Cache.Entry 没有获取 json 数据

Question

我正在使用 volly 来检索数据并且它的工作很完美，除了我的 json 数组没有存储在缓存中。

这是我的代码：

private void getCacheValue() {
    Cache cache = AppController.getInstance().getRequestQueue().getCache();
    Cache.Entry entry = cache.get(Endpoints.product_url);


    if(entry != null){
        Log.w("Logdata:", ""+ entry.toString());
        try {
            String data = new String(entry.data, "UTF-8");
            JSONArray jsonArray = new JSONArray(data);

            // handle data, like converting it to xml, json, bitmap etc.,
            Log.v("Hello", data);
            listProduct.clear();
            for (int i = 0; i < jsonArray.length(); i++) {
                try {
                    JSONObject object = jsonArray.getJSONObject(i);
                    ItemCategories image = new ItemCategories();
                    image.setCategoryItem(object.getString(key_title));
                    image.setUrlThumb(object.getString(key_image));

                    listProduct.add(image);
                    } catch (JSONException e) {
                        Log.e(TAG, "Json parsing error: " + e.getMessage());
                    }
                }

                adapterProductList.notifyDataSetChanged();
                progressBarMain.setVisibility(View.GONE);
                internetError.setVisibility(View.GONE);
                recycleProductList.setVisibility(View.VISIBLE);

            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
                Log.v("JSON EXCEPTION", "DECLINE");
            }
    } else {
        if(isNetworkAvailable()) {
            fetchImages();
        } else {
            progressBarMain.setVisibility(View.GONE);
            internetError.setVisibility(View.VISIBLE);
        }
    }
}

private void fetchImages() {
    JsonObjectRequest jsObjRequest = 
        new JsonObjectRequest(Request.Method.GET, Endpoints.product_url(String) null, 
            new Response.Listener<JSONObject>() {    
                @Override
                public void onResponse(JSONObject response) {
                    listProduct.clear();
                    try {
                        JSONArray routes = response.getJSONArray(key_product);
                        for (int i = 0; i < routes.length(); i++) {
                            JSONObject object = routes.getJSONObject(i);
                            ItemCategories categories = new ItemCategories();
                            categories.setCategoryItem(object.getString(key_title));
                            categories.setUrlThumb(object.getString(key_image)); 

                            listProduct.add(categories);
                        }
                        adapterProductList.notifyDataSetChanged();
                        progressBarMain.setVisibility(View.GONE);
                        internetError.setVisibility(View.GONE);
                        recycleProductList.setVisibility(View.VISIBLE);
                        } catch (JSONException e) {
                            e.printStackTrace();
                            Toast.makeText(getApplicationContext(),
                                           "Error: " + e.getMessage(),
                                            Toast.LENGTH_LONG).show();
                         }
                     }
                  }, 
                  new Response.ErrorListener() {
                      @Override
                      public void onErrorResponse(VolleyError error) {
                          VolleyLog.d(TAG, "Error: " + error.getMessage());
                      }
                  });
    // Adding request to request queue
    AppController.getInstance().addToRequestQueue(jsObjRequest);
}

我已经在清单中声明了Appcontroller，但我不知道为什么缓存不起作用。

这是我的json_data

fetchImage() 正在工作，因为recyclerview 中有数据。但是，当我尝试离线检索数据时，它会显示任何数据，因为我的缓存无法存储任何数据。

Answer 1

默认情况下，Volley 仅在 Response Header 允许的情况下缓存数据。 Volley 根据以下响应头缓存响应：
1. 缓存控制
2. 过期
3. 最大年龄

详情见以下功能：

public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
    long now = System.currentTimeMillis();

    Map<String, String> headers = response.headers;

    long serverDate = 0;
    long lastModified = 0;
    long serverExpires = 0;
    long softExpire = 0;
    long finalExpire = 0;
    long maxAge = 0;
    long staleWhileRevalidate = 0;
    boolean hasCacheControl = false;
    boolean mustRevalidate = false;

    String serverEtag = null;
    String headerValue;

    headerValue = headers.get("Date");
    if (headerValue != null) {
        serverDate = parseDateAsEpoch(headerValue);
    }

    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.startsWith("stale-while-revalidate=")) {
                try {
                    staleWhileRevalidate = Long.parseLong(token.substring(23));
                } catch (Exception e) {
                }
            } else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
                mustRevalidate = true;
            }
        }
    }

    headerValue = headers.get("Expires");
    if (headerValue != null) {
        serverExpires = parseDateAsEpoch(headerValue);
    }

    headerValue = headers.get("Last-Modified");
    if (headerValue != null) {
        lastModified = 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;
        finalExpire = mustRevalidate
                ? softExpire
                : softExpire + staleWhileRevalidate * 1000;
    } else if (serverDate > 0 && serverExpires >= serverDate) {
        // Default semantic for Expire header in HTTP specification is softExpire.
        softExpire = now + (serverExpires - serverDate);
        finalExpire = softExpire;
    }

    Cache.Entry entry = new Cache.Entry();
    entry.data = response.data;
    entry.etag = serverEtag;
    entry.softTtl = softExpire;
    entry.ttl = finalExpire;
    entry.serverDate = serverDate;
    entry.lastModified = lastModified;
    entry.responseHeaders = headers;

    return entry;
}

您可以通过覆盖请求对象来更改默认缓存策略。 您可以像这样覆盖 JsonObjectRequest：

public class CustomJsonObjectRequest extends JsonObjectRequest {
    public CustomJsonObjectRequest(int method, String url, JSONObject jsonRequest, Response.Listener<JSONObject> listener, Response.ErrorListener errorListener) {
        super(method, url, jsonRequest, listener, errorListener);
    }

    public CustomJsonObjectRequest(String url, JSONObject jsonRequest, Response.Listener<JSONObject> listener, Response.ErrorListener errorListener) {
        super(url, jsonRequest, listener, errorListener);
    }

    @Override
    protected Response<JSONObject> parseNetworkResponse(NetworkResponse response) {
        try {
            String jsonString = new String(response.data,
                    HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET));
            return Response.success(new JSONObject(jsonString),
                    parseIgnoreCacheHeaders(response));
        } catch (UnsupportedEncodingException e) {
            return Response.error(new ParseError(e));
        } catch (JSONException je) {
            return Response.error(new ParseError(je));
        }
    }

    public static Cache.Entry parseIgnoreCacheHeaders(NetworkResponse response) {
        long now = System.currentTimeMillis();
        Map<String, String> headers = response.headers;
        long serverDate = 0;
        String serverEtag = null;
        String headerValue;

        headerValue = headers.get("Date");
        if (headerValue != null) {
            serverDate = HttpHeaderParser.parseDateAsEpoch(headerValue);
        }

        serverEtag = headers.get("ETag");

        final long cacheHitButRefreshed = 3 * 60 * 1000; // in 3 minutes cache will be hit, but also refreshed on background
        final long cacheExpired = 24 * 60 * 60 * 1000; // in 24 hours this cache entry expires completely
        final long softExpire = now + cacheHitButRefreshed;
        final long ttl = now + cacheExpired;

        Cache.Entry entry = new Cache.Entry();
        entry.data = response.data;
        entry.etag = serverEtag;
        entry.softTtl = softExpire;
        entry.ttl = ttl;
        entry.serverDate = serverDate;
        entry.responseHeaders = headers;

        return entry;
    }
}

将您的 fetchImage 函数更新为：

private void fetchImages() {

    CustomJsonObjectRequest jsObjRequest = new CustomJsonObjectRequest()
            (Request.Method.GET, Endpoints.product_url, (String) null, new Response.Listener<JSONObject>() {

                @Override
                public void onResponse(JSONObject response) {
                    listProduct.clear();
                    try {
                        JSONArray routes = response.getJSONArray(key_product);
                        for (int i = 0; i < routes.length(); i++) {
                            JSONObject object = routes.getJSONObject(i);
                            ItemCategories categories = new ItemCategories();

                            categories.setCategoryItem(object.getString(key_title));
                            categories.setUrlThumb(object.getString(key_image));

                            listProduct.add(categories);
                        }
                        adapterProductList.notifyDataSetChanged();
                        progressBarMain.setVisibility(View.GONE);
                        internetError.setVisibility(View.GONE);
                        recycleProductList.setVisibility(View.VISIBLE);
                    } catch (JSONException e) {
                        e.printStackTrace();
                        Toast.makeText(getApplicationContext(),
                                "Error: " + e.getMessage(),
                                Toast.LENGTH_LONG).show();
                    }
                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    VolleyLog.d(TAG, "Error: " + error.getMessage());

                }
            });
    // Adding request to request queue
    AppController.getInstance().addToRequestQueue(jsObjRequest);
}

供参考，请查看Android Volley + JSONObjectRequest Caching