content: // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  package com.google.protobuf;
  
  /**
   * Grab-bag of utility functions useful when dealing with RPCs.
   *
   * @author kenton@google.com Kenton Varda
   */
  public final class RpcUtil {
    private RpcUtil() {}
  
    /**
     * Take an {@code RpcCallback<Message>} and convert it to an {@code RpcCallback} accepting a
     * specific message type. This is always type-safe (parameter type contravariance).
     */
    @SuppressWarnings("unchecked")
    public static <Type extends Message> RpcCallback<Type> specializeCallback(
        final RpcCallback<Message> originalCallback) {
      return (RpcCallback<Type>) originalCallback;
      // The above cast works, but only due to technical details of the Java
      // implementation.  A more theoretically correct -- but less efficient --
      // implementation would be as follows:
      //   return new RpcCallback<Type>() {
      //     public void run(Type parameter) {
      //       originalCallback.run(parameter);
      //     }
      //   };
    }
  
    /**
     * Take an {@code RpcCallback} accepting a specific message type and convert it to an {@code
     * RpcCallback<Message>}. The generalized callback will accept any message object which has the
     * same descriptor, and will convert it to the correct class before calling the original callback.
     * However, if the generalized callback is given a message with a different descriptor, an
     * exception will be thrown.
     */
    public static <Type extends Message> RpcCallback<Message> generalizeCallback(
        final RpcCallback<Type> originalCallback,
        final Class<Type> originalClass,
        final Type defaultInstance) {
      return new RpcCallback<Message>() {
        @Override
        public void run(final Message parameter) {
          Type typedParameter;
          try {
            typedParameter = originalClass.cast(parameter);
          } catch (ClassCastException ignored) {
            typedParameter = copyAsType(defaultInstance, parameter);
          }
          originalCallback.run(typedParameter);
        }
      };
    }
  
    /**
     * Creates a new message of type "Type" which is a copy of "source". "source" must have the same
     * descriptor but may be a different class (e.g. DynamicMessage).
     */
    @SuppressWarnings("unchecked")
    private static <Type extends Message> Type copyAsType(
        final Type typeDefaultInstance, final Message source) {
      return (Type) typeDefaultInstance.newBuilderForType().mergeFrom(source).build();
    }
  
    /**
     * Creates a callback which can only be called once. This may be useful for security, when passing
     * a callback to untrusted code: most callbacks do not expect to be called more than once, so
     * doing so may expose bugs if it is not prevented.
     */
    public static <ParameterType> RpcCallback<ParameterType> newOneTimeCallback(
        final RpcCallback<ParameterType> originalCallback) {
      return new RpcCallback<ParameterType>() {
        private boolean alreadyCalled = false;
  
        @Override
        public void run(final ParameterType parameter) {
          synchronized (this) {
            if (alreadyCalled) {
              throw new AlreadyCalledException();
            }
            alreadyCalled = true;
          }
  
          originalCallback.run(parameter);
        }
      };
    }
  
    /** Exception thrown when a one-time callback is called more than once. */
    public static final class AlreadyCalledException extends RuntimeException {
      private static final long serialVersionUID = 5469741279507848266L;
  
      public AlreadyCalledException() {
        super("This RpcCallback was already called and cannot be called multiple times.");
      }
    }
  }
  