凌空請求隊列返回null

Question

這是我的單身類：

public class RequestSingleton { 

    private static RequestSingleton mInstance ; 
    private RequestQueue mRequestQueue; 
    private ImageLoader mImageLoader; 
    private static Context mCtx; 

     private RequestSingleton(Context context){ 
      mCtx = context; 
      mRequestQueue = getRequestQueue(); 

      mImageLoader = new ImageLoader(mRequestQueue, new ImageLoader.ImageCache(){ 

       private final LruCache<String, Bitmap> cache = new LruCache<String, Bitmap>(20); 

       @Override 
       public Bitmap getBitmap(String url) { 
        return cache.get(url); 
       } 

       @Override 
       public void putBitmap(String url, Bitmap bitmap) { 
        cache.put(url, bitmap); 
       } 
      }); 
     } 

    public static synchronized RequestSingleton getInstance(Context context){ 
     if (mInstance == null){ 
      mInstance = new RequestSingleton(context); 
     } 
     return mInstance; 
    } 

    private RequestQueue getRequestQueue(){ 
     if(mRequestQueue == null){ 
      mRequestQueue = Volley.newRequestQueue(mCtx.getApplicationContext()); 
     } 
     return mRequestQueue; 
    } 

    public <T> void addToRequestQueue(Request<T> req){ 
     getRequestQueue().add(req); 
    } 
    public ImageLoader getImageLoader(){ 
     return mImageLoader; 
    } 

這是從主要活動的方法來獲得職位：

public void getJsonPosts(){ 

     String url = "http://linagrey.blogspot.com/feeds/posts/default?alt=json"; 


     JsonObjectRequest jsonRequest = new JsonObjectRequest(Request.Method.GET,url,null, new Response.Listener<JSONObject>() { 
      @Override 
      public void onResponse(JSONObject response) { 
       try { 
        JSONObject feedResponse = response.getJSONObject("feed"); 
        JSONArray entryArrayPosts = feedResponse.getJSONArray("entry"); 
        BlogPost bp; 
        for (int i = 0; i < entryArrayPosts.length(); i++){ 
         bp = new BlogPost(); 
         //Get the entry as an array and loop through 
         JSONObject entryBlogPost = entryArrayPosts.getJSONObject(i); 

         //Get the title from the array 
         JSONObject postObject = entryBlogPost.getJSONObject("title"); 
         //JSONObject postHtmlContent = entryBlogPost.getJSONObject(KEYS.POST_JSON_OBJECT_HTML_CONTENT); 

         //GET LINK ARRAY FROM JSON 
         JSONArray linkAddress = entryBlogPost.getJSONArray(KEYS.POST_JSON_ARRAY_OBJECT); 

         if(entryBlogPost.has(KEYS.MEDIA_THUMBNAIL)) { 
          JSONObject imageObjectUrl = entryBlogPost.getJSONObject(KEYS.MEDIA_THUMBNAIL); 
          String thumbnailUrl = imageObjectUrl.getString(KEYS.MEDIA_THUMBNAIL_URL); 
          bp.setmUrlThumbnail(thumbnailUrl); 
         } 

         //for loop for link array object 
         for (int j = 0;j < linkAddress.length(); j++){ 
          JSONObject eachLink = linkAddress.getJSONObject(4); 
          String htmlUrl = eachLink.getString(KEYS.POST_HTML_LINK); 
          bp.setmLinkContent(htmlUrl); 
         } 


         //Get title string contained in Title Object 
         String title = postObject.getString(KEYS.POST_TITLE); 
         Log.v(TAG, "OBJECTS HAVE THIS PROPERTIES: " + i + " " + title); 


         //Set Properties Gotten 
         bp.setmTitle(title); 

         Log.v(TAG, "OBJECTS HAVE THIS PROPERTIES: " + i + " " + bp.toString()); 
         //Add a blog post Object to the Arraylist 
         postArrayList.add(bp); 


        } 
        // Log.v(TAG, "Arraylist SIZE " + postArrayList.size()); 

        for(BlogPost posts : postArrayList){ 
         Log.v(TAG, posts + " "); 
        } 

        // Toast.makeText(this.,response.toString(),Toast.LENGTH_LONG).show(); 

       }catch(JSONException e){ 
        Log.v(TAG, e.toString()); 
       } 
      } 
     },new Response.ErrorListener(){ 
      @Override 
      public void onErrorResponse(VolleyError error) { 
       Log.v(TAG, error.toString()); 
      } 
     }); 
     RequestSingleton.getInstance(this).addToRequestQueue(jsonRequest); 
    } 

這是錯誤輸出，我不知道關於如何解釋這些錯誤信息：

05-31 08:44:14.880 2653-2653/wait.com.linasblog E/AndroidRuntime: FATAL EXCEPTION: main 
                    Process: wait.com.linasblog, PID: 2653 
                    java.lang.RuntimeException: Unable to start activity ComponentInfo{wait.com.linasblog/wait.com.linasblog.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference 
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) 
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                     at android.os.Looper.loop(Looper.java:148) 
                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                     at java.lang.reflect.Method.invoke(Native Method) 
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
                    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference 
                     at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:107) 
                     at wait.com.linasblog.RequestSingleton.getRequestQueue(RequestSingleton.java:51) 
                     at wait.com.linasblog.RequestSingleton.<init>(RequestSingleton.java:24) 
                     at wait.com.linasblog.RequestSingleton.getInstance(RequestSingleton.java:44) 
                     at wait.com.linasblog.MainActivity.getJsonPosts(MainActivity.java:204) 
                     at wait.com.linasblog.MainActivity.onCreate(MainActivity.java:41) 
                     at android.app.Activity.performCreate(Activity.java:6237) 
                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) 
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) 
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)  
                     at android.app.ActivityThread.-wrap11(ActivityThread.java)  
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  
                     at android.os.Handler.dispatchMessage(Handler.java:102)  
                     at android.os.Looper.loop(Looper.java:148)  
                     at android.app.ActivityThread.main(ActivityThread.java:5417)  
                     at java.lang.reflect.Method.invoke(Native Method)  
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

我發佈了類似的問題，但沒有得到答案。

Answer 1

我不認爲請求singleton類正在初始化。所有的字段都是靜態的，但是它們沒有被設置爲任何東西，所以它們仍然是空的。嘗試添加到請求隊列之前，請嘗試RequestSingleton rs = new RequestSingleton();。更好的重寫應用程序類，以便它在啓動時初始化請求singleton類。

編輯： 我不知道這是否是一種最佳做法，但如果您仍有問題，至少應嘗試其他方法。這將覆蓋Application類並使用自定義類。它會在你的應用程序啓動時初始化Volley。

public class VolleyApplication extends Application { 
    private static final String TAG = VolleyNetworkTask.class.getSimpleName(); 
    private static VolleyNetworkTask sInstance; 
    private static RequestQueue mRequestQueue; 

    @Override 
    public void onCreate() { 
     super.onCreate(); 
     mRequestQueue = Volley.newRequestQueue(this); 
     sInstance = this; 
    } 

    public synchronized static VolleyNetworkTask getInstance() { 
     return sInstance; 
    } 

    public static RequestQueue getRequestQueue() { 
     return mRequestQueue; 
    } 
} 

而且在清單標籤

<application 
     android:name=".VolleyApplication" 
     .../> 

的把這個在您的清單獲取請求隊列 VolleyApplication.getRequestQueue();