httpclient 发送http请求过程

socket建立示例

http请求是在应用层，建立在tcp/ip之上，httpclient的实现也是建立在socket操作上的，先看socket客户端发送消息示例：

// 1.建立端到端的连接
Socket socket = new Socket("127.0.0.1", 8080);
// 2.往输出流发送字节
OutputStream outputStream = socket.getOutputStream();
outputStream.write(1);
// 3.往输入流读取字节
InputStream inputStream = socket.getInputStream();
inputStream.read();

httpclient建立连接

httpclient就是把上述示例封装，先实例化PoolingHttpClientConnectionManager 构造函数默认传入DefaultHttpClientConnectionOperator实例，然后每次执行发送请求，先执行连接过程。

// 1.dns解析
// 2.解析端口，ip 建立连接
// 3.将建立的连接的socket对象赋值给conn对象
// 4.conn对象write数据，其实操作的就是socket的outputStream 对象
// 5.conn对象读取数据，其实操作的就是socket的inputStream对象
@Override
public void connect(
        final ManagedHttpClientConnection conn,
        final HttpHost host,
        final InetSocketAddress localAddress,
        final int connectTimeout,
        final SocketConfig socketConfig,
        final HttpContext context) throws IOException {
    final Lookup<ConnectionSocketFactory> registry = getSocketFactoryRegistry(context);
    final ConnectionSocketFactory sf = registry.lookup(host.getSchemeName());

    final InetAddress[] addresses = host.getAddress() != null ?new InetAddress[] { host.getAddress() } :   
    this.dnsResolver.resolve(host.getHostName());
    final int port = this.schemePortResolver.resolve(host);
    for (int i = 0; i < addresses.length; i++) {
        final InetAddress address = addresses[i];
        final boolean last = i == addresses.length - 1;

        Socket sock = sf.createSocket(context);
        sock.setSoTimeout(socketConfig.getSoTimeout());
        sock.setReuseAddress(socketConfig.isSoReuseAddress());
        sock.setTcpNoDelay(socketConfig.isTcpNoDelay());
        sock.setKeepAlive(socketConfig.isSoKeepAlive());
        final int linger = socketConfig.getSoLinger();
        if (linger >= 0) {
            sock.setSoLinger(true, linger);
        }
        conn.bind(sock);

        final InetSocketAddress remoteAddress = new InetSocketAddress(address, port);
        try {
            sock = sf.connectSocket(connectTimeout, sock, host, remoteAddress, localAddress, context);
            conn.bind(sock);
            if (this.log.isDebugEnabled()) {
                this.log.debug("Connection established " + conn);
            }
            return;
        } catch (final SocketTimeoutException ex) {
            if (last) {
                throw new ConnectTimeoutException(ex, host, addresses);
            }
        } 
    }
}