在之前的一篇博文中，我简略记录了，Volley的请求队列和线程管理的实现。这一次来记录一下HttpStack的工作过程
/** * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it. * * @param context A {@link Context} to use for creating the cache dir. * @param stack An {@link HttpStack} to use for the network, or null for default. * @return A started {@link RequestQueue} instance. */ public static RequestQueue newRequestQueue(Context context, HttpStack stack) { File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR); String userAgent = "volley/0"; try { String packageName = context.getPackageName(); PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0); userAgent = packageName + "/" + info.versionCode; } catch (NameNotFoundException e) { } if (stack == null) { if (Build.VERSION.SDK_INT >= 9) { stack = new HurlStack(); } else { // Prior to Gingerbread, HttpUrlConnection was unreliable. // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent)); } } Network network = new BasicNetwork(stack); RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); queue.start(); return queue; }
这段代码在上一篇中贴过，是新建一个请求队列的大致流程。我们可以看到，当SDK版本大于9时，就会使用HurlStack作为HttpStack。下面我们来看看，HurlStack是如何工作的。
1 public HurlStack() { 2 this(null); 3 } 4 5 /** 6 * @param urlRewriter Rewriter to use for request URLs 7 */ 8 public HurlStack(UrlRewriter urlRewriter) { 9 this(urlRewriter, null); 10 } 11 12 /** 13 * @param urlRewriter Rewriter to use for request URLs 14 * @param sslSocketFactory SSL factory to use for HTTPS connections 15 */ 16 public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) { 17 mUrlRewriter = urlRewriter; 18 mSslSocketFactory = sslSocketFactory; 19 }
我们发现，在创建HurlStack的代码中，我们参数全部传入的null，即在默认条件下，mUrlRewriter和mSslSocketFactory都是为空的。那么这个初始化似乎什么都没有做。其实，真正起的主要作用的是HurlStack中的performRequest方法。在BasicNetwork中，HurlStack只有performRequest会被调用。
performRequest的实现也不算短，所以我们依然分段来阅读。
@Override 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()); map.putAll(additionalHeaders); if (mUrlRewriter != null) { String rewritten = mUrlRewriter.rewriteUrl(url); if (rewritten == null) { throw new IOException("URL blocked by rewriter: " + url); } url = rewritten; } URL parsedUrl = new URL(url); HttpURLConnection connection = openConnection(parsedUrl, request); for (String headerName : map.keySet()) { connection.addRequestProperty(headerName, map.get(headerName)); } setConnectionParametersForRequest(connection, request);
首先，从request中获取url，和http的包头map，我们在发送http请求时，cookie这些东西就放在headers中，在新建时我们已经读过，在默认状态下mUrlRewriter是为null的，所以我们可以直接跳过if判断。然后我们建立HttpURLConnection，并将我们的请求包头一一设置到connection的RequstProperty中。
接下来，我们来看看其中一些方法的实现。
openConnection：
/** * Opens an {@link HttpURLConnection} with parameters. * @param url * @return an open connection * @throws IOException */ 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; }
设置超时，设置缓存使能，设置InputStream的读使能。
setConnectionParametersForRequest:
@SuppressWarnings("deprecation")
/* package */ static void setConnectionParametersForRequest(HttpURLConnection connection,
Request<?> request) throws IOException, AuthFailureError {
switch (request.getMethod()) {
case Method.DEPRECATED_GET_OR_POST:
// This is the deprecated way that needs to be handled for backwards compatibility.
// If the request's post body is null, then the assumption is that the request is
// GET. Otherwise, it is assumed that the request is a POST.
byte[] postBody = request.getPostBody();
if (postBody != null) {
// Prepare output. There is no need to set Content-Length explicitly,
// since this is handled by HttpURLConnection using the size of the prepared
// output stream.
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:
// Not necessary to set the request method because connection defaults to GET but
// being explicit here.
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.");
}
}
代码行数不少，但其实内容比较简单，就是request.getMethod()获取请求类型，然后connection.setRequestMethod("GET");设置相应的方法类型。
如果像POST或PATCH这些类型的请求，则还需要加上Body信息。
addBodyIfExists：
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(); } }
这个方法中，将post部分，写入请求的body中，我们在自己的post请求时，只需要重写getParams方法，即可修改Body的params。
到这里，就完成了HttpURLConnection对象的各种设置。接下来，就是创建和设置HttpResponse:
1 // Initialize HttpResponse with data from the HttpURLConnection. 2 ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1); 3 int responseCode = connection.getResponseCode(); 4 if (responseCode == -1) { 5 // -1 is returned by getResponseCode() if the response code could not be retrieved. 6 // Signal to the caller that something was wrong with the connection. 7 throw new IOException("Could not retrieve response code from HttpUrlConnection."); 8 } 9 StatusLine responseStatus = new BasicStatusLine(protocolVersion, 10 connection.getResponseCode(), connection.getResponseMessage()); 11 BasicHttpResponse response = new BasicHttpResponse(responseStatus); 12 if (hasResponseBody(request.getMethod(), responseStatus.getStatusCode())) { 13 response.setEntity(entityFromConnection(connection)); 14 } 15 for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) { 16 if (header.getKey() != null) { 17 Header h = new BasicHeader(header.getKey(), header.getValue().get(0)); 18 response.addHeader(h); 19 } 20 } 21 return response;
我们仔细观察可以发现，这一段代码主要是将connect中的各种信息提取出来，然后设置到response中。其实设置提取connection的Entity单独拉出一个方法，我们可以看一看：
/** * Initializes an {@link HttpEntity} from the given {@link HttpURLConnection}. * @param connection * @return an HttpEntity populated with data from <code>connection</code>. */ 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; }
这里读取了connection的InputStream，这里我们可以联系到前面的代码，connection.setDoInput(true);这就是为什么在初始化时要设置这个使能。
自此，HurlStack的基本实现，就阅读完毕。在这个类中，Volley创建并设置了HttpURLConnection 和 HttpResponse。这些对象是为了后面BasicNetWork做准备，我将会在下一篇中阅读BasicNetWork。
Done
