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解析Android框架之Volley源碼

 更新時間：2021年06月08日 09:59:06   作者：handsome黃  
我們知道Volley是在2013年Google I/O大會上推出了一個新的網(wǎng)絡通信框架,他的設計目的就是為了那些請求數(shù)據(jù)量不是特別大，但是又是特別頻繁的網(wǎng)絡操作非常適合。但是對于數(shù)據(jù)量較大的請求，比如說下載一個較大的文件，Volley可能相比于其他的框架，就有點不足了。

Volley簡單使用

我這里是以依賴架包的形式 ，大家也可以以gradle的形式進行依賴。

好了，接下來上代碼了.....

//獲取volley的請求對象
        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);

從代碼可以看出，首先newRequestQueue來獲取到一個請求隊列，然后在將StringRequest這個請求添加到請求隊列中，就可以了，就是這么簡單。當然請求不值StringRequest，還有JsonObjectRequest ，ImageRequest等等但是用法都是一樣的，這里就不貼代碼了。Volley的簡單使用就這樣可以進行請求了。是不是很簡單

Volley執(zhí)行原理

但是這個不是本篇的重點，重點是分析一下這些是怎么執(zhí)行的。先上一張圖

我們先看看newRequestQueue這個內(nèi)部是怎么執(zhí)行的，代碼一開始連續(xù)執(zhí)行了幾個重載方法，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) {
        ;
    }

    //這里進行了一個版本的判斷 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;
}

在這里，我們看到了一個版本判斷，是不是瞬間感覺有點熟悉，沒錯，我們前面說的，volley2.3之前用的是HTTPClient，2.3之后使用的是HttpURLConnection就是在這里進行判斷的。接著看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是緩存調(diào)度線程，NetworkDispatcher是網(wǎng)絡調(diào)度線程，而這個this.mDispatchers.length系統(tǒng)默認的大小為4，也就是說，在這里總共啟動了5個線程在后臺運行。

好了，到這里，就可以了，看源碼不要每一行都弄懂，不然，出不來了。到這里就拿到了這個RequestQueue對象?；剡^頭來看前面使用的代碼

//獲取volley的請求對象
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);

我們拿到這個RequestQueue對象以后，然后就把這個請求通過add方法添加到隊列中，我們看看這個add()方法是怎么執(zhí)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()) { //如果不能緩存
            this.mNetworkQueue.add(request);
            return request;
        } else {
            Map var7 = this.mWaitingRequests;
            synchronized(this.mWaitingRequests) {
                String cacheKey = request.getCacheKey();
                if (this.mWaitingRequests.containsKey(cacheKey)) { //判斷之前是否執(zhí)行過，但是還沒有返回結果
                    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 {
                //沒有的話就將請求加入緩存隊列mCacheQueue，同時加入mWaitingRequests中用來做下次同樣請求來時的重復判斷依據(jù)
                    this.mWaitingRequests.put(cacheKey, (Object)null);
                    this.mCacheQueue.add(request);
                }

                return request;
            }
        }
    }

從代碼中可以看出，首先判斷是否可以緩存，當然，默認是可以緩存的。如果不能緩存的話，則通過this.mNetworkQueue.add(request);將請求添加到網(wǎng)絡請求隊列中。如果可以緩存，則還會判斷一次這個請求是否請求，如果執(zhí)行過就就通過this.mWaitingRequests.put(cacheKey, stagedRequests);添加到mWaitingRequests隊列，不在重復請求。否則就加入到緩存隊列。

大體的流程是這樣?，F(xiàn)在我們看看緩存的，和網(wǎng)絡的是怎么執(zhí)行的。我們找到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(); //從緩存隊列中獲取到一個請求
                                request.addMarker("cache-queue-take");
                                if (request.isCanceled()) { //判斷請求是否取消，如果取消了，那就將該請求finish掉
                                    request.finish("cache-discard-canceled");
                                } else {
                                    Entry entry = this.mCache.get(request.getCacheKey());
                                    if (entry == null) {//如果從緩存中取出來的內(nèi)容為空，則將請求加入到網(wǎng)絡線程中再次請求
                                        request.addMarker("cache-miss");
                                        this.mNetworkQueue.put(request);
                                    } else if (entry.isExpired()) { //如果請求過期了，則將請求加入到網(wǎng)絡線程中再次請求
                                        request.addMarker("cache-hit-expired");
                                        request.setCacheEntry(entry);
                                        this.mNetworkQueue.put(request);
                                    } else { //將數(shù)據(jù)回調(diào)到主線程
                                        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;
                            }
                        }
                    }
                }
            }
        }
    }

這里嵌套了幾個循環(huán)，有點凌亂啊，但是慢慢分析的話，就會發(fā)現(xiàn)，其實很清晰。我在注釋上面寫了，這里就不重復了

我們在看看NetworkDispatcher，看看網(wǎng)絡線程是怎么執(zhí)行的。一樣找到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(); //獲取到一個請求
                    break;
                } catch (InterruptedException var6) {
                    if (this.mQuit) {
                        return;
                    }
                }
            }

            try {
                request.addMarker("network-queue-take");
                if (request.isCanceled()) { //如果請求取消了，則將請求finish掉
                    request.finish("network-discard-cancelled");
                } else {//進行網(wǎng)絡請求
                    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);
            }
        }
    }

代碼比較多，我們直接找到NetworkResponse networkResponse = this.mNetwork.performRequest(request);這句代碼，這句代碼就是請求網(wǎng)絡的代碼，最核心的。performRequest是一個接口，我們看看這個performRequest()方法。Network在最開始說版本判斷的時候里面有一句代碼Network network = new BasicNetwork((HttpStack)stack); 從這句代碼，我們可以知道BasicNetwork才是最終實現(xiàn)網(wǎng)絡請求的類，我們找到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));
                }
            }
        }
    }

代碼比較多，但是大多數(shù)代碼是判斷狀態(tài)返回碼的，不需要理會。

我們直接看httpResponse = this.mHttpStack.performRequest(request, headers);這一句代碼，HttpStack這個有沒有很熟悉。沒有？？沒關系我在復制一次代碼

if (stack == null) {
    if (VERSION.SDK_INT >= 9) {
        stack = new HurlStack();
    } else {
        stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
    }
}

還是在這個版本判斷這里，這里就是HurlStack就是真正的網(wǎng)絡請求的類了，網(wǎng)絡請求，就是寫在這個類里面的。好了，volley整個流程大概就是這樣了?，F(xiàn)在大家回過頭看最初的哪一張圖，是不是明了很多。

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