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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.");
}
}
}
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