RPC，即 Remote Procedure Call（远程过程调用），说得通俗一点就是：调用远程计算机上的服务，就像调用本地服务一样。
RPC 可基于 HTTP 或 TCP 协议，Web Service 就是基于 HTTP 协议的 RPC，它具有良好的跨平台性，但其性能却不如基于 TCP 协议的 RPC。会两方面会直接影响 RPC 的性能，一是传输方式，二是序列化。
轻量级分布式 RPC 框架的具体过程，该框架基于 TCP 协议
- Spring：它是最强大的依赖注入框架，也是业界的权威标准。
- Netty：它使 NIO 编程更加容易，屏蔽了 Java 底层的 NIO 细节。
- Protostuff：它基于 Protobuf 序列化框架，面向 POJO，无需编写 .proto 文件。
- ZooKeeper：提供服务注册与发现功能，开发分布式系统的必备选择，同时它也具备天生的集群能力。
第一步：编写服务接口
<!-- lang: java --> public interface HelloService { String hello(String name); }
第二步：编写服务接口的实现类
<!-- lang: java --> @RpcService(HelloService.class) // 指定远程接口
//使用RpcService注解定义在服务接口的实现类上，需要对该实现类指定远程接口，因为实现类可能会实现多个接口，一定要告诉框架哪个才是远程接口。
public class HelloServiceImpl implements HelloService { @Override public String hello(String name) { return "Hello! " + name; } }
<!-- lang: java --> @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Component // 表明可被 Spring 扫描 public @interface RpcService { Class<?> value(); }
第三步：配置服务端
服务端 Spring 配置文件名为spring.xml，内容如下：
<!-- lang: xml --> <beans ...> <context:component-scan base-package="com.xxx.rpc.sample.server"/> <context:property-placeholder location="classpath:config.properties"/> <!-- 配置服务注册组件 --> <bean id="serviceRegistry" class="com.xxx.rpc.registry.ServiceRegistry"> <constructor-arg name="registryAddress" value="${registry.address}"/> </bean> <!-- 配置 RPC 服务器 --> <bean id="rpcServer" class="com.xxx.rpc.server.RpcServer"> <constructor-arg name="serverAddress" value="${server.address}"/> <constructor-arg name="serviceRegistry" ref="serviceRegistry"/> </bean> </beans>
具体的配置参数在config.properties文件中，连接本地的 ZooKeeper 服务器，并在 8000 端口上发布 RPC 服务。
<!-- lang: java --> # ZooKeeper 服务器 registry.address=127.0.0.1:2181 # RPC 服务器 server.address=127.0.0.1:8000
第四步：启动服务器并发布服务
为了加载 Spring 配置文件来发布服务，只需编写一个引导程序即可：
<!-- lang: java --> public class RpcBootstrap { public static void main(String[] args) { new ClassPathXmlApplicationContext("spring.xml"); } }
运行RpcBootstrap类的main方法即可启动服务端，但还有两个重要的组件尚未实现，它们分别是：ServiceRegistry与RpcServer
第五步：实现服务注册
使用 ZooKeeper 客户端可轻松实现服务注册功能，ServiceRegistry代码如下：
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，通过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"; }
首先需要使用 ZooKeeper 客户端命令行创建/registry永久节点，用于存放所有的服务临时节点
第六步：实现 RPC 服务器
使用 Netty 可实现一个支持 NIO 的 RPC 服务器，需要使用ServiceRegistry注册服务地址，RpcServer代码如下：
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<>(); // 存放接口名与服务对象之间的映射关系 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); // 获取所有带有 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)) // 将 RPC 请求进行解码（为了处理请求） .addLast(new RpcEncoder(RpcResponse.class)) // 将 RPC 响应进行编码（为了返回响应） .addLast(new RpcHandler(handlerMap)); // 处理 RPC 请求 } }) .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); // 注册服务地址 } future.channel().closeFuture().sync(); } finally { workerGroup.shutdownGracefully(); bossGroup.shutdownGracefully(); } } }
以上代码中，有两个重要的 POJO 需要描述一下，它们分别是RpcRequest与RpcResponse。
使用RpcRequest封装 RPC 请求，代码如下：
public class RpcRequest { private String requestId; private String className; private String methodName; private Class<?>[] parameterTypes; private Object[] parameters; // getter/setter... }
使用RpcResponse封装 RPC 响应，代码如下：
public class RpcResponse { private String requestId; private Throwable error; private Object result; // getter/setter... }
使用RpcDecoder提供 RPC 解码，只需扩展 Netty 的ByteToMessageDecoder抽象类的decode方法即可，代码如下：
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 编码，只需扩展 Netty 的MessageToByteEncoder抽象类的encode方法即可，代码如下：
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); } } }
编写一个SerializationUtil工具类，使用Protostuff实现序列化：
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 来实例化对象，它是比 Java 反射更加强大。
注意：如需要替换其它序列化框架，只需修改SerializationUtil即可。当然，更好的实现方式是提供配置项来决定使用哪种序列化方式。
使用RpcHandler中处理 RPC 请求，只需扩展 Netty 的SimpleChannelInboundHandler抽象类即可，代码如下：
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(); } }
为了避免使用 Java 反射带来的性能问题，我们可以使用 CGLib 提供的反射 API，如上面用到的FastClass与FastMethod。
第七步：配置客户端
同样使用 Spring 配置文件来配置 RPC 客户端，spring.xml代码如下：
<!-- lang: java --> <beans ...> <context:property-placeholder location="classpath:config.properties"/> <!-- 配置服务发现组件 --> <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>
<!-- lang: java --> # ZooKeeper 服务器 registry.address=127.0.0.1:2181
第八步：实现服务发现
同样使用 ZooKeeper 实现服务发现功能，见如下代码：
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：实现 RPC 代理
这里使用 Java 提供的动态代理技术实现 RPC 代理（当然也可以使用 CGLib 来实现），具体代码如下：
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(); // 创建并初始化 RPC 请求 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(); // 发现服务 } String[] array = serverAddress.split(":"); String host = array[0]; int port = Integer.parseInt(array[1]); RpcClient client = new RpcClient(host, port); // 初始化 RPC 客户端 RpcResponse response = client.send(request); // 通过 RPC 客户端发送 RPC 请求并获取 RPC 响应 if (response.isError()) { throw response.getError(); } else { return response.getResult(); } } } ); } }
使用RpcClient类实现 RPC 客户端，只需扩展 Netty 提供的SimpleChannelInboundHandler抽象类即可，代码如下：
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(); // 收到响应，唤醒线程 } } @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)) // 将 RPC 请求进行编码（为了发送请求） .addLast(new RpcDecoder(RpcResponse.class)) // 将 RPC 响应进行解码（为了处理响应） .addLast(RpcClient.this); // 使用 RpcClient 发送 RPC 请求 } }) .option(ChannelOption.SO_KEEPALIVE, true); ChannelFuture future = bootstrap.connect(host, port).sync(); future.channel().writeAndFlush(request).sync(); synchronized (obj) { obj.wait(); // 未收到响应，使线程等待 } if (response != null) { future.channel().closeFuture().sync(); } return response; } finally { group.shutdownGracefully(); } } }
第十步：发送 RPC 请求
使用 JUnit 结合 Spring 编写一个单元测试，代码如下：
@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); } }
本文通过 Spring + Netty + Protostuff + ZooKeeper 实现了一个轻量级 RPC 框架，使用 Spring 提供依赖注入与参数配置，使用 Netty 实现 NIO 方式的数据传输，使用 Protostuff 实现对象序列化，使用 ZooKeeper 实现服务注册与发现。使用该框架，可将服务部署到分布式环境中的任意节点上，客户端通过远程接口来调用服务端的具体实现，让服务端与客户端的开发完全分离，为实现大规模分布式应用提供了基础支持。
附录：Maven 依赖
<!-- 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>