1 // Protocol Buffers - Google's data interchange format
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30 
31 package com.google.protobuf;
32 
33 /**
34  * Grab-bag of utility functions useful when dealing with RPCs.
35  *
36  * @author kenton@google.com Kenton Varda
37  */
38 public final class RpcUtil {
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>
specializeCallback(final RpcCallback<Message> originalCallback)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>
generalizeCallback( final RpcCallback<Type> originalCallback, final Class<Type> originalClass, final Type defaultInstance)69   RpcCallback<Message> generalizeCallback(
70       final RpcCallback<Type> originalCallback,
71       final Class<Type> originalClass,
72       final Type defaultInstance) {
73     return new RpcCallback<Message>() {
74       @Override
75       public void run(final Message parameter) {
76         Type typedParameter;
77         try {
78           typedParameter = originalClass.cast(parameter);
79         } catch (ClassCastException ignored) {
80           typedParameter = copyAsType(defaultInstance, parameter);
81         }
82         originalCallback.run(typedParameter);
83       }
84     };
85   }
86 
87   /**
88    * Creates a new message of type "Type" which is a copy of "source".  "source"
89    * must have the same descriptor but may be a different class (e.g.
90    * DynamicMessage).
91    */
92   @SuppressWarnings("unchecked")
93   private static <Type extends Message> Type copyAsType(
94       final Type typeDefaultInstance, final Message source) {
95     return (Type) typeDefaultInstance
96         .newBuilderForType().mergeFrom(source).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       @Override
112       public void run(final ParameterType parameter) {
113         synchronized (this) {
114           if (alreadyCalled) {
115             throw new AlreadyCalledException();
116           }
117           alreadyCalled = true;
118         }
119 
120         originalCallback.run(parameter);
121       }
122     };
123   }
124 
125   /**
126    * Exception thrown when a one-time callback is called more than once.
127    */
128   public static final class AlreadyCalledException extends RuntimeException {
129     private static final long serialVersionUID = 5469741279507848266L;
130 
131     public AlreadyCalledException() {
132       super("This RpcCallback was already called and cannot be called " +
133             "multiple times.");
134     }
135   }
136 }
137