什么是RPC

RPC(全稱為 Remote Procedure Call)即遠(yuǎn)程過程調(diào)用
在維基百科中這樣解釋

遠(yuǎn)程過程調(diào)用（英語：Remote Procedure Call邑彪，縮寫為 RPC）是一個(gè)計(jì)算機(jī)通信協(xié)議漂羊。該協(xié)議允許運(yùn)行于一臺(tái)計(jì)算機(jī)的程序調(diào)用另一臺(tái)計(jì)算機(jī)的子程序搬卒，而程序員無需額外地為這個(gè)交互作用編程召嘶。

RPC解決的事情

• 進(jìn)程間通訊
• 提供和本地方法調(diào)用一樣的調(diào)用機(jī)制
• 屏蔽程序員對(duì)遠(yuǎn)程調(diào)用的細(xì)節(jié)實(shí)現(xiàn)

簡(jiǎn)單的RPC結(jié)構(gòu)

簡(jiǎn)單的RPC可以分為五個(gè)部分：

1. client
2. client-stub
3. network-service
4. server-stub
5. server

這5個(gè)部分的關(guān)系及基本流程如下圖

關(guān)系及基本流程圖

RPC基本過程

1. 服務(wù)消費(fèi)方（client）以本地調(diào)用方式調(diào)用服務(wù)胡诗；
2. client stub接收到調(diào)用后負(fù)責(zé)將方法摊趾、參數(shù)等組裝成能夠進(jìn)行網(wǎng)絡(luò)傳輸?shù)南Ⅲw顽频；
3. client stub找到服務(wù)地址肛著，并將消息發(fā)送到服務(wù)端圆兵；
4. server stub收到消息后進(jìn)行解碼；
5. server stub根據(jù)解碼結(jié)果通過反射調(diào)用本地的服務(wù)枢贿；
6. 本地服務(wù)執(zhí)行并將結(jié)果返回給server stub殉农；
7. server stub將返回結(jié)果打包成消息并發(fā)送至消費(fèi)方；
8. client stub接收到消息局荚，并進(jìn)行解碼超凳；
9. 服務(wù)消費(fèi)方得到最終結(jié)果愈污。

一般情況下我們的RPC就是對(duì)我們的上面過程的2～8進(jìn)行封裝然后，給client的程序員感覺就像調(diào)用本地的方法一樣轮傍。

簡(jiǎn)單實(shí)現(xiàn)

首先是Client端的應(yīng)用層怎么發(fā)起RPC暂雹，CallerApp

public class CallerApp {
    public static void main(String[] args) {
        Calculator calculator = new CalculatorRemoteImpl();
        int result = calculator.add(3, 3);
        System.out.println(result);
    }
}

通過一個(gè)CalculatorRemoteImpl，我們把RPC的邏輯封裝進(jìn)去了创夜，客戶端調(diào)用時(shí)感知不到遠(yuǎn)程調(diào)用的麻煩杭跪。將上面的2、3驰吓、4進(jìn)行封裝

public class CalculatorRemoteImpl implements Calculator {
    public static final int PORT = 9090;
    private static Logger log = LoggerFactory.getLogger(CalculatorRemoteImpl.class);

    public int add(int a, int b) {
        List<String> addressList = lookupProviders("Calculator.add");
        String address = chooseTarget(addressList);
        try {
            Socket socket = new Socket(address, PORT);

            // 將請(qǐng)求序列化
            CalculateRpcRequest calculateRpcRequest = generateRequest(a, b);
            ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());

            // 將請(qǐng)求發(fā)給服務(wù)提供方
            objectOutputStream.writeObject(calculateRpcRequest);

            // 將響應(yīng)體反序列化
            ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
            Object response = objectInputStream.readObject();

            log.info("response is {}", response);

            if (response instanceof Integer) {
                return (Integer) response;
            } else {
                throw new InternalError();
            }

        } catch (Exception e) {
            log.error("fail", e);
            throw new InternalError();
        }
    }

    private CalculateRpcRequest generateRequest(int a, int b) {
        CalculateRpcRequest calculateRpcRequest = new CalculateRpcRequest();
        calculateRpcRequest.setA(a);
        calculateRpcRequest.setB(b);
        calculateRpcRequest.setMethod("add");
        return calculateRpcRequest;
    }

    private String chooseTarget(List<String> providers) {
        if (null == providers || providers.size() == 0) {
            throw new IllegalArgumentException();
        }
        return providers.get(0);
    }

    public static List<String> lookupProviders(String name) {
        List<String> strings = new ArrayList();
        strings.add("127.0.0.1");
        return strings;
    }
}

這里用到了Socket來進(jìn)行遠(yuǎn)程通訊涧尿，同時(shí)利用ObjectOutputStream的writeObject和ObjectInputStream的readObject，來實(shí)現(xiàn)序列化和反序列化棚瘟。
下面是service端的實(shí)現(xiàn)

public class CalleeApp {
    private static Logger log = LoggerFactory.getLogger(CalleeApp.class);

    private Calculator calculator = new CalculatorImpl();

    public static void main(String[] args) throws IOException {
        new CalleeApp().run();
    }

    private void run() throws IOException {
        ServerSocket listener = new ServerSocket(9090);
        try {
            while (true) {
                Socket socket = listener.accept();
                try {
                    // 將請(qǐng)求反序列化
                    ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
                    Object object = objectInputStream.readObject();

                    log.info("request is {}", object);

                    // 調(diào)用服務(wù)
                    int result = 0;
                    if (object instanceof CalculateRpcRequest) {
                        CalculateRpcRequest calculateRpcRequest = (CalculateRpcRequest) object;
                        if ("add".equals(calculateRpcRequest.getMethod())) {
                            result = calculator.add(calculateRpcRequest.getA(), calculateRpcRequest.getB());
                        } else {
                            throw new UnsupportedOperationException();
                        }
                    }

                    // 返回結(jié)果
                    ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
                    objectOutputStream.writeObject(new Integer(result));
                } catch (Exception e) {
                    log.error("fail", e);
                } finally {
                    socket.close();
                }
            }
        } finally {
            listener.close();
        }
    }

}

示例代碼在GitHub上下載