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  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

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  *

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  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

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  *

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 package java.lang.invoke;

 /**

  * A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed.

  * An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently

  * bound to the call site's target.

  * @author John Rose, JSR 292 EG

  */

 public class ConstantCallSite extends CallSite {

     private final boolean isFrozen;

     /**

      * Creates a call site with a permanent target.

      * @param target the target to be permanently associated with this call site

      * @throws NullPointerException if the proposed target is null

      */

     public ConstantCallSite(MethodHandle target) {

         super(target);

         isFrozen = true;

     }

     /**

      * Creates a call site with a permanent target, possibly bound to the call site itself.

      * <p>

      * During construction of the call site, the {@code createTargetHook} is invoked to

      * produce the actual target, as if by a call of the form

      * {@code (MethodHandle) createTargetHook.invoke(this)}.

      * <p>

      * Note that user code cannot perform such an action directly in a subclass constructor,

      * since the target must be fixed before the {@code ConstantCallSite} constructor returns.

      * <p>

      * The hook is said to bind the call site to a target method handle,

      * and a typical action would be {@code someTarget.bindTo(this)}.

      * However, the hook is free to take any action whatever,

      * including ignoring the call site and returning a constant target.

      * <p>

      * The result returned by the hook must be a method handle of exactly

      * the same type as the call site.

      * <p>

      * While the hook is being called, the new {@code ConstantCallSite}

      * object is in a partially constructed state.

      * In this state,

      * a call to {@code getTarget}, or any other attempt to use the target,

      * will result in an {@code IllegalStateException}.

      * It is legal at all times to obtain the call site's type using the {@code type} method.

      *

      * @param targetType the type of the method handle to be permanently associated with this call site

      * @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target

      * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,

      *         or if the target returned by the hook is not of the given {@code targetType}

      * @throws NullPointerException if the hook returns a null value

      * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}

      * @throws Throwable anything else thrown by the hook function

      */

     protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {

         super(targetType, createTargetHook);

         isFrozen = true;

     }

     /**

      * Returns the target method of the call site, which behaves

      * like a {@code final} field of the {@code ConstantCallSite}.

      * That is, the target is always the original value passed

      * to the constructor call which created this instance.

      *

      * @return the immutable linkage state of this call site, a constant method handle

      * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed

      */

     @Override public final MethodHandle getTarget() {

         if (!isFrozen)  throw new IllegalStateException();

         return target;

     }

     /**

      * Always throws an {@link UnsupportedOperationException}.

      * This kind of call site cannot change its target.

      * @param ignore a new target proposed for the call site, which is ignored

      * @throws UnsupportedOperationException because this kind of call site cannot change its target

      */

     @Override public final void setTarget(MethodHandle ignore) {

         throw new UnsupportedOperationException();

     }

     /**

      * Returns this call site's permanent target.

      * Since that target will never change, this is a correct implementation

      * of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.

      * @return the immutable linkage state of this call site, a constant method handle

      * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed

      */

     @Override

     public final MethodHandle dynamicInvoker() {

         return getTarget();

     }

 }