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package com.google.protobuf;

Grab-bag of utility functions useful when dealing with RPCs.
Author:
kenton@google.com Kenton Varda/**
 * Grab-bag of utility functions useful when dealing with RPCs.
 *
 * @author kenton@google.com Kenton Varda
 */
public final class RpcUtil {
  private RpcUtil() {}

  
Take an RpcCallback<Message> and convert it to an RpcCallback accepting a specific message type. This is always type-safe (parameter type contravariance). /**
   * Take an {@code RpcCallback<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 RpcCallback accepting a specific message type and convert it to an 
RpcCallback<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. /**
   * Take an {@code RpcCallback} accepting a specific message type and convert it to an {@code
   * RpcCallback<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>() {
      @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).
/**
   * 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(
      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.
/**
   * 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>() {
      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. /** 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.");
    }
  }
}