【源码分析】Volley用法及源码分析

学了这么久的android开发，总算到了这一天。网络框架。

工欲善其事，必先利其器。

写一个框架，之前先看volley究竟是怎么写的。

看volley之前，先学会如何使用。ok，来看看如何使用。

一、Volley使用：

1）网络请求代码如下：

RequestQueue mQueue = Volley.newRequestQueue(this);        StringRequest stringRequest = new StringRequest("http://182.61.37.49:8080/spring-mvc-study/hello/login",                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);            }        });        mQueue.add(stringRequest);

2）看看我写的demo的效果：

二.阅读源码

读前辈读烂的源码最简单的方法：先把代码书写逻辑弄懂。打个比方，就好像读书的时候写作文，先知道个中心思想大纲，再做平铺扩充。（参考自郭神博客）

《一》中心思想

写写关于我自己的理解： 

• 先画一个思路总体图。

• 按标注说明：

1.维护一个“请求队列”；

2.向队列中添加请求（4.队列添加请求的具体方法）；

3.队列执行请求（5.缓存分发线程；6.网络分发线程）；

• 总结各个对象间关系说明：

Volley初始化维护了一个请求队列（包含缓存队列，网络请求队列两种）。该队列包含两种线程，一是缓存分发线程，二是网络请求分发线程。见名知意，缓存分发线程用来从缓存队列取出缓存（请求）分发到网络请求队列，网络请求分发线程用来将网络请求队列的数据（请求）取出并通过网络请求得到返回结果。

• 流程：

当网络请求进来，添加进队列，队列按顺序执行请求，获得返回结果。

• 核心：

该队列包含两种线程，一是缓存分发线程，二是网络请求分发线程。缓存分发线程用来从缓存队列取出缓存（请求）分发到网络请求队列，网络请求分发线程用来将网络请求队列的数据（请求）取出并通过网络请求得到返回结果。

《二》核心代码对应：

1）初始化4个对象

先初始化缓存队列、网络请求队列，然后初始化缓存分发器并start，网络请求分发器并start，按照如下先后顺序执行。

• 缓存队列

/** The cache triage queue. */    private final PriorityBlockingQueue<Request<?>> mCacheQueue =        new PriorityBlockingQueue<Request<?>>();

• 网络请求队列

/** The queue of requests that are actually going out to the network. */    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =        new PriorityBlockingQueue<Request<?>>();

• 缓存分发器（Thread）

mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);

• 网络请求分发器（Thread）

NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,                    mCache, mDelivery);

2）当添加请求，分几种情况。

public <T> Request<T> add(Request<T> request) {        ...        // If the request is uncacheable, skip the cache queue and go straight to the network.        if (!request.shouldCache()) {            mNetworkQueue.add(request);            return request;        }        ...    }

先判断是否需要缓存，如果不需要直接加入网络请求队列，over。

如果需要缓存，则判断是否等待请求Map中是否有该请求的缓存。

如果有，则获取到请求缓存链表，添加该请求到缓存链表。并更新等待请求Map。

如果没有，则在等待请求Map中添加该请求缓存key（即URL），缓存队列添加该请求。

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);            }

3）执行请求逻辑（核心）

缓存分发线程CacheDispatcher执行请求：

整体代码附上：（不用看，后紧跟着具体分析）

@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();        while (true) {            try {                // Get a request from the cache triage queue, blocking until                // at least one is available.                final Request<?> request = mCacheQueue.take();                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.                    mDelivery.postResponse(request, response, new Runnable() {                        @Override                        public void run() {                            try {                                mNetworkQueue.put(request);                            } catch (InterruptedException e) {                                // Not much we can do about this.                            }                        }                    });                }            } catch (InterruptedException e) {                // We may have been interrupted because it was time to quit.                if (mQuit) {                    return;                }                continue;            }        }    }

<1>取本地缓存结果，如果本地缓存结果为空，则将请求添加到网络请求队列。

// 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;                }

<2>如果结果已经过期，则将请求添加到网络请求队列。

// 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;                }

<3>如果结果存在，并且不过期，则通过请求解析结果

// 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");

<4>如果结果不需要刷新，将结果发送给response，否则添加到网络请求队列。（网络请求具体见后续分析）（response返回见后续分析）

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.                    mDelivery.postResponse(request, response, new Runnable() {                        @Override                        public void run() {                            try {                                mNetworkQueue.put(request);                            } catch (InterruptedException e) {                                // Not much we can do about this.                            }                        }                    });                }

网络分发线程NetworkDispatcher执行请求：

整体代码如下：（不用细看，后紧跟着具体分析）

@Override    public void run() {        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);        while (true) {            long startTimeMs = SystemClock.elapsedRealtime();            Request<?> request;            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);            }        }    }

1）从网络请求队列取请求

request = mQueue.take();

2）执行网络请求并解析网络请求数据。（网络请求具体见后续分析）

// Perform the network request.                NetworkResponse networkResponse = mNetwork.performRequest(request);

其中：解析response代码：

// Parse the response here on the worker thread.                Response<?> response = request.parseNetworkResponse(networkResponse);

3）将请求结果networkResponse存入缓存

if (request.shouldCache() && response.cacheEntry != null) {                    mCache.put(request.getCacheKey(), response.cacheEntry);                    request.addMarker("network-cache-written");                }

4）将结果返回（response返回见后续分析）

mDelivery.postResponse(request, response);

4.网络请求

思路总体图中的内容分析差不多了。然后把网络请求部分具体说下。请求网络的时候，我们使用了如下代码：

mNetwork.performRequest(request)

mNetwork是什么？见Volley初始化代码newRequestQueue中：层层传递，mNetwork即network。

Network network = new BasicNetwork(stack);

找到BasicNetwork的performRequest方法：

@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);                }                // Some responses such as 204s do not have content.  We must check.                if (httpResponse.getEntity() != null) {                  responseContents = entityToBytes(httpResponse.getEntity());                } 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, statusLine);                if (statusCode < 200 || statusCode > 299) {                    throw new IOException();                }                return new NetworkResponse(statusCode, responseContents, responseHeaders, false,                        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);                }                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 {                        // TODO: Only throw ServerError for 5xx status codes.                        throw new ServerError(networkResponse);                    }                } else {                    throw new NetworkError(networkResponse);                }            }        }    }

代码挺多，其实关键请求代码就一句话：

httpResponse = mHttpStack.performRequest(request, headers);

mHttpStack是上级构造方法传递过来的参数stack。stack就是httpClient和httpUrlConnection的不同选择实现。

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));            }        }

5.结果返回（response返回）

mDelivery.postResponse(request, response);

mDelivery即默认的new ExecutorDelivery(new Handler(Looper.getMainLooper()))

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);            }        };    }

postResponse

@Override    public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {        request.markDelivered();        request.addMarker("post-response");        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));    }

ResponseDeliveryRunnable

/**     * A Runnable used for delivering network responses to a listener on the     * main thread.     */    @SuppressWarnings("rawtypes")    private 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() {            // 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();            }       }    }

mRequest.deliverResponse(mResponse.result);以StringRequest为例：其中mListener接口为外部传入接口，见文章最开始使用。至于这个parseNetworkResponse则是用来解析网络请求的返回数据response的。

@Override    protected void deliverResponse(String response) {        mListener.onResponse(response);    }    @Override    protected Response<String> parseNetworkResponse(NetworkResponse response) {        String parsed;        try {            parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));        } catch (UnsupportedEncodingException e) {            parsed = new String(response.data);        }        return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));    }

over～