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      1 // Protocol Buffers - Google's data interchange format
      2 // Copyright 2008 Google Inc.  All rights reserved.
      3 // http://code.google.com/p/protobuf/
      4 //
      5 // Redistribution and use in source and binary forms, with or without
      6 // modification, are permitted provided that the following conditions are
      7 // met:
      8 //
      9 //     * Redistributions of source code must retain the above copyright
     10 // notice, this list of conditions and the following disclaimer.
     11 //     * Redistributions in binary form must reproduce the above
     12 // copyright notice, this list of conditions and the following disclaimer
     13 // in the documentation and/or other materials provided with the
     14 // distribution.
     15 //     * Neither the name of Google Inc. nor the names of its
     16 // contributors may be used to endorse or promote products derived from
     17 // this software without specific prior written permission.
     18 //
     19 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     20 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     21 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     22 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     23 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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     25 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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     29 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     30 
     31 package com.google.protobuf;
     32 
     33 /**
     34  * Grab-bag of utility functions useful when dealing with RPCs.
     35  *
     36  * @author kenton (at) google.com Kenton Varda
     37  */
     38 public final class RpcUtil {
     39   private RpcUtil() {}
     40 
     41   /**
     42    * Take an {@code RpcCallback<Message>} and convert it to an
     43    * {@code RpcCallback} accepting a specific message type.  This is always
     44    * type-safe (parameter type contravariance).
     45    */
     46   @SuppressWarnings("unchecked")
     47   public static <Type extends Message> RpcCallback<Type>
     48   specializeCallback(final RpcCallback<Message> originalCallback) {
     49     return (RpcCallback<Type>)originalCallback;
     50     // The above cast works, but only due to technical details of the Java
     51     // implementation.  A more theoretically correct -- but less efficient --
     52     // implementation would be as follows:
     53     //   return new RpcCallback<Type>() {
     54     //     public void run(Type parameter) {
     55     //       originalCallback.run(parameter);
     56     //     }
     57     //   };
     58   }
     59 
     60   /**
     61    * Take an {@code RpcCallback} accepting a specific message type and convert
     62    * it to an {@code RpcCallback<Message>}.  The generalized callback will
     63    * accept any message object which has the same descriptor, and will convert
     64    * it to the correct class before calling the original callback.  However,
     65    * if the generalized callback is given a message with a different descriptor,
     66    * an exception will be thrown.
     67    */
     68   public static <Type extends Message>
     69   RpcCallback<Message> generalizeCallback(
     70       final RpcCallback<Type> originalCallback,
     71       final Class<Type> originalClass,
     72       final Type defaultInstance) {
     73     return new RpcCallback<Message>() {
     74       public void run(final Message parameter) {
     75         Type typedParameter;
     76         try {
     77           typedParameter = originalClass.cast(parameter);
     78         } catch (ClassCastException ignored) {
     79           typedParameter = copyAsType(defaultInstance, parameter);
     80         }
     81         originalCallback.run(typedParameter);
     82       }
     83     };
     84   }
     85 
     86   /**
     87    * Creates a new message of type "Type" which is a copy of "source".  "source"
     88    * must have the same descriptor but may be a different class (e.g.
     89    * DynamicMessage).
     90    */
     91   @SuppressWarnings("unchecked")
     92   private static <Type extends Message> Type copyAsType(
     93       final Type typeDefaultInstance, final Message source) {
     94     return (Type)typeDefaultInstance.newBuilderForType()
     95                                     .mergeFrom(source)
     96                                     .build();
     97   }
     98 
     99   /**
    100    * Creates a callback which can only be called once.  This may be useful for
    101    * security, when passing a callback to untrusted code:  most callbacks do
    102    * not expect to be called more than once, so doing so may expose bugs if it
    103    * is not prevented.
    104    */
    105   public static <ParameterType>
    106     RpcCallback<ParameterType> newOneTimeCallback(
    107       final RpcCallback<ParameterType> originalCallback) {
    108     return new RpcCallback<ParameterType>() {
    109       private boolean alreadyCalled = false;
    110 
    111       public void run(final ParameterType parameter) {
    112         synchronized(this) {
    113           if (alreadyCalled) {
    114             throw new AlreadyCalledException();
    115           }
    116           alreadyCalled = true;
    117         }
    118 
    119         originalCallback.run(parameter);
    120       }
    121     };
    122   }
    123 
    124   /**
    125    * Exception thrown when a one-time callback is called more than once.
    126    */
    127   public static final class AlreadyCalledException extends RuntimeException {
    128     private static final long serialVersionUID = 5469741279507848266L;
    129 
    130     public AlreadyCalledException() {
    131       super("This RpcCallback was already called and cannot be called " +
    132             "multiple times.");
    133     }
    134   }
    135 }
    136