A simple RPC mechanism.A protocol  is a Java interface.  All parameters and return types must be one of:a primitive type（这个注意是9个基本类型，包括void），a String ; or a  Writable or an array of the above types All methods in the protocol should throw only IOException.  No field data of the protocol instance is transmitted.

1、这个RPC有5个内部类。

分别是ClientCache，Invocation，Invoker，Server，VersionMismatch。

更奇葩的是它的构造器是私有的。

2、Invocation

A method invocation, including the method name and its parameters

private String methodName;
private Class[] parameterClasses;
private Object[] parameters;
private Configuration conf;
3、ClientCache

Cache a client using its socket factory as the hash key

private Map<SocketFactory, Client> clients =new HashMap<SocketFactory, Client>();
4、Invoker

private Client.ConnectionId remoteId;
private Client client;
private boolean isClosed = false;
5、VersionMismatch

private String interfaceName;the name of the protocol mismatch 不协调，不搭配
private long clientVersion; the client‘s version of the protocol
private long serverVersion;the server‘s version of the protocol

Get the client‘s preferred version

Get the server‘s agreed to version.

6、Server

An RPC Server.

public static class Server extends org.apache.hadoop.ipc.Server

private Object instance;
private boolean verbose;

踏着前人的脚印学Hadoop——RPC源码