if (!TRACE_METHOD_LINKAGE)
 366             return linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
 367         return linkMethodTracing(callerClass, refKind, defc, name, type, appendixResult);
 368     }
 369     static MemberName linkMethodImpl(Class<?> callerClass, int refKind,
 370                                      Class<?> defc, String name, Object type,
 371                                      Object[] appendixResult) {
 372         try {
 373             if (refKind == REF_invokeVirtual) {
 374                 if (defc == MethodHandle.class) {
 375                     return Invokers.methodHandleInvokeLinkerMethod(
 376                             name, fixMethodType(callerClass, type), appendixResult);
 377                 } else if (defc == VarHandle.class) {
 378                     return varHandleOperationLinkerMethod(
 379                             name, fixMethodType(callerClass, type), appendixResult);
 380                 }
 381             }
 382         } catch (Error e) {
 383             // Pass through an Error, including say StackOverflowError or
 384             // OutOfMemoryError
 385             throw e;
 386         } catch (Throwable ex) {
 387             // Wrap anything else in LinkageError
 388             throw new LinkageError(ex.getMessage(), ex);
 389         }
 390         throw new LinkageError("no such method "+defc.getName()+"."+name+type);
 391     }
 392     private static MethodType fixMethodType(Class<?> callerClass, Object type) {
 393         if (type instanceof MethodType)
 394             return (MethodType) type;
 395         else
 396             return MethodType.fromDescriptor((String)type, callerClass.getClassLoader());
 397     }
 398     // Tracing logic:
 399     static MemberName linkMethodTracing(Class<?> callerClass, int refKind,
 400                                         Class<?> defc, String name, Object type,
 401                                         Object[] appendixResult) {
 402         System.out.println("linkMethod "+defc.getName()+"."+
 403                            name+type+"/"+Integer.toHexString(refKind));
 404         try {
 405             MemberName res = linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
 406             System.out.println("linkMethod => "+res+" + "+appendixResult[0]);
 407             return res;
 408         } catch (Throwable ex) {
 409             System.out.println("linkMethod => throw "+ex);
 410             throw ex;
 411         }
 412     }
 413 
 414     /**
 415      * Obtain the method to link to the VarHandle operation.
 416      * This method is located here and not in Invokers to avoid
 417      * intializing that and other classes early on in VM bootup.
 418      */
 419     private static MemberName varHandleOperationLinkerMethod(String name,
 420                                                              MethodType mtype,
 421                                                              Object[] appendixResult) {
 422         // Get the signature method type
 423         final MethodType sigType = mtype.basicType();
 424 
 425         // Get the access kind from the method name
 426         VarHandle.AccessMode ak;
 427         try {
 428             ak = VarHandle.AccessMode.valueFromMethodName(name);
 429         } catch (IllegalArgumentException e) {
 430             throw MethodHandleStatics.newInternalError(e);
 431         }
 432 
 433         // Create the appendix descriptor constant
 434         VarHandle.AccessDescriptor ad = new VarHandle.AccessDescriptor(mtype, ak.at.ordinal(), ak.ordinal());
 435         appendixResult[0] = ad;
 436 
 437         if (MethodHandleStatics.VAR_HANDLE_GUARDS) {
 438             // If not polymorphic in the return type, such as the compareAndSet
 439             // methods that return boolean
 440             Class<?> guardReturnType = sigType.returnType();
 441             if (ak.at.isMonomorphicInReturnType) {
 442                 if (ak.at.returnType != mtype.returnType()) {
 443                     // The caller contains a different return type than that
 444                     // defined by the method
 445                     throw newNoSuchMethodErrorOnVarHandle(name, mtype);
 446                 }
 447                 // Adjust the return type of the signature method type
 448                 guardReturnType = ak.at.returnType;
 449             }
 450 
 451             // Get the guard method type for linking
 452             final Class<?>[] guardParams = new Class<?>[sigType.parameterCount() + 2];
 453             // VarHandle at start
 454             guardParams[0] = VarHandle.class;
 455             for (int i = 0; i < sigType.parameterCount(); i++) {
 456                 guardParams[i + 1] = sigType.parameterType(i);
 457             }
 458             // Access descriptor at end
 459             guardParams[guardParams.length - 1] = VarHandle.AccessDescriptor.class;
 460             MethodType guardType = MethodType.makeImpl(guardReturnType, guardParams, true);
 461 
 462             MemberName linker = new MemberName(
 463                     VarHandleGuards.class, getVarHandleGuardMethodName(guardType),
 464                     guardType, REF_invokeStatic);
 465 
 466             linker = MemberName.getFactory().resolveOrNull(REF_invokeStatic, linker,
 467                                                            VarHandleGuards.class);
 468             if (linker != null) {
 469                 return linker;
 470             }
 471             // Fall back to lambda form linkage if guard method is not available
 472             // TODO Optionally log fallback ?
 473         }
 474         return Invokers.varHandleInvokeLinkerMethod(ak, mtype);
 475     }
 476     static String getVarHandleGuardMethodName(MethodType guardType) {
 477         String prefix = "guard_";
 478         StringBuilder sb = new StringBuilder(prefix.length() + guardType.parameterCount());
 479 
 480         sb.append(prefix);
 481         for (int i = 1; i < guardType.parameterCount() - 1; i++) {
 482             Class<?> pt = guardType.parameterType(i);
 483             sb.append(getCharType(pt));
 484         }
 485         sb.append('_').append(getCharType(guardType.returnType()));
 486         return sb.toString();
 487     }
 488     static char getCharType(Class<?> pt) {
 489         return Wrapper.forBasicType(pt).basicTypeChar();
 490     }
 491     static NoSuchMethodError newNoSuchMethodErrorOnVarHandle(String name, MethodType mtype) {
 492         return new NoSuchMethodError("VarHandle." + name + mtype);
 493     }
 494 
 495     /**
 496      * The JVM is resolving a CONSTANT_MethodHandle CP entry.  And it wants our help.
 497      * It will make an up-call to this method.  (Do not change the name or signature.)
 498      * The type argument is a Class for field requests and a MethodType for non-fields.
 499      * <p>
 500      * Recent versions of the JVM may also pass a resolved MemberName for the type.
 501      * In that case, the name is ignored and may be null.
 502      */
 503     static MethodHandle linkMethodHandleConstant(Class<?> callerClass, int refKind,
 504                                                  Class<?> defc, String name, Object type) {
 505         try {
 506             Lookup lookup = IMPL_LOOKUP.in(callerClass);
 507             assert(refKindIsValid(refKind));
 508             return lookup.linkMethodHandleConstant((byte) refKind, defc, name, type);
 509         } catch (IllegalAccessException ex) {
 510             Throwable cause = ex.getCause();
 511             if (cause instanceof AbstractMethodError) {
 512                 throw (AbstractMethodError) cause;
 513             } else {
 514                 Error err = new IllegalAccessError(ex.getMessage());
 515                 throw initCauseFrom(err, ex);
 516             }
 517         } catch (NoSuchMethodException ex) {
 518             Error err = new NoSuchMethodError(ex.getMessage());
 519             throw initCauseFrom(err, ex);
 520         } catch (NoSuchFieldException ex) {
 521             Error err = new NoSuchFieldError(ex.getMessage());
 522             throw initCauseFrom(err, ex);
 523         } catch (ReflectiveOperationException ex) {
 524             Error err = new IncompatibleClassChangeError();
 525             throw initCauseFrom(err, ex);
 526         }
 527     }
 528 
 529     /**
 530      * Use best possible cause for err.initCause(), substituting the
 531      * cause for err itself if the cause has the same (or better) type.
 532      */
 533     private static Error initCauseFrom(Error err, Exception ex) {
 534         Throwable th = ex.getCause();
 535         if (err.getClass().isInstance(th))
 536            return (Error) th;
 537         err.initCause(th == null ? ex : th);
 538         return err;
 539     }
 540 
 541     /**
 542      * Is this method a caller-sensitive method?
 543      * I.e., does it call Reflection.getCallerClass or a similar method
 544      * to ask about the identity of its caller?
 545      */
 546     static boolean isCallerSensitive(MemberName mem) {
 547         if (!mem.isInvocable())  return false;  // fields are not caller sensitive
 548 
 549         return mem.isCallerSensitive() || canBeCalledVirtual(mem);
 550     }
 551 
 552     static boolean canBeCalledVirtual(MemberName mem) {
 553         assert(mem.isInvocable());
 554         Class<?> defc = mem.getDeclaringClass();
 555         switch (mem.getName()) {
 556         case "checkMemberAccess":
 557             return canBeCalledVirtual(mem, java.lang.SecurityManager.class);
 558         case "getContextClassLoader":
 559             return canBeCalledVirtual(mem, java.lang.Thread.class);
 560         }
 561         return false;
 562     }
 563 
 564     static boolean canBeCalledVirtual(MemberName symbolicRef, Class<?> definingClass) {
 565         Class<?> symbolicRefClass = symbolicRef.getDeclaringClass();
 566         if (symbolicRefClass == definingClass)  return true;
 567         if (symbolicRef.isStatic() || symbolicRef.isPrivate())  return false;
 568         return (definingClass.isAssignableFrom(symbolicRefClass) ||  // Msym overrides Mdef
 569                 symbolicRefClass.isInterface());                     // Mdef implements Msym
 570     }
 571 }