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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * 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
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
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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();
      }
  }
  