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   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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   25   
   26   package java.lang.invoke;
   27   
   28   import sun.invoke.empty.Empty;
   29   import sun.misc.Unsafe;
   30   import static java.lang.invoke.MethodHandleStatics.*;
   31   import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
   32   
   33   /**
   34    * A {@code CallSite} is a holder for a variable {@link MethodHandle},
   35    * which is called its {@code target}.
   36    * An {@code invokedynamic} instruction linked to a {@code CallSite} delegates
   37    * all calls to the site's current target.
   38    * A {@code CallSite} may be associated with several {@code invokedynamic}
   39    * instructions, or it may be "free floating", associated with none.
   40    * In any case, it may be invoked through an associated method handle
   41    * called its {@linkplain #dynamicInvoker dynamic invoker}.
   42    * <p>
   43    * {@code CallSite} is an abstract class which does not allow
   44    * direct subclassing by users.  It has three immediate,
   45    * concrete subclasses that may be either instantiated or subclassed.
   46    * <ul>
   47    * <li>If a mutable target is not required, an {@code invokedynamic} instruction
   48    * may be permanently bound by means of a {@linkplain ConstantCallSite constant call site}.
   49    * <li>If a mutable target is required which has volatile variable semantics,
   50    * because updates to the target must be immediately and reliably witnessed by other threads,
   51    * a {@linkplain VolatileCallSite volatile call site} may be used.
   52    * <li>Otherwise, if a mutable target is required,
   53    * a {@linkplain MutableCallSite mutable call site} may be used.
   54    * </ul>
   55    * <p>
   56    * A non-constant call site may be <em>relinked</em> by changing its target.
   57    * The new target must have the same {@linkplain MethodHandle#type() type}
   58    * as the previous target.
   59    * Thus, though a call site can be relinked to a series of
   60    * successive targets, it cannot change its type.
   61    * <p>
   62    * Here is a sample use of call sites and bootstrap methods which links every
   63    * dynamic call site to print its arguments:
   64   <blockquote><pre><!-- see indy-demo/src/PrintArgsDemo.java -->
   65   static void test() throws Throwable {
   66       // THE FOLLOWING LINE IS PSEUDOCODE FOR A JVM INSTRUCTION
   67       InvokeDynamic[#bootstrapDynamic].baz("baz arg", 2, 3.14);
   68   }
   69   private static void printArgs(Object... args) {
   70     System.out.println(java.util.Arrays.deepToString(args));
   71   }
   72   private static final MethodHandle printArgs;
   73   static {
   74     MethodHandles.Lookup lookup = MethodHandles.lookup();
   75     Class thisClass = lookup.lookupClass();  // (who am I?)
   76     printArgs = lookup.findStatic(thisClass,
   77         "printArgs", MethodType.methodType(void.class, Object[].class));
   78   }
   79   private static CallSite bootstrapDynamic(MethodHandles.Lookup caller, String name, MethodType type) {
   80     // ignore caller and name, but match the type:
   81     return new ConstantCallSite(printArgs.asType(type));
   82   }
   83   </pre></blockquote>
   84    * @author John Rose, JSR 292 EG
   85    */
   86   abstract
   87   public class CallSite {
   88       static { MethodHandleImpl.initStatics(); }
   89   
   90       // Fields used only by the JVM.  Do not use or change.
   91       private MemberName vmmethod; // supplied by the JVM (ref. to calling method)
   92       private int        vmindex;  // supplied by the JVM (BCI within calling method)
   93   
   94       // The actual payload of this call site:
   95       /*package-private*/
   96       MethodHandle target;
   97   
   98       /**
   99        * Make a blank call site object with the given method type.
  100        * An initial target method is supplied which will throw
  101        * an {@link IllegalStateException} if called.
  102        * <p>
  103        * Before this {@code CallSite} object is returned from a bootstrap method,
  104        * it is usually provided with a more useful target method,
  105        * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
  106        * @throws NullPointerException if the proposed type is null
  107        */
  108       /*package-private*/
  109       CallSite(MethodType type) {
  110           target = type.invokers().uninitializedCallSite();
  111       }
  112   
  113       /**
  114        * Make a call site object equipped with an initial target method handle.
  115        * @param target the method handle which will be the initial target of the call site
  116        * @throws NullPointerException if the proposed target is null
  117        */
  118       /*package-private*/
  119       CallSite(MethodHandle target) {
  120           target.type();  // null check
  121           this.target = target;
  122       }
  123   
  124       /**
  125        * Make a call site object equipped with an initial target method handle.
  126        * @param targetType the desired type of the call site
  127        * @param createTargetHook a hook which will bind the call site to the target method handle
  128        * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,
  129        *         or if the target returned by the hook is not of the given {@code targetType}
  130        * @throws NullPointerException if the hook returns a null value
  131        * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
  132        * @throws Throwable anything else thrown by the the hook function
  133        */
  134       /*package-private*/
  135       CallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
  136           this(targetType);
  137           ConstantCallSite selfCCS = (ConstantCallSite) this;
  138           MethodHandle boundTarget = (MethodHandle) createTargetHook.invokeWithArguments(selfCCS);
  139           checkTargetChange(this.target, boundTarget);
  140           this.target = boundTarget;
  141       }
  142   
  143       /**
  144        * Returns the type of this call site's target.
  145        * Although targets may change, any call site's type is permanent, and can never change to an unequal type.
  146        * The {@code setTarget} method enforces this invariant by refusing any new target that does
  147        * not have the previous target's type.
  148        * @return the type of the current target, which is also the type of any future target
  149        */
  150       public MethodType type() {
  151           // warning:  do not call getTarget here, because CCS.getTarget can throw IllegalStateException
  152           return target.type();
  153       }
  154   
  155       /** Called from JVM (or low-level Java code) after the BSM returns the newly created CallSite.
  156        *  The parameters are JVM-specific.
  157        */
  158       void initializeFromJVM(String name,
  159                              MethodType type,
  160                              MemberName callerMethod,
  161                              int        callerBCI) {
  162           if (this.vmmethod != null) {
  163               // FIXME
  164               throw new BootstrapMethodError("call site has already been linked to an invokedynamic instruction");
  165           }
  166           if (!this.type().equals(type)) {
  167               throw wrongTargetType(target, type);
  168           }
  169           this.vmindex  = callerBCI;
  170           this.vmmethod = callerMethod;
  171       }
  172   
  173       /**
  174        * Returns the target method of the call site, according to the
  175        * behavior defined by this call site's specific class.
  176        * The immediate subclasses of {@code CallSite} document the
  177        * class-specific behaviors of this method.
  178        *
  179        * @return the current linkage state of the call site, its target method handle
  180        * @see ConstantCallSite
  181        * @see VolatileCallSite
  182        * @see #setTarget
  183        * @see ConstantCallSite#getTarget
  184        * @see MutableCallSite#getTarget
  185        * @see VolatileCallSite#getTarget
  186        */
  187       public abstract MethodHandle getTarget();
  188   
  189       /**
  190        * Updates the target method of this call site, according to the
  191        * behavior defined by this call site's specific class.
  192        * The immediate subclasses of {@code CallSite} document the
  193        * class-specific behaviors of this method.
  194        * <p>
  195        * The type of the new target must be {@linkplain MethodType#equals equal to}
  196        * the type of the old target.
  197        *
  198        * @param newTarget the new target
  199        * @throws NullPointerException if the proposed new target is null
  200        * @throws WrongMethodTypeException if the proposed new target
  201        *         has a method type that differs from the previous target
  202        * @see CallSite#getTarget
  203        * @see ConstantCallSite#setTarget
  204        * @see MutableCallSite#setTarget
  205        * @see VolatileCallSite#setTarget
  206        */
  207       public abstract void setTarget(MethodHandle newTarget);
  208   
  209       void checkTargetChange(MethodHandle oldTarget, MethodHandle newTarget) {
  210           MethodType oldType = oldTarget.type();
  211           MethodType newType = newTarget.type();  // null check!
  212           if (!newType.equals(oldType))
  213               throw wrongTargetType(newTarget, oldType);
  214       }
  215   
  216       private static WrongMethodTypeException wrongTargetType(MethodHandle target, MethodType type) {
  217           return new WrongMethodTypeException(String.valueOf(target)+" should be of type "+type);
  218       }
  219   
  220       /**
  221        * Produces a method handle equivalent to an invokedynamic instruction
  222        * which has been linked to this call site.
  223        * <p>
  224        * This method is equivalent to the following code:
  225        * <blockquote><pre>
  226        * MethodHandle getTarget, invoker, result;
  227        * getTarget = MethodHandles.publicLookup().bind(this, "getTarget", MethodType.methodType(MethodHandle.class));
  228        * invoker = MethodHandles.exactInvoker(this.type());
  229        * result = MethodHandles.foldArguments(invoker, getTarget)
  230        * </pre></blockquote>
  231        *
  232        * @return a method handle which always invokes this call site's current target
  233        */
  234       public abstract MethodHandle dynamicInvoker();
  235   
  236       /*non-public*/ MethodHandle makeDynamicInvoker() {
  237           MethodHandle getTarget = MethodHandleImpl.bindReceiver(GET_TARGET, this);
  238           MethodHandle invoker = MethodHandles.exactInvoker(this.type());
  239           return MethodHandles.foldArguments(invoker, getTarget);
  240       }
  241   
  242       private static final MethodHandle GET_TARGET;
  243       static {
  244           try {
  245               GET_TARGET = IMPL_LOOKUP.
  246                   findVirtual(CallSite.class, "getTarget", MethodType.methodType(MethodHandle.class));
  247           } catch (ReflectiveOperationException ignore) {
  248               throw new InternalError();
  249           }
  250       }
  251   
  252       /** This guy is rolled into the default target if a MethodType is supplied to the constructor. */
  253       /*package-private*/
  254       static Empty uninitializedCallSite() {
  255           throw new IllegalStateException("uninitialized call site");
  256       }
  257   
  258       // unsafe stuff:
  259       private static final Unsafe unsafe = Unsafe.getUnsafe();
  260       private static final long TARGET_OFFSET;
  261   
  262       static {
  263           try {
  264               TARGET_OFFSET = unsafe.objectFieldOffset(CallSite.class.getDeclaredField("target"));
  265           } catch (Exception ex) { throw new Error(ex); }
  266       }
  267   
  268       /*package-private*/
  269       void setTargetNormal(MethodHandle newTarget) {
  270           target = newTarget;
  271       }
  272       /*package-private*/
  273       MethodHandle getTargetVolatile() {
  274           return (MethodHandle) unsafe.getObjectVolatile(this, TARGET_OFFSET);
  275       }
  276       /*package-private*/
  277       void setTargetVolatile(MethodHandle newTarget) {
  278           unsafe.putObjectVolatile(this, TARGET_OFFSET, newTarget);
  279       }
  280   
  281       // this implements the upcall from the JVM, MethodHandleNatives.makeDynamicCallSite:
  282       static CallSite makeSite(MethodHandle bootstrapMethod,
  283                                // Callee information:
  284                                String name, MethodType type,
  285                                // Extra arguments for BSM, if any:
  286                                Object info,
  287                                // Caller information:
  288                                MemberName callerMethod, int callerBCI) {
  289           Class<?> callerClass = callerMethod.getDeclaringClass();
  290           Object caller = IMPL_LOOKUP.in(callerClass);
  291           CallSite site;
  292           try {
  293               Object binding;
  294               info = maybeReBox(info);
  295               if (info == null) {
  296                   binding = bootstrapMethod.invoke(caller, name, type);
  297               } else if (!info.getClass().isArray()) {
  298                   binding = bootstrapMethod.invoke(caller, name, type, info);
  299               } else {
  300                   Object[] argv = (Object[]) info;
  301                   maybeReBoxElements(argv);
  302                   if (3 + argv.length > 255)
  303                       throw new BootstrapMethodError("too many bootstrap method arguments");
  304                   MethodType bsmType = bootstrapMethod.type();
  305                   if (bsmType.parameterCount() == 4 && bsmType.parameterType(3) == Object[].class)
  306                       binding = bootstrapMethod.invoke(caller, name, type, argv);
  307                   else
  308                       binding = MethodHandles.spreadInvoker(bsmType, 3)
  309                           .invoke(bootstrapMethod, caller, name, type, argv);
  310               }
  311               //System.out.println("BSM for "+name+type+" => "+binding);
  312               if (binding instanceof CallSite) {
  313                   site = (CallSite) binding;
  314               }  else {
  315                   throw new ClassCastException("bootstrap method failed to produce a CallSite");
  316               }
  317               if (!site.getTarget().type().equals(type))
  318                   throw new WrongMethodTypeException("wrong type: "+site.getTarget());
  319           } catch (Throwable ex) {
  320               BootstrapMethodError bex;
  321               if (ex instanceof BootstrapMethodError)
  322                   bex = (BootstrapMethodError) ex;
  323               else
  324                   bex = new BootstrapMethodError("call site initialization exception", ex);
  325               throw bex;
  326           }
  327           return site;
  328       }
  329   
  330       private static Object maybeReBox(Object x) {
  331           if (x instanceof Integer) {
  332               int xi = (int) x;
  333               if (xi == (byte) xi)
  334                   x = xi;  // must rebox; see JLS 5.1.7
  335           }
  336           return x;
  337       }
  338       private static void maybeReBoxElements(Object[] xa) {
  339           for (int i = 0; i < xa.length; i++) {
  340               xa[i] = maybeReBox(xa[i]);
  341           }
  342       }
  343   }

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