网络库简单总结

网络库的介绍

1.HttpURLConnection

API简单,体积较小,因而非常适用于Android项目,但是在android 2.2及以下版本中HttpUrlConnection存在着一些bug,所以建议在android 2.3以后使用HttpUrlConnection,在这之前使用的是HttpClient。

2.HttpClient (Apache )

高效稳定,但是维护成本高昂,故android 开发团队不愿意维护该库更青睐轻便的HttpUrlConnection。Android 5.0后已废弃该库

3.OKHttp

Square公司产品,OkHttp相比HttpURLConnection和HttpClient功能更加强大。

4.Volley

Volley是在2013年Google I/O大会上推出了一个新的网络通信框架,内部封装了HttpURLConnection和HttpClient, 解决了网络数据解析和线程切换的问题。
主要用于解决通讯频率高,但传输数据量小的情景而对于大数据量的网络操作,比如说下载文件等,Volley的表现就会非常糟糕。

其实Volley的使用是很简单的,总的来说就是发送一个http的请求,将请求加入到RequestQueue(请求队列)中,这里的RequestQueue是一个请求队列对象,它可以缓存所有的HTTP请求,然后按照一定的算法并发地发出这些请求。RequestQueue内部的设计就是非常合适高并发的,因此我们不必为每一次HTTP请求都创建一个RequestQueue对象,这是非常浪费资源的,基本上在每一个需要和网络交互的Activity中创建一个RequestQueue对象就足够了。

总的来说我们常用的Volley就下面三个步骤:Volley.newRequestQueue(context).add(request);
1. 创建一个RequestQueue对象。
2. 创建一个StringRequest对象。
3. 将StringRequest对象添加到RequestQueue里面。
下面我们主要通过这三句话来分析一下Volley源码中的实现原理

volley.png

这是官方的关于Volley工作流程图 , 其中蓝色部分代表主线程,绿色部分代表缓存线程,橙色部分代表网络线程。我们在主线程中调用RequestQueue的add()方法来添加一条网络请求,这条请求会先被加入到缓存队列当中,如果发现可以找到相应的缓存结果就直接读取缓存并解析,然后回调给主线程。如果在缓存中没有找到结果,则将这条请求加入到网络请求队列中,然后处理发送HTTP请求,解析响应结果,写入缓存,并回调主线程。

第一步:newRequestQueue(context)

 public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);  //执行带两个参数的构造方法
    }

这个方法仅仅只有一行代码,只是调用了newRequestQueue()的方法重载,并给第二个参数传入null。那我们看下带有两个参数的newRequestQueue()方法中的代码,如下所示:

 public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        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) {   //判断如果stack是等于null的,则去创建一个HttpStack对象
            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 = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

在上面的代码中可以看出,如果stack是等于null的,则去创建一个HttpStack对象,这里会判断如果手机系统版本号是大于9(这里指SDK版本)的,则创建一个HurlStack的实例,否则就创建一个HttpClientStack的实例。实际上HurlStack的内部就是使用HttpURLConnection进行网络通讯的,而HttpClientStack的内部则是使用HttpClient进行网络通讯的,至于为什么说需要大于或等于9,是因为SDK为9时对应的系统为android2.3,版本2.3以后推荐使用HttpURLConnection.
创建好了HttpStack之后,接下来又创建了一个Network对象,它是用于根据传入的HttpStack对象来处理网络请求的,紧接着new出一个RequestQueue对象,并调用它的start()方法进行启动,然后将RequestQueue返回,这样newRequestQueue()的方法就执行结束了。接下啦我们看看RequestQueue的start()方法内部执行内容:

    /** Number of network request dispatcher threads to start. */
    private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;//默认网络请求线程数量

    /**
     * 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();
        }
    }

这里先是创建了一个CacheDispatcher的实例,然后调用了它的start()方法,接着在一个for循环里去创建NetworkDispatcher的实例,并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的,而默认情况下for循环会执行四次,也就是说当调用了Volley.newRequestQueue(context)之后,就会有五个线程一直在后台运行,不断等待网络请求的到来,其中CacheDispatcher是缓存线程,NetworkDispatcher是网络请求线程。

上面得到了RequestQueue之后,我们只需要构建出相应的Request,然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了,下面我们分析一下add()方法的内部的逻辑

 /**
     * 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()) {  //判断当前的请求是否可以缓存
            mNetworkQueue.add(request);  //如果不能缓存则直接将这条请求加入网络请求队列
            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);  //可以缓存的话则在将这条请求加入缓存队列
            }
            return request;
        }
    }

可以看到,开始的时候会判断当前的请求是否可以缓存,如果不能缓存则直接将这条请求加入网络请求队列,可以缓存的话则将这条请求加入缓存队列。在默认情况下,每条请求都是可以缓存的,当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。
既然默认每条请求都是可以缓存的,自然就被添加到了缓存队列中,于是一直在后台等待的缓存线程就要开始运行起来了,我们看下CacheDispatcher中的run()方法,代码如下所示:

public class CacheDispatcher extends Thread {
    ......  //省略代码
    @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) {   //死循环
            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());  //从缓存当中取出响应结果
                if (entry == null) {  //如何为空的话则把这条请求加入到网络请求队列中
                    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()) {  //如果不为空的但该缓存已过期,则同样把这条请求加入到网络请求队列中
                    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));   //对数据进行解析
                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;
            }
        }
    }
}

可以看到一个while(true)循环,说明缓存线程始终是在运行的,接着会尝试从缓存当中取出响应结果,如何为空的话则把这条请求加入到网络请求队列中,如果不为空的话再判断该缓存是否已过期,如果已经过期了则同样把这条请求加入到网络请求队列中,否则就认为不需要重发网络请求,直接使用缓存中的数据即可。之后会调用Request的parseNetworkResponse()方法来对数据进行解析,再往后就是将解析出来的数据进行回调了,这部分代码的逻辑和NetworkDispatcher后半部分的逻辑是基本相同的,那么我们等下合并在一起看就好了,先来看一下NetworkDispatcher中是怎么处理网络请求队列的,代码如下所示:

public class NetworkDispatcher extends Thread {
    .......//省略代码
    @Override
    public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        while (true) {  //死循环
            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);  //执行网络请求
                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);  //解析响应的网络数据
                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);
            }
        }
    }

    private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
        error = request.parseNetworkError(error);
        mDelivery.postError(request, error);
    }
}

同样地,我们看到了类似的while(true)循环,说明网络请求线程也是在不断运行的。在死循环中会调用Network的performRequest()方法来去发送网络请求,而Network是一个接口,我们在上面创建 stack是通过这段代码(Network network = new BasicNetwork(stack);)实现网络请求的,所以这里具体的实现是BasicNetwork,所以我们在BasicNetwork类中查看performRequest()方法,如下所示:

public class BasicNetwork implements Network {
   .....//省略代码
    @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); //调用了HttpStack的performRequest()方法
                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);  //组装成一个NetworkResponse对象进行返回
                    }
                    // 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); //组装成一个NetworkResponse对象进行返回
                }

                // 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) {
                    ...... //省略代码
            }
        }
    }
}

这段代码主要是一些网络请求细节方面的东西,需要注意的是调用了HttpStack的performRequest()方法,这里的HttpStack就是在一开始调用newRequestQueue()方法是创建的实例,默认情况下如果系统版本号大于9就创建的HurlStack对象,否则创建HttpClientStack对象。这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的,之后会将服务器返回的数据组装成一个NetworkResponse对象进行返回。

在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据,以及将数据写入到缓存,这个方法的实现是交给Request的子类来完成的,因为不同种类的Request解析的方式也肯定不同。如果想自定义Request的方式,其中parseNetworkResponse()这个方法就是必须要重写的
在解析完了NetworkResponse中的数据之后,又会调用ExecutorDelivery的postResponse()方法来回调解析出的数据,代码如下所示:

1.在NetworkDispatcher类中
       private final ResponseDelivery mDelivery;
 // Parse the response here on the worker thread.
                Response<?> response = request.parseNetworkResponse(networkResponse);   //解析数据
                request.addMarker("network-parse-complete");

                mDelivery.postResponse(request, response);  //回调解析出的数据,具体实现在下面代码中

2.在ExecutorDelivery类中回调解析出来的数据
public class ExecutorDelivery implements ResponseDelivery {
    @Override
    public void postResponse(Request<?> request, Response<?> response) {
        postResponse(request, response, null);
    }

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

其中,在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象,这样就可以保证该对象中的run()方法就是在主线程当中运行的了,我们看下run()方法中的代码是什么样的:

private class ResponseDeliveryRunnable implements 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();
            }
       }

分析重点代码即可,主要看Request的deliverResponse()方法这个方法也是我们在自定义Request时需要重写的另外一个方法,每一条网络请求的响应都是回调到这个方法中,最后我们再在这个方法中将响应的数据回调到Response.Listener的onResponse()方法中就可以了。

5.Retrofit.

Square公司产品,内部封装了OKhttp, 解决了网络数据解析和线程切换的问题。

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