原文出處： [黃勇](http://my.oschina.net/huangyong/blog/361751)
RPC，即 Remote Procedure Call（遠(yuǎn)程過程調(diào)用），說得通俗一點(diǎn)就是：調(diào)用遠(yuǎn)程計(jì)算機(jī)上的服務(wù)，就像調(diào)用本地服務(wù)一樣。
RPC 可基于 HTTP 或 TCP 協(xié)議，Web Service 就是基于 HTTP 協(xié)議的 RPC，它具有良好的跨平臺性，但其性能卻不如基于 TCP 協(xié)議的 RPC。會兩方面會直接影響 RPC 的性能，一是傳輸方式，二是序列化。
眾所周知，TCP 是傳輸層協(xié)議，HTTP 是應(yīng)用層協(xié)議，而傳輸層較應(yīng)用層更加底層，在數(shù)據(jù)傳輸方面，越底層越快，因此，在一般情況下，TCP 一定比 HTTP 快。就序列化而言，Java 提供了默認(rèn)的序列化方式，但在高并發(fā)的情況下，這種方式將會帶來一些性能上的瓶頸，于是市面上出現(xiàn)了一系列優(yōu)秀的序列化框架，比如：Protobuf、Kryo、Hessian、Jackson 等，它們可以取代 Java 默認(rèn)的序列化，從而提供更高效的性能。
為了支持高并發(fā)，傳統(tǒng)的阻塞式 IO 顯然不太合適，因此我們需要異步的 IO，即 NIO。Java 提供了 NIO 的解決方案，Java 7 也提供了更優(yōu)秀的 NIO.2 支持，用 Java 實(shí)現(xiàn) NIO 并不是遙不可及的事情，只是需要我們熟悉 NIO 的技術(shù)細(xì)節(jié)。
我們需要將服務(wù)部署在分布式環(huán)境下的不同節(jié)點(diǎn)上，通過服務(wù)注冊的方式，讓客戶端來自動發(fā)現(xiàn)當(dāng)前可用的服務(wù)，并調(diào)用這些服務(wù)。這需要一種服務(wù)注冊表（Service Registry）的組件，讓它來注冊分布式環(huán)境下所有的服務(wù)地址（包括：主機(jī)名與端口號）。
應(yīng)用、服務(wù)、服務(wù)注冊表之間的關(guān)系見下圖：
每臺 Server 上可發(fā)布多個(gè) Service，這些 Service 共用一個(gè) host 與 port，在分布式環(huán)境下會提供 Server 共同對外提供 Service。此外，為防止 Service Registry 出現(xiàn)單點(diǎn)故障，因此需要將其搭建為集群環(huán)境。
本文將為您揭曉開發(fā)輕量級分布式 RPC 框架的具體過程，該框架基于 TCP 協(xié)議，提供了 NIO 特性，提供高效的序列化方式，同時(shí)也具備服務(wù)注冊與發(fā)現(xiàn)的能力。
根據(jù)以上技術(shù)需求，我們可使用如下技術(shù)選型：
? ? Spring：它是最強(qiáng)大的依賴注入框架，也是業(yè)界的權(quán)威標(biāo)準(zhǔn)。
? ? Netty：它使 NIO 編程更加容易，屏蔽了 Java 底層的 NIO 細(xì)節(jié)。
? ? Protostuff：它基于 Protobuf 序列化框架，面向 POJO，無需編寫 .proto 文件。
? ? ZooKeeper：提供服務(wù)注冊與發(fā)現(xiàn)功能，開發(fā)分布式系統(tǒng)的必備選擇，同時(shí)它也具備天生的集群能力。
相關(guān) Maven 依賴請見附錄。
第一步：編寫服務(wù)接口
public interface HelloService {
??? String hello(String name);
}
將該接口放在獨(dú)立的客戶端 jar 包中，以供應(yīng)用使用。
第二步：編寫服務(wù)接口的實(shí)現(xiàn)類
@RpcService(HelloService.class) // 指定遠(yuǎn)程接口
public class HelloServiceImpl implements HelloService {
??? @Override
??? public String hello(String name) {
??????? return "Hello! " + name;
??? }
}
使用RpcService注解定義在服務(wù)接口的實(shí)現(xiàn)類上，需要對該實(shí)現(xiàn)類指定遠(yuǎn)程接口，因?yàn)閷?shí)現(xiàn)類可能會實(shí)現(xiàn)多個(gè)接口，一定要告訴框架哪個(gè)才是遠(yuǎn)程接口。
RpcService代碼如下：
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Component // 表明可被 Spring 掃描
public @interface RpcService {
??? Class<?> value();
}
該注解具備 Spring 的Component注解的特性，可被 Spring 掃描。
該實(shí)現(xiàn)類放在服務(wù)端 jar 包中，該 jar 包還提供了一些服務(wù)端的配置文件與啟動服務(wù)的引導(dǎo)程序。
第三步：配置服務(wù)端
服務(wù)端 Spring 配置文件名為spring.xml，內(nèi)容如下：
<!-- lang: xml -->
<beans ...>
??? <context:component-scan base-package="com.xxx.rpc.sample.server"/>
??? <context:property-placeholder location="classpath:config.properties"/>
??? <!-- 配置服務(wù)注冊組件 -->
??? <bean id="serviceRegistry" class="com.xxx.rpc.registry.ServiceRegistry">
??????? <constructor-arg name="registryAddress" value="${registry.address}"/>
??? </bean>
??? <!-- 配置 RPC 服務(wù)器 -->
??? <bean id="rpcServer" class="com.xxx.rpc.server.RpcServer">
??????? <constructor-arg name="serverAddress" value="${server.address}"/>
??????? <constructor-arg name="serviceRegistry" ref="serviceRegistry"/>
??? </bean>
</beans>
具體的配置參數(shù)在config.properties文件中，內(nèi)容如下：
# ZooKeeper 服務(wù)器
registry.address=127.0.0.1:2181
# RPC 服務(wù)器
server.address=127.0.0.1:8000
以上配置表明：連接本地的 ZooKeeper 服務(wù)器，并在 8000 端口上發(fā)布 RPC 服務(wù)。
第四步：啟動服務(wù)器并發(fā)布服務(wù)
為了加載 Spring 配置文件來發(fā)布服務(wù)，只需編寫一個(gè)引導(dǎo)程序即可：
public class RpcBootstrap {
??? public static void main(String[] args) {
??????? new ClassPathXmlApplicationContext("spring.xml");
??? }
}
運(yùn)行RpcBootstrap類的main方法即可啟動服務(wù)端，但還有兩個(gè)重要的組件尚未實(shí)現(xiàn)，它們分別是：ServiceRegistry與RpcServer，下文會給出具體實(shí)現(xiàn)細(xì)節(jié)。
第五步：實(shí)現(xiàn)服務(wù)注冊
使用 ZooKeeper 客戶端可輕松實(shí)現(xiàn)服務(wù)注冊功能，ServiceRegistry代碼如下：
public class ServiceRegistry {
??? private static final Logger LOGGER = LoggerFactory.getLogger(ServiceRegistry.class);
??? private CountDownLatch latch = new CountDownLatch(1);
??? private String registryAddress;
??? public ServiceRegistry(String registryAddress) {
??????? this.registryAddress = registryAddress;
??? }
??? public void register(String data) {
??????? if (data != null) {
??????????? ZooKeeper zk = connectServer();
??????????? if (zk != null) {
??????????????? createNode(zk, data);
??????????? }
??????? }
??? }
??? private ZooKeeper connectServer() {
??????? ZooKeeper zk = null;
??????? try {
??????????? zk = new ZooKeeper(registryAddress, Constant.ZK_SESSION_TIMEOUT, new Watcher() {
??????????????? @Override
??????????????? public void process(WatchedEvent event) {
??????????????????? if (event.getState() == Event.KeeperState.SyncConnected) {
??????????????????????? latch.countDown();
??????????????????? }
??????????????? }
??????????? });
??????????? latch.await();
??????? } catch (IOException | InterruptedException e) {
??????????? LOGGER.error("", e);
??????? }
??????? return zk;
??? }
??? private void createNode(ZooKeeper zk, String data) {
??????? try {
??????????? byte[] bytes = data.getBytes();
??????????? String path = zk.create(Constant.ZK_DATA_PATH, bytes, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
??????????? LOGGER.debug("create zookeeper node ({} => {})", path, data);
??????? } catch (KeeperException | InterruptedException e) {
??????????? LOGGER.error("", e);
??????? }
??? }
}
其中，通過Constant配置了所有的常量：
public interface Constant {
??? int ZK_SESSION_TIMEOUT = 5000;
??? String ZK_REGISTRY_PATH = "/registry";
??? String ZK_DATA_PATH = ZK_REGISTRY_PATH + "/data";
}
注意：首先需要使用 ZooKeeper 客戶端命令行創(chuàng)建/registry永久節(jié)點(diǎn)，用于存放所有的服務(wù)臨時(shí)節(jié)點(diǎn)。
第六步：實(shí)現(xiàn) RPC 服務(wù)器
使用 Netty 可實(shí)現(xiàn)一個(gè)支持 NIO 的 RPC 服務(wù)器，需要使用ServiceRegistry注冊服務(wù)地址，RpcServer代碼如下：
public class RpcServer implements ApplicationContextAware, InitializingBean {
??? private static final Logger LOGGER = LoggerFactory.getLogger(RpcServer.class);
??? private String serverAddress;
??? private ServiceRegistry serviceRegistry;
??? private Map<String, Object> handlerMap = new HashMap<>(); // 存放接口名與服務(wù)對象之間的映射關(guān)系
??? public RpcServer(String serverAddress) {
??????? this.serverAddress = serverAddress;
??? }
??? public RpcServer(String serverAddress, ServiceRegistry serviceRegistry) {
??????? this.serverAddress = serverAddress;
??????? this.serviceRegistry = serviceRegistry;
??? }
??? @Override
??? public void setApplicationContext(ApplicationContext ctx) throws BeansException {
??????? Map<String, Object> serviceBeanMap = ctx.getBeansWithAnnotation(RpcService.class); // 獲取所有帶有 RpcService 注解的 Spring Bean
??????? if (MapUtils.isNotEmpty(serviceBeanMap)) {
??????????? for (Object serviceBean : serviceBeanMap.values()) {
??????????????? String interfaceName = serviceBean.getClass().getAnnotation(RpcService.class).value().getName();
??????????????? handlerMap.put(interfaceName, serviceBean);
??????????? }
??????? }
??? }
??? @Override
??? public void afterPropertiesSet() throws Exception {
??????? EventLoopGroup bossGroup = new NioEventLoopGroup();
??????? EventLoopGroup workerGroup = new NioEventLoopGroup();
??????? try {
??????????? ServerBootstrap bootstrap = new ServerBootstrap();
??????????? bootstrap.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)
??????????????? .childHandler(new ChannelInitializer<SocketChannel>() {
??????????????????? @Override
??????????????????? public void initChannel(SocketChannel channel) throws Exception {
??????????????????????? channel.pipeline()
??????????????????????????? .addLast(new RpcDecoder(RpcRequest.class)) // 將 RPC 請求進(jìn)行解碼（為了處理請求）
??????????????????????????? .addLast(new RpcEncoder(RpcResponse.class)) // 將 RPC 響應(yīng)進(jìn)行編碼（為了返回響應(yīng)）
??????????????????????????? .addLast(new RpcHandler(handlerMap)); // 處理 RPC 請求
??????????????????? }
??????????????? })
??????????????? .option(ChannelOption.SO_BACKLOG, 128)
??????????????? .childOption(ChannelOption.SO_KEEPALIVE, true);
??????????? String[] array = serverAddress.split(":");
??????????? String host = array[0];
??????????? int port = Integer.parseInt(array[1]);
??????????? ChannelFuture future = bootstrap.bind(host, port).sync();
??????????? LOGGER.debug("server started on port {}", port);
??????????? if (serviceRegistry != null) {
??????????????? serviceRegistry.register(serverAddress); // 注冊服務(wù)地址
??????????? }
??????????? future.channel().closeFuture().sync();
??????? } finally {
??????????? workerGroup.shutdownGracefully();
??????????? bossGroup.shutdownGracefully();
??????? }
??? }
}
以上代碼中，有兩個(gè)重要的 POJO 需要描述一下，它們分別是RpcRequest與RpcResponse。
使用RpcRequest封裝 RPC 請求，代碼如下：
public class RpcRequest {
??? private String requestId;
??? private String className;
??? private String methodName;
??? private Class<?>[] parameterTypes;
??? private Object[] parameters;
??? // getter/setter...
}
使用RpcResponse封裝 RPC 響應(yīng)，代碼如下：
public class RpcResponse {
??? private String requestId;
??? private Throwable error;
??? private Object result;
??? // getter/setter...
}
使用RpcDecoder提供 RPC 解碼，只需擴(kuò)展 Netty 的ByteToMessageDecoder抽象類的decode方法即可，代碼如下：
public class RpcDecoder extends ByteToMessageDecoder {
??? private Class<?> genericClass;
??? public RpcDecoder(Class<?> genericClass) {
??????? this.genericClass = genericClass;
??? }
??? @Override
??? public void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
??????? if (in.readableBytes() < 4) {
??????????? return;
??????? }
??????? in.markReaderIndex();
??????? int dataLength = in.readInt();
??????? if (dataLength < 0) {
??????????? ctx.close();
??????? }
??????? if (in.readableBytes() < dataLength) {
??????????? in.resetReaderIndex();
??????????? return;
??????? }
??????? byte[] data = new byte[dataLength];
??????? in.readBytes(data);
??????? Object obj = SerializationUtil.deserialize(data, genericClass);
??????? out.add(obj);
??? }
}
使用RpcEncoder提供 RPC 編碼，只需擴(kuò)展 Netty 的MessageToByteEncoder抽象類的encode方法即可，代碼如下：
public class RpcEncoder extends MessageToByteEncoder {
??? private Class<?> genericClass;
??? public RpcEncoder(Class<?> genericClass) {
??????? this.genericClass = genericClass;
??? }
??? @Override
??? public void encode(ChannelHandlerContext ctx, Object in, ByteBuf out) throws Exception {
??????? if (genericClass.isInstance(in)) {
??????????? byte[] data = SerializationUtil.serialize(in);
??????????? out.writeInt(data.length);
??????????? out.writeBytes(data);
??????? }
??? }
}
編寫一個(gè)SerializationUtil工具類，使用Protostuff實(shí)現(xiàn)序列化：
public class SerializationUtil {
??? private static Map<Class<?>, Schema<?>> cachedSchema = new ConcurrentHashMap<>();
??? private static Objenesis objenesis = new ObjenesisStd(true);
??? private SerializationUtil() {
??? }
??? @SuppressWarnings("unchecked")
??? private static <T> Schema<T> getSchema(Class<T> cls) {
??????? Schema<T> schema = (Schema<T>) cachedSchema.get(cls);
??????? if (schema == null) {
??????????? schema = RuntimeSchema.createFrom(cls);
??????????? if (schema != null) {
??????????????? cachedSchema.put(cls, schema);
??????????? }
??????? }
??????? return schema;
??? }
??? @SuppressWarnings("unchecked")
??? public static <T> byte[] serialize(T obj) {
??????? Class<T> cls = (Class<T>) obj.getClass();
??????? LinkedBuffer buffer = LinkedBuffer.allocate(LinkedBuffer.DEFAULT_BUFFER_SIZE);
??????? try {
??????????? Schema<T> schema = getSchema(cls);
??????????? return ProtostuffIOUtil.toByteArray(obj, schema, buffer);
??????? } catch (Exception e) {
??????????? throw new IllegalStateException(e.getMessage(), e);
??????? } finally {
??????????? buffer.clear();
??????? }
??? }
??? public static <T> T deserialize(byte[] data, Class<T> cls) {
??????? try {
??????????? T message = (T) objenesis.newInstance(cls);
??????????? Schema<T> schema = getSchema(cls);
??????????? ProtostuffIOUtil.mergeFrom(data, message, schema);
??????????? return message;
??????? } catch (Exception e) {
??????????? throw new IllegalStateException(e.getMessage(), e);
??????? }
??? }
}
以上了使用 Objenesis 來實(shí)例化對象，它是比 Java 反射更加強(qiáng)大。
注意：如需要替換其它序列化框架，只需修改SerializationUtil即可。當(dāng)然，更好的實(shí)現(xiàn)方式是提供配置項(xiàng)來決定使用哪種序列化方式。
使用RpcHandler中處理 RPC 請求，只需擴(kuò)展 Netty 的SimpleChannelInboundHandler抽象類即可，代碼如下：
public class RpcHandler extends SimpleChannelInboundHandler<RpcRequest> {
??? private static final Logger LOGGER = LoggerFactory.getLogger(RpcHandler.class);
??? private final Map<String, Object> handlerMap;
??? public RpcHandler(Map<String, Object> handlerMap) {
??????? this.handlerMap = handlerMap;
??? }
??? @Override
??? public void channelRead0(final ChannelHandlerContext ctx, RpcRequest request) throws Exception {
??????? RpcResponse response = new RpcResponse();
??????? response.setRequestId(request.getRequestId());
??????? try {
??????????? Object result = handle(request);
??????????? response.setResult(result);
??????? } catch (Throwable t) {
??????????? response.setError(t);
??????? }
??????? ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
??? }
??? private Object handle(RpcRequest request) throws Throwable {
??????? String className = request.getClassName();
??????? Object serviceBean = handlerMap.get(className);
??????? Class<?> serviceClass = serviceBean.getClass();
??????? String methodName = request.getMethodName();
??????? Class<?>[] parameterTypes = request.getParameterTypes();
??????? Object[] parameters = request.getParameters();
??????? /*Method method = serviceClass.getMethod(methodName, parameterTypes);
??????? method.setAccessible(true);
??????? return method.invoke(serviceBean, parameters);*/
??????? FastClass serviceFastClass = FastClass.create(serviceClass);
??????? FastMethod serviceFastMethod = serviceFastClass.getMethod(methodName, parameterTypes);
??????? return serviceFastMethod.invoke(serviceBean, parameters);
??? }
??? @Override
??? public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
??????? LOGGER.error("server caught exception", cause);
??????? ctx.close();
??? }
}
為了避免使用 Java 反射帶來的性能問題，我們可以使用 CGLib 提供的反射 API，如上面用到的FastClass與FastMethod。
第七步：配置客戶端
同樣使用 Spring 配置文件來配置 RPC 客戶端，spring.xml代碼如下：
<beans ...>
??? <context:property-placeholder location="classpath:config.properties"/>
??? <!-- 配置服務(wù)發(fā)現(xiàn)組件 -->
??? <bean id="serviceDiscovery" class="com.xxx.rpc.registry.ServiceDiscovery">
??????? <constructor-arg name="registryAddress" value="${registry.address}"/>
??? </bean>
??? <!-- 配置 RPC 代理 -->
??? <bean id="rpcProxy" class="com.xxx.rpc.client.RpcProxy">
??????? <constructor-arg name="serviceDiscovery" ref="serviceDiscovery"/>
??? </bean>
</beans>
其中config.properties提供了具體的配置：
# ZooKeeper 服務(wù)器
registry.address=127.0.0.1:2181
第八步：實(shí)現(xiàn)服務(wù)發(fā)現(xiàn)
同樣使用 ZooKeeper 實(shí)現(xiàn)服務(wù)發(fā)現(xiàn)功能，見如下代碼：
public class ServiceDiscovery {
??? private static final Logger LOGGER = LoggerFactory.getLogger(ServiceDiscovery.class);
??? private CountDownLatch latch = new CountDownLatch(1);
??? private volatile List<String> dataList = new ArrayList<>();
??? private String registryAddress;
??? public ServiceDiscovery(String registryAddress) {
??????? this.registryAddress = registryAddress;
??????? ZooKeeper zk = connectServer();
??????? if (zk != null) {
??????????? watchNode(zk);
??????? }
??? }
??? public String discover() {
??????? String data = null;
??????? int size = dataList.size();
??????? if (size > 0) {
??????????? if (size == 1) {
??????????????? data = dataList.get(0);
??????????????? LOGGER.debug("using only data: {}", data);
??????????? } else {
??????????????? data = dataList.get(ThreadLocalRandom.current().nextInt(size));
??????????????? LOGGER.debug("using random data: {}", data);
??????????? }
??????? }
??????? return data;
??? }
??? private ZooKeeper connectServer() {
??????? ZooKeeper zk = null;
??????? try {
??????????? zk = new ZooKeeper(registryAddress, Constant.ZK_SESSION_TIMEOUT, new Watcher() {
??????????????? @Override
??????????????? public void process(WatchedEvent event) {
??????????????????? if (event.getState() == Event.KeeperState.SyncConnected) {
??????????????????????? latch.countDown();
??????????????????? }
??????????????? }
??????????? });
??????????? latch.await();
??????? } catch (IOException | InterruptedException e) {
??????????? LOGGER.error("", e);
??????? }
??????? return zk;
??? }
??? private void watchNode(final ZooKeeper zk) {
??????? try {
??????????? List<String> nodeList = zk.getChildren(Constant.ZK_REGISTRY_PATH, new Watcher() {
??????????????? @Override
??????????????? public void process(WatchedEvent event) {
??????????????????? if (event.getType() == Event.EventType.NodeChildrenChanged) {
??????????????????????? watchNode(zk);
??????????????????? }
??????????????? }
??????????? });
??????????? List<String> dataList = new ArrayList<>();
??????????? for (String node : nodeList) {
??????????????? byte[] bytes = zk.getData(Constant.ZK_REGISTRY_PATH + "/" + node, false, null);
??????????????? dataList.add(new String(bytes));
??????????? }
??????????? LOGGER.debug("node data: {}", dataList);
??????????? this.dataList = dataList;
??????? } catch (KeeperException | InterruptedException e) {
??????????? LOGGER.error("", e);
??????? }
??? }
}
第九步：實(shí)現(xiàn) RPC 代理
這里使用 Java 提供的動態(tài)代理技術(shù)實(shí)現(xiàn) RPC 代理（當(dāng)然也可以使用 CGLib 來實(shí)現(xiàn)），具體代碼如下：
public class RpcProxy {
??? private String serverAddress;
??? private ServiceDiscovery serviceDiscovery;
??? public RpcProxy(String serverAddress) {
??????? this.serverAddress = serverAddress;
??? }
??? public RpcProxy(ServiceDiscovery serviceDiscovery) {
??????? this.serviceDiscovery = serviceDiscovery;
??? }
??? @SuppressWarnings("unchecked")
??? public <T> T create(Class<?> interfaceClass) {
??????? return (T) Proxy.newProxyInstance(
??????????? interfaceClass.getClassLoader(),
??????????? new Class<?>[]{interfaceClass},
??????????? new InvocationHandler() {
??????????????? @Override
??????????????? public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
??????????????????? RpcRequest request = new RpcRequest(); // 創(chuàng)建并初始化 RPC 請求
??????????????????? request.setRequestId(UUID.randomUUID().toString());
??????????????????? request.setClassName(method.getDeclaringClass().getName());
??????????????????? request.setMethodName(method.getName());
??????????????????? request.setParameterTypes(method.getParameterTypes());
??????????????????? request.setParameters(args);
??????????????????? if (serviceDiscovery != null) {
??????????????????????? serverAddress = serviceDiscovery.discover(); // 發(fā)現(xiàn)服務(wù)
??????????????????? }
??????????????????? String[] array = serverAddress.split(":");
??????????????????? String host = array[0];
??????????????????? int port = Integer.parseInt(array[1]);
??????????????????? RpcClient client = new RpcClient(host, port); // 初始化 RPC 客戶端
??????????????????? RpcResponse response = client.send(request); // 通過 RPC 客戶端發(fā)送 RPC 請求并獲取 RPC 響應(yīng)
??????????????????? if (response.isError()) {
??????????????????????? throw response.getError();
??????????????????? } else {
??????????????????????? return response.getResult();
??????????????????? }
??????????????? }
??????????? }
??????? );
??? }
}
使用RpcClient類實(shí)現(xiàn) RPC 客戶端，只需擴(kuò)展 Netty 提供的SimpleChannelInboundHandler抽象類即可，代碼如下：
public class RpcClient extends SimpleChannelInboundHandler<RpcResponse> {
??? private static final Logger LOGGER = LoggerFactory.getLogger(RpcClient.class);
??? private String host;
??? private int port;
??? private RpcResponse response;
??? private final Object obj = new Object();
??? public RpcClient(String host, int port) {
??????? this.host = host;
??????? this.port = port;
??? }
??? @Override
??? public void channelRead0(ChannelHandlerContext ctx, RpcResponse response) throws Exception {
??????? this.response = response;
??????? synchronized (obj) {
??????????? obj.notifyAll(); // 收到響應(yīng)，喚醒線程
??????? }
??? }
??? @Override
??? public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
??????? LOGGER.error("client caught exception", cause);
??????? ctx.close();
??? }
??? public RpcResponse send(RpcRequest request) throws Exception {
??????? EventLoopGroup group = new NioEventLoopGroup();
??????? try {
??????????? Bootstrap bootstrap = new Bootstrap();
??????????? bootstrap.group(group).channel(NioSocketChannel.class)
??????????????? .handler(new ChannelInitializer<SocketChannel>() {
??????????????????? @Override
??????????????????? public void initChannel(SocketChannel channel) throws Exception {
??????????????????????? channel.pipeline()
??????????????????????????? .addLast(new RpcEncoder(RpcRequest.class)) // 將 RPC 請求進(jìn)行編碼（為了發(fā)送請求）
??????????????????????????? .addLast(new RpcDecoder(RpcResponse.class)) // 將 RPC 響應(yīng)進(jìn)行解碼（為了處理響應(yīng)）
??????????????????????????? .addLast(RpcClient.this); // 使用 RpcClient 發(fā)送 RPC 請求
??????????????????? }
??????????????? })
??????????????? .option(ChannelOption.SO_KEEPALIVE, true);
??????????? ChannelFuture future = bootstrap.connect(host, port).sync();
??????????? future.channel().writeAndFlush(request).sync();
??????????? synchronized (obj) {
??????????????? obj.wait(); // 未收到響應(yīng)，使線程等待
??????????? }
??????????? if (response != null) {
??????????????? future.channel().closeFuture().sync();
??????????? }
??????????? return response;
??????? } finally {
??????????? group.shutdownGracefully();
??????? }
??? }
}
第十步：發(fā)送 RPC 請求
使用 JUnit 結(jié)合 Spring 編寫一個(gè)單元測試，代碼如下：
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:spring.xml")
public class HelloServiceTest {
??? @Autowired
??? private RpcProxy rpcProxy;
??? @Test
??? public void helloTest() {
??????? HelloService helloService = rpcProxy.create(HelloService.class);
??????? String result = helloService.hello("World");
??????? Assert.assertEquals("Hello! World", result);
??? }
}
運(yùn)行以上單元測試，如果不出意外的話，您應(yīng)該會看到綠條。
總結(jié)
本文通過 Spring + Netty + Protostuff + ZooKeeper 實(shí)現(xiàn)了一個(gè)輕量級 RPC 框架，使用
Spring 提供依賴注入與參數(shù)配置，使用 Netty 實(shí)現(xiàn) NIO 方式的數(shù)據(jù)傳輸，使用 Protostuff 實(shí)現(xiàn)對象序列化，使用
ZooKeeper
實(shí)現(xiàn)服務(wù)注冊與發(fā)現(xiàn)。使用該框架，可將服務(wù)部署到分布式環(huán)境中的任意節(jié)點(diǎn)上，客戶端通過遠(yuǎn)程接口來調(diào)用服務(wù)端的具體實(shí)現(xiàn)，讓服務(wù)端與客戶端的開發(fā)完全分離，為實(shí)現(xiàn)大規(guī)模分布式應(yīng)用提供了基礎(chǔ)支持。
附錄：Maven 依賴
<!-- lang: xml -->
<!-- JUnit -->
<dependency>
??? <groupId>junit</groupId>
??? <artifactId>junit</artifactId>
??? <version>4.11</version>
??? <scope>test</scope>
</dependency>
<!-- SLF4J -->
<dependency>
??? <groupId>org.slf4j</groupId>
??? <artifactId>slf4j-log4j12</artifactId>
??? <version>1.7.7</version>
</dependency>
<!-- Spring -->
<dependency>
??? <groupId>org.springframework</groupId>
??? <artifactId>spring-context</artifactId>
??? <version>3.2.12.RELEASE</version>
</dependency>
<dependency>
??? <groupId>org.springframework</groupId>
??? <artifactId>spring-test</artifactId>
??? <version>3.2.12.RELEASE</version>
??? <scope>test</scope>
</dependency>
<!-- Netty -->
<dependency>
??? <groupId>io.netty</groupId>
??? <artifactId>netty-all</artifactId>
??? <version>4.0.24.Final</version>
</dependency>
<!-- Protostuff -->
<dependency>
??? <groupId>com.dyuproject.protostuff</groupId>
??? <artifactId>protostuff-core</artifactId>
??? <version>1.0.8</version>
</dependency>
<dependency>
??? <groupId>com.dyuproject.protostuff</groupId>
??? <artifactId>protostuff-runtime</artifactId>
??? <version>1.0.8</version>
</dependency>
<!-- ZooKeeper -->
<dependency>
??? <groupId>org.apache.zookeeper</groupId>
??? <artifactId>zookeeper</artifactId>
??? <version>3.4.6</version>
</dependency>
<!-- Apache Commons Collections -->
<dependency>
??? <groupId>org.apache.commons</groupId>
??? <artifactId>commons-collections4</artifactId>
??? <version>4.0</version>
</dependency>
<!-- Objenesis -->
<dependency>
??? <groupId>org.objenesis</groupId>
??? <artifactId>objenesis</artifactId>
??? <version>2.1</version>
</dependency>
<!-- CGLib -->
<dependency>
??? <groupId>cglib</groupId>
??? <artifactId>cglib</artifactId>
??? <version>3.1</version>
</dependency>
