1 /*
  2  * Copyright (c) 2008, 2023, 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.misc.VM;
 29 import jdk.internal.ref.CleanerFactory;
 30 import sun.invoke.util.Wrapper;
 31 
 32 import java.lang.invoke.MethodHandles.Lookup;
 33 import java.lang.reflect.Field;
 34 
 35 import static java.lang.invoke.MethodHandleNatives.Constants.*;
 36 import static java.lang.invoke.MethodHandleStatics.TRACE_METHOD_LINKAGE;
 37 import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
 38 
 39 /**
 40  * The JVM interface for the method handles package is all here.
 41  * This is an interface internal and private to an implementation of JSR 292.
 42  * <em>This class is not part of the JSR 292 standard.</em>
 43  * @author jrose
 44  */
 45 class MethodHandleNatives {
 46 
 47     private MethodHandleNatives() { } // static only
 48 
 49     /// MemberName support
 50 
 51     static native void init(MemberName self, Object ref);
 52     static native void expand(MemberName self);
 53     static native MemberName resolve(MemberName self, Class<?> caller, int lookupMode,
 54             boolean speculativeResolve) throws LinkageError, ClassNotFoundException;
 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     static native void copyOutBootstrapArguments(Class<?> caller, int[] indexInfo,
 69                                                  int start, int end,
 70                                                  Object[] buf, int pos,
 71                                                  boolean resolve,
 72                                                  Object ifNotAvailable);
 73 
 74     /** Represents a context to track nmethod dependencies on CallSite instance target. */
 75     static class CallSiteContext implements Runnable {
 76         //@Injected JVM_nmethodBucket* vmdependencies;
 77         //@Injected jlong last_cleanup;
 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 object constructor)
114             MN_IS_OBJECT_CONSTRUCTOR = 0x00020000, // object constructor
115             MN_IS_FIELD              = 0x00040000, // field
116             MN_IS_TYPE               = 0x00080000, // nested type
117             MN_CALLER_SENSITIVE      = 0x00100000, // @CallerSensitive annotation detected
118             MN_TRUSTED_FINAL         = 0x00200000, // trusted final field
119             MN_HIDDEN_MEMBER         = 0x00400000, // members defined in a hidden class or with @Hidden
120             MN_FLAT_FIELD            = 0x00800000, // flat field
121             MN_NULL_RESTRICTED       = 0x01000000, // null-restricted field
122             MN_REFERENCE_KIND_SHIFT  = 26, // refKind
123             MN_REFERENCE_KIND_MASK   = 0x3C000000 >> MN_REFERENCE_KIND_SHIFT;
124 
125         /**
126          * Constant pool reference-kind codes, as used by CONSTANT_MethodHandle CP entries.
127          */
128         static final byte
129             REF_NONE                    = 0,  // null value
130             REF_getField                = 1,
131             REF_getStatic               = 2,
132             REF_putField                = 3,
133             REF_putStatic               = 4,
134             REF_invokeVirtual           = 5,
135             REF_invokeStatic            = 6,
136             REF_invokeSpecial           = 7,
137             REF_newInvokeSpecial        = 8,
138             REF_invokeInterface         = 9,
139             REF_LIMIT                  = 10;
140 
141         /**
142          * Flags for Lookup.ClassOptions
143          */
144         static final int
145             NESTMATE_CLASS            = 0x00000001,
146             HIDDEN_CLASS              = 0x00000002,
147             STRONG_LOADER_LINK        = 0x00000004,
148             ACCESS_VM_ANNOTATIONS     = 0x00000008;
149 
150         /**
151          * Lookup modes
152          */
153         static final int
154             LM_MODULE        = Lookup.MODULE,
155             LM_UNCONDITIONAL = Lookup.UNCONDITIONAL,
156             LM_TRUSTED       = -1;
157 
158     }
159 
160     static boolean refKindIsValid(int refKind) {
161         return (refKind > REF_NONE && refKind < REF_LIMIT);
162     }
163     static boolean refKindIsField(byte refKind) {
164         assert(refKindIsValid(refKind));
165         return (refKind <= REF_putStatic);
166     }
167     static boolean refKindIsGetter(byte refKind) {
168         assert(refKindIsValid(refKind));
169         return (refKind <= REF_getStatic);
170     }
171     static boolean refKindIsSetter(byte refKind) {
172         return refKindIsField(refKind) && !refKindIsGetter(refKind);
173     }
174     static boolean refKindIsMethod(byte refKind) {
175         return !refKindIsField(refKind) && (refKind != REF_newInvokeSpecial);
176     }
177     static boolean refKindIsObjectConstructor(byte refKind) {
178         return (refKind == REF_newInvokeSpecial);
179     }
180     static boolean refKindHasReceiver(byte refKind) {
181         assert(refKindIsValid(refKind));
182         return (refKind & 1) != 0;
183     }
184     static boolean refKindIsStatic(byte refKind) {
185         return !refKindHasReceiver(refKind) && (refKind != REF_newInvokeSpecial);
186     }
187     static boolean refKindDoesDispatch(byte refKind) {
188         assert(refKindIsValid(refKind));
189         return (refKind == REF_invokeVirtual ||
190                 refKind == REF_invokeInterface);
191     }
192     static {
193         final int HR_MASK = ((1 << REF_getField) |
194                              (1 << REF_putField) |
195                              (1 << REF_invokeVirtual) |
196                              (1 << REF_invokeSpecial) |
197                              (1 << REF_invokeInterface)
198                             );
199         for (byte refKind = REF_NONE+1; refKind < REF_LIMIT; refKind++) {
200             assert(refKindHasReceiver(refKind) == (((1<<refKind) & HR_MASK) != 0)) : refKind;
201         }
202     }
203     static String refKindName(byte refKind) {
204         assert(refKindIsValid(refKind));
205         return switch (refKind) {
206             case REF_getField         -> "getField";
207             case REF_getStatic        -> "getStatic";
208             case REF_putField         -> "putField";
209             case REF_putStatic        -> "putStatic";
210             case REF_invokeVirtual    -> "invokeVirtual";
211             case REF_invokeStatic     -> "invokeStatic";
212             case REF_invokeSpecial    -> "invokeSpecial";
213             case REF_newInvokeSpecial -> "newInvokeSpecial";
214             case REF_invokeInterface  -> "invokeInterface";
215             default -> "REF_???";
216         };
217     }
218 
219     private static native int getNamedCon(int which, Object[] name);
220     static boolean verifyConstants() {
221         Object[] box = { null };
222         for (int i = 0; ; i++) {
223             box[0] = null;
224             int vmval = getNamedCon(i, box);
225             if (box[0] == null)  break;
226             String name = (String) box[0];
227             try {
228                 Field con = Constants.class.getDeclaredField(name);
229                 int jval = con.getInt(null);
230                 if (jval == vmval)  continue;
231                 String err = (name+": JVM has "+vmval+" while Java has "+jval);
232                 if (name.equals("CONV_OP_LIMIT")) {
233                     System.err.println("warning: "+err);
234                     continue;
235                 }
236                 throw new InternalError(err);
237             } catch (NoSuchFieldException | IllegalAccessException ex) {
238                 String err = (name+": JVM has "+vmval+" which Java does not define");
239                 // ignore exotic ops the JVM cares about; we just won't issue them
240                 //System.err.println("warning: "+err);
241                 continue;
242             }
243         }
244         return true;
245     }
246     static {
247         VM.setJavaLangInvokeInited();
248         assert(verifyConstants());
249     }
250 
251     // Up-calls from the JVM.
252     // These must NOT be public.
253 
254     /**
255      * The JVM is linking an invokedynamic instruction.  Create a reified call site for it.
256      */
257     static MemberName linkCallSite(Object callerObj,
258                                    Object bootstrapMethodObj,
259                                    Object nameObj, Object typeObj,
260                                    Object staticArguments,
261                                    Object[] appendixResult) {
262         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
263         Class<?> caller = (Class<?>)callerObj;
264         String name = nameObj.toString().intern();
265         MethodType type = (MethodType)typeObj;
266         if (!TRACE_METHOD_LINKAGE)
267             return linkCallSiteImpl(caller, bootstrapMethod, name, type,
268                                     staticArguments, appendixResult);
269         return linkCallSiteTracing(caller, bootstrapMethod, name, type,
270                                    staticArguments, appendixResult);
271     }
272     static MemberName linkCallSiteImpl(Class<?> caller,
273                                        MethodHandle bootstrapMethod,
274                                        String name, MethodType type,
275                                        Object staticArguments,
276                                        Object[] appendixResult) {
277         CallSite callSite = CallSite.makeSite(bootstrapMethod,
278                                               name,
279                                               type,
280                                               staticArguments,
281                                               caller);
282         if (TRACE_METHOD_LINKAGE) {
283             MethodHandle target = callSite.getTarget();
284             System.out.println("linkCallSite target class => " + target.getClass().getName());
285             System.out.println("linkCallSite target => " + target.debugString(0));
286         }
287 
288         if (callSite instanceof ConstantCallSite) {
289             appendixResult[0] = callSite.dynamicInvoker();
290             return Invokers.linkToTargetMethod(type);
291         } else {
292             appendixResult[0] = callSite;
293             return Invokers.linkToCallSiteMethod(type);
294         }
295     }
296     // Tracing logic:
297     static MemberName linkCallSiteTracing(Class<?> caller,
298                                           MethodHandle bootstrapMethod,
299                                           String name, MethodType type,
300                                           Object staticArguments,
301                                           Object[] appendixResult) {
302         Object bsmReference = bootstrapMethod.internalMemberName();
303         if (bsmReference == null)  bsmReference = bootstrapMethod;
304         String staticArglist = staticArglistForTrace(staticArguments);
305         System.out.println("linkCallSite "+getCallerInfo(caller)+" "+
306                            bsmReference+" "+
307                            name+type+"/"+staticArglist);
308         try {
309             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
310                                               staticArguments, appendixResult);
311             System.out.println("linkCallSite linkage => "+res+" + "+appendixResult[0]);
312             return res;
313         } catch (Throwable ex) {
314             ex.printStackTrace(); // print now in case exception is swallowed
315             System.out.println("linkCallSite => throw "+ex);
316             throw ex;
317         }
318     }
319 
320     /**
321      * Return a human-readable description of the caller. Something like
322      * "java.base/java.security.Security.<clinit>(Security.java:82)"
323      */
324     private static String getCallerInfo(Class<?> caller) {
325         for (StackTraceElement e : Thread.currentThread().getStackTrace()) {
326             if (e.getClassName().equals(caller.getName())) {
327                 return e.toString();
328             }
329         }
330         // fallback if the caller is somehow missing from the stack.
331         return caller.getName();
332     }
333 
334     // this implements the upcall from the JVM, MethodHandleNatives.linkDynamicConstant:
335     static Object linkDynamicConstant(Object callerObj,
336                                       Object bootstrapMethodObj,
337                                       Object nameObj, Object typeObj,
338                                       Object staticArguments) {
339         MethodHandle bootstrapMethod = (MethodHandle)bootstrapMethodObj;
340         Class<?> caller = (Class<?>)callerObj;
341         String name = nameObj.toString().intern();
342         Class<?> type = (Class<?>)typeObj;
343         if (!TRACE_METHOD_LINKAGE)
344             return linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
345         return linkDynamicConstantTracing(caller, bootstrapMethod, name, type, staticArguments);
346     }
347 
348     static Object linkDynamicConstantImpl(Class<?> caller,
349                                           MethodHandle bootstrapMethod,
350                                           String name, Class<?> type,
351                                           Object staticArguments) {
352         return ConstantBootstraps.makeConstant(bootstrapMethod, name, type, staticArguments, caller);
353     }
354 
355     private static String staticArglistForTrace(Object staticArguments) {
356         if (staticArguments instanceof Object[] array)
357             return "BSA="+java.util.Arrays.asList(array);
358         if (staticArguments instanceof int[] array)
359             return "BSA@"+java.util.Arrays.toString(array);
360         if (staticArguments == null)
361             return "BSA0=null";
362         return "BSA1="+staticArguments;
363     }
364 
365     // Tracing logic:
366     static Object linkDynamicConstantTracing(Class<?> caller,
367                                              MethodHandle bootstrapMethod,
368                                              String name, Class<?> type,
369                                              Object staticArguments) {
370         Object bsmReference = bootstrapMethod.internalMemberName();
371         if (bsmReference == null)  bsmReference = bootstrapMethod;
372         String staticArglist = staticArglistForTrace(staticArguments);
373         System.out.println("linkDynamicConstant "+caller.getName()+" "+
374                            bsmReference+" "+
375                            name+type+"/"+staticArglist);
376         try {
377             Object res = linkDynamicConstantImpl(caller, bootstrapMethod, name, type, staticArguments);
378             System.out.println("linkDynamicConstantImpl => "+res);
379             return res;
380         } catch (Throwable ex) {
381             ex.printStackTrace(); // print now in case exception is swallowed
382             System.out.println("linkDynamicConstant => throw "+ex);
383             throw ex;
384         }
385     }
386 
387     /** The JVM is requesting pull-mode bootstrap when it provides
388      *  a tuple of the form int[]{ argc, vmindex }.
389      *  The BSM is expected to call back to the JVM using the caller
390      *  class and vmindex to resolve the static arguments.
391      */
392     static boolean staticArgumentsPulled(Object staticArguments) {
393         return staticArguments instanceof int[];
394     }
395 
396     /** A BSM runs in pull-mode if and only if its sole arguments
397      * are (Lookup, BootstrapCallInfo), or can be converted pairwise
398      * to those types, and it is not of variable arity.
399      * Excluding error cases, we can just test that the arity is a constant 2.
400      *
401      * NOTE: This method currently returns false, since pulling is not currently
402      * exposed to a BSM. When pull mode is supported the method block will be
403      * replaced with currently commented out code.
404      */
405     static boolean isPullModeBSM(MethodHandle bsm) {
406         return false;
407 //        return bsm.type().parameterCount() == 2 && !bsm.isVarargsCollector();
408     }
409 
410     /**
411      * The JVM wants a pointer to a MethodType.  Oblige it by finding or creating one.
412      */
413     static MethodType findMethodHandleType(Class<?> rtype, Class<?>[] ptypes) {
414         return MethodType.methodType(rtype, ptypes, true);
415     }
416 
417     /**
418      * The JVM wants to link a call site that requires a dynamic type check.
419      * Name is a type-checking invoker, invokeExact or invoke.
420      * Return a JVM method (MemberName) to handle the invoking.
421      * The method assumes the following arguments on the stack:
422      * 0: the method handle being invoked
423      * 1-N: the arguments to the method handle invocation
424      * N+1: an optional, implicitly added argument (typically the given MethodType)
425      * <p>
426      * The nominal method at such a call site is an instance of
427      * a signature-polymorphic method (see @PolymorphicSignature).
428      * Such method instances are user-visible entities which are
429      * "split" from the generic placeholder method in {@code MethodHandle}.
430      * (Note that the placeholder method is not identical with any of
431      * its instances.  If invoked reflectively, is guaranteed to throw an
432      * {@code UnsupportedOperationException}.)
433      * If the signature-polymorphic method instance is ever reified,
434      * it appears as a "copy" of the original placeholder
435      * (a native final member of {@code MethodHandle}) except
436      * that its type descriptor has shape required by the instance,
437      * and the method instance is <em>not</em> varargs.
438      * The method instance is also marked synthetic, since the
439      * method (by definition) does not appear in Java source code.
440      * <p>
441      * The JVM is allowed to reify this method as instance metadata.
442      * For example, {@code invokeBasic} is always reified.
443      * But the JVM may instead call {@code linkMethod}.
444      * If the result is an * ordered pair of a {@code (method, appendix)},
445      * the method gets all the arguments (0..N inclusive)
446      * plus the appendix (N+1), and uses the appendix to complete the call.
447      * In this way, one reusable method (called a "linker method")
448      * can perform the function of any number of polymorphic instance
449      * methods.
450      * <p>
451      * Linker methods are allowed to be weakly typed, with any or
452      * all references rewritten to {@code Object} and any primitives
453      * (except {@code long}/{@code float}/{@code double})
454      * rewritten to {@code int}.
455      * A linker method is trusted to return a strongly typed result,
456      * according to the specific method type descriptor of the
457      * signature-polymorphic instance it is emulating.
458      * This can involve (as necessary) a dynamic check using
459      * data extracted from the appendix argument.
460      * <p>
461      * The JVM does not inspect the appendix, other than to pass
462      * it verbatim to the linker method at every call.
463      * This means that the JDK runtime has wide latitude
464      * for choosing the shape of each linker method and its
465      * corresponding appendix.
466      * Linker methods should be generated from {@code LambdaForm}s
467      * so that they do not become visible on stack traces.
468      * <p>
469      * The {@code linkMethod} call is free to omit the appendix
470      * (returning null) and instead emulate the required function
471      * completely in the linker method.
472      * As a corner case, if N==255, no appendix is possible.
473      * In this case, the method returned must be custom-generated to
474      * perform any needed type checking.
475      * <p>
476      * If the JVM does not reify a method at a call site, but instead
477      * calls {@code linkMethod}, the corresponding call represented
478      * in the bytecodes may mention a valid method which is not
479      * representable with a {@code MemberName}.
480      * Therefore, use cases for {@code linkMethod} tend to correspond to
481      * special cases in reflective code such as {@code findVirtual}
482      * or {@code revealDirect}.
483      */
484     static MemberName linkMethod(Class<?> callerClass, int refKind,
485                                  Class<?> defc, String name, Object type,
486                                  Object[] appendixResult) {
487         if (!TRACE_METHOD_LINKAGE)
488             return linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
489         return linkMethodTracing(callerClass, refKind, defc, name, type, appendixResult);
490     }
491     static MemberName linkMethodImpl(Class<?> callerClass, int refKind,
492                                      Class<?> defc, String name, Object type,
493                                      Object[] appendixResult) {
494         try {
495             if (refKind == REF_invokeVirtual) {
496                 if (defc == MethodHandle.class) {
497                     return Invokers.methodHandleInvokeLinkerMethod(
498                             name, fixMethodType(callerClass, type), appendixResult);
499                 } else if (defc == VarHandle.class) {
500                     return varHandleOperationLinkerMethod(
501                             name, fixMethodType(callerClass, type), appendixResult);
502                 }
503             }
504         } catch (Error e) {
505             // Pass through an Error, including say StackOverflowError or
506             // OutOfMemoryError
507             throw e;
508         } catch (Throwable ex) {
509             // Wrap anything else in LinkageError
510             throw new LinkageError(ex.getMessage(), ex);
511         }
512         throw new LinkageError("no such method "+defc.getName()+"."+name+type);
513     }
514     private static MethodType fixMethodType(Class<?> callerClass, Object type) {
515         if (type instanceof MethodType mt)
516             return mt;
517         else
518             return MethodType.fromDescriptor((String)type, callerClass.getClassLoader());
519     }
520     // Tracing logic:
521     static MemberName linkMethodTracing(Class<?> callerClass, int refKind,
522                                         Class<?> defc, String name, Object type,
523                                         Object[] appendixResult) {
524         System.out.println("linkMethod "+defc.getName()+"."+
525                            name+type+"/"+Integer.toHexString(refKind));
526         try {
527             MemberName res = linkMethodImpl(callerClass, refKind, defc, name, type, appendixResult);
528             System.out.println("linkMethod => "+res+" + "+appendixResult[0]);
529             return res;
530         } catch (Throwable ex) {
531             System.out.println("linkMethod => throw "+ex);
532             throw ex;
533         }
534     }
535 
536     /**
537      * Obtain the method to link to the VarHandle operation.
538      * This method is located here and not in Invokers to avoid
539      * initializing that and other classes early on in VM bootup.
540      */
541     private static MemberName varHandleOperationLinkerMethod(String name,
542                                                              MethodType mtype,
543                                                              Object[] appendixResult) {
544         // Get the signature method type
545         final MethodType sigType = mtype.basicType();
546 
547         // Get the access kind from the method name
548         VarHandle.AccessMode ak;
549         try {
550             ak = VarHandle.AccessMode.valueFromMethodName(name);
551         } catch (IllegalArgumentException e) {
552             throw MethodHandleStatics.newInternalError(e);
553         }
554 
555         // Create the appendix descriptor constant
556         VarHandle.AccessDescriptor ad = new VarHandle.AccessDescriptor(mtype, ak.at.ordinal(), ak.ordinal());
557         appendixResult[0] = ad;
558 
559         if (MethodHandleStatics.VAR_HANDLE_GUARDS) {
560             // If not polymorphic in the return type, such as the compareAndSet
561             // methods that return boolean
562             Class<?> guardReturnType = sigType.returnType();
563             if (ak.at.isMonomorphicInReturnType) {
564                 if (ak.at.returnType != mtype.returnType()) {
565                     // The caller contains a different return type than that
566                     // defined by the method
567                     throw newNoSuchMethodErrorOnVarHandle(name, mtype);
568                 }
569                 // Adjust the return type of the signature method type
570                 guardReturnType = ak.at.returnType;
571             }
572 
573             // Get the guard method type for linking
574             final Class<?>[] guardParams = new Class<?>[sigType.parameterCount() + 2];
575             // VarHandle at start
576             guardParams[0] = VarHandle.class;
577             for (int i = 0; i < sigType.parameterCount(); i++) {
578                 guardParams[i + 1] = sigType.parameterType(i);
579             }
580             // Access descriptor at end
581             guardParams[guardParams.length - 1] = VarHandle.AccessDescriptor.class;
582             MethodType guardType = MethodType.methodType(guardReturnType, guardParams, true);
583 
584             MemberName linker = new MemberName(
585                     VarHandleGuards.class, getVarHandleGuardMethodName(guardType),
586                     guardType, REF_invokeStatic);
587 
588             linker = MemberName.getFactory().resolveOrNull(REF_invokeStatic, linker,
589                                                            VarHandleGuards.class, LM_TRUSTED);
590             if (linker != null) {
591                 return linker;
592             }
593             // Fall back to lambda form linkage if guard method is not available
594             // TODO Optionally log fallback ?
595         }
596         return Invokers.varHandleInvokeLinkerMethod(mtype);
597     }
598     static String getVarHandleGuardMethodName(MethodType guardType) {
599         String prefix = "guard_";
600         StringBuilder sb = new StringBuilder(prefix.length() + guardType.parameterCount());
601 
602         sb.append(prefix);
603         for (int i = 1; i < guardType.parameterCount() - 1; i++) {
604             Class<?> pt = guardType.parameterType(i);
605             sb.append(getCharErasedType(pt));
606         }
607         sb.append('_').append(getCharErasedType(guardType.returnType()));
608         return sb.toString();
609     }
610     static char getCharErasedType(Class<?> pt) {
611         return Wrapper.forBasicType(pt).basicTypeChar();
612     }
613     static NoSuchMethodError newNoSuchMethodErrorOnVarHandle(String name, MethodType mtype) {
614         return new NoSuchMethodError("VarHandle." + name + mtype);
615     }
616 
617     /**
618      * The JVM is resolving a CONSTANT_MethodHandle CP entry.  And it wants our help.
619      * It will make an up-call to this method.  (Do not change the name or signature.)
620      * The type argument is a Class for field requests and a MethodType for non-fields.
621      * <p>
622      * Recent versions of the JVM may also pass a resolved MemberName for the type.
623      * In that case, the name is ignored and may be null.
624      */
625     static MethodHandle linkMethodHandleConstant(Class<?> callerClass, int refKind,
626                                                  Class<?> defc, String name, Object type) {
627         try {
628             Lookup lookup = IMPL_LOOKUP.in(callerClass);
629             assert(refKindIsValid(refKind));
630             return lookup.linkMethodHandleConstant((byte) refKind, defc, name, type);
631         } catch (ReflectiveOperationException ex) {
632             throw mapLookupExceptionToError(ex);
633         }
634     }
635 
636     /**
637      * Map a reflective exception to a linkage error.
638      */
639     static LinkageError mapLookupExceptionToError(ReflectiveOperationException ex) {
640         LinkageError err;
641         if (ex instanceof IllegalAccessException) {
642             Throwable cause = ex.getCause();
643             if (cause instanceof AbstractMethodError ame) {
644                 return ame;
645             } else {
646                 err = new IllegalAccessError(ex.getMessage());
647             }
648         } else if (ex instanceof NoSuchMethodException) {
649             err = new NoSuchMethodError(ex.getMessage());
650         } else if (ex instanceof NoSuchFieldException) {
651             err = new NoSuchFieldError(ex.getMessage());
652         } else {
653             err = new IncompatibleClassChangeError();
654         }
655         return initCauseFrom(err, ex);
656     }
657 
658     /**
659      * Use best possible cause for err.initCause(), substituting the
660      * cause for err itself if the cause has the same (or better) type.
661      */
662     static <E extends Error> E initCauseFrom(E err, Exception ex) {
663         Throwable th = ex.getCause();
664         @SuppressWarnings("unchecked")
665         final Class<E> Eclass = (Class<E>) err.getClass();
666         if (Eclass.isInstance(th))
667            return Eclass.cast(th);
668         err.initCause(th == null ? ex : th);
669         return err;
670     }
671 
672     /**
673      * Is this method a caller-sensitive method?
674      * I.e., does it call Reflection.getCallerClass or a similar method
675      * to ask about the identity of its caller?
676      */
677     static boolean isCallerSensitive(MemberName mem) {
678         if (!mem.isInvocable())  return false;  // fields are not caller sensitive
679 
680         return mem.isCallerSensitive() || canBeCalledVirtual(mem);
681     }
682 
683     static boolean canBeCalledVirtual(MemberName mem) {
684         assert(mem.isInvocable());
685         return mem.getName().equals("getContextClassLoader") && canBeCalledVirtual(mem, java.lang.Thread.class);
686     }
687 
688     static boolean canBeCalledVirtual(MemberName symbolicRef, Class<?> definingClass) {
689         Class<?> symbolicRefClass = symbolicRef.getDeclaringClass();
690         if (symbolicRefClass == definingClass)  return true;
691         if (symbolicRef.isStatic() || symbolicRef.isPrivate())  return false;
692         return (definingClass.isAssignableFrom(symbolicRefClass) ||  // Msym overrides Mdef
693                 symbolicRefClass.isInterface());                     // Mdef implements Msym
694     }
695 }