解析Android框架之Volley源码
作者：handsome黄
我们知道Volley是在2013年Google I/O大会上推出了一个新的网络通信框架,他的设计目的就是为了那些请求数据量不是特别大，但是又是特别频繁的网络操作非常适合。但是对于数据量较大的请求，比如说下载一个较大的文件，Volley可能相比于其他的框架，就有点不足了。
Volley简单使用
我这里是以依赖架包的形式 ，大家也可以以gradle的形式进行依赖。
好了，接下来上代码了.....
//获取volley的请求对象
RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());
StringRequest stringRequest = new StringRequest(StringRequest.Method.GET, "http://www.baidu.com", new Response.Listener<String>() {
@Override
public void onResponse(String s) {
Log.d("MainActivity", "----->" + s);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError volleyError) {
Log.d("MainActivity", "---volleyError-->" + volleyError);
}
});
requestQueue.add(stringRequest);
从代码可以看出，首先newRequestQueue来获取到一个请求队列，然后在将StringRequest这个请求添加到请求队列中，就可以了，就是这么简单。当然请求不值StringRequest，还有JsonObjectRequest ，ImageRequest等等但是用法都是一样的，这里就不贴代码了。Volley的简单使用就这样可以进行请求了。是不是很简单
Volley执行原理
但是这个不是本篇的重点，重点是分析一下这些是怎么执行的。先上一张图
我们先看看newRequestQueue这个内部是怎么执行的，代码一开始连续执行了几个重载方法，最后走到newRequestQueue
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
File cacheDir = new File(context.getCacheDir(), "volley");
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException var7) {
;
}
//这里进行了一个版本的判断 2.3之前用的是HTTPClient，2.3之后使用的是HttpURLConnection
if (stack == null) {
if (VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork((HttpStack)stack);
RequestQueue queue;
if (maxDiskCacheBytes <= -1) {
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
} else {
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start();
return queue;
}
在这里，我们看到了一个版本判断，是不是瞬间感觉有点熟悉，没错，我们前面说的，volley2.3之前用的是HTTPClient，2.3之后使用的是HttpURLConnection就是在这里进行判断的。接着看queue.start();
public void start() {
this.stop();
this.mCacheDispatcher = new CacheDispatcher(this.mCacheQueue, this.mNetworkQueue, this.mCache, this.mDelivery);
this.mCacheDispatcher.start();
for(int i = 0; i < this.mDispatchers.length; ++i) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(this.mNetworkQueue, this.mNetwork, this.mCache, this.mDelivery);
this.mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
mCacheDispatcher是缓存调度线程，NetworkDispatcher是网络调度线程，而这个this.mDispatchers.length系统默认的大小为4，也就是说，在这里总共启动了5个线程在后台运行。
好了，到这里，就可以了，看源码不要每一行都弄懂，不然，出不来了。到这里就拿到了这个RequestQueue对象。回过头来看前面使用的代码
//获取volley的请求对象
RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());
StringRequest stringRequest = new StringRequest(StringRequest.Method.GET, "http://www.baidu.com", new Response.Listener<String>() {
@Override
public void onResponse(String s) {
Log.d("MainActivity", "----->" + s);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError volleyError) {
Log.d("MainActivity", "---volleyError-->" + volleyError);
}
});
requestQueue.add(stringRequest);
我们拿到这个RequestQueue对象以后，然后就把这个请求通过add方法添加到队列中，我们看看这个add()方法是怎么执行的。
public <T> Request<T> add(Request<T> request) {
request.setRequestQueue(this);
Set var2 = this.mCurrentRequests;
synchronized(this.mCurrentRequests) {
this.mCurrentRequests.add(request);
}
request.setSequence(this.getSequenceNumber());
request.addMarker("add-to-queue");
if (!request.shouldCache()) { //如果不能缓存
this.mNetworkQueue.add(request);
return request;
} else {
Map var7 = this.mWaitingRequests;
synchronized(this.mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (this.mWaitingRequests.containsKey(cacheKey)) { //判断之前是否执行过，但是还没有返回结果
Queue<Request<?>> stagedRequests = (Queue)this.mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList();
}
((Queue)stagedRequests).add(request);
this.mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", new Object[]{cacheKey});
}
} else {
//没有的话就将请求加入缓存队列mCacheQueue，同时加入mWaitingRequests中用来做下次同样请求来时的重复判断依据
this.mWaitingRequests.put(cacheKey, (Object)null);
this.mCacheQueue.add(request);
}
return request;
}
}
}
从代码中可以看出，首先判断是否可以缓存，当然，默认是可以缓存的。如果不能缓存的话，则通过this.mNetworkQueue.add(request);将请求添加到网络请求队列中。如果可以缓存，则还会判断一次这个请求是否请求，如果执行过就就通过this.mWaitingRequests.put(cacheKey, stagedRequests);添加到mWaitingRequests队列，不在重复请求。否则就加入到缓存队列。
大体的流程是这样。现在我们看看缓存的，和网络的是怎么执行的。我们找到start()方法
public void start() {
this.stop();
this.mCacheDispatcher = new CacheDispatcher(this.mCacheQueue, this.mNetworkQueue, this.mCache, this.mDelivery);
this.mCacheDispatcher.start();
for(int i = 0; i < this.mDispatchers.length; ++i) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(this.mNetworkQueue, this.mNetwork, this.mCache, this.mDelivery);
this.mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
先看CacheDispatcher，找到run()方法
public void run() {
if (DEBUG) {
VolleyLog.v("start new dispatcher", new Object[0]);
}
Process.setThreadPriority(10);
this.mCache.initialize();
while(true) {
while(true) {
while(true) {
while(true) {
try {
while(true) {
final Request<?> request = (Request)this.mCacheQueue.take(); //从缓存队列中获取到一个请求
request.addMarker("cache-queue-take");
if (request.isCanceled()) { //判断请求是否取消，如果取消了，那就将该请求finish掉
request.finish("cache-discard-canceled");
} else {
Entry entry = this.mCache.get(request.getCacheKey());
if (entry == null) {//如果从缓存中取出来的内容为空，则将请求加入到网络线程中再次请求
request.addMarker("cache-miss");
this.mNetworkQueue.put(request);
} else if (entry.isExpired()) { //如果请求过期了，则将请求加入到网络线程中再次请求
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
this.mNetworkQueue.put(request);
} else { //将数据回调到主线程
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
if (entry.refreshNeeded()) {
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);
response.intermediate = true;
this.mDelivery.postResponse(request, response, new Runnable() {
public void run() {
try {
CacheDispatcher.this.mNetworkQueue.put(request);
} catch (InterruptedException var2) {
;
}
}
});
} else {
this.mDelivery.postResponse(request, response);
}
}
}
}
} catch (InterruptedException var4) {
if (this.mQuit) {
return;
}
}
}
}
}
}
}
这里嵌套了几个循环，有点凌乱啊，但是慢慢分析的话，就会发现，其实很清晰。我在注释上面写了，这里就不重复了
我们在看看NetworkDispatcher，看看网络线程是怎么执行的。一样找到run()方法
public void run() {
Process.setThreadPriority(10);
while(true) {
long startTimeMs;
Request request;
while(true) {
startTimeMs = SystemClock.elapsedRealtime();
try {
request = (Request)this.mQueue.take(); //获取到一个请求
break;
} catch (InterruptedException var6) {
if (this.mQuit) {
return;
}
}
}
try {
request.addMarker("network-queue-take");
if (request.isCanceled()) { //如果请求取消了，则将请求finish掉
request.finish("network-discard-cancelled");
} else {//进行网络请求
this.addTrafficStatsTag(request);
NetworkResponse networkResponse = this.mNetwork.performRequest(request);
request.addMarker("network-http-complete");
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
} else {
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
if (request.shouldCache() && response.cacheEntry != null) {
this.mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
request.markDelivered();
this.mDelivery.postResponse(request, response);
}
}
} catch (VolleyError var7) {
var7.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
this.parseAndDeliverNetworkError(request, var7);
} catch (Exception var8) {
VolleyLog.e(var8, "Unhandled exception %s", new Object[]{var8.toString()});
VolleyError volleyError = new VolleyError(var8);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
this.mDelivery.postError(request, volleyError);
}
}
}
代码比较多，我们直接找到NetworkResponse networkResponse = this.mNetwork.performRequest(request);这句代码，这句代码就是请求网络的代码，最核心的。performRequest是一个接口，我们看看这个performRequest()方法。Network在最开始说版本判断的时候里面有一句代码Network network = new BasicNetwork((HttpStack)stack); 从这句代码，我们可以知道BasicNetwork才是最终实现网络请求的类，我们找到performRequest方法
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while(true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map responseHeaders = Collections.emptyMap();
try {
Map<String, String> headers = new HashMap();
this.addCacheHeaders(headers, request.getCacheEntry());
httpResponse = this.mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());
if (statusCode == 304) {
Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(304, (byte[])null, responseHeaders, true, SystemClock.elapsedRealtime() - requestStart);
}
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(304, entry.data, entry.responseHeaders, true, SystemClock.elapsedRealtime() - requestStart);
}
if (statusCode == 301 || statusCode == 302) {
String newUrl = (String)responseHeaders.get("Location");
request.setRedirectUrl(newUrl);
}
byte[] responseContents;
if (httpResponse.getEntity() != null) {
responseContents = this.entityToBytes(httpResponse.getEntity());
} else {
responseContents = new byte[0];
}
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
this.logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode >= 200 && statusCode <= 299) {
return new NetworkResponse(statusCode, responseContents, responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
}
throw new IOException();
} catch (SocketTimeoutException var12) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException var13) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException var14) {
throw new RuntimeException("Bad URL " + request.getUrl(), var14);
} catch (IOException var15) {
int statusCode = false;
NetworkResponse networkResponse = null;
if (httpResponse == null) {
throw new NoConnectionError(var15);
}
int statusCode = httpResponse.getStatusLine().getStatusCode();
if (statusCode != 301 && statusCode != 302) {
VolleyLog.e("Unexpected response code %d for %s", new Object[]{statusCode, request.getUrl()});
} else {
VolleyLog.e("Request at %s has been redirected to %s", new Object[]{request.getOriginUrl(), request.getUrl()});
}
if (responseContents == null) {
throw new NetworkError(networkResponse);
}
networkResponse = new NetworkResponse(statusCode, (byte[])responseContents, responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
if (statusCode != 401 && statusCode != 403) {
if (statusCode != 301 && statusCode != 302) {
throw new ServerError(networkResponse);
}
attemptRetryOnException("redirect", request, new AuthFailureError(networkResponse));
} else {
attemptRetryOnException("auth", request, new AuthFailureError(networkResponse));
}
}
}
}
代码比较多，但是大多数代码是判断状态返回码的，不需要理会。
我们直接看httpResponse = this.mHttpStack.performRequest(request, headers);这一句代码，HttpStack这个有没有很熟悉。没有？？没关系我在复制一次代码
if (stack == null) {
if (VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
还是在这个版本判断这里，这里就是HurlStack就是真正的网络请求的类了，网络请求，就是写在这个类里面的。好了，volley整个流程大概就是这样了。现在大家回过头看最初的哪一张图，是不是明了很多。
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