Android每周一轮子：HttpUrlConnection

序言

接着上一篇的Volley，本篇原定计划是OkHttp的，但是在分析道OKhttp底层时，对于IO的包装等等特性，需要一个可参照的对比的例子，比如HttpURLConnection等，通过这种对比，才可以看的出其优势。对于Volley，其实只是对于底层网络库的封装，真正的网络请求的发起还是通过HttpStack来执行，HttpStack在此之前可选的为HttpClient和HttpURLConnection。这里针对HttpURLConnection展开进行分析。分析这样的一个网络库是如何实现的。本代码基于Android 4.3，4.4和其之后底层的实现采用了OkHttp。

基础使用

URL url = new URL("http://www.android.com/");
HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
try {
     InputStream in = new BufferedInputStream(urlConnection.getInputStream());
     readStream(in);
} finally {
     urlConnection.disconnect();
}

建立连接之后，获得一个InputStream，我们就可以从中读取连接建立后的返回数据。如果我们需要在请求中添加参数也可以通过获取一个输出流，在输出流中写入我们的请求数据。而其底层背后就是建立的一个Socket。

源码实现

首先是获取HttpURLConnection

首先调用了URL的openConnection方法。

public URLConnection openConnection() throws java.io.IOException {
      return handler.openConnection(this);
 }

通过源码可以看出，对于连接的操作都是通过URLStreamHandler来进行的，对于URLStreamHandler的创建是在URL的构造函数之中。

public URL(URL context, String spec, URLStreamHandler handler) throws MalformedURLException {
    ....
    if (streamHandler == null) {
        setupStreamHandler();
        if (streamHandler == null) {
            throw new MalformedURLException("Unknown protocol: " + protocol);
        }
    }
   try {
        streamHandler.parseURL(this, spec, schemeSpecificPartStart, spec.length());
    } catch (Exception e) {
        throw new MalformedURLException(e.toString());
    }
}

对于StreamHandler，有多个子类，分别可以用来进行http，https，ftp等协议流的处理。下面是HttpHandler的创建过程。

void setupStreamHandler() {
  //检查是否有缓存的处理相应协议的StreamHandler
    streamHandler = streamHandlers.get(protocol);
    if (streamHandler != null) {
        return;
    }

    //如果streamHandlerFactory不为空，通过其创建streamHandler，并将其缓存下来
    if (streamHandlerFactory != null) {
        streamHandler = streamHandlerFactory.createURLStreamHandler(protocol);
        if (streamHandler != null) {
            streamHandlers.put(protocol, streamHandler);
            return;
        }
    }

    //从用户提供的包中加载相应的StreamHandler，创建相应的实例，并加入到内存缓存中，按照制定的路径
    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) {
            }
        }
    }

    // 如果用户没有提供，则会根据协议的要求，加载相应的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();
    }
    //将Handler加入到缓存
    if (streamHandler != null) {
        streamHandlers.put(protocol, streamHandler);
    }
}

首先判断是否已经有创建，从Handler列表中获取，如果没有判断Handler工厂是否存在，如果不存在，加载本地的指定路径，从中加载并创建相应的实例，最后如果本地路径也没有，则根据协议的类型，创建相应的协议Handler。

• HttpHandler

这里我们只针对Http协议来看，跟进下HttpHandler的源码，来了解一下其实现。Handler的连接建立，通过HttpURLConnectionImpl实例来进行。

protected URLConnection openConnection(URL u) throws IOException {
    return new HttpURLConnectionImpl(u, getDefaultPort());
}

• 获取连接的输出流

public final InputStream getInputStream() throws IOException {
    if (!doInput) {
        throw new ProtocolException("This protocol does not support input");
    }

    HttpEngine response = getResponse();

    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;
}

• HttpEngine的建立

调用getResponse获得HttpEngine对象，从中获取请求的内容。

private HttpEngine getResponse() throws IOException {
    //初始化HttpEngine
    initHttpEngine();
    //判断HttpEngine如果有响应直接返回
    if (httpEngine.hasResponse()) {
        return httpEngine;
    }

    while (true) {
        try {
            httpEngine.sendRequest();
            httpEngine.readResponse();
        } catch (IOException e) {
            OutputStream requestBody = httpEngine.getRequestBody();
            if (httpEngine.hasRecycledConnection()
                    && (requestBody == null || requestBody instanceof RetryableOutputStream)) {
                httpEngine.release(false);
                httpEngine = newHttpEngine(method, rawRequestHeaders, null,
                        (RetryableOutputStream) requestBody);
                continue;
            }
            httpEngineFailure = e;
            throw e;
        }

        Retry retry = processResponseHeaders();
        if (retry == Retry.NONE) {
            httpEngine.automaticallyReleaseConnectionToPool();
            return httpEngine;
        }

        String retryMethod = method;
        OutputStream requestBody = httpEngine.getRequestBody();

        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();
        } else {
            httpEngine.markConnectionAsRecycled();
        }

        httpEngine.release(true);
        //创建HttpEngine
        httpEngine = newHttpEngine(retryMethod, rawRequestHeaders,
                httpEngine.getConnection(), (RetryableOutputStream) requestBody);
    }

}

根据上述方法中的核心调用，逐步展开，首先是初始化HttpEngine，并创建其实例。

private void initHttpEngine() throws IOException {
    if (httpEngineFailure != null) {
        throw httpEngineFailure;
    } else if (httpEngine != null) {
        return;
    }

    connected = true;
    try {
        if (doOutput) {
            if (method == HttpEngine.GET) {
                // they are requesting a stream to write to. This implies a POST method
                method = HttpEngine.POST;
            } else if (method != HttpEngine.POST && method != HttpEngine.PUT) {
                // If the request method is neither POST nor PUT, then you're not writing
                throw new ProtocolException(method + " does not support writing");
            }
        }
        httpEngine = newHttpEngine(method, rawRequestHeaders, null, null);
    } catch (IOException e) {
        httpEngineFailure = e;
        throw e;
    }
}

protected HttpEngine newHttpEngine(String method, RawHeaders requestHeaders,
        HttpConnection connection, RetryableOutputStream requestBody) throws IOException {
    return new HttpEngine(this, method, requestHeaders, connection, requestBody);
}

在newHttpEngie中，new了一个HttpEngine的实例。

public HttpEngine(HttpURLConnectionImpl policy, String method, RawHeaders requestHeaders,
        HttpConnection connection, RetryableOutputStream requestBodyOut) throws IOException {
    this.policy = policy;
    this.method = method;
    this.connection = connection;
    this.requestBodyOut = requestBodyOut;

    try {
        uri = policy.getURL().toURILenient();
    } catch (URISyntaxException e) {
        throw new IOException(e);
    }

    this.requestHeaders = new RequestHeaders(uri, new RawHeaders(requestHeaders));
}

在执行完了HttpEngine的初始化方法之后，调用了其sendRequest方法，首先会进行缓存的判断，最后会判断其是否需要连接，如果需要，则会调用相应的连接方法：sendSocketRequest。

public final void sendRequest() throws IOException {
    if (responseSource != null) {
        return;
    }

    prepareRawRequestHeaders();
    initResponseSource();
    if (responseCache instanceof ExtendedResponseCache) {
        ((ExtendedResponseCache) responseCache).trackResponse(responseSource);
    }


    if (requestHeaders.isOnlyIfCached() && responseSource.requiresConnection()) {
        if (responseSource == ResponseSource.CONDITIONAL_CACHE) {
            IoUtils.closeQuietly(cachedResponseBody);
        }
        this.responseSource = ResponseSource.CACHE;
        this.cacheResponse = GATEWAY_TIMEOUT_RESPONSE;
        RawHeaders rawResponseHeaders = RawHeaders.fromMultimap(cacheResponse.getHeaders());
        setResponse(new ResponseHeaders(uri, rawResponseHeaders), cacheResponse.getBody());
    }

    if (responseSource.requiresConnection()) {
        //放松socket建立请求
        sendSocketRequest();
    } else if (connection != null) {
        HttpConnectionPool.INSTANCE.recycle(connection);
        connection = null;
    }
}

首先判断connection是否为空，如果为空，调用connect方法，然后获得该连接的OutputStream，InputStream。

private void sendSocketRequest() throws IOException {
    if (connection == null) {
        connect();
    }

    if (socketOut != null || requestOut != null || socketIn != null) {
        throw new IllegalStateException();
    }
    //建立socket后，返回其读写流
    socketOut = connection.getOutputStream();
    requestOut = socketOut;
    socketIn = connection.getInputStream();

    if (hasRequestBody()) {
        initRequestBodyOut();
    }
}

实际连接过程调用

protected void connect() throws IOException {
    if (connection == null) {
        connection = openSocketConnection();
    }
}

开启Socket连接，这里调用了HttpConnect的连接函数。

protected final HttpConnection openSocketConnection() throws IOException {
    HttpConnection result = HttpConnection.connect(uri, getSslSocketFactory(),
            policy.getProxy(), requiresTunnel(), policy.getConnectTimeout());
    Proxy proxy = result.getAddress().getProxy();
    if (proxy != null) {
        policy.setProxy(proxy);
    }
    result.setSoTimeout(policy.getReadTimeout());
    return result;
}

对于具体的连接任务交给了HttpConnection来处理，调用其连接方法。会从连接池中获取相应的连接，调用其get方法。

public static HttpConnection connect(URI uri, SSLSocketFactory sslSocketFactory,
        Proxy proxy, boolean requiresTunnel, int connectTimeout) throws IOException {

    if (proxy != null) {
        Address address = (proxy.type() == Proxy.Type.DIRECT)
                ? new Address(uri, sslSocketFactory)
                : new Address(uri, sslSocketFactory, proxy, requiresTunnel);
        return HttpConnectionPool.INSTANCE.get(address, connectTimeout);
    }

    /*
     * Try connecting to each of the proxies provided by the ProxySelector
     * until a connection succeeds.
     */
    ProxySelector selector = ProxySelector.getDefault();
    List<Proxy> proxyList = selector.select(uri);
    if (proxyList != null) {
        for (Proxy selectedProxy : proxyList) {
            if (selectedProxy.type() == Proxy.Type.DIRECT) {
                continue;
            }
            try {
                Address address = new Address(uri, sslSocketFactory,
                        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);
            }
        }
    }

    /*
     * Try a direct connection. If this fails, this method will throw.
     */
    return HttpConnectionPool.INSTANCE.get(new Address(uri, sslSocketFactory), connectTimeout);
}

根据传递的请求，从HttpConnectionPool中获取HttpConnection连接。当其中不存在该连接的时候，重新创建一个实例，然后返回。

public HttpConnection get(HttpConnection.Address address, int connectTimeout)
        throws IOException {
    // First try to reuse an existing HTTP connection.
    synchronized (connectionPool) {
        List<HttpConnection> connections = connectionPool.get(address);
        while (connections != null) {
            HttpConnection connection = connections.remove(connections.size() - 1);
            if (connections.isEmpty()) {
                connectionPool.remove(address);
                connections = null;
            }
            if (connection.isEligibleForRecycling()) {
                // Since Socket is recycled, re-tag before using
                Socket socket = connection.getSocket();
                SocketTagger.get().tag(socket);
                return connection;
            }
        }
    }

    //当我们无法找到一个可用的连接，这个时候，我们需要重新创建一个新的连接
    return address.connect(connectTimeout);
}

Address 为HttpConnection的一个内部类。

public HttpConnection connect(int connectTimeout) throws IOException {
    return new HttpConnection(this, connectTimeout);
}

此时会再创建一个新的连接 ，在HttpConnection的构造函数之中是真正的socket连接建立的地方。

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 {
            socketCandidate.connect(
                    new InetSocketAddress(addresses[i], config.socketPort), connectTimeout);
            break;
        } catch (IOException e) {
            if (i == addresses.length - 1) {
                throw e;
            }
        }
    }

    this.socket = socketCandidate;
}

请求体的写入，获得Socket的写入流，然后将我们的请求数据写入

private void writeRequestHeaders(int contentLength) throws IOException {
    if (sentRequestMillis != -1) {
        throw new IllegalStateException();
    }

    RawHeaders headersToSend = getNetworkRequestHeaders();
    byte[] bytes = headersToSend.toHeaderString().getBytes(Charsets.ISO_8859_1);

    if (contentLength != -1 && bytes.length + contentLength <= MAX_REQUEST_BUFFER_LENGTH) {
        requestOut = new BufferedOutputStream(socketOut, bytes.length + contentLength);
    }

    sentRequestMillis = System.currentTimeMillis();
    requestOut.write(bytes);
}

小结

对于HttpURLConnection库，是一个相对比较简单的网络库，最开始通过根据设置的URL信息，创建一个Socket连接，然后获得Socket连接后得到Socket的InputStream和OutputStream，然后通过其获取数据和写入数据，其内部提供的功能比较少，仅限于帮助我们做一些简单的http的包装，核心类是HttpConnection，HttpEngine两个类。