https://developers.google.com/protocol-buffers/protobuf-java/: Core Protocol Buffers library. Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. 
// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package com.google.protobuf;


Abstract base interface for protocol-buffer-based RPC services. Services themselves are abstract
classes (implemented either by servers or as stubs), but they subclass this base interface. The
methods of this interface can be used to call the methods of the service without knowing its
exact type at compile time (analogous to the Message interface).

Starting with version 2.3.0, RPC implementations should not try to build on this, but should
instead provide code generator plugins which generate code specific to the particular RPC
implementation. This way the generated code can be more appropriate for the implementation in use
and can avoid unnecessary layers of indirection.

Author:kenton@google.com Kenton Varda
/**
 * Abstract base interface for protocol-buffer-based RPC services. Services themselves are abstract
 * classes (implemented either by servers or as stubs), but they subclass this base interface. The
 * methods of this interface can be used to call the methods of the service without knowing its
 * exact type at compile time (analogous to the Message interface).
 *
 * <p>Starting with version 2.3.0, RPC implementations should not try to build on this, but should
 * instead provide code generator plugins which generate code specific to the particular RPC
 * implementation. This way the generated code can be more appropriate for the implementation in use
 * and can avoid unnecessary layers of indirection.
 *
 * @author kenton@google.com Kenton Varda
 */
public interface Service {
  
Get the ServiceDescriptor describing this service and its methods. 
/** Get the {@code ServiceDescriptor} describing this service and its methods. */
  Descriptors.ServiceDescriptor getDescriptorForType();

  
Call a method of the service specified by MethodDescriptor. This is normally implemented as a simple switch() that calls the standard definitions of the service's methods. 
Preconditions:

    
  
  • method.getService() == getDescriptorForType() 
  • request is of the exact same class as the object returned by 
      getRequestPrototype(method). 
  • controller is of the correct type for the RPC implementation being used by this Service. For stubs, the "correct type" depends on the RpcChannel which the stub is using. Server-side Service implementations are expected to accept whatever type of 
      RpcController the server-side RPC implementation uses. 


Postconditions:

    
  
  • done will be called when the method is complete. This may be before 
      callMethod() returns or it may be at some point in the future. 
  • The parameter to done is the response. It must be of the exact same type as would be returned by getResponsePrototype(method). 
  • If the RPC failed, the parameter to done will be null. Further details about the failure can be found by querying controller. 


/**
   * Call a method of the service specified by MethodDescriptor. This is normally implemented as a
   * simple {@code switch()} that calls the standard definitions of the service's methods.
   *
   * <p>Preconditions:
   *
   * <ul>
   *   <li>{@code method.getService() == getDescriptorForType()}
   *   <li>{@code request} is of the exact same class as the object returned by {@code
   *       getRequestPrototype(method)}.
   *   <li>{@code controller} is of the correct type for the RPC implementation being used by this
   *       Service. For stubs, the "correct type" depends on the RpcChannel which the stub is using.
   *       Server-side Service implementations are expected to accept whatever type of {@code
   *       RpcController} the server-side RPC implementation uses.
   * </ul>
   *
   * <p>Postconditions:
   *
   * <ul>
   *   <li>{@code done} will be called when the method is complete. This may be before {@code
   *       callMethod()} returns or it may be at some point in the future.
   *   <li>The parameter to {@code done} is the response. It must be of the exact same type as would
   *       be returned by {@code getResponsePrototype(method)}.
   *   <li>If the RPC failed, the parameter to {@code done} will be {@code null}. Further details
   *       about the failure can be found by querying {@code controller}.
   * </ul>
   */
  void callMethod(
      Descriptors.MethodDescriptor method,
      RpcController controller,
      Message request,
      RpcCallback<Message> done);

  
callMethod() requires that the request passed in is of a particular subclass of 
Message. getRequestPrototype() gets the default instances of this type for a given method. You can then call Message.newBuilderForType() on this instance to construct a builder to build an object which you can then pass to callMethod(). 
Example:

  MethodDescriptor method =
    service.getDescriptorForType().findMethodByName("Foo");
  Message request =
    stub.getRequestPrototype(method).newBuilderForType()
        .mergeFrom(input).build();
  service.callMethod(method, request, callback);



/**
   * {@code callMethod()} requires that the request passed in is of a particular subclass of {@code
   * Message}. {@code getRequestPrototype()} gets the default instances of this type for a given
   * method. You can then call {@code Message.newBuilderForType()} on this instance to construct a
   * builder to build an object which you can then pass to {@code callMethod()}.
   *
   * <p>Example:
   *
   * <pre>
   *   MethodDescriptor method =
   *     service.getDescriptorForType().findMethodByName("Foo");
   *   Message request =
   *     stub.getRequestPrototype(method).newBuilderForType()
   *         .mergeFrom(input).build();
   *   service.callMethod(method, request, callback);
   * </pre>
   */
  Message getRequestPrototype(Descriptors.MethodDescriptor method);

  
Like getRequestPrototype(), but gets a prototype of the response message. 
getResponsePrototype() is generally not needed because the Service implementation constructs the response message itself, but it may be useful in some cases to know ahead of time what type of object will be returned. 
/**
   * Like {@code getRequestPrototype()}, but gets a prototype of the response message. {@code
   * getResponsePrototype()} is generally not needed because the {@code Service} implementation
   * constructs the response message itself, but it may be useful in some cases to know ahead of
   * time what type of object will be returned.
   */
  Message getResponsePrototype(Descriptors.MethodDescriptor method);
}