if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        ...
        httpResponse = mHttpStack.performRequest(request, headers);

由于HurlStack和HttpClientStack的实现机制是一样的，只是使用的类不一样，我们这篇文章就只讲解HurlStack了。

下面是HurlStack中performRequest方法，

    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        String url = request.getUrl();
        HashMap<String, String> map = new HashMap<String, String>();
        map.putAll(request.getHeaders());//默认为null
        map.putAll(additionalHeaders);//添加头部,主要是缓存相关的头部信息
        if (mUrlRewriter != null) {
            ...//代码不执行
        }
        URL parsedUrl = new URL(url);
        HttpURLConnection connection = openConnection(parsedUrl, request);//打开Connection
        for (String headerName : map.keySet()) {
            connection.addRequestProperty(headerName, map.get(headerName));
        }//将Map的对象添加到Connection的属性中
        setConnectionParametersForRequest(connection, request);//设置connection方法，主要是设置Method属性和Content(for post/put)
        // Initialize HttpResponse with data from the HttpURLConnection.
        ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);//Http 1.1 协议
        int responseCode = connection.getResponseCode();
        if (responseCode == -1) {
            // -1 is returned by getResponseCode() if the response code could not be retrieved.
            // Signal to the caller that something was wrong with the connection.
            throw new IOException("Could not retrieve response code from HttpUrlConnection.");
        }
        StatusLine responseStatus = new BasicStatusLine(protocolVersion,
                connection.getResponseCode(), connection.getResponseMessage());
        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        response.setEntity(entityFromConnection(connection));将返回的内容解析成response的Entity对象
        for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
            if (header.getKey() != null) {
                Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
                response.addHeader(h);
            }
        }
        return response;
    }

HttpURLConnection是Android3.0以后才提供的一个网络访问类，而HurlStack类，也正是H(ttp)URL的缩写，所以这个类，其实就是基于HttpUrlConnection的实现，其步骤如下：

1）从Request中获得url参数，根据url参数构造URL对象，而URL对象是java提供的获取网络资源的一个封装好的实用类。

2）从URL对象打开Connection，并设置connection的超时，缓存，让网络资源写入等属性。

    private HttpURLConnection openConnection(URL url, Request<?> request) throws IOException {
        HttpURLConnection connection = createConnection(url);

        int timeoutMs = request.getTimeoutMs();
        connection.setConnectTimeout(timeoutMs);
        connection.setReadTimeout(timeoutMs);
        connection.setUseCaches(false);
        connection.setDoInput(true);

        // use caller-provided custom SslSocketFactory, if any, for HTTPS
        if ("https".equals(url.getProtocol()) && mSslSocketFactory != null) {
            ((HttpsURLConnection)connection).setSSLSocketFactory(mSslSocketFactory);
        }

        return connection;
    }

3）调用方法 setConnectionParametersForRequest 来设置 Method属性，如果是Post或者Put的话，还要设置Content内容。

4）设置Http 协议，这里基本上是1.1了。

5）获得Response的流，并将其解析成对应的HttpEntity对象，设置给Response.entity字段，返回给BasicNetwork。

    private static HttpEntity entityFromConnection(HttpURLConnection connection) {
        BasicHttpEntity entity = new BasicHttpEntity();
        InputStream inputStream;
        try {
            inputStream = connection.getInputStream();
        } catch (IOException ioe) {
            inputStream = connection.getErrorStream();
        }
        entity.setContent(inputStream);
        entity.setContentLength(connection.getContentLength());
        entity.setContentEncoding(connection.getContentEncoding());
        entity.setContentType(connection.getContentType());
        return entity;
    }

6）BasicNetwork获得返回来的Response对象，就会由Request去解析这个Response对象，因为不同的请求返回来的对象是不一样的，所以这个解析的过程必须由各个请求的实现类自己去实现，也即如ImageRequest，JsonObjectRequest对象等，都要实现自己的parseNetworkResponse方法。

HurlStack提供了三个构造函数，如下：

    public HurlStack() {
        this(null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     */
    public HurlStack(UrlRewriter urlRewriter) {
        this(urlRewriter, null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     * @param sslSocketFactory SSL factory to use for HTTPS connections
     */
    public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
    }

其中第一个就是Volley类中使用的构造函数，但其实最终调用的都是

public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
    }

默认这两个类都是为null的，但是如果我们要实现对Url的拦截，对url进行一些处理的话，或者利用Https来保证数据传输的安全性的话，我们就可以传入自己实现的UrlRewriterc对象，或者添加SSlSocketFactory。不过在我们一般的项目中，也是用不到的啦，只要稍微了解一下就好了。

结束。