OKHttp源码解析（二）

上一篇文章里我们大致分析了OkHttp整个请求的流程，重点分析了具体发送请求前都做了哪些操作，这篇文章我们将继续上篇的内容，看看在发送请求过程中做了什么，看了上篇文章的应该都知道，我们将从HttpEngine的sendRequest入手看是如何操作的

public void sendRequest() throws RequestException, RouteException, IOException {
        if(this.cacheStrategy == null) {
            if(this.transport != null) {
                throw new IllegalStateException();
            } else {
                Request request = this.networkRequest(this.userRequest);
                //读取用户设置的缓存
                InternalCache responseCache = Internal.instance.internalCache(this.client);
                //从缓存中读取之前相同请求得到的Response
                Response cacheCandidate = responseCache != null?responseCache.get(request):null;
                long now = System.currentTimeMillis();
                //根据请求和缓存结果（可能为null）去得到缓存策略
                this.cacheStrategy = (new Factory(now, request, cacheCandidate)).get();
                //如果请求策略为只要缓存则networkRequest,cacheResponse都为空，大部分条件都会得到networkRequest就是request，cacheResponse为空
                this.networkRequest = this.cacheStrategy.networkRequest;
                this.cacheResponse = this.cacheStrategy.cacheResponse;
                if(responseCache != null) {
                    responseCache.trackResponse(this.cacheStrategy);
                }

                if(cacheCandidate != null && this.cacheResponse == null) {
                    Util.closeQuietly(cacheCandidate.body());
                }

                if(this.networkRequest != null) {
                    if(this.connection == null) {
                        //建立连接
                        this.connect();
                    }
                    //得到HttpTransport(Http请求)
                    this.transport = Internal.instance.newTransport(this.connection, this);
                    //根据条件将一些执行请求头部的写入，具体的写入会调动到httpConnection的sink（就是一个socket请求的outputstream,具体后面分析）去write
                    if(this.callerWritesRequestBody && this.permitsRequestBody() && this.requestBodyOut == null) {
                        long contentLength = OkHeaders.contentLength(request);
                        if(this.bufferRequestBody) {
                            if(contentLength > 2147483647L) {
                                throw new IllegalStateException("Use setFixedLengthStreamingMode() or setChunkedStreamingMode() for requests larger than 2 GiB.");
                            }

                            if(contentLength != -1L) {
                                this.transport.writeRequestHeaders(this.networkRequest);
                                this.requestBodyOut = new RetryableSink((int)contentLength);
                            } else {
                                this.requestBodyOut = new RetryableSink();
                            }
                        } else {
                        //进入这个判断说明不需要走网络请求，直接读取缓存                                                  
                        this.transport.writeRequestHeaders(this.networkRequest);
                            this.requestBodyOut = this.transport.createRequestBody(this.networkRequest, contentLength);
                        }
                    }
                } else {
                    if(this.connection != null) {
                        Internal.instance.recycle(this.client.getConnectionPool(), this.connection);
                        this.connection = null;
                    }

                    if(this.cacheResponse != null) {
                        //组装缓存数据成Response
                        this.userResponse = this.cacheResponse.newBuilder().request(this.userRequest).priorResponse(stripBody(this.priorResponse)).cacheResponse(stripBody(this.cacheResponse)).build();
                    } else {
                        //只要缓存的数据但之前又没有缓存则抛出 504的Response
                        this.userResponse = (new Builder()).request(this.userRequest).priorResponse(stripBody(this.priorResponse)).protocol(Protocol.HTTP_1_1).code(504).message("Unsatisfiable Request (only-if-cached)").body(EMPTY_BODY).build();
                    }
                    //解压请求结果
                    this.userResponse = this.unzip(this.userResponse);
                }

            }
        }
    }

上面的代码先是针对请求策略去判断是否走网络，若不走网络networkResponse为空直接去生成userReponse，否则就进入网络请求状态并且在第28行进行建立连接操作，这个操作比较重要，我们进去看看

private void connect() throws RequestException, RouteException {
        if(this.connection != null) {
            throw new IllegalStateException();
        } else {
            if(this.routeSelector == null) {
            //建立一个Address,用来记录web服务器，以及要连接服务器需要的一些静态配置比如端口号，网络协议等
                this.address = createAddress(this.client, this.networkRequest);

                try {
                  //得到路由选择器，用于记录连接服务器的一些动态配置，比如查询DNS的ip，代理服务器,TLS协议版本
                    this.routeSelector = RouteSelector.get(this.address, this.networkRequest, this.client);
                } catch (IOException var2) {
                    throw new RequestException(var2);
                }
            }
            //得到httpConnection
            this.connection = this.nextConnection();
            this.route = this.connection.getRoute();
        }
    }
    private Connection nextConnection() throws RouteException {
        Connection connection = this.createNextConnection();
        //将connection设置到okhttpclient
        Internal.instance.connectAndSetOwner(this.client, connection, this, this.networkRequest);
        return connection;
    }

    private Connection createNextConnection() throws RouteException {
    //拿到连接池，如果用户没有设置pool，将得到默认的ConnectionPool 
        ConnectionPool pool = this.client.getConnectionPool();

        Connection e;
        //根据address和存活时间等条件找到是否有之前的connection可用
        while((e = pool.get(this.address)) != null) {
            if(this.networkRequest.method().equals("GET") || Internal.instance.isReadable(e)) {
                return e;
            }

            Util.closeQuietly(e.getSocket());
        }

        try {
            Route e1 = this.routeSelector.next();
            //没有满足条件的Connection建立新的connection
            return new Connection(pool, e1);
        } catch (IOException var3) {
            throw new RouteException(var3);
        }
    }

上面的代码是跟服务器建立链接的过程，在createNextConnection里先去线程池里找是否有之前请求过该Address且还在存活时间里的connection否则新建一个，并调用Internal.instance.connectAndSetOwner(this.client, connection, this, this.networkRequest);，这个代码最后会执行到Connection里的

    void connect(int connectTimeout, int readTimeout, int writeTimeout, Request request, List<ConnectionSpec> connectionSpecs, boolean connectionRetryEnabled) throws RouteException {
        if(this.connected) {
            throw new IllegalStateException("already connected");
        } else {
        //建立Socket连接器
            SocketConnector socketConnector = new SocketConnector(this, this.pool);
            ConnectedSocket connectedSocket;
            if(this.route.address.getSslSocketFactory() != null) {
                connectedSocket = socketConnector.connectTls(connectTimeout, readTimeout, writeTimeout, request, this.route, connectionSpecs, connectionRetryEnabled);
            } else {
                if(!connectionSpecs.contains(ConnectionSpec.CLEARTEXT)) {
                    throw new RouteException(new UnknownServiceException("CLEARTEXT communication not supported: " + connectionSpecs));
                }
                //建立socket并进行socket.connect发起对服务器的连接
                connectedSocket = socketConnector.connectCleartext(connectTimeout, readTimeout, this.route);
            }

            this.socket = connectedSocket.socket;
            this.handshake = connectedSocket.handshake;
            //这里我们最终会得到protocol为Protocol.HTTP_1_1
            this.protocol = connectedSocket.alpnProtocol == null?Protocol.HTTP_1_1:connectedSocket.alpnProtocol;

            try {
                if(this.protocol != Protocol.SPDY_3 && this.protocol != Protocol.HTTP_2) {
                //建立httpConnection
                    this.httpConnection = new HttpConnection(this.pool, this, this.socket);
                } else {
                    this.socket.setSoTimeout(0);
                    this.spdyConnection = (new Builder(this.route.address.uriHost, true, this.socket)).protocol(this.protocol).build();
                    this.spdyConnection.sendConnectionPreface();
                }
            } catch (IOException var10) {
                throw new RouteException(var10);
            }

            this.connected = true;
        }
    }

上面26行处我们建立的httpconnection，它的构造函数将根据传入的socket生成source（socket的输入流用来读）和s**ink（socket的输出流用来写）**

public HttpConnection(ConnectionPool pool, Connection connection, Socket socket) throws IOException {
        this.pool = pool;
        this.connection = connection;
        this.socket = socket;
        this.source = Okio.buffer(Okio.source(socket));
        this.sink = Okio.buffer(Okio.sink(socket));
    }

代码看到这里我们知道了connect的整个代码流程，得到socket，连接，建立输入输出流。

然后我们继续回到sendRequest看看connect后做了什么，sendRequest(）代码31行新建HttpEngine，并在第41行去写请求，其实这里我们大致可以猜到是根据前面得到的sink去把请求安装一定的格式写到socket里

public void writeRequestHeaders(Request request) throws IOException {
        this.httpEngine.writingRequestHeaders();
        //组装请求的信息，比如url，请求方式，请求协议
        String requestLine = RequestLine.get(request, this.httpEngine.getConnection().getRoute().getProxy().type(), this.httpEngine.getConnection().getProtocol());
        //将请求头和请求体写入socket
        this.httpConnection.writeRequest(request.headers(), requestLine);
    }

public void writeRequest(Headers headers, String requestLine) throws IOException {
        if(this.state != 0) {
            throw new IllegalStateException("state: " + this.state);
        } else {
            this.sink.writeUtf8(requestLine).writeUtf8("\r\n");
            int i = 0;

            for(int size = headers.size(); i < size; ++i) {
                this.sink.writeUtf8(headers.name(i)).writeUtf8(": ").writeUtf8(headers.value(i)).writeUtf8("\r\n");
            }

            this.sink.writeUtf8("\r\n");
            this.state = 1;
        }
    }

到这里我们基本把整个发送请求的过程分析完了，又是大段大段贴代码，因为这部分其实逻辑比较清楚，代码也比较容易看懂，重要的部分都注释出来，下一篇我们将继续来讲解readResponse。