基于Netty和SpringBoot实现一个轻量级RPC框架-Client篇

前提

前置文章：

前一篇文章相对简略地介绍了RPC服务端的编写，而这篇博文最要介绍服务端（Client）的实现。RPC调用一般是面向契约编程的，而Client的核心功能就是：把契约接口方法的调用抽象为使用Netty向RPC服务端通过私有协议发送一个请求。这里最底层的实现依赖于动态代理，因此动态代理是动态实现接口的最简单方式（如果字节码研究得比较深入，可以通过字节码编程实现接口）。需要的依赖如下：

JDK1.8+
Netty:4.1.44.Final
SpringBoot:2.2.2.RELEASE

动态代理的简单使用

一般可以通过JDK动态代理或者Cglib的字节码增强来实现此功能，为了简单起见，不引入额外的依赖，这里选用JDK动态代理。这里重新搬出前面提到的契约接口HelloService：

public interface HelloService {

    String sayHello(String name);

}

接下来需要通过动态代理为此接口添加一个实现：

public class TestDynamicProxy {

    public static void main(String[] args) throws Exception {

        Class<HelloService> interfaceKlass = HelloService.class;

        InvocationHandler handler = new HelloServiceImpl(interfaceKlass);

        HelloService helloService = (HelloService)

                Proxy.newProxyInstance(interfaceKlass.getClassLoader(), new Class[]{interfaceKlass}, handler);

        System.out.println(helloService.sayHello("throwable"));

    }

    @RequiredArgsConstructor

    private static class HelloServiceImpl implements InvocationHandler {

        private final Class<?> interfaceKlass;

        @Override

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

            // 这里应该根据方法的返回值类型去决定返回结果

            return String.format("[%s#%s]方法被调用,参数列表:%s", interfaceKlass.getName(), method.getName(),

                    JSON.toJSONString(args));

        }

    }

}

// 控制台输出结果

[club.throwable.contract.HelloService#sayHello]方法被调用,参数列表:["throwable"]

这里可以确认两点：

InvocationHandler实现后会对被代理接口生成一个动态实现类。
动态实现类（接口）方法被调用的时候，实际上是调用InvocationHandler对应实例的invoke()方法，传入的参数就是当前方法调用的元数据。

Client端代码实现

Client端需要通过动态代理为契约接口生成一个动态实现类，然后提取契约接口调用方法时候所能提供的元数据，通过这些元数据和Netty客户端的支持（例如Netty的Channel）基于私有RPC协议组装请求信息并且发送请求。这里先定义一个请求参数提取器接口RequestArgumentExtractor：

@Data

public class RequestArgumentExtractInput {

    private Class<?> interfaceKlass;

    private Method method;

}

@Data

public class RequestArgumentExtractOutput {

    private String interfaceName;

    private String methodName;

    private List<String> methodArgumentSignatures;

}

// 接口

public interface RequestArgumentExtractor {

    RequestArgumentExtractOutput extract(RequestArgumentExtractInput input);

}

简单实现一下，解析结果添加到缓存中，实现类DefaultRequestArgumentExtractor代码如下：

public class DefaultRequestArgumentExtractor implements RequestArgumentExtractor {

    private final ConcurrentMap<CacheKey, RequestArgumentExtractOutput> cache = Maps.newConcurrentMap();

    @Override

    public RequestArgumentExtractOutput extract(RequestArgumentExtractInput input) {

        Class<?> interfaceKlass = input.getInterfaceKlass();

        Method method = input.getMethod();

        String methodName = method.getName();

        Class<?>[] parameterTypes = method.getParameterTypes();

        return cache.computeIfAbsent(new CacheKey(interfaceKlass.getName(), methodName,

                Lists.newArrayList(parameterTypes)), x -> {

            RequestArgumentExtractOutput output = new RequestArgumentExtractOutput();

            output.setInterfaceName(interfaceKlass.getName());

            List<String> methodArgumentSignatures = Lists.newArrayList();

            for (Class<?> klass : parameterTypes) {

                methodArgumentSignatures.add(klass.getName());

            }

            output.setMethodArgumentSignatures(methodArgumentSignatures);

            output.setMethodName(methodName);

            return output;

        });

    }

    @RequiredArgsConstructor

    private static class CacheKey {

        private final String interfaceName;

        private final String methodName;

        private final List<Class<?>> parameterTypes;

        @Override

        public boolean equals(Object o) {

            if (this == o) return true;

            if (o == null || getClass() != o.getClass()) return false;

            CacheKey cacheKey = (CacheKey) o;

            return Objects.equals(interfaceName, cacheKey.interfaceName) &&

                    Objects.equals(methodName, cacheKey.methodName) &&

                    Objects.equals(parameterTypes, cacheKey.parameterTypes);

        }

        @Override

        public int hashCode() {

            return Objects.hash(interfaceName, methodName, parameterTypes);

        }

    }

}

在不考虑重连、断连等情况下，新增一个类ClientChannelHolder用于保存Netty客户端的Channel实例：

public class ClientChannelHolder {

    public static final AtomicReference<Channel> CHANNEL_REFERENCE = new AtomicReference<>();

}

接着新增一个契约动态代理工厂（工具类）ContractProxyFactory，用于为契约接口生成代理类实例：

public class ContractProxyFactory {

    private static final RequestArgumentExtractor EXTRACTOR = new DefaultRequestArgumentExtractor();

    private static final ConcurrentMap<Class<?>, Object> CACHE = Maps.newConcurrentMap();

    @SuppressWarnings("unchecked")

    public static <T> T ofProxy(Class<T> interfaceKlass) {

        // 缓存契约接口的代理类实例

        return (T) CACHE.computeIfAbsent(interfaceKlass, x ->

                Proxy.newProxyInstance(interfaceKlass.getClassLoader(), new Class[]{interfaceKlass}, (target, method, args) -> {

                    RequestArgumentExtractInput input = new RequestArgumentExtractInput();

                    input.setInterfaceKlass(interfaceKlass);

                    input.setMethod(method);

                    RequestArgumentExtractOutput output = EXTRACTOR.extract(input);

                    // 封装请求参数

                    RequestMessagePacket packet = new RequestMessagePacket();

                    packet.setMagicNumber(ProtocolConstant.MAGIC_NUMBER);

                    packet.setVersion(ProtocolConstant.VERSION);

                    packet.setSerialNumber(SerialNumberUtils.X.generateSerialNumber());

                    packet.setMessageType(MessageType.REQUEST);

                    packet.setInterfaceName(output.getInterfaceName());

                    packet.setMethodName(output.getMethodName());

                    packet.setMethodArgumentSignatures(output.getMethodArgumentSignatures().toArray(new String[0]));

                    packet.setMethodArguments(args);

                    Channel channel = ClientChannelHolder.CHANNEL_REFERENCE.get();

                    // 发起请求

                    channel.writeAndFlush(packet);

                    // 这里方法返回值需要进行同步处理,相对复杂,后面专门开一篇文章讲解,暂时统一返回字符串

                    // 如果契约接口的返回值类型不是字符串,这里方法返回后会抛出异常

                    return String.format("[%s#%s]调用成功,发送了[%s]到NettyServer[%s]", output.getInterfaceName(),

                            output.getMethodName(), JSON.toJSONString(packet), channel.remoteAddress());

                }));

    }

}

最后编写客户端ClientApplication的代码:

@Slf4j

public class ClientApplication {

    public static void main(String[] args) throws Exception {

        int port = 9092;

        EventLoopGroup workerGroup = new NioEventLoopGroup();

        Bootstrap bootstrap = new Bootstrap();

        try {

            bootstrap.group(workerGroup);

            bootstrap.channel(NioSocketChannel.class);

            bootstrap.option(ChannelOption.SO_KEEPALIVE, Boolean.TRUE);

            bootstrap.option(ChannelOption.TCP_NODELAY, Boolean.TRUE);

            bootstrap.handler(new ChannelInitializer<SocketChannel>() {

                @Override

                protected void initChannel(SocketChannel ch) throws Exception {

                    ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(1024, 0, 4, 0, 4));

                    ch.pipeline().addLast(new LengthFieldPrepender(4));

                    ch.pipeline().addLast(new LoggingHandler(LogLevel.DEBUG));

                    ch.pipeline().addLast(new RequestMessagePacketEncoder(FastJsonSerializer.X));

                    ch.pipeline().addLast(new ResponseMessagePacketDecoder());

                    ch.pipeline().addLast(new SimpleChannelInboundHandler<ResponseMessagePacket>() {

                        @Override

                        protected void channelRead0(ChannelHandlerContext ctx, ResponseMessagePacket packet) throws Exception {

                            Object targetPayload = packet.getPayload();

                            if (targetPayload instanceof ByteBuf) {

                                ByteBuf byteBuf = (ByteBuf) targetPayload;

                                int readableByteLength = byteBuf.readableBytes();

                                byte[] bytes = new byte[readableByteLength];

                                byteBuf.readBytes(bytes);

                                targetPayload = FastJsonSerializer.X.decode(bytes, String.class);

                                byteBuf.release();

                            }

                            packet.setPayload(targetPayload);

                            log.info("接收到来自服务端的响应消息,消息内容:{}", JSON.toJSONString(packet));

                        }

                    });

                }

            });

            ChannelFuture future = bootstrap.connect("localhost", port).sync();

            // 保存Channel实例,暂时不考虑断连重连

            ClientChannelHolder.CHANNEL_REFERENCE.set(future.channel());

            // 构造契约接口代理类实例

            HelloService helloService = ContractProxyFactory.ofProxy(HelloService.class);

            String result = helloService.sayHello("throwable");

            log.info(result);

            future.channel().closeFuture().sync();

        } finally {

            workerGroup.shutdownGracefully();

        }

    }

}

先启动《基于Netty和SpringBoot实现一个轻量级RPC框架-Server篇》一文中的ServerApplication，再启动ClientApplication，控制台输出如下：

// 服务端日志

2020-01-16 22:34:51 [main] INFO  c.throwable.server.ServerApplication - 启动NettyServer[9092]成功...

2020-01-16 22:36:35 [nioEventLoopGroup-3-1] INFO  club.throwable.server.ServerHandler - 服务端接收到:RequestMessagePacket(interfaceName=club.throwable.contract.HelloService, methodName=sayHello, methodArgumentSignatures=[java.lang.String], methodArguments=[PooledUnsafeDirectByteBuf(ridx: 0, widx: 11, cap: 11/144)])

2020-01-16 22:36:35 [nioEventLoopGroup-3-1] INFO  club.throwable.server.ServerHandler - 查找目标实现方法成功,目标类:club.throwable.server.contract.DefaultHelloService,宿主类:club.throwable.server.contract.DefaultHelloService,宿主方法:sayHello

2020-01-16 22:36:35 [nioEventLoopGroup-3-1] INFO  club.throwable.server.ServerHandler - 服务端输出:{"attachments":{},"errorCode":200,"magicNumber":10086,"message":"Success","messageType":"RESPONSE","payload":"\"throwable say hello!\"","serialNumber":"63d386214d30410c9e5f04de03d8b2da","version":1}

// 客户端日志

2020-01-16 22:36:35 [main] INFO  c.throwable.client.ClientApplication - [club.throwable.contract.HelloService#sayHello]调用成功,发送了[{"attachments":{},"interfaceName":"club.throwable.contract.HelloService","magicNumber":10086,"messageType":"REQUEST","methodArgumentSignatures":["java.lang.String"],"methodArguments":["throwable"],"methodName":"sayHello","serialNumber":"63d386214d30410c9e5f04de03d8b2da","version":1}]到NettyServer[localhost/127.0.0.1:9092]

2020-01-16 22:36:35 [nioEventLoopGroup-2-1] INFO  c.throwable.client.ClientApplication - 接收到来自服务端的响应消息,消息内容:{"attachments":{},"errorCode":200,"magicNumber":10086,"message":"Success","messageType":"RESPONSE","payload":"\"throwable say hello!\"","serialNumber":"63d386214d30410c9e5f04de03d8b2da","version":1}

小结

Client端主要负责契约接口调用转换为发送RPC协议请求这一步，核心技术就是动态代理，在不进行模块封装优化的前提下实现是相对简单的。这里其实Client端还有一个比较大的技术难题没有解决，上面例子中客户端日志输出如果眼尖的伙伴会发现，Client端发送RPC请求的线程（main线程）和Client端接收Server端RPC响应处理的线程（nioEventLoopGroup-2-1线程）并不相同，这一点是Netty处理网络请求之所以能够如此高效的根源（简单来说就是请求和响应是异步的，两个流程本来是互不感知的）。但是更多情况下，我们希望外部请求是同步的，希望发送RPC请求的线程得到响应结果再返回（这里请求和响应有可能依然是异步流程）。下一篇文章会详细分析一下如果对请求-响应做同步化处理。

Demo项目地址：

ch0-custom-rpc-protocol

（c-2-d e-a-20200116）