com.google.proto4pingcap.RpcUtil Maven / Gradle / Ivy
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package com.google.proto4pingcap;
/**
* Grab-bag of utility functions useful when dealing with RPCs.
*
* @author [email protected] Kenton Varda
*/
public final class RpcUtil {
private RpcUtil() {}
/**
* Take an {@code RpcCallback} and convert it to an
* {@code RpcCallback} accepting a specific message type. This is always
* type-safe (parameter type contravariance).
*/
@SuppressWarnings("unchecked")
public static RpcCallback
specializeCallback(final RpcCallback originalCallback) {
return (RpcCallback)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() {
// public void run(Type parameter) {
// originalCallback.run(parameter);
// }
// };
}
/**
* Take an {@code RpcCallback} accepting a specific message type and convert
* it to an {@code RpcCallback}. 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
RpcCallback generalizeCallback(
final RpcCallback originalCallback,
final Class originalClass,
final Type defaultInstance) {
return new RpcCallback() {
@Override
public void run(final Message parameter) {
Type typedParameter;
try {
typedParameter = originalClass.cast(parameter);
} catch (ClassCastException ignored) {
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 copyAsType(
final Type typeDefaultInstance, final 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
RpcCallback newOneTimeCallback(
final RpcCallback originalCallback) {
return new RpcCallback() {
private boolean alreadyCalled = false;
@Override
public void run(final 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 {
private static final long serialVersionUID = 5469741279507848266L;
public AlreadyCalledException() {
super("This RpcCallback was already called and cannot be called " +
"multiple times.");
}
}
}