RpcUtil.java revision fbaaef999ba563838ebd00874ed8a1c01fbf286d 
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30
31package 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 */
38public 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