什么是RPC

RPC (Remote Procedure Call Protocol), 远程过程调用，通俗的解释就是：客户端在不知道调用细节的情况下，调用存在于远程计算机上的某个对象，就像调用本地应用程序中的对象一样，不需要了解底层网络技术的协议。

简单的整体工作流程
请求端发送一个调用的数据包，该包中包含有调用标识，参数等协议要求的参数。当响应端接收到这个数据包，对应的程序被调起，然后返回结果数据包，返回的数据包含了和请求的数据包中同样的请求标识，结果等。

性能影响因素

1. 利用的网络协议。可以使用应用层协议，例如HTTP或者HTTP/2协议；也可以利用传输层协议，例如TCP协议，但是主流的RPC还没有采用UDP传输协议。
2. 消息封装格式。选择或设计一种协议来封装信息进行组装发送。比如，dubbo中消息体数据包含dubbo版本号、接口名称、接口版本、方法名称、参数类型列表、参数、附加信息等。
3. 序列化。信息在网络传输中要以二进制格式进行传输。序列化和反序列化，是对象到而二进制数据的转换。常见的序列化方法有JSON、Hessian、Protostuff等。
4. 网络IO模型。可以采用非阻塞式同步IO,也可以在服务器上实现对多路IO模型的支持。
5. 线程管理方式。在高并发请求下，可以使用单个线程运行服务的具体实现，但是会出现请求阻塞等待现象。也可以为每一个RPC具体服务的实现开启一个独立的线程运行，最大线程数有限制，可以使用线程池来管理多个线程的分配和调度。

第一版RPC

第一个版本简单实现了RPC的最基本功能，即服务信息的发送与接收、序列化方式和动态代理等。
项目利用Springboot来实现依赖注入与参数配置，使用netty实现NIO方式的数据传输，使用Hessian来实现对象序列化。
动态代理
这里要提到代理模式，它的特征是代理类与委托类有同样的接口，代理类主要负责为委托类预处理消息、过滤消息、把消息转发给委托类，以及事后处理消息等。代理类与委托类之间通常会存在关联关系。
根据创建代理类的时间点，又可以分为静态代理和动态代理。
在以往的静态代理中需要手动为每一个目标编写对应的代理类。如果系统已经有了成百上千个类，工作量太大了。
静态代理由程序员创建或特定工具自动生成源代码，也就是在编译时就已经将接口与被代理类，代理类等确定下来。在程序运行之前，代理类的.class文件就已经生成。
代理类在程序运行时创建的代理方式被称为代理模式。在静态代理中，代理类是自己定义好的，在运行之前就已经编译完成了。而在动态代理中，可以很方便地对代理类的函数进行统一的处理，而不用修改每个代理类中的方法。可以通过InvocationHandler接口来实现。

客户端的动态代理

public class ProxyFactory {
    public static <T> T create(Class<T> interfaceClass) throws Exception {
        return (T) Proxy.newProxyInstance(interfaceClass.getClassLoader(), new Class<?>[]{interfaceClass}, new LwRpcClientDynamicProxy<T>(interfaceClass));
    }
}

@Slf4j
public class LwRpcClientDynamicProxy<T> implements InvocationHandler {
    private Class<T> clazz;
    public LwRpcClientDynamicProxy(Class<T> clazz) throws Exception {
        this.clazz = clazz;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        LwRequest lwRequest = new LwRequest();
        String requestId = UUID.randomUUID().toString();
        String className = method.getDeclaringClass().getName();
        String methodName = method.getName();
        Class<?>[] parameterTypes = method.getParameterTypes();

        lwRequest.setRequestId(requestId);
        lwRequest.setClassName(className);
        lwRequest.setMethodName(methodName);
        lwRequest.setParameterTypes(parameterTypes);
        lwRequest.setParameters(args);
        NettyClient nettyClient = new NettyClient("127.0.0.1", 8888);
        log.info("开始连接服务器端:{}", new Date());
        LwResponse send = nettyClient.send(lwRequest);
        log.info("请求后返回的结果:{}", send.getResult());
        return send.getResult();
    }
}

在服务端会利用在客户端获取到的类名。参数等信息利用反射机制进行调用。

Class<?>[] parameterTypes = request.getParameterTypes();
        Object[] paramethers = request.getParameters();
        // 使用CGLIB 反射
        FastClass fastClass = FastClass.create(serviceClass);
        FastMethod fastMethod = fastClass.getMethod(methodName, parameterTypes);
        return fastMethod.invoke(serviceBean, paramethers);

Netty客户端

@Slf4j
public class NettyClient  {
    private String host;
    private Integer port;
    private LwResponse response;
    private EventLoopGroup group;
    private ChannelFuture future = null;
    private Object obj = new Object();
    private NettyClientHandler nettyClientHandler;
    public NettyClient(String host, Integer port) {
        this.host = host;
        this.port = port;
    }


    public LwResponse send(LwRequest request) throws Exception{
        nettyClientHandler = new NettyClientHandler(request);
        group = new NioEventLoopGroup();
        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.SO_KEEPALIVE, true)
                .option(ChannelOption.TCP_NODELAY, true)
                .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 5000)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel socketChannel) throws Exception {
                        ChannelPipeline pipeline = socketChannel.pipeline();
                        pipeline.addLast(new LengthFieldBasedFrameDecoder(65535, 0, 4));
                        pipeline.addLast(new LwRpcEncoder(LwRequest.class, new HessianSerializer()));
                        pipeline.addLast(new LwRpcDecoder(LwResponse.class, new HessianSerializer()));
                        pipeline.addLast(nettyClientHandler);
                    }
                });
        future = bootstrap.connect(host, port).sync();
        nettyClientHandler.getCountDownLatch().await();
        this.response = nettyClientHandler.getLwResponse();
        return this.response;
    }

    @PreDestroy
    public void close() {
        group.shutdownGracefully();
        future.channel().closeFuture().syncUninterruptibly();
    }

}

@Slf4j
public class NettyClientHandler extends ChannelInboundHandlerAdapter {
    private final CountDownLatch countDownLatch = new CountDownLatch(1);
    private LwResponse response = null;
    private LwRequest request;

    public NettyClientHandler(LwRequest request) {
        this.request = request;
    }


    public CountDownLatch getCountDownLatch() {
        return countDownLatch;
    }

    public LwResponse getLwResponse() {
        return this.response;
    }
    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        log.info("客户端向客户端发送消息");
        ctx.writeAndFlush(request);
        log.info("客户端请求成功");
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        LwResponse lwResponse = (LwResponse) msg;
        log.info("收到服务端的信息:{}", lwResponse.getResult());
        this.response = lwResponse;
        this.countDownLatch.countDown();
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.close();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx,
                                Throwable cause) {
        cause.printStackTrace();
        ctx.close();
    }

}

在客户端发送服务信息时，用LwQuest类进行封装，返回的结果用LwResponse进行封装，当客户端读取到服务器端返回的响应时，在NettyClientHandler中进行处理，并利用CountDownLatch进行线程的阻塞和运行。
Netty服务端

@Component
@Slf4j
public class NettyServer {
    private EventLoopGroup boss = null;
    private EventLoopGroup worker = null;
    @Autowired
    private ServerHandler serverHandler;
    @Value("${server.address}")
    private String address;
    public void start() throws Exception {
        log.info("成功");
        boss = new NioEventLoopGroup();
        worker = new NioEventLoopGroup();
        try {
            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(boss, worker)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 1024)
                    .childOption(ChannelOption.SO_KEEPALIVE, true)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel socketChannel) throws Exception {
                            ChannelPipeline pipeline = socketChannel.pipeline();
                            pipeline.addLast(new LengthFieldBasedFrameDecoder(65535, 0, 4));
                            pipeline.addLast(new LwRpcEncoder(LwResponse.class, new HessianSerializer()));
                            pipeline.addLast(new LwRpcDecoder(LwRequest.class, new HessianSerializer()));
                            pipeline.addLast(serverHandler);
                        }
                    });
            String[] strs = address.split(":");
            String addr = strs[0];
            int port = Integer.valueOf(strs[1]);
            ChannelFuture future = serverBootstrap.bind(addr, port).sync();
            future.channel().closeFuture().sync();
        } finally {
            worker.shutdownGracefully();
            boss.shutdownGracefully();
        }
    }

    @PreDestroy
    public void destory() throws InterruptedException {
        boss.shutdownGracefully().sync();
        worker.shutdownGracefully().sync();
        log.info("关闭netty");
    }
}

@Component
@Slf4j
@ChannelHandler.Sharable
public class ServerHandler extends SimpleChannelInboundHandler<LwRequest> implements ApplicationContextAware {
    private ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

    @Override
    protected void channelRead0(ChannelHandlerContext channelHandlerContext, LwRequest msg) throws Exception {
        LwResponse lwResponse = new LwResponse();
        lwResponse.setRequestId(msg.getRequestId());
        log.info("从客户端接收到请求信息:{}", msg);
        try {
            Object result = handler(msg);
            lwResponse.setResult(result);
        } catch (Throwable throwable) {
            lwResponse.setCause(throwable);
            throwable.printStackTrace();

        }
        channelHandlerContext.writeAndFlush(lwResponse);
    }

    private Object handler(LwRequest request) throws ClassNotFoundException, InvocationTargetException {

        Class<?> clazz = Class.forName(request.getClassName());
        Object serviceBean = applicationContext.getBean(clazz);
        Class<?> serviceClass = serviceBean.getClass();
        String methodName = request.getMethodName();
        log.info("获取到的服务类:{}", serviceBean);
        Class<?>[] parameterTypes = request.getParameterTypes();
        Object[] paramethers = request.getParameters();
        // 使用CGLIB 反射
        FastClass fastClass = FastClass.create(serviceClass);
        FastMethod fastMethod = fastClass.getMethod(methodName, parameterTypes);
        return fastMethod.invoke(serviceBean, paramethers);
    }
}

在Netty服务端中，会利用`serverHandler来处理从客户端中接收的信息，并利用反射的思想调用本地的方法，并将处理的结构封装在LwResponse中。

LwRequest和LwRespnse要想在网络中进行传输，需要转化为二进制转换。具体方法如下：

public class HessianSerializer implements Serializer {
    @Override
    public byte[] serialize(Object object) throws IOException {
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        Hessian2Output output = new Hessian2Output(byteArrayOutputStream);
        output.writeObject(object);
        output.flush();
        return byteArrayOutputStream.toByteArray();
    }

    public <T> T deserialize(Class<T> clazz, byte[] bytes) throws IOException {
        Hessian2Input input = new Hessian2Input(new ByteArrayInputStream(bytes));
        return (T) input.readObject(clazz);
    }
}

public class LwRpcDecoder extends ByteToMessageDecoder {

    private Class<?> clazz;
    private Serializer serializer;

    public LwRpcDecoder(Class<?> clazz, Serializer serializer) {
        this.clazz = clazz;
        this.serializer = serializer;
    }


    @Override
    protected void decode(ChannelHandlerContext channelHandlerContext, ByteBuf byteBuf, List<Object> list) throws Exception {
        if (byteBuf.readableBytes() < 4)
            return;
        byteBuf.markReaderIndex();
        int dataLength = byteBuf.readInt();
        if (dataLength < 0) {
            channelHandlerContext.close();
        }
        if (byteBuf.readableBytes() < dataLength) {
            byteBuf.resetReaderIndex();
        }
        byte[] data = new byte[dataLength];
        byteBuf.readBytes(data);

        Object obj = serializer.deserialize(clazz, data);
        list.add(obj);
    }
}

public class LwRpcEncoder extends MessageToByteEncoder<Object> {
    private Class<?> clazz;
    private Serializer serializer;

    public LwRpcEncoder(Class<?> clazz, Serializer serializer) {
        this.clazz = clazz;
        this.serializer = serializer;
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, Object in, ByteBuf out) throws Exception {
        if (clazz.isInstance(in)) {
            byte[] data = serializer.serialize(in);
            out.writeInt(data.length);
            out.writeBytes(data);
        }

    }

}