1 /*
   2  * Copyright (c) 2008, 2018, 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,
  53             boolean speculativeResolve) throws LinkageError, ClassNotFoundException;
  54     static native int getMembers(Class<?> defc, String matchName, String matchSig,
  55             int matchFlags, Class<?> caller, int skip, MemberName[] results);
  56 
  57     /// Field layout queries parallel to jdk.internal.misc.Unsafe:
  58     static native long objectFieldOffset(MemberName self);  // e.g., returns vmindex
  59     static native long staticFieldOffset(MemberName self);  // e.g., returns vmindex
  60     static native Object staticFieldBase(MemberName self);  // e.g., returns clazz
  61     static native Object getMemberVMInfo(MemberName self);  // returns {vmindex,vmtarget}
  62 
  63     /// CallSite support
  64 
  65     /** Tell the JVM that we need to change the target of a CallSite. */
  66     static native void setCallSiteTargetNormal(CallSite site, MethodHandle target);
  67     static native void setCallSiteTargetVolatile(CallSite site, MethodHandle target);
  68 
  69     static native void copyOutBootstrapArguments(Class<?> caller, int[] indexInfo,
  70                                                  int start, int end,
  71                                                  Object[] buf, int pos,
  72                                                  boolean resolve,
  73                                                  Object ifNotAvailable);
  74 
  75     /** Represents a context to track nmethod dependencies on CallSite instance target. */
  76     static class CallSiteContext implements Runnable {
  77         //@Injected JVM_nmethodBucket* vmdependencies;
  78 
  79         static CallSiteContext make(CallSite cs) {
  80             final CallSiteContext newContext = new CallSiteContext();
  81             // CallSite instance is tracked by a Cleanable which clears native
  82             // structures allocated for CallSite context. Though the CallSite can
  83             // become unreachable, its Context is retained by the Cleanable instance
  84             // (which is referenced from Cleaner instance which is referenced from
  85             // CleanerFactory class) until cleanup is performed.
  86             CleanerFactory.cleaner().register(cs, newContext);
  87             return newContext;
  88         }
  89 
  90         @Override
  91         public void run() {
  92             MethodHandleNatives.clearCallSiteContext(this);
  93         }
  94     }
  95 
  96     /** Invalidate all recorded nmethods. */
  97     private static native void clearCallSiteContext(CallSiteContext context);
  98 
  99     private static native void registerNatives();
 100     static {
 101         registerNatives();
 102     }
 103 
 104     /**
 105      * Compile-time constants go here. This collection exists not only for
 106      * reference from clients, but also for ensuring the VM and JDK agree on the
 107      * values of these constants (see {@link #verifyConstants()}).
 108      */
 109     static class Constants {
 110         Constants() { } // static only
 111 
 112         static final int
 113             MN_IS_METHOD           = 0x00010000, // method (not constructor)
 114             MN_IS_CONSTRUCTOR      = 0x00020000, // constructor
 115             MN_IS_FIELD            = 0x00040000, // field
 116             MN_IS_TYPE             = 0x00080000, // nested type
 117             MN_CALLER_SENSITIVE    = 0x00100000, // @CallerSensitive annotation detected
 118             MN_REFERENCE_KIND_SHIFT = 24, // refKind
 119             MN_REFERENCE_KIND_MASK = 0x0F000000 >> MN_REFERENCE_KIND_SHIFT,
 120             // The SEARCH_* bits are not for MN.flags but for the matchFlags argument of MHN.getMembers:
 121             MN_SEARCH_SUPERCLASSES = 0x00100000,
 122             MN_SEARCH_INTERFACES   = 0x00200000;
 123 
 124         /**
 125          * Constant pool reference-kind codes, as used by CONSTANT_MethodHandle CP entries.
 126          */
 127         static final byte
 128             REF_NONE                    = 0,  // null value
 129             REF_getField                = 1,
 130             REF_getStatic               = 2,
 131             REF_putField                = 3,
 132             REF_putStatic               = 4,
 133             REF_invokeVirtual           = 5,
 134             REF_invokeStatic            = 6,
 135             REF_invokeSpecial           = 7,
 136             REF_newInvokeSpecial        = 8,
 137             REF_invokeInterface         = 9,
 138             REF_LIMIT                  = 10;
 139     }
 140 
 141     static boolean refKindIsValid(int refKind) {
 142         return (refKind > REF_NONE && refKind < REF_LIMIT);
 143     }
 144     static boolean refKindIsField(byte refKind) {
 145         assert(refKindIsValid(refKind));
 146         return (refKind <= REF_putStatic);
 147     }
 148     static boolean refKindIsGetter(byte refKind) {
 149         assert(refKindIsValid(refKind));
 150         return (refKind <= REF_getStatic);
 151     }
 152     static boolean refKindIsSetter(byte refKind) {
 153         return refKindIsField(refKind) && !refKindIsGetter(refKind);
 154     }
 155     static boolean refKindIsMethod(byte refKind) {
 156         return !refKindIsField(refKind) && (refKind != REF_newInvokeSpecial);
 157     }
 158     static boolean refKindIsConstructor(byte refKind) {
 159         return (refKind == REF_newInvokeSpecial);
 160     }
 161     static boolean refKindHasReceiver(byte refKind) {
 162         assert(refKindIsValid(refKind));
 163         return (refKind & 1) != 0;
 164     }
 165     static boolean refKindIsStatic(byte refKind) {
 166         return !refKindHasReceiver(refKind) && (refKind != REF_newInvokeSpecial);
 167     }
 168     static boolean refKindDoesDispatch(byte refKind) {
 169         assert(refKindIsValid(refKind));
 170         return (refKind == REF_invokeVirtual ||
 171                 refKind == REF_invokeInterface);
 172     }
 173     static {
 174         final int HR_MASK = ((1 << REF_getField) |
 175                              (1 << REF_putField) |
 176                              (1 << REF_invokeVirtual) |
 177                              (1 << REF_invokeSpecial) |
 178                              (1 << REF_invokeInterface)
 179                             );
 180         for (byte refKind = REF_NONE+1; refKind < REF_LIMIT; refKind++) {
 181             assert(refKindHasReceiver(refKind) == (((1<<refKind) & HR_MASK) != 0)) : refKind;
 182         }
 183     }
 184     static String refKindName(byte refKind) {
 185         assert(refKindIsValid(refKind));
 186         switch (refKind) {
 187         case REF_getField:          return "getField";
 188         case REF_getStatic:         return "getStatic";
 189         case REF_putField:          return "putField";
 190         case REF_putStatic:         return "putStatic";
 191         case REF_invokeVirtual:     return "invokeVirtual";
 192         case REF_invokeStatic:      return "invokeStatic";
 193         case REF_invokeSpecial:     return "invokeSpecial";
 194         case REF_newInvokeSpecial:  return "newInvokeSpecial";
 195         case REF_invokeInterface:   return "invokeInterface";
 196         default:                    return "REF_???";
 197         }
 198     }
 199 
 200     private static native int getNamedCon(int which, Object[] name);
 201     static boolean verifyConstants() {
 202         Object[] box = { null };
 203         for (int i = 0; ; i++) {
 204             box[0] = null;
 205             int vmval = getNamedCon(i, box);
 206             if (box[0] == null)  break;
 207             String name = (String) box[0];
 208             try {
 209                 Field con = Constants.class.getDeclaredField(name);
 210                 int jval = con.getInt(null);
 211                 if (jval == vmval)  continue;
 212                 String err = (name+": JVM has "+vmval+" while Java has "+jval);
 213                 if (name.equals("CONV_OP_LIMIT")) {
 214                     System.err.println("warning: "+err);
 215                     continue;
 216                 }
 217                 throw new InternalError(err);
 218             } catch (NoSuchFieldException | IllegalAccessException ex) {
 219                 String err = (name+": JVM has "+vmval+" which Java does not define");
 220                 // ignore exotic ops the JVM cares about; we just wont issue them
 221                 //System.err.println("warning: "+err);
 222                 continue;
 223             }
 224         }
 225         return true;
 226     }
 227     static {
 228         assert(verifyConstants());
 229     }
 230 
 231     // Up-calls from the JVM.
 232     // These must NOT be public.
 233 
 234     /**
 235      * The JVM is linking an invokedynamic instruction.  Create a reified call site for it.
 236      */
 237     static MemberName linkCallSite(Object callerObj,
 238                                    int indexInCP,
 239                                    Object bootstrapMethodObj,
 240                                    Object nameObj, Object typeObj,
 241                                    Object staticArguments,
 242                                    Object[] appendixResult) {
 243         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
 244         Class<?> caller = (Class<?>)callerObj;
 245         String name = nameObj.toString().intern();
 246         MethodType type = (MethodType)typeObj;
 247         if (!TRACE_METHOD_LINKAGE)
 248             return linkCallSiteImpl(caller, bootstrapMethod, name, type,
 249                                     staticArguments, appendixResult);
 250         return linkCallSiteTracing(caller, bootstrapMethod, name, type,
 251                                    staticArguments, appendixResult);
 252     }
 253     static MemberName linkCallSiteImpl(Class<?> caller,
 254                                        MethodHandle bootstrapMethod,
 255                                        String name, MethodType type,
 256                                        Object staticArguments,
 257                                        Object[] appendixResult) {
 258         CallSite callSite = CallSite.makeSite(bootstrapMethod,
 259                                               name,
 260                                               type,
 261                                               staticArguments,
 262                                               caller);
 263         if (callSite instanceof ConstantCallSite) {
 264             appendixResult[0] = callSite.dynamicInvoker();
 265             return Invokers.linkToTargetMethod(type);
 266         } else {
 267             appendixResult[0] = callSite;
 268             return Invokers.linkToCallSiteMethod(type);
 269         }
 270     }
 271     // Tracing logic:
 272     static MemberName linkCallSiteTracing(Class<?> caller,
 273                                           MethodHandle bootstrapMethod,
 274                                           String name, MethodType type,
 275                                           Object staticArguments,
 276                                           Object[] appendixResult) {
 277         Object bsmReference = bootstrapMethod.internalMemberName();
 278         if (bsmReference == null)  bsmReference = bootstrapMethod;
 279         String staticArglist = staticArglistForTrace(staticArguments);
 280         System.out.println("linkCallSite "+caller.getName()+" "+
 281                            bsmReference+" "+
 282                            name+type+"/"+staticArglist);
 283         try {
 284             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
 285                                               staticArguments, appendixResult);
 286             System.out.println("linkCallSite => "+res+" + "+appendixResult[0]);
 287             return res;
 288         } catch (Throwable ex) {
 289             ex.printStackTrace(); // print now in case exception is swallowed
 290             System.out.println("linkCallSite => throw "+ex);
 291             throw ex;
 292         }
 293     }
 294 
 295     // this implements the upcall from the JVM, MethodHandleNatives.linkDynamicConstant:
 296     static Object linkDynamicConstant(Object callerObj,
 297                                       int indexInCP,
 298                                       Object bootstrapMethodObj,
 299                                       Object nameObj, Object typeObj,
 300                                       Object staticArguments) {
 301         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
 302         Class<?> caller = (Class<?>)callerObj;
 303         String name = nameObj.toString().intern();
 304         Class<?> type = (Class<?>)typeObj;
 305         if (!TRACE_METHOD_LINKAGE)
 306             return linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
 307         return linkDynamicConstantTracing(caller, bootstrapMethod, name, type, staticArguments);
 308     }
 309 
 310     static Object linkDynamicConstantImpl(Class<?> caller,
 311                                           MethodHandle bootstrapMethod,
 312                                           String name, Class<?> type,
 313                                           Object staticArguments) {
 314         return ConstantBootstraps.makeConstant(bootstrapMethod, name, type, staticArguments, caller);
 315     }
 316 
 317     private static String staticArglistForTrace(Object staticArguments) {
 318         if (staticArguments instanceof Object[])
 319             return "BSA="+java.util.Arrays.asList((Object[]) staticArguments);
 320         if (staticArguments instanceof int[])
 321             return "BSA@"+java.util.Arrays.toString((int[]) staticArguments);
 322         if (staticArguments == null)
 323             return "BSA0=null";
 324         return "BSA1="+staticArguments;
 325     }
 326 
 327     // Tracing logic:
 328     static Object linkDynamicConstantTracing(Class<?> caller,
 329                                              MethodHandle bootstrapMethod,
 330                                              String name, Class<?> type,
 331                                              Object staticArguments) {
 332         Object bsmReference = bootstrapMethod.internalMemberName();
 333         if (bsmReference == null)  bsmReference = bootstrapMethod;
 334         String staticArglist = staticArglistForTrace(staticArguments);
 335         System.out.println("linkDynamicConstant "+caller.getName()+" "+
 336                            bsmReference+" "+
 337                            name+type+"/"+staticArglist);
 338         try {
 339             Object res = linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
 340             System.out.println("linkDynamicConstantImpl => "+res);
 341             return res;
 342         } catch (Throwable ex) {
 343             ex.printStackTrace(); // print now in case exception is swallowed
 344             System.out.println("linkDynamicConstant => throw "+ex);
 345             throw ex;
 346         }
 347     }
 348 
 349     /** The JVM is requesting pull-mode bootstrap when it provides
 350      *  a tuple of the form int[]{ argc, vmindex }.
 351      *  The BSM is expected to call back to the JVM using the caller
 352      *  class and vmindex to resolve the static arguments.
 353      */
 354     static boolean staticArgumentsPulled(Object staticArguments) {
 355         return staticArguments instanceof int[];
 356     }
 357 
 358     /** A BSM runs in pull-mode if and only if its sole arguments
 359      * are (Lookup, BootstrapCallInfo), or can be converted pairwise
 360      * to those types, and it is not of variable arity.
 361      * Excluding error cases, we can just test that the arity is a constant 2.
 362      *
 363      * NOTE: This method currently returns false, since pulling is not currently
 364      * exposed to a BSM. When pull mode is supported the method block will be
 365      * replaced with currently commented out code.
 366      */
 367     static boolean isPullModeBSM(MethodHandle bsm) {
 368         return false;
 369 //        return bsm.type().parameterCount() == 2 && !bsm.isVarargsCollector();
 370     }
 371 
 372     /**
 373      * The JVM wants a pointer to a MethodType.  Oblige it by finding or creating one.
 374      */
 375     static MethodType findMethodHandleType(Class<?> rtype, Class<?>[] ptypes) {
 376         return MethodType.makeImpl(rtype, ptypes, true);
 377     }
 378 
 379     /**
 380      * The JVM wants to link a call site that requires a dynamic type check.
 381      * Name is a type-checking invoker, invokeExact or invoke.
 382      * Return a JVM method (MemberName) to handle the invoking.
 383      * The method assumes the following arguments on the stack:
 384      * 0: the method handle being invoked
 385      * 1-N: the arguments to the method handle invocation
 386      * N+1: an optional, implicitly added argument (typically the given MethodType)
 387      * <p>
 388      * The nominal method at such a call site is an instance of
 389      * a signature-polymorphic method (see @PolymorphicSignature).
 390      * Such method instances are user-visible entities which are
 391      * "split" from the generic placeholder method in {@code MethodHandle}.
 392      * (Note that the placeholder method is not identical with any of
 393      * its instances.  If invoked reflectively, is guaranteed to throw an
 394      * {@code UnsupportedOperationException}.)
 395      * If the signature-polymorphic method instance is ever reified,
 396      * it appears as a "copy" of the original placeholder
 397      * (a native final member of {@code MethodHandle}) except
 398      * that its type descriptor has shape required by the instance,
 399      * and the method instance is <em>not</em> varargs.
 400      * The method instance is also marked synthetic, since the
 401      * method (by definition) does not appear in Java source code.
 402      * <p>
 403      * The JVM is allowed to reify this method as instance metadata.
 404      * For example, {@code invokeBasic} is always reified.
 405      * But the JVM may instead call {@code linkMethod}.
 406      * If the result is an * ordered pair of a {@code (method, appendix)},
 407      * the method gets all the arguments (0..N inclusive)
 408      * plus the appendix (N+1), and uses the appendix to complete the call.
 409      * In this way, one reusable method (called a "linker method")
 410      * can perform the function of any number of polymorphic instance
 411      * methods.
 412      * <p>
 413      * Linker methods are allowed to be weakly typed, with any or
 414      * all references rewritten to {@code Object} and any primitives
 415      * (except {@code long}/{@code float}/{@code double})
 416      * rewritten to {@code int}.
 417      * A linker method is trusted to return a strongly typed result,
 418      * according to the specific method type descriptor of the
 419      * signature-polymorphic instance it is emulating.
 420      * This can involve (as necessary) a dynamic check using
 421      * data extracted from the appendix argument.
 422      * <p>
 423      * The JVM does not inspect the appendix, other than to pass
 424      * it verbatim to the linker method at every call.
 425      * This means that the JDK runtime has wide latitude
 426      * for choosing the shape of each linker method and its
 427      * corresponding appendix.
 428      * Linker methods should be generated from {@code LambdaForm}s
 429      * so that they do not become visible on stack traces.
 430      * <p>
 431      * The {@code linkMethod} call is free to omit the appendix
 432      * (returning null) and instead emulate the required function
 433      * completely in the linker method.
 434      * As a corner case, if N==255, no appendix is possible.
 435      * In this case, the method returned must be custom-generated to
 436      * to perform any needed type checking.
 437      * <p>
 438      * If the JVM does not reify a method at a call site, but instead
 439      * calls {@code linkMethod}, the corresponding call represented
 440      * in the bytecodes may mention a valid method which is not
 441      * representable with a {@code MemberName}.
 442      * Therefore, use cases for {@code linkMethod} tend to correspond to
 443      * special cases in reflective code such as {@code findVirtual}
 444      * or {@code revealDirect}.
 445      */
 446     static MemberName linkMethod(Class<?> callerClass, int refKind,
 447                                  Class<?> defc, String name, Object type,
 448                                  Object[] appendixResult) {
 449         if (!TRACE_METHOD_LINKAGE)
 450             return linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
 451         return linkMethodTracing(callerClass, refKind, defc, name, type, appendixResult);
 452     }
 453     static MemberName linkMethodImpl(Class<?> callerClass, int refKind,
 454                                      Class<?> defc, String name, Object type,
 455                                      Object[] appendixResult) {
 456         try {
 457             if (refKind == REF_invokeVirtual) {
 458                 if (defc == MethodHandle.class) {
 459                     return Invokers.methodHandleInvokeLinkerMethod(
 460                             name, fixMethodType(callerClass, type), appendixResult);
 461                 } else if (defc == VarHandle.class) {
 462                     return varHandleOperationLinkerMethod(
 463                             name, fixMethodType(callerClass, type), appendixResult);
 464                 }
 465             }
 466         } catch (Error e) {
 467             // Pass through an Error, including say StackOverflowError or
 468             // OutOfMemoryError
 469             throw e;
 470         } catch (Throwable ex) {
 471             // Wrap anything else in LinkageError
 472             throw new LinkageError(ex.getMessage(), ex);
 473         }
 474         throw new LinkageError("no such method "+defc.getName()+"."+name+type);
 475     }
 476     private static MethodType fixMethodType(Class<?> callerClass, Object type) {
 477         if (type instanceof MethodType)
 478             return (MethodType) type;
 479         else
 480             return MethodType.fromDescriptor((String)type, callerClass.getClassLoader());
 481     }
 482     // Tracing logic:
 483     static MemberName linkMethodTracing(Class<?> callerClass, int refKind,
 484                                         Class<?> defc, String name, Object type,
 485                                         Object[] appendixResult) {
 486         System.out.println("linkMethod "+defc.getName()+"."+
 487                            name+type+"/"+Integer.toHexString(refKind));
 488         try {
 489             MemberName res = linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
 490             System.out.println("linkMethod => "+res+" + "+appendixResult[0]);
 491             return res;
 492         } catch (Throwable ex) {
 493             System.out.println("linkMethod => throw "+ex);
 494             throw ex;
 495         }
 496     }
 497 
 498     /**
 499      * Obtain the method to link to the VarHandle operation.
 500      * This method is located here and not in Invokers to avoid
 501      * intializing that and other classes early on in VM bootup.
 502      */
 503     private static MemberName varHandleOperationLinkerMethod(String name,
 504                                                              MethodType mtype,
 505                                                              Object[] appendixResult) {
 506         // Get the signature method type
 507         final MethodType sigType = mtype.basicType();
 508 
 509         // Get the access kind from the method name
 510         VarHandle.AccessMode ak;
 511         try {
 512             ak = VarHandle.AccessMode.valueFromMethodName(name);
 513         } catch (IllegalArgumentException e) {
 514             throw MethodHandleStatics.newInternalError(e);
 515         }
 516 
 517         // Create the appendix descriptor constant
 518         VarHandle.AccessDescriptor ad = new VarHandle.AccessDescriptor(mtype, ak.at.ordinal(), ak.ordinal());
 519         appendixResult[0] = ad;
 520 
 521         if (MethodHandleStatics.VAR_HANDLE_GUARDS) {
 522             // If not polymorphic in the return type, such as the compareAndSet
 523             // methods that return boolean
 524             Class<?> guardReturnType = sigType.returnType();
 525             if (ak.at.isMonomorphicInReturnType) {
 526                 if (ak.at.returnType != mtype.returnType()) {
 527                     // The caller contains a different return type than that
 528                     // defined by the method
 529                     throw newNoSuchMethodErrorOnVarHandle(name, mtype);
 530                 }
 531                 // Adjust the return type of the signature method type
 532                 guardReturnType = ak.at.returnType;
 533             }
 534 
 535             // Get the guard method type for linking
 536             final Class<?>[] guardParams = new Class<?>[sigType.parameterCount() + 2];
 537             // VarHandle at start
 538             guardParams[0] = VarHandle.class;
 539             for (int i = 0; i < sigType.parameterCount(); i++) {
 540                 guardParams[i + 1] = sigType.parameterType(i);
 541             }
 542             // Access descriptor at end
 543             guardParams[guardParams.length - 1] = VarHandle.AccessDescriptor.class;
 544             MethodType guardType = MethodType.makeImpl(guardReturnType, guardParams, true);
 545 
 546             MemberName linker = new MemberName(
 547                     VarHandleGuards.class, getVarHandleGuardMethodName(guardType),
 548                     guardType, REF_invokeStatic);
 549 
 550             linker = MemberName.getFactory().resolveOrNull(REF_invokeStatic, linker,
 551                                                            VarHandleGuards.class);
 552             if (linker != null) {
 553                 return linker;
 554             }
 555             // Fall back to lambda form linkage if guard method is not available
 556             // TODO Optionally log fallback ?
 557         }
 558         return Invokers.varHandleInvokeLinkerMethod(ak, mtype);
 559     }
 560     static String getVarHandleGuardMethodName(MethodType guardType) {
 561         String prefix = "guard_";
 562         StringBuilder sb = new StringBuilder(prefix.length() + guardType.parameterCount());
 563 
 564         sb.append(prefix);
 565         for (int i = 1; i < guardType.parameterCount() - 1; i++) {
 566             Class<?> pt = guardType.parameterType(i);
 567             sb.append(getCharType(pt));
 568         }
 569         sb.append('_').append(getCharType(guardType.returnType()));
 570         return sb.toString();
 571     }
 572     static char getCharType(Class<?> pt) {
 573         return Wrapper.forBasicType(pt).basicTypeChar();
 574     }
 575     static NoSuchMethodError newNoSuchMethodErrorOnVarHandle(String name, MethodType mtype) {
 576         return new NoSuchMethodError("VarHandle." + name + mtype);
 577     }
 578 
 579     /**
 580      * The JVM is resolving a CONSTANT_MethodHandle CP entry.  And it wants our help.
 581      * It will make an up-call to this method.  (Do not change the name or signature.)
 582      * The type argument is a Class for field requests and a MethodType for non-fields.
 583      * <p>
 584      * Recent versions of the JVM may also pass a resolved MemberName for the type.
 585      * In that case, the name is ignored and may be null.
 586      */
 587     static MethodHandle linkMethodHandleConstant(Class<?> callerClass, int refKind,
 588                                                  Class<?> defc, String name, Object type) {
 589         try {
 590             Lookup lookup = IMPL_LOOKUP.in(callerClass);
 591             assert(refKindIsValid(refKind));
 592             return lookup.linkMethodHandleConstant((byte) refKind, defc, name, type);
 593         } catch (ReflectiveOperationException ex) {
 594             throw mapLookupExceptionToError(ex);
 595         }
 596     }
 597 
 598     /**
 599      * Map a reflective exception to a linkage error.
 600      */
 601     static LinkageError mapLookupExceptionToError(ReflectiveOperationException ex) {
 602         LinkageError err;
 603         if (ex instanceof IllegalAccessException) {
 604             Throwable cause = ex.getCause();
 605             if (cause instanceof AbstractMethodError) {
 606                 return (AbstractMethodError) cause;
 607             } else {
 608                 err = new IllegalAccessError(ex.getMessage());
 609             }
 610         } else if (ex instanceof NoSuchMethodException) {
 611             err = new NoSuchMethodError(ex.getMessage());
 612         } else if (ex instanceof NoSuchFieldException) {
 613             err = new NoSuchFieldError(ex.getMessage());
 614         } else {
 615             err = new IncompatibleClassChangeError();
 616         }
 617         return initCauseFrom(err, ex);
 618     }
 619 
 620     /**
 621      * Use best possible cause for err.initCause(), substituting the
 622      * cause for err itself if the cause has the same (or better) type.
 623      */
 624     static <E extends Error> E initCauseFrom(E err, Exception ex) {
 625         Throwable th = ex.getCause();
 626         @SuppressWarnings("unchecked")
 627         final Class<E> Eclass = (Class<E>) err.getClass();
 628         if (Eclass.isInstance(th))
 629            return Eclass.cast(th);
 630         err.initCause(th == null ? ex : th);
 631         return err;
 632     }
 633 
 634     /**
 635      * Is this method a caller-sensitive method?
 636      * I.e., does it call Reflection.getCallerClass or a similar method
 637      * to ask about the identity of its caller?
 638      */
 639     static boolean isCallerSensitive(MemberName mem) {
 640         if (!mem.isInvocable())  return false;  // fields are not caller sensitive
 641 
 642         return mem.isCallerSensitive() || canBeCalledVirtual(mem);
 643     }
 644 
 645     static boolean canBeCalledVirtual(MemberName mem) {
 646         assert(mem.isInvocable());
 647         switch (mem.getName()) {
 648         case "getContextClassLoader":
 649             return canBeCalledVirtual(mem, java.lang.Thread.class);
 650         }
 651         return false;
 652     }
 653 
 654     static boolean canBeCalledVirtual(MemberName symbolicRef, Class<?> definingClass) {
 655         Class<?> symbolicRefClass = symbolicRef.getDeclaringClass();
 656         if (symbolicRefClass == definingClass)  return true;
 657         if (symbolicRef.isStatic() || symbolicRef.isPrivate())  return false;
 658         return (definingClass.isAssignableFrom(symbolicRefClass) ||  // Msym overrides Mdef
 659                 symbolicRefClass.isInterface());                     // Mdef implements Msym
 660     }
 661 }