Android常用開源網路框架（一）—— Volley篇
阿新 • • 發佈：2018-12-21
在Android開發中，網路請求是最重要的模組之一，Android中大部分網路請求使用的是HTTP連線，包括原生的HttpClient和HttpUrlConnection兩種訪問網路方式。需要注意的是，HttpClient方式在Android6.0以後，很多類已經不支援了。
目前主流的開源網路框架，主要有OkHttp，Volley，Retrofit三種，我本人在短暫的開發經歷中基本也只接觸過這幾個，在此簡單分析這三個框架，僅作為本人記錄。
Volley框架
Volley框架是由Google在2013年釋出的較為輕量級的網路框架，主要適用於處理請求次數較多，量級較小的網路請求。
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：在專案的build.gradle 新增依賴
implementation 'com.mcxiaoke.volley:library:1.0.+'
方法二：引入volley.jar包
方法三：通過git下載volley包，之後新增為專案的module，併為主工程新增依賴
需要注意的是，如果Volley返回Object，需要對其進行解析，轉換為Gson物件，那麼還需要引入Gson依賴
implementation 'com.google.code.gson:gson:2.8.2' 注：引入的版本根據sdk版本而定
Step2: 建立請求佇列例項
requestQueue = Volley.newRequestQueue(context);
一般而言，請求佇列不需要每次進行網路請求時建立，通常一個Activity建立一個，或者對於較少請求的輕量級的應用，也可以一個應用只建立一個請求佇列，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，可以方便地支援圖片的獲取和載入，不需要額外自己新增圖片的載入
以StringRequest為例，建立Request的程式碼也十分簡單：
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: 將請求加入請求佇列
requestQueue.add(stringRequest);
經過以上幾個步驟，就可以實現網路請求，成功和失敗結果均返回。
二、Volley原始碼分析
Volley作為一個輕量級的網路框架，原始碼實際上並不複雜，接下來將針對其主要的程式碼進行分析。
1. Volley.java 的主要程式碼分析
圖中截出的就是我們在使用Volley時的第一步建立請求佇列的程式碼，事實上Volley檔案中也只有這一個重要的函式，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;
}
- 建立了一個用於儲存Volley快取的檔案，使用的是預設的快取路徑，然後獲取包名、包的資訊以及使用者資訊等
- 根據sdk版本，sdk版本大於等於9的，即Android系統2.3版本以上，建立HurlStack，9以下建立HttpClientStack
- 建立了請求佇列，maxDiskCacheBytes指的是快取的最大容量，在建立佇列時如果定義了該引數，則按指定的容量，否則快取容量為按照預設的-1
- RequestQueue.start()
2. RequestQueue的程式碼分析部分
RequestQueue最重要的是add和start兩個函式。
start函式：
/**
* 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並啟動了兩種分發器，其中mNetworkDispatchers包含多個NetworkDispatcher，數量預設為4。
add函式的主要程式碼如圖所示：
/**
* 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;
}
}
- 將要add的請求加入當前請求的列表，並設定一個序列號碼和一個已加入佇列的tag，用來讓分發器按請求的順序處理請求。
- 如果請求設定了不快取（預設是快取），那麼將其加入網路請求佇列。
- 用cachekey來作為請求的唯一標識，如果有相同key的請求在waitingRequests中，標識有相同的請求已經執行並且還沒有返回結果，為避免重複請求，則將該請求存入；如果沒有，則將該請求加入快取佇列中。
3. CacheDispatcher 和 NetworkDispatcher
在RequestQueue的程式碼分析中，我們可以看到佇列並沒有對網路請求進行處理，接下來我們看看這兩個分發器是如何處理加入佇列的網路請求的。
① CacheDispatcher
run()函式解析：
@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) {
...
}
}
- 設定執行緒的優先順序為最高，並初始化快取
- 接下來進入一個無限迴圈，按序取出列表中的request，對request佇列中的每一個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());
}
}
- 從請求列表中取出request，判斷是否已取消，如未取消，則通過request的cacheKey從快取中獲取
- 如果快取中沒有對應key的request或者已經過期，則將該request傳送到mNetworkQueue，等待NetworkDispatcher進行處理
- 如果有對應的快取request，並且沒有過期，那麼CacheDispatcher會將該請求傳送到主執行緒（傳送到主執行緒的具體方法後面會說明）。
② NetworkDispatcher
同樣，NetworkDispatcher的run()函式也有一個無限迴圈對request進行處理，在確認request未被中斷（通常發生在請求超時的情況）並且未被取消後，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請求網路
- 在獲取到response後，通過parseNetworkResponse對response進行解析
- 如果request需要快取，則儲存到mCache中，等待CacheDispatcher進行處理
- 最後將獲取到的response返回給主執行緒並標記request完成即可
4. NetworkDispatcher 請求網路方式
上面run函式中可以看到請求網路是通過mNetwork.performRequest函式請求網路，該函式在BasicNetwork中實現，實現方式如下圖：
@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)