在Android開發中，網絡請求是最重要的子產品之一，Android中大部分網絡請求使用的是HTTP連接配接，包括原生的HttpClient和HttpUrlConnection兩種通路網絡方式。需要注意的是，HttpClient方式在Android6.0以後，很多類已經不支援了。
目前主流的開源網絡架構，主要有OkHttp，Volley，Retrofit三種，我本人在短暫的開發經曆中基本也隻接觸過這幾個，在此簡單分析這三個架構，僅作為本人記錄。
Volley架構
Volley架構是由Google在2013年釋出的較為輕量級的網絡架構，主要适用于處理請求次數較多，量級較小的網絡請求。
一、Volley使用方式
Volley的使用方式十分簡單，主要分為以下幾步：
Step1: 引入Volley（Android Studio）
方法一：在項目的build.gradle 添加依賴
方法二：引入volley.jar包
方法三：通過git下載下傳volley包，之後添加為項目的module，并為主工程添加依賴
需要注意的是，如果Volley傳回Object，需要對其進行解析，轉換為Gson對象，那麼還需要引入Gson依賴
注：引入的版本根據sdk版本而定
Step2: 建立請求隊列執行個體
一般而言，請求隊列不需要每次進行網絡請求時建立，通常一個Activity建立一個，或者對于較少請求的輕量級的應用，也可以一個應用隻建立一個請求隊列，主要視請求的多少而定。
Step3: 建立請求Request
Volley本身已封裝好幾種常用的Request，包括StringRequest，JsonRequest，JsonObjectRequest，JsonArrayRequest
值得一提的是，Volley封裝了ImageLoader和ImageRequest，可以友善地支援圖檔的擷取和加載，不需要額外自己添加圖檔的加載
以StringRequest為例，建立Request的代碼也十分簡單：
StringRequest stringRequest = new StringRequest(url, new Listener<String>() {
@Override
public void onResponse(String response) {
Log.e("xxxx", "onStringResponse: " + response);
}
}, new ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("xxxx", "onStringErrorResponse: " + error);
}
});
Step4: 将請求加入請求隊列
經過以上幾個步驟，就可以實作網絡請求，成功和失敗結果均傳回。
二、Volley源碼分析
Volley作為一個輕量級的網絡架構，源碼實際上并不複雜，接下來将針對其主要的代碼進行分析。
1. Volley.java 的主要代碼分析
圖中截出的就是我們在使用Volley時的第一步建立請求隊列的代碼，事實上Volley檔案中也隻有這一個重要的函數，主要步驟如下：
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
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;
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start();
return queue;
}
- 建立了一個用于存儲Volley緩存的檔案，使用的是預設的緩存路徑，然後擷取包名、包的資訊以及使用者資訊等
- 根據sdk版本，sdk版本大于等于9的，即Android系統2.3版本以上，建立HurlStack，9以下建立HttpClientStack
- 建立了請求隊列，maxDiskCacheBytes指的是緩存的最大容量，在建立隊列時如果定義了該參數，則按指定的容量，否則緩存容量為按照預設的-1
- RequestQueue.start()
2. RequestQueue的代碼分析部分
RequestQueue最重要的是add和start兩個函數。
start函數：
/**
* Starts the dispatchers in this queue.
*/
public void start() {
stop(); // Make sure any currently running dispatchers are stopped.
// Create the cache dispatcher and start it.
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// Create network dispatchers (and corresponding threads) up to the pool size.
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
可以看到主要的步驟是建立并啟動了兩種分發器，其中mNetworkDispatchers包含多個NetworkDispatcher，數量預設為4。
add函數的主要代碼如圖所示：
/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
public <T> Request<T> add(Request<T> request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else {
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}
- 将要add的請求加入目前請求的清單，并設定一個序列号碼和一個已加入隊列的tag，用來讓分發器按請求的順序處理請求。
- 如果請求設定了不緩存（預設是緩存），那麼将其加入網絡請求隊列。
- 用cachekey來作為請求的唯一辨別，如果有相同key的請求在waitingRequests中，辨別有相同的請求已經執行并且還沒有傳回結果，為避免重複請求，則将該請求存入；如果沒有，則将該請求加入緩存隊列中。
3. CacheDispatcher 和 NetworkDispatcher
在RequestQueue的代碼分析中，我們可以看到隊列并沒有對網絡請求進行處理，接下來我們看看這兩個分發器是如何處理加入隊列的網絡請求的。
① CacheDispatcher
run()函數解析：
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// Make a blocking call to initialize the cache.
mCache.initialize();
Request<?> request;
while (true) {
...
}
}
- 設定線程的優先級為最高，并初始化緩存
- 接下來進入一個無限循環，按序取出清單中的request，對request隊列中的每一個request進行處理
對request進行處理的步驟：
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);
continue;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}
// We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
if (!entry.refreshNeeded()) {
// Completely unexpired cache hit. Just deliver the response.
mDelivery.postResponse(request, response);
} else {
// Soft-expired cache hit. We can deliver the cached response,
// but we need to also send the request to the network for
// refreshing.
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);
// Mark the response as intermediate.
response.intermediate = true;
// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
- 從請求清單中取出request，判斷是否已取消，如未取消，則通過request的cacheKey從緩存中擷取
- 如果緩存中沒有對應key的request或者已經過期，則将該request發送到mNetworkQueue，等待NetworkDispatcher進行處理
- 如果有對應的緩存request，并且沒有過期，那麼CacheDispatcher會将該請求發送到主線程（發送到主線程的具體方法後面會說明）。
② NetworkDispatcher
同樣，NetworkDispatcher的run()函數也有一個無限循環對request進行處理，在确認request未被中斷（通常發生在請求逾時的情況）并且未被取消後，進行處理。
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("network-queue-take");
// If the request was cancelled already, do not perform the
// network request.
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
addTrafficStatsTag(request);
// Perform the network request.
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
// If the server returned 304 AND we delivered a response already,
// we're done -- don't deliver a second identical response.
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}
// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
// Write to cache if applicable.
// TODO: Only update cache metadata instead of entire record for 304s.
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
}
}
}
- 取出request後，通過Network的performRequest請求網絡
- 在擷取到response後，通過parseNetworkResponse對response進行解析
- 如果request需要緩存，則存儲到mCache中，等待CacheDispatcher進行處理
- 最後将擷取到的response傳回給主線程并标記request完成即可
4. NetworkDispatcher 請求網絡方式
上面run函數中可以看到請求網絡是通過mNetwork.performRequest函數請求網絡，該函數在BasicNetwork中實作，實作方式如下圖：
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());
// Handle cache validation.
if (statusCode == HttpStatus.SC_NOT_MODIFIED) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
// A HTTP 304 response does not have all header fields. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}
...
} catch (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode = 0;
NetworkResponse networkResponse = null;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
VolleyLog.e("Request at %s has been redirected to %s", request.getOriginUrl(), request.getUrl());
} else {
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
}
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
attemptRetryOnException("redirect",
request, new RedirectError(networkResponse));
} else {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(e);
}
}
}
}
- 擷取headers，之後通過mHttpStack請求網絡
- mHttpStack也是一個接口類，通過之前提到過的HurlStack和HttpClientStack實作
- 擷取到 httpResponse後，根據響應的狀态碼，傳回不同的NetworkResponse給NetworkDispatcher
5. 傳回response給主線程的方式
在請求網絡擷取到NetworkResponse後，通過mDeliver.postResponse将結果傳回給主線程，代碼如下圖：
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
ResponseRunnable代碼：
@SuppressWarnings("unchecked")
@Override
public void run() {
// If this request has canceled, finish it and don't deliver.
if (mRequest.isCanceled()) {
mRequest.finish("canceled-at-delivery");
return;
}
// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
// If this is an intermediate response, add a marker, otherwise we're done
// and the request can be finished.
if (mResponse.intermediate) {
mRequest.addMarker("intermediate-response");
} else {
mRequest.finish("done");
}
// If we have been provided a post-delivery runnable, run it.
if (mRunnable != null) {
mRunnable.run();
}
}
根據Request請求的結果，Runnable分别調用了Request不同的函數。
以StringRequest為例：
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}
請求成功時調用mListener（建立StringRequest時的參數）的回調函數，在回調函數中對擷取到的response進行需要的操作即可。
至此，Volley實作網絡請求的基本流程就梳理完成了，最後再通過流程圖進行總結：
對Volley架構的總結就到這裡，下一篇将繼續總結另一種重要的網絡架構——OkHttp的相關内容。
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