第六章：小朱笔记hadoop之源码分析-ipc分析 第四节：RPC类分析

第六章：小朱笔记hadoop之源码分析-ipc分析

第四节：RPC类分析

RPC类是对Server、Client的具体化。在RPC类中规定，客户程序发出请求调用时，参数类型必须是Invocation；从服务器返回的值类型必须是ObjectWritable。
RPC类是对Server、Client的包装，简化用户的使用。如果一个类需充当服务器，只需通过RPC类的静态方法getServer获得Server实例，然后start。同时此类提供协议接口的实现。如果一个类充当客户端，可以通过getProxy或者waitForProxy获得一个实现了协议接口的proxy object，与服务器端交互。

RPC类中有5个静态内部类，分别为：

（1）Invocation

/** A method invocation, including the method name and its parameters. */
    private static class Invocation implements Writable, Configurable {
        private String methodName; // 方法名
        private Class[] parameterClasses; // 参数类型集合
        private Object[] parameters; // 参数值
        private Configuration conf; // 配置类实例
        ......
    }

（2）ClientCache

/* Cache a client using its socket factory as the hash key */
    static private class ClientCache {
        // 该内部类定义了一个缓存Map
        private Map<SocketFactory, Client> clients = new HashMap<SocketFactory, Client>();

        /**
         * Construct & cache an IPC client with the user-provided SocketFactory
         * if no cached client exists.
         * 通过客户端org.apache.hadoop.ipc.Client的SocketFactory可以快速取出对应的Client实例
         * 
         * @param conf
         *            Configuration
         * @return an IPC client
         */
        /**
         * 从缓存Map中取出一个IPC Client实例，如果缓存够中不存在，就创建一个兵加入到缓存Map中
         */
        private synchronized Client getClient(Configuration conf, SocketFactory factory) {
            // Construct & cache client. The configuration is only used for
            // timeout,
            // and Clients have connection pools. So we can either (a) lose some
            // connection pooling and leak sockets, or (b) use the same timeout
            // for all
            // configurations. Since the IPC is usually intended globally, not
            // per-job, we choose (a).
            Client client = clients.get(factory);
            if (client == null) {
                client = new Client(ObjectWritable.class, conf, factory); // 通过反射实例化一个ObjectWritable对象，构造Client实例
                clients.put(factory, client);
            } else {
                client.incCount();
            }
            return client;
        }

        /**
         * Construct & cache an IPC client with the default SocketFactory if no
         * cached client exists.
         * 
         * @param conf
         *            Configuration
         * @return an IPC client
         */
        private synchronized Client getClient(Configuration conf) {
            return getClient(conf, SocketFactory.getDefault());
        }

        /**
         * Stop a RPC client connection A RPC client is closed only when its
         * reference count becomes zero.
         */
        private void stopClient(Client client) {
            synchronized (this) {
                client.decCount(); // 该client实例的引用计数减1
                if (client.isZeroReference()) { // 如果client实例的引用计数此时为0
                    clients.remove(client.getSocketFactory()); // 从缓存中删除
                }
            }
            if (client.isZeroReference()) { // 如果client实例引用计数为0，需要关闭
                client.stop(); // 停止所有与该client实例相关的线程
            }
        }
    }

（3）Invoker

private static class Invoker implements InvocationHandler {
        private Client.ConnectionId remoteId;
        private Client client;
        private boolean isClosed = false;

        public Invoker(Class<? extends VersionedProtocol> protocol, InetSocketAddress address, UserGroupInformation ticket,
                Configuration conf, SocketFactory factory, int rpcTimeout) throws IOException {
            this.remoteId = Client.ConnectionId.getConnectionId(address, protocol, ticket, rpcTimeout, conf);
            this.client = CLIENTS.getClient(conf, factory);
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            final boolean logDebug = LOG.isDebugEnabled();
            long startTime = 0;
            if (logDebug) {
                startTime = System.currentTimeMillis();
            }
            // 构造一个RPC.Invocation实例作为参数传递给调用程序，执行调用，返回值为value
            ObjectWritable value = (ObjectWritable) client.call(new Invocation(method, args), remoteId);
            if (logDebug) {
                long callTime = System.currentTimeMillis() - startTime;
                LOG.debug("Call: " + method.getName() + " " + callTime);
            }
            return value.get();
        }

        /* close the IPC client that's responsible for this invoker's RPCs */
        synchronized private void close() {
            if (!isClosed) {
                isClosed = true;
                CLIENTS.stopClient(client);
            }
        }
    }

（4）Server

public Writable call(Class<?> protocol, Writable param, long receivedTime)throws IOException {
Invocation call = (Invocation)param;

// 通过反射，根据调用方法名和方法参数类型得到Method实例  
Method method =protocol.getMethod(call.getMethodName(),
call.getParameterClasses()); 

method.setAccessible(true);// 设置反射的对象在使用时取消Java语言访问检查，提高效率  
        
Object value = method.invoke(instance, call.getParameters());// 执行调用（instance是调用底层方法的对象，第二个参数是方法调用的参数）     
}