Volley-HttpStack

Android控件跟框架 Android源码分析

HttpStack是一个接口，只有一个方法。主要用于请求网络数据，并返回结果。

public interface HttpStack {
   public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
        throws IOException, AuthFailureError;
}

HttpStack有两个实现类，HurlStack和HttpClientStack，HurlStack是利用HttpUrlConnection实现的，HttpClientStack是利用HttpClient实现的。

在Volley的newRequestQueue(Context context, HttpStack stack)方法中，如果用户传入的HttpStack为空的话，会自动的根据当前系统的版本来创建HttpStack的实现类，如果SDK大于等于9，也就是Android 2.3以后，会创建HurlStack，否则会创建HttpClientStack。

关于HurlStack和HttpClientStack的优劣的问题，建议阅读：Android访问网络，使用HttpURLConnection还是HttpClient？

在Android 2.2版本之前，HttpClient拥有较少的bug，因此使用它是最好的选择。
而在Android 2.3版本及以后，HttpURLConnection则是最佳的选择。它的API简单，体积较小，因而非常适用于Android项目。压缩和缓存机制可以有效地减少网络访问的流量，在提升速度和省电方面也起到了较大的作用。对于新的应用程序应该更加偏向于使用HttpURLConnection，因为在以后的工作当中我们也会将更多的时间放在优化HttpURLConnection上面。

若用户想要使用其他的网络请求类库，比如okhttp等就可以实现HttpStack接口，并在performRequest方法中调用okhttp进行网络请求，并把请求的结果封装成一个HttpResponse返回即可，HttpResponse中包含了状态码，响应头，body信息。

HttpStack的两个子类HurlStack&HttpClientStack。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。

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，一般用不到，只要稍微了解一下就好。
接下来，看最主要的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()) {  
        //将Map的对象添加到Connection的属性中  
        connection.addRequestProperty(headerName, map.get(headerName));  
    }
    //设置connection方法，主要是设置Method属性和Content(for post/put)  
    setConnectionParametersForRequest(connection, request);
    // 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);  
    //将返回的内容解析成response的Entity对象  
    response.setEntity(entityFromConnection(connection));
    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;  
}  
protected HttpURLConnection createConnection(URL url) throws IOException {
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        //设置所有的http连接是否自动处理重定向；
       // public static void HttpURLConnection.setFollowRedirects(boolean //followRedirects)
        //设置本次连接是否自动处理重定向。设置成true，系统自动处理重定向；设置成false则需要自己从http reply中分析新的url自己重新连接。
        connection.setInstanceFollowRedirects(HttpURLConnection.getFollowRedirects());
        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。

static void setConnectionParametersForRequest(HttpURLConnection connection,
            Request<?> request) throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST:
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    connection.setDoOutput(true);
                    connection.setRequestMethod("POST");
                    connection.addRequestProperty(HEADER_CONTENT_TYPE,
                            request.getPostBodyContentType());
                    DataOutputStream out = new DataOutputStream(connection.getOutputStream());
                    out.write(postBody);
                    out.close();
                }
                break;
            case Method.GET:
                connection.setRequestMethod("GET");
                break;
            case Method.DELETE:
                connection.setRequestMethod("DELETE");
                break;
            case Method.POST:
                connection.setRequestMethod("POST");
                addBodyIfExists(connection, request);
                break;
            case Method.PUT:
                connection.setRequestMethod("PUT");
                addBodyIfExists(connection, request);
                break;
            case Method.HEAD:
                connection.setRequestMethod("HEAD");
                break;
            case Method.OPTIONS:
                connection.setRequestMethod("OPTIONS");
                break;
            case Method.TRACE:
                connection.setRequestMethod("TRACE");
                break;
            case Method.PATCH:
                connection.setRequestMethod("PATCH");
                addBodyIfExists(connection, request);
                break;
            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }
    private static void addBodyIfExists(HttpURLConnection connection, Request<?> request)
            throws IOException, AuthFailureError {
        byte[] body = request.getBody();
        if (body != null) {
            connection.setDoOutput(true);
            connection.addRequestProperty(HEADER_CONTENT_TYPE, request.getBodyContentType());
            DataOutputStream out = new DataOutputStream(connection.getOutputStream());
            out.write(body);
            out.close();
        }
    }

4）设置Http 协议，这里基本上是1.1了。

ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);

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。

        int responseCode = connection.getResponseCode();
        if (responseCode == -1) {
            throw new IOException("Could not retrieve response code from HttpUrlConnection.");
        }
        StatusLine responseStatus = new BasicStatusLine(protocolVersion,
                connection.getResponseCode(), connection.getResponseMessage());
        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        if (hasResponseBody(request.getMethod(), responseStatus.getStatusCode())) {
            response.setEntity(entityFromConnection(connection));
        }
        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);
            }
        }
private static boolean hasResponseBody(int requestMethod, int responseCode) {
        return requestMethod != Request.Method.HEAD
            && !(HttpStatus.SC_CONTINUE <= responseCode && responseCode < HttpStatus.SC_OK)
            && responseCode != HttpStatus.SC_NO_CONTENT
            && responseCode != HttpStatus.SC_NOT_MODIFIED;
}        
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。

相关HTTP知识：

Http status code : 304
HTTP状态码

HttpUrlConnection相关知识：

关于HttpURLConnection.setFollowRedirects
如何通过HttpURLConnection得到http 302的跳转地址

HttpUrlConnection的setDoOutput与setDoInput的区别
HttpURLConnection用法详解

Android中TrafficStats流量监控类

Android访问网络，使用HttpURLConnection还是HttpClient？

参考链接：

Android中关于Volley的使用（九）认识HurlStack（HttpClientStack）
volley源码解析(六)--HurlStack与HttpClientStack之争
Android中的volley_3_网络请求HttpStack、HttpClientStack和HurlStack