解析Android框架之Volley源码

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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