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diff --git a/java/src/main/java/com/google/protobuf/RpcUtil.java b/java/src/main/java/com/google/protobuf/RpcUtil.java
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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.
+// http://code.google.com/p/protobuf/
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+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 RcpCallabck<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 RcpCallabck} accepting a specific message type and convert
+   * it to an {@code RcpCallabck<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>() {
+      public void run(Message parameter) {
+        Type typedParameter;
+        try {
+          typedParameter = originalClass.cast(parameter);
+        } catch (ClassCastException e) {
+          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(
+      Type typeDefaultInstance, 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>() {
+      boolean alreadyCalled = false;
+      public void run(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 {
+    public AlreadyCalledException() {
+      super("This RpcCallback was already called and cannot be called " +
+            "multiple times.");
+    }
+  }
+}