Android框架原始碼解析之（一）Volley
阿新 • • 發佈：2018-10-31
前幾天面試CVTE，HR面掛了。讓內部一個學長幫我查看了一下面試官評價，發現二面面試官的評價如下：
- 廣度OK，但缺乏深究能力，深度與實踐不足
- 原始碼：只能說流程，細節程式碼不清楚，retrofit和volley都是。
感覺自己一方面：自己面試技巧有待提高吧（框架只說了流程，而沒說原始碼，原始碼實在是不知道怎麼說）
另一方面：原始碼雖然說系統的看過，但是細節不夠深入。
所以，就開個專欄，分析框架原始碼吧
第一篇Volley解析，雖然Volley已經很少有人使用了，但是內部設計三級快取機制值得我們學習。
先看一下volley的基本使用：
//1、建立RequestQueen Volley的原始碼解析：
下載地址：https://github.com/google/volley
下載後直接匯入Android studio中即可
1、從RequestQueen入手
RequestQueue mQueue = Volley.newRequestQueue(context);原始碼：
public class Volley {
/** Default on-disk cache directory. */
// /預設快取資料夾，檔名為Volley，預設最大為5M
private static final String DEFAULT_CACHE_DIR = "volley";
/**
* 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 A {@link BaseHttpStack} to use for the network, or null for default.
* @return A started {@link RequestQueue} instance.
*/
public static RequestQueue newRequestQueue(Context context, BaseHttpStack stack) {
//BasicNetwork 網路請求類
BasicNetwork network;
//建立網路請求實現類
// stack 預設網路請求實現類，如果Build.VERSION.SDK_INT >= 9 則stack =HttpURLConnecton，否則 stack =HttpClient
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
network = new BasicNetwork(new HurlStack());
} else {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
// At some point in the future we'll move our minSdkVersion past Froyo and can
// delete this fallback (along with all Apache HTTP code).
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info =
context.getPackageManager().getPackageInfo(packageName, /* flags= */ 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
network =
new BasicNetwork(
new HttpClientStack(AndroidHttpClient.newInstance(userAgent)));
}
} else {
network = new BasicNetwork(stack);
}
//呼叫下面的回撥方法
return newRequestQueue(context, network);
}
/**
* 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.
* @deprecated Use {@link #newRequestQueue(Context, BaseHttpStack)} instead to avoid depending
* on Apache HTTP. This method may be removed in a future release of Volley.
*/
@Deprecated
@SuppressWarnings("deprecation")
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
if (stack == null) {
return newRequestQueue(context, (BaseHttpStack) null);
}
return newRequestQueue(context, new BasicNetwork(stack));
}
private static RequestQueue newRequestQueue(Context context, Network network) {
//建立磁碟快取檔案
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
//建立請求佇列
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
//開啟請求佇列
queue.start();
return queue;
}
/**
* 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.
* @return A started {@link RequestQueue} instance.
*/
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, (BaseHttpStack) null);
}
}
可以看到，Volley.newRequestQueue(context);中主要做了以下操作：
1、建立網路請求實現類
2、建立快取
3、建立請求佇列，並開啟請求佇列
2、接下來看RequestQueen的start()方法
/** 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.
//建立一個快取排程執行緒，並開啟
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// Create network dispatchers (and corresponding threads) up to the pool size.
//建立4個網路排程執行緒，並開啟
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher =
new NetworkDispatcher(mNetworkQueue, mNetwork, mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}做了以下操作：
1、建立一個快取排程執行緒和4個網路排程執行緒
2、開啟這些執行緒
一會分析而這些執行緒
3、Request.add(stringRequest);
原始碼：
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.
//按照新增順序，設定請求順序
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
//判斷request是否可以快取，如果不可以快取，直接加入網路請求佇列
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
//如果可以快取，將請求加入快取請求佇列
mCacheQueue.add(request);
return request;
}判斷request是否可以快取，如果不能快取，直接將請求新增到網路請求佇列。若能，將請求新增到快取請求佇列。預設是可以快取的。
如何判斷request是否可以快取：
看原始碼：
Request.java
/** Whether or not responses to this request should be cached. */
// TODO(#190): Turn this off by default for anything other than GET requests.
//預設是可以快取的
private boolean mShouldCache = true;
/** Returns true if responses to this request should be cached. */
public final boolean shouldCache() {
return mShouldCache;
}4、CacheDispatcher快取排程執行緒
先看一下類的定義
public class CacheDispatcher extends Thread {說明CacheDispatcher是一個執行緒類，那直接看run方法
@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.
//初始化快取
mCache.initialize();
while (true) {
try {
//重點在這
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
//如果quit，則中斷執行緒
Thread.currentThread().interrupt();
return;
}
VolleyLog.e(
"Ignoring spurious interrupt of CacheDispatcher thread; "
+ "use quit() to terminate it");
}
}
}
初始化了快取，呼叫了processRequest();
檢視processRequest();原始碼
private void processRequest() throws InterruptedException {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request<?> request = mCacheQueue.take();
//呼叫下面回撥
processRequest(request);
}
@VisibleForTesting
void processRequest(final Request<?> request) throws InterruptedException {
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
//如果request請求取消，則停止分發
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
return;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
//嘗試讀取快取，如果快取為空，則將Request新增到網路請求佇列
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// If it is completely expired, just send it to the network.
//如果快取過期，則將Request新增到網路請求佇列
/*
判斷快取過期的條件 Cache.java
/*
public boolean isExpired() {
return this.ttl < System.currentTimeMillis();
面試被問到，無奈，過期時間是否小於當前時間
}
*/
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// We have a cache hit; parse its data for delivery back to the request.
//存在快取，且快取沒有過期，並命中，則將快取結構解析，並回調給主執行緒
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.
//回撥給主執行緒，一會分析這個類
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;
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
mDelivery.postResponse(
request,
response,
new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Restore the interrupted status
Thread.currentThread().interrupt();
}
}
});
} else {
// request has been added to list of waiting requests
// to receive the network response from the first request once it returns.
mDelivery.postResponse(request, response);
}
}
}總結：
首先從快取佇列中取出請求，判斷請求是否取消，若果沒有取消，則判斷快取是否有響應，若果有響應，並且沒有過期，則解析快取並回調給主執行緒。如果沒有快取或者快取過期，則將請求新增到網路排程執行緒。接下來檢視網路排程執行緒。
5、網路排程執行緒
public class NetworkDispatcher extends Thread {同樣整合Thread，直接檢視run方法；
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
try {
//重點在這
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
//如果quit，則中斷執行緒
Thread.currentThread().interrupt();
return;
}
VolleyLog.e(
"Ignoring spurious interrupt of NetworkDispatcher thread; "
+ "use quit() to terminate it");
}
}
}可以看出呼叫了processRequest();方法
processRequest()原始碼：
private void processRequest() throws InterruptedException {
// Take a request from the queue.
//從佇列中取出請求
Request<?> request = mQueue.take();
processRequest(request);
}
@VisibleForTesting
void processRequest(Request<?> request) {
long startTimeMs = SystemClock.elapsedRealtime();
try {
request.addMarker("network-queue-take");
// If the request was cancelled already, do not perform the
// network request.
//如果request被取消，中斷網路請求
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
request.notifyListenerResponseNotUsable();
return;
}
addTrafficStatsTag(request);
// Perform the network request.
//具體的網路請求在這裡實現mNetwork.performRequest(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.
//如果伺服器返回304並且，request已經分發出去，則不進行第二次網路請求
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
request.notifyListenerResponseNotUsable();
return;
}
// Parse the response here on the worker thread.
//解析請求
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.
//更新快取
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// Post the response back.
request.markDelivered();
//回撥給主執行緒進行顯示
mDelivery.postResponse(request, response);
request.notifyListenerResponseReceived(response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
request.notifyListenerResponseNotUsable();
} 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);
request.notifyListenerResponseNotUsable();
}
}總結：
判斷請求是否取消，若沒被取消，則請求網路，得到響應，回撥給主執行緒並將快取存入快取。
6、真正的網路請求實現類BasicNetwork實現方法為performRequest()
看原始碼：
@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
List<Header> responseHeaders = Collections.emptyList();
try {
// Gather headers.
Map<String, String> additionalRequestHeaders =
getCacheHeaders(request.getCacheEntry());
//真真真 的網路請求實現
httpResponse = mBaseHttpStack.executeRequest(request, additionalRequestHeaders);
int statusCode = httpResponse.getStatusCode();
responseHeaders = httpResponse.getHeaders();
// Handle cache validation.
if (statusCode == HttpURLConnection.HTTP_NOT_MODIFIED) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(
HttpURLConnection.HTTP_NOT_MODIFIED,
/* data= */ null,
/* notModified= */ true,
SystemClock.elapsedRealtime() - requestStart,
responseHeaders);
}
// Combine cached and response headers so the response will be complete.
List<Header> combinedHeaders = combineHeaders(responseHeaders, entry);
return new NetworkResponse(
HttpURLConnection.HTTP_NOT_MODIFIED,
entry.data,
/* notModified= */ true,
SystemClock.elapsedRealtime() - requestStart,
combinedHeaders);
}
// Some responses such as 204s do not have content. We must check.
InputStream inputStream = httpResponse.getContent();
if (inputStream != null) {
responseContents =
inputStreamToBytes(inputStream, httpResponse.getContentLength());
} else {
// Add 0 byte response as a way of honestly representing a
// no-content request.
responseContents = new byte[0];
}
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusCode);
if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}
return new NetworkResponse(
statusCode,
responseContents,
/* notModified= */ false,
SystemClock.elapsedRealtime() - requestStart,
responseHeaders);
} catch (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode;
if (httpResponse != null) {
statusCode = httpResponse.getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
NetworkResponse networkResponse;
if (responseContents != null) {
networkResponse =
new NetworkResponse(
statusCode,
responseContents,
/* notModified= */ false,
SystemClock.elapsedRealtime() - requestStart,
responseHeaders);
if (statusCode == HttpURLConnection.HTTP_UNAUTHORIZED
|| statusCode == HttpURLConnection.HTTP_FORBIDDEN) {
attemptRetryOnException(
"auth", request, new AuthFailureError(networkResponse));
} else if (statusCode >= 400 && statusCode <= 499) {
// Don't retry other client errors.
throw new ClientError(networkResponse);
} else if (statusCode >= 500 && statusCode <= 599) {
if (request.shouldRetryServerErrors()) {
attemptRetryOnException(
"server", request, new ServerError(networkResponse));
} else {
throw new ServerError(networkResponse);
}
} else {
// 3xx? No reason to retry.
throw new ServerError(networkResponse);
}
} else {
attemptRetryOnException("network", request, new NetworkError());
}
}
}
}主要是通過mBaseHttpStack.executeRequest(request, additionalRequestHeaders);請求網路，
接下來根據不同相應的餓狀態碼，返回不同的Response。
HttpStack具體的實現類是HttpUrlConnection和HttpClient,因此Volley具體底層的實現操作還是HttpUrlConnection和HttpClient。
7、如何將response回撥到主執行緒 ？ResponseDelivery的postResponse()方法
public interface ResponseDelivery {
/** Parses a response from the network or cache and delivers it. */
void postResponse(Request<?> request, Response<?> response);
/**
* Parses a response from the network or cache and delivers it. The provided Runnable will be
* executed after delivery.
*/
void postResponse(Request<?> request, Response<?> response, Runnable runnable);
/** Posts an error for the given request. */
void postError(Request<?> request, VolleyError error);
}ResponseDelivery 是一個介面，來看它的實現的類ExecutorDelivery
原始碼：
/** Delivers responses and errors. */
public class ExecutorDelivery implements ResponseDelivery {
/** Used for posting responses, typically to the main thread. */
private final Executor mResponsePoster;
/**
* Creates a new response delivery interface.
*
* @param handler {@link Handler} to post responses on
*/
//內部例項化了Handler，用於工作執行緒和UI執行緒的切換
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster =
new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}
/**
* Creates a new response delivery interface, mockable version for testing.
*
* @param executor For running delivery tasks
*/
public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}
@Override
public void postResponse(Request<?> request, Response<?> response) {
postResponse(request, response, null);
}
//利用handler進行轉發 handler.post(Runable)
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
//利用handler進行轉發 handler.post(Runable)
@Override
public void postError(Request<?> request, VolleyError error) {
request.addMarker("post-error");
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}
/** A Runnable used for delivering network responses to a listener on the main thread. */
@SuppressWarnings("rawtypes")
private static class ResponseDeliveryRunnable implements Runnable {
private final Request mRequest;
private final Response mResponse;
private final Runnable mRunnable;
public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
mRequest = request;
mResponse = response;
mRunnable = runnable;
}
@SuppressWarnings("unchecked")
@Override
public void run() {
// NOTE: If cancel() is called off the thread that we're currently running in (by
// default, the main thread), we cannot guarantee that deliverResponse()/deliverError()
// won't be called, since it may be canceled after we check isCanceled() but before we
// deliver the response. Apps concerned about this guarantee must either call cancel()
// from the same thread or implement their own guarantee about not invoking their
// listener after cancel() has been called.
// 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()) {
//呼叫Request進行轉發
mRequest.deliverResponse(mResponse.result);
} else {
//呼叫Request進行轉發
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();
}
}
}
}呼叫Request.deliverResponse(mResponse.result)進行轉發
Request是一個抽象類，看看他的子類StringRequest 的實現方法
8、StringRequest原始碼
@Override
protected void deliverResponse(String response) {
Response.Listener<String> listener;
synchronized (mLock) {
listener = mListener;
}
if (listener != null) {
//重點在這
listener.onResponse(response);
//
}
}在deliverResponse方法中呼叫了listener.onResponse(response);，最終將Response回撥給了Response.Listener的onResponse()方法。
看看我們剛開始的網路請求操作
StringRequest stringRequest = new StringRequest("http://www.baidu.com",
//最終會將事件回撥到這塊
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
Log.d("TAG", response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("TAG", error.getMessage(), error);
}
});最後來張圖總結一下吧
總結：
Volley分為三類執行緒，分別是主執行緒，快取排程執行緒和網路排程執行緒，其中網路排程執行緒預設開啟四個。首先請求會加入快取佇列，快取排程執行緒從快取佇列中取出請求。如果找到該請求的快取響應就直接讀取快取響應並回調給主執行緒。如果沒有快取效應，或者快取已過期，則將請求加入到網路佇列，然後網路排程執行緒會輪詢取出網路佇列中的請求，取出後傳送HTTP請求，解析響應回撥給主執行緒，並將相應存入主執行緒。