Android volley 解析（四）之缓存篇

这是 volley 的第四篇 blog 了，写完这篇，volley 的大部分用法也都算写了一遍，所以暂时不会写 volley 的文章了，如果想看我前面写的文章，可以点这里 Android volley 解析（三）之文件上传篇

为什么要用缓存

我们知道，当客户端在请求网络数据的时候，是需要消耗流量的，特别是对于移动端用户来说，对于流量的控制要求很高。所以在做网络请求的时候，如果对数据更新要求不是特别高，往往都会用到缓存机制，一方面能减少对服务端的请求，控制流量；另一方面，当客户端在没有网络的情况下也能看到上一次请求的数据，这样使用户体验更佳，如下图，微信朋友圈在没有网络的情况下，依然能看到朋友们的动态

volley 缓存的原理

看了我前面blog 的朋友一定还记得，我在讲 get 请求的时候，讲到了volley 的基本流程，首先启动的是缓存线程mCacheDispatcher来获取缓存；
那我们就从如何获取缓存开始分析；
1、初始化缓存

  @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.这里对缓存做初始化操作        mCache.initialize();        while (true) {        ...

2、获取缓存

        while (true) {            try {                // Get a request from the cache triage queue, blocking until                // at least one is available.从缓存队列中获取缓存                final Request request = mCacheQueue.take();                request.addMarker("cache-queue-take");                // If the request has been canceled, don't bother dispatching it.                ...

这里有一个问题，缓存队列在什么时候添加缓存请求呢？我们回到最开始请求队列添加请求的地方

    public Request add(Request 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.如果请求没有设置缓存，则把请求添加到网络队列中        if (!request.shouldCache()) {            mNetworkQueue.add(request);            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);                //这里添加请求到缓存队列中                mCacheQueue.add(request);            }            return request;        }    }

代码不长，也很好理解，如果我们的请求没有设置了缓存，则请求添加到网络请求队列中，并直接返回了，不往下执行了，这时缓存队列中无法获取请求，所以这里我们知道了，想要用缓存则需要
在 Request 中把

    //设置是否启用缓存    public final void setShouldCache(boolean shouldCache) {        mShouldCache = shouldCache;    }

设为 true，当然，我们看mShouldCache 的默认值

    /**     * Whether or not responses to this request should be cached.     */    private boolean mShouldCache = true;

volley默认启用缓存的。再往下看

                // If the request has been canceled, don't bother dispatching it.如果已经取消请求，则结束本次请求的所有操作                if (request.isCanceled()) {                    request.finish("cache-discard-canceled");                    continue;                }                // Attempt to retrieve this item from cache. 通过缓存类获取缓存                Cache.Entry entry = mCache.get(request.getCacheKey());                //如果获取的缓存为空，这里有两种情况为空，一，第一次换取，没有情求过网络；二，缓存的数据达到了最大限度，此缓存已经被删除。                if (entry == null) {                    request.addMarker("cache-miss");                    Log.i("CacheDispatcher", "没有缓存数据：" + request.getUrl());                    mNetworkQueue.put(request);                    continue;                }                // If it is completely expired, just send it to the network.缓存已经过期，则重新请求网络                if (entry.isExpired()) {                    request.addMarker("cache-hit-expired");                    request.setCacheEntry(entry);                    Log.i("CacheDispatcher", "缓存数据过期：" + request.getUrl());                    mNetworkQueue.put(request);                    continue;                }                // We have a cache hit; parse its data for delivery back to the request.                request.addMarker("cache-hit");                //到这里表示已经成功获取缓存数据                Response<?> response = request.parseNetworkResponse(                        new NetworkResponse(entry.data, entry.responseHeaders));                request.addMarker("cache-hit-parsed");                //如果缓存需要刷新，则请求网络                if (!entry.refreshNeeded()) {                    // Completely unexpired cache hit. Just deliver the response.                    Log.i("CacheDispatcher", "获取缓存数据：" + request.getUrl());                    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;            }        }

直接从缓存类Cache获取，并经过几次验证，如果缓存合法则解析然后交给 UI线程分发出去。下面来看看具体的流程

存储缓存
如果是第一次请求，或者缓存已过期，肯定是无法获取到缓存的，这时可根据上图分析，将会进入网络请求线程NetworkDispatcher，储存缓存毫无疑问也是在这里面实现的。

        //省略部分代码                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.如果需要缓存，并且用户已经把网络请求的数据转换成一份为缓存数据，则通过 Cache 类把缓存存储。                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) {                parseAndDeliverNetworkError(request, volleyError);            } catch (Exception e) {                VolleyLog.e(e, "Unhandled exception %s", e.toString());                mDelivery.postError(request, new VolleyError(e));            }            //省略部分代码

通过以上代码可以知道，在网络请求线程请求到数据以后，会交给用户解析，并把数据转换一份成缓存数据，通过 Cache 缓存操作类，把数据缓存起来。

网络数据转换成缓存数据
上面提到了，把网络数据转化成缓存数据，那么，volley 是如何转换的？

     */    public static Cache.Entry parseCacheHeaders(NetworkResponse response) {        long now = System.currentTimeMillis();        //获取网络请求数据的头信息        Map<String, String> headers = response.headers;        long serverDate = 0;        long serverExpires = 0;        long softExpire = 0;        long maxAge = 0;        boolean hasCacheControl = false;        String serverEtag = null;        String headerValue;        //从头信息中获取 Date 数据        headerValue = headers.get("Date");        if (headerValue != null) {            serverDate = parseDateAsEpoch(headerValue);        }        //从头信息中获取 Cache-Control 数据，来控制缓存        headerValue = headers.get("Cache-Control");        if (headerValue != null) {            hasCacheControl = true;            String[] tokens = headerValue.split(",");            for (int i = 0; i < tokens.length; i++) {                String token = tokens[i].trim();                if (token.equals("no-cache") || token.equals("no-store")) {                    return null;                } else if (token.startsWith("max-age=")) {                    try {                        maxAge = Long.parseLong(token.substring(8));                    } catch (Exception e) {                    }                } else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {                    maxAge = 0;                }            }        }        headerValue = headers.get("Expires");        if (headerValue != null) {            serverExpires = parseDateAsEpoch(headerValue);        }        serverEtag = headers.get("ETag");        // Cache-Control takes precedence over an Expires header, even if both exist and Expires        // is more restrictive.        if (hasCacheControl) {            softExpire = now + maxAge * 1000;        } else if (serverDate > 0 && serverExpires >= serverDate) {            // Default semantic for Expire header in HTTP specification is softExpire.            softExpire = now + (serverExpires - serverDate);        }        Cache.Entry entry = new Cache.Entry();        entry.data = response.data;        entry.etag = serverEtag;        entry.softTtl = softExpire;        entry.ttl = entry.softTtl;        entry.serverDate = serverDate;        entry.responseHeaders = headers;        return entry;    }

前面我们提到的缓存是否过期和是否需要刷新，都是通过 response 的头部信息来判断，但是在BasicNetwork中只实现了缓存是否过期的操作，没有实现缓存是否需要刷新

    @Override    public NetworkResponse performRequest(Request<?> request) throws VolleyError {        long requestStart = SystemClock.elapsedRealtime();        while (true) {            HttpResponse httpResponse = null;            byte[] responseContents = null;            Map<String, String> responseHeaders = new HashMap<String, String>();            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());                //从这里开始设置缓存信息，如果设置了缓存的缓存时间，则把它添加到头部信息中，但是没有实现是否需要刷新缓存的操作，如果有需要，也可以在这里实现，这是就需要修改源码。                if(request.getCacheTime() != 0){                    responseHeaders.put("Cache-Control","max-age=" + request.getCacheTime());                }                // Handle cache validation.                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,                            request.getCacheEntry().data, responseHeaders, true);                }                responseContents = entityToBytes(httpResponse.getEntity());                // if the request is slow, log it.                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;                logSlowRequests(requestLifetime, request, responseContents, statusLine);                if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {                    throw new IOException();                }                return new NetworkResponse(statusCode, responseContents, responseHeaders, false);            } 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);                    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);                }            }        }    }

如何使用缓存

根据上面分析不难发现，要使用缓存，得具备两个条件，
1、启用缓存

public final void setShouldCache(boolean shouldCache) {        mShouldCache = shouldCache;    }

不过这个条件默认情况下是开启的。
2、设置缓存的时间

    public void setCacheTime(long cacheTime) {        mCacheTime = cacheTime;    }

这里 cacheTime 的单位是秒。
接下来看看具体用法

public class CacheRequestActivity extends ActionBarActivity {    /*数据显示的View*/    private TextView mIdTxt,mNameTxt,mDownloadTxt,mLogoTxt,mVersionTxt ;    /*弹出等待对话框*/    private ProgressDialog mDialog ;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_get);        mIdTxt = (TextView) findViewById(R.id.id_id) ;        mNameTxt = (TextView) findViewById(R.id.id_name) ;        mDownloadTxt = (TextView) findViewById(R.id.id_download) ;        mLogoTxt = (TextView) findViewById(R.id.id_logo) ;        mVersionTxt = (TextView) findViewById(R.id.id_version) ;        mDialog = new ProgressDialog(this) ;        mDialog.setMessage("get请求中...");        mDialog.show();        /*请求网络获取数据*/        MiNongApi.CacheObjectMiNongApi("minongbang", new ResponseListener<TestBean>() {            @Override            public void onErrorResponse(VolleyError error) {                mDialog.dismiss();            }            @Override            public void onResponse(TestBean response) {                Log.v("zgy","=======response======="+response) ;                mDialog.dismiss();                /*显示数据*/                mIdTxt.setText(response.getId() + "");                mNameTxt.setText(response.getName());                mDownloadTxt.setText(response.getDownload() + "");                mLogoTxt.setText(response.getLogo());                mVersionTxt.setText(response.getVersion() + "");            }        });    }}

cache api

    public static void CacheObjectMiNongApi(String value,ResponseListener listener){        String url ;        try {            url = Constant.MinongHost +"?test="+ URLEncoder.encode(value, "utf-8") ;        } catch (UnsupportedEncodingException e) {            e.printStackTrace();            url = Constant.MinongHost +"?test="+ URLEncoder.encode(value) ;        }        Request request = new GetObjectRequest(url,new TypeToken<TestBean>(){}.getType(),listener) ;        //请用缓存        request.setShouldCache(true);        //设置缓存时间10分钟        request.setCacheTime(10*60);        VolleyUtil.getRequestQueue().add(request) ;    }

再来看看效果图，在缓存存在的情况下当把网络连接断开的时候，依然能够获取到数据

有一种情况需要注意：当需要获取缓存，且希望缓存刷新的时候，这种情况就需要修改 Volley 的源码，前面已经提到一点点，相信大家都能实现的。

源码下载地址