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多年以前Android的网络请求只有Apache开源的HttpClient和JDK的HttpUrlConnection，近几年随着OkHttp的流行Android在高版本的SDK中加入了OkHttp。但在Android官方文档中推荐使用HttpUrlConnection并且其会一直被维护，所以在学习Android网络相关的知识时我们队HttpUrlConnection要有足够的了解。。。。

前几天因为时间的关系只画了图 HttpUrlConnection和Socket的关系图，本来说好的第二天续写，结果一直拖到了周末晚上。幸好时间还来的及，趁这短时间影响深刻，将自己解析代码过程记录下来。（PS：解析的过程有什么地方不明白的可以看看 HttpUrlConnection和Socket的关系图图中讲出的过程和这次代码分析的过程是一样的，只不过代码讲述更加详细。所有源码都是来自Android4.0.4。有代码就有真相！！）

类结构图

先给大家展示一张相关类的结构图：

HttpUrlConnection和Socket关系类图

HttpUrlConnection 使用

在分析代码的时候我希望首相脑海中要有一个URL的请求过程。
这是我在网上摘的一个HttpUrlConnection请求小Demo：

public class EsmTest {    /**     * 通过HttpURLConnection模拟post表单提交     * @throws Exception     */    @Test    public void sendEms() throws Exception {        String wen = "MS2201828";        String btnSearch = "EMS快递查询";        URL url = new URL("http://www.kd185.com/ems.php");        HttpURLConnection conn = (HttpURLConnection) url.openConnection();        conn.setRequestMethod("POST");// 提交模式        // conn.setConnectTimeout(10000);//连接超时 单位毫秒        // conn.setReadTimeout(2000);//读取超时 单位毫秒        conn.setDoOutput(true);// 是否输入参数        StringBuffer params = new StringBuffer();        // 表单参数与get形式一样        params.append("wen").append("=").append(wen).append("&")              .append("btnSearch").append("=").append(btnSearch);        byte[] bypes = params.toString().getBytes();        conn.getOutputStream().write(bypes);// 输入参数        InputStream inStream=conn.getInputStream();        System.out.println(new String(StreamTool.readInputStream(inStream), "gbk"));     }    public void sendSms() throws Exception{        String message="货已发到";        message=URLEncoder.encode(message, "UTF-8");        System.out.println(message);        String path ="http://localhost:8083/DS_Trade/mobile/sim!add.do?message="+message;        URL url =new URL(path);        HttpURLConnection conn = (HttpURLConnection)url.openConnection();        conn.setConnectTimeout(5*1000);        conn.setRequestMethod("GET");        InputStream inStream = conn.getInputStream();            byte[] data = StreamTool.readInputStream(inStream);        String result=new String(data, "UTF-8");        System.out.println(result);    }}

URL产生请求

/*****************URL.java************************//** * 创建一个新的URL实例 */public URL(String spec) throws MalformedURLException {    this((URL) null, spec, null);}public URL(URL context, String spec, URLStreamHandler handler) throws MalformedURLException {    if (spec == null) {        throw new MalformedURLException();    }    if (handler != null) {        streamHandler = handler;    }    spec = spec.trim();    //获取url的协议类型，http,https    protocol = UrlUtils.getSchemePrefix(spec);    //请求开始部分的位置    int schemeSpecificPartStart = protocol != null ? (protocol.length() + 1) : 0;    if (protocol != null && context != null && !protocol.equals(context.protocol)) {        context = null;    }    if (context != null) {        set(context.protocol, context.getHost(), context.getPort(), context.getAuthority(),                context.getUserInfo(), context.getPath(), context.getQuery(),                context.getRef());        if (streamHandler == null) {            streamHandler = context.streamHandler;        }    } else if (protocol == null) {        throw new MalformedURLException("Protocol not found: " + spec);    }    //这里为重点，获取StreamHandler    if (streamHandler == null) {        setupStreamHandler();        if (streamHandler == null) {            throw new MalformedURLException("Unknown protocol: " + protocol);        }    }    try {        //对url的处理        streamHandler.parseURL(this, spec, schemeSpecificPartStart, spec.length());    } catch (Exception e) {        throw new MalformedURLException(e.toString());    }}void setupStreamHandler() {    //从缓存中获取    streamHandler = streamHandlers.get(protocol);    if (streamHandler != null) {        return;    }    //通过工厂方法创建    if (streamHandlerFactory != null) {        streamHandler = streamHandlerFactory.createURLStreamHandler(protocol);        if (streamHandler != null) {            streamHandlers.put(protocol, streamHandler);            return;        }    }    //在同名包下检测一个可用的hadnler    String packageList = System.getProperty("java.protocol.handler.pkgs");    ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();    if (packageList != null && contextClassLoader != null) {        for (String packageName : packageList.split("\\|")) {            String className = packageName + "." + protocol + ".Handler";            try {                Class<?> c = contextClassLoader.loadClass(className);                streamHandler = (URLStreamHandler) c.newInstance();                if (streamHandler != null) {                    streamHandlers.put(protocol, streamHandler);                }                return;            } catch (IllegalAccessException ignored) {            } catch (InstantiationException ignored) {            } catch (ClassNotFoundException ignored) {            }        }    }    //如果还是没有创建成功那么new一个handler    if (protocol.equals("file")) {        streamHandler = new FileHandler();    } else if (protocol.equals("ftp")) {        streamHandler = new FtpHandler();    } else if (protocol.equals("http")) {        streamHandler = new HttpHandler();    } else if (protocol.equals("https")) {        streamHandler = new HttpsHandler();    } else if (protocol.equals("jar")) {        streamHandler = new JarHandler();    }    if (streamHandler != null) {        streamHandlers.put(protocol, streamHandler);    }}

/** * streamHandler实现类为HttpURLConnectionImpl */public final class HttpHandler extends URLStreamHandler {    @Override protected URLConnection openConnection(URL u) throws IOException {        return new HttpURLConnectionImpl(u, getDefaultPort());    }    @Override protected URLConnection openConnection(URL url, Proxy proxy) throws IOException {        if (url == null || proxy == null) {            throw new IllegalArgumentException("url == null || proxy == null");        }        return new HttpURLConnectionImpl(url, getDefaultPort(), proxy);    }    @Override protected int getDefaultPort() {        return 80;    }}

创建连接请求准备

/*****************HttpURLConnectionImpl.java start************************//** * 无论是get还是post都需要建立连接 * post */@Override public final OutputStream getOutputStream() throws IOException {    connect();    OutputStream result = httpEngine.getRequestBody();    if (result == null) {        throw new ProtocolException("method does not support a request body: " + method);    } else if (httpEngine.hasResponse()) {        throw new ProtocolException("cannot write request body after response has been read");    }    return result;}/** * 无论是get还是post都需要建立连接 * get */@Override public final InputStream getInputStream() throws IOException {    if (!doInput) {        throw new ProtocolException("This protocol does not support input");    }    //获取http响应    HttpEngine response = getResponse();    //返回400抛异常    if (getResponseCode() >= HTTP_BAD_REQUEST) {        throw new FileNotFoundException(url.toString());    }    InputStream result = response.getResponseBody();    if (result == null) {        throw new IOException("No response body exists; responseCode=" + getResponseCode());    }    return result;}private HttpEngine getResponse() throws IOException {    //初始化http引擎    initHttpEngine();    //是否有响应头信息    if (httpEngine.hasResponse()) {        return httpEngine;    }    try {        while (true) {            //发送请求            httpEngine.sendRequest();            httpEngine.readResponse();            //为下次请求做准备            Retry retry = processResponseHeaders();            if (retry == Retry.NONE) {                httpEngine.automaticallyReleaseConnectionToPool();                break;            }            //如果一个请求不能完成那么接下来为下次请求做准备            String retryMethod = method;            OutputStream requestBody = httpEngine.getRequestBody();            /*             * Although RFC 2616 10.3.2 specifies that a HTTP_MOVED_PERM             * redirect should keep the same method, Chrome, Firefox and the             * RI all issue GETs when following any redirect.             */            int responseCode = getResponseCode();            if (responseCode == HTTP_MULT_CHOICE || responseCode == HTTP_MOVED_PERM                    || responseCode == HTTP_MOVED_TEMP || responseCode == HTTP_SEE_OTHER) {                retryMethod = HttpEngine.GET;                requestBody = null;            }            if (requestBody != null && !(requestBody instanceof RetryableOutputStream)) {                throw new HttpRetryException("Cannot retry streamed HTTP body",                        httpEngine.getResponseCode());            }            if (retry == Retry.DIFFERENT_CONNECTION) {                httpEngine.automaticallyReleaseConnectionToPool();            }            httpEngine.release(true);            httpEngine = newHttpEngine(retryMethod, rawRequestHeaders,                    httpEngine.getConnection(), (RetryableOutputStream) requestBody);        }        return httpEngine;    } catch (IOException e) {        httpEngineFailure = e;        throw e;    }}@Override public final void connect() throws IOException {    initHttpEngine();    try {        httpEngine.sendRequest();    } catch (IOException e) {        httpEngineFailure = e;        throw e;    }}/** * 无论是get还是post都需要初始化Http引擎 */private void initHttpEngine() throws IOException {    if (httpEngineFailure != null) {        throw httpEngineFailure;    } else if (httpEngine != null) {        return;    }    connected = true;    try {        if (doOutput) {            if (method == HttpEngine.GET) {                //如果要写入那么这就是一个post请求                method = HttpEngine.POST;            } else if (method != HttpEngine.POST && method != HttpEngine.PUT) {                //如果你要写入，那么不是post请求也不是put请求那就抛异常吧。                throw new ProtocolException(method + " does not support writing");            }        }        httpEngine = newHttpEngine(method, rawRequestHeaders, null, null);    } catch (IOException e) {        httpEngineFailure = e;        throw e;    }}

创建Socket连接

/********************HttpEngine.java**************//** * Figures out what the response source will be, and opens a socket to that * source if necessary. Prepares the request headers and gets ready to start * writing the request body if it exists. */public final void sendRequest() throws IOException {    if (responseSource != null) {        return;    }    //填充请求头和cookies    prepareRawRequestHeaders();    //初始化响应资源，计算缓存过期时间，判断是否读取缓冲中数据，或者进行网络请求    //responseSource = ?    //CACHE:返回缓存信息    //CONDITIONAL_CACHE：进行网络请求如果网络请求结果无效则使用缓存    //NETWORK:返回网络请求    initResponseSource();    //请求行为记录    if (responseCache instanceof HttpResponseCache) {        ((HttpResponseCache) responseCache).trackResponse(responseSource);    }    //请求资源需要访问网络，但请求头部禁止请求。在这种情况下使用BAD_GATEWAY_RESPONSE替代    if (requestHeaders.isOnlyIfCached() && responseSource.requiresConnection()) {        if (responseSource == ResponseSource.CONDITIONAL_CACHE) {            IoUtils.closeQuietly(cachedResponseBody);        }        this.responseSource = ResponseSource.CACHE;        this.cacheResponse = BAD_GATEWAY_RESPONSE;        RawHeaders rawResponseHeaders = RawHeaders.fromMultimap(cacheResponse.getHeaders());        setResponse(new ResponseHeaders(uri, rawResponseHeaders), cacheResponse.getBody());    }    if (responseSource.requiresConnection()) {        //socket网络连接        sendSocketRequest();    } else if (connection != null) {        HttpConnectionPool.INSTANCE.recycle(connection);        connection = null;    }}private void sendSocketRequest() throws IOException {    if (connection == null) {        connect();    }    if (socketOut != null || requestOut != null || socketIn != null) {        throw new IllegalStateException();    }    socketOut = connection.getOutputStream();    requestOut = socketOut;    socketIn = connection.getInputStream();    if (hasRequestBody()) {        initRequestBodyOut();    }}//打开Socket连接protected void connect() throws IOException {    if (connection == null) {        connection = openSocketConnection();    }}protected final HttpConnection openSocketConnection() throws IOException {    HttpConnection result = HttpConnection.connect(            uri, policy.getProxy(), requiresTunnel(), policy.getConnectTimeout());    Proxy proxy = result.getAddress().getProxy();    if (proxy != null) {        policy.setProxy(proxy);    }    result.setSoTimeout(policy.getReadTimeout());    return result;}

/********************HttpConnection.java**************/public static HttpConnection connect(URI uri, Proxy proxy, boolean requiresTunnel,        int connectTimeout) throws IOException {    //代理直连    if (proxy != null) {        Address address = (proxy.type() == Proxy.Type.DIRECT)                ? new Address(uri)                : new Address(uri, proxy, requiresTunnel);        return HttpConnectionPool.INSTANCE.get(address, connectTimeout);    }    //寻找代理直连    ProxySelector selector = ProxySelector.getDefault();    List proxyList = selector.select(uri);    if (proxyList != null) {        for (Proxy selectedProxy : proxyList) {            if (selectedProxy.type() == Proxy.Type.DIRECT) {                // the same as NO_PROXY                // TODO: if the selector recommends a direct connection, attempt that?                continue;            }            try {                Address address = new Address(uri, selectedProxy, requiresTunnel);                return HttpConnectionPool.INSTANCE.get(address, connectTimeout);            } catch (IOException e) {                // failed to connect, tell it to the selector                selector.connectFailed(uri, selectedProxy.address(), e);            }        }    }    //创建一个直连接    return HttpConnectionPool.INSTANCE.get(new Address(uri), connectTimeout);}private HttpConnection(Address config, int connectTimeout) throws IOException {    this.address = config;    Socket socketCandidate = null;    InetAddress[] addresses = InetAddress.getAllByName(config.socketHost);    for (int i = 0; i < addresses.length; i++) {        socketCandidate = (config.proxy != null && config.proxy.type() != Proxy.Type.HTTP)                ? new Socket(config.proxy)                : new Socket();        try {            //DNS解析，socket连接（这块不做详细分析）            socketCandidate.connect(                    new InetSocketAddress(addresses[i], config.socketPort), connectTimeout);            break;        } catch (IOException e) {            if (i == addresses.length - 1) {                throw e;            }        }    }    this.socket = socketCandidate;}

/********************HttpConnectionPool.java**************/public HttpConnection get(HttpConnection.Address address, int connectTimeout)        throws IOException {    //首先尝试重用现有的HTTP连接。    synchronized (connectionPool) {        List connections = connectionPool.get(address);        if (connections != null) {            while (!connections.isEmpty()) {                HttpConnection connection = connections.remove(connections.size() - 1);                if (!connection.isStale()) { // TODO: this op does I/O!                    // Since Socket is recycled, re-tag before using                    final Socket socket = connection.getSocket();                    SocketTagger.get().tag(socket);                    return connection;                }            }            connectionPool.remove(address);        }    }    //无法找到可以复用的链接是，创建一个新的链接    return address.connect(connectTimeout);}/********************HttpConnection.Address.java**************/public HttpConnection connect(int connectTimeout) throws IOException {    return new HttpConnection(this, connectTimeout);}

输出内容获取

/********************HttpEngine.java**************/public final void readResponse() throws IOException {    //如果有响应头    if (hasResponse()) {        return;    }    //readResponse之前是否sendRequest    if (responseSource == null) {        throw new IllegalStateException("readResponse() without sendRequest()");    }    //如果不进行网络请求直接返回    if (!responseSource.requiresConnection()) {        return;    }    //刷新请求头    if (sentRequestMillis == -1) {        int contentLength = requestBodyOut instanceof RetryableOutputStream                ? ((RetryableOutputStream) requestBodyOut).contentLength()                : -1;        writeRequestHeaders(contentLength);    }    //刷新请求体    if (requestBodyOut != null) {        requestBodyOut.close();        if (requestBodyOut instanceof RetryableOutputStream) {            ((RetryableOutputStream) requestBodyOut).writeToSocket(requestOut);        }    }    requestOut.flush();    requestOut = socketOut;    //解析响应头    readResponseHeaders();    responseHeaders.setLocalTimestamps(sentRequestMillis, System.currentTimeMillis());    //判断响应体类型    if (responseSource == ResponseSource.CONDITIONAL_CACHE) {        if (cachedResponseHeaders.validate(responseHeaders)) {            if (responseCache instanceof HttpResponseCache) {                ((HttpResponseCache) responseCache).trackConditionalCacheHit();            }            //释放资源            release(true);            //返回缓存信息            setResponse(cachedResponseHeaders.combine(responseHeaders), cachedResponseBody);            return;        } else {            IoUtils.closeQuietly(cachedResponseBody);        }    }    if (hasResponseBody()) {        maybeCache(); // reentrant. this calls into user code which may call back into this!    }        initContentStream(getTransferStream());}private InputStream getTransferStream() throws IOException {    if (!hasResponseBody()) {        return new FixedLengthInputStream(socketIn, cacheRequest, this, 0);    }    if (responseHeaders.isChunked()) {        return new ChunkedInputStream(socketIn, cacheRequest, this);    }    if (responseHeaders.getContentLength() != -1) {        return new FixedLengthInputStream(socketIn, cacheRequest, this,                responseHeaders.getContentLength());    }    return new UnknownLengthHttpInputStream(socketIn, cacheRequest, this);}private void initContentStream(InputStream transferStream) throws IOException {    //是否gzip压缩    if (transparentGzip && responseHeaders.isContentEncodingGzip()) {        responseHeaders.stripContentEncoding();        responseBodyIn = new GZIPInputStream(transferStream);    } else {        responseBodyIn = transferStream;    }}

整个请求的响应流程大概就是这样子的，其中的涉及的路由信息获取，DNS解析与缓存，请求的缓存过期等都还没有仔细研读。不过这些也够自己消化一段时间了_，相信自己现在回过头来看OkHttp的实现应该不是那么困难了。

默默肃立的路灯，像等待检阅的哨兵，站姿笔挺，瞪着炯炯有神的眼睛，时刻守护着这城市的安宁。一排排、一行行路灯不断向远方延伸，汇聚成了一支支流光溢彩的河流，偶尔有汽车疾驰而去，也是一尾尾鱼儿在河里游动。夜已深！！~！

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Android网络之HttpUrlConnection和Socket关系解析