使用Singleton模式的Android Volley错误

Question

I am trying to follow this guide on how to work with Volley using a Singleton. The goal is to use this Singleton to have my RequestQueue in an Application context so it won't be affected by shifting from landscape to portrait and such.

But I get the following error:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getCacheDir()' on a null object reference at com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:45) at com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:105) at com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:115) at se.ccconsulting.arenaui3.VolleySingleton.(VolleySingleton.java:20) at se.ccconsulting.arenaui3.VolleySingleton.getInstance(VolleySingleton.java:34) at se.ccconsulting.arenaui3.AdventureFragment.onCreateView(AdventureFragment.java:62) ...

And it points towards this line:

mRequestQueue = Volley.newRequestQueue(HelperApplication.getAppContext());

I am not sure what is wrong here as from what I can see in the code the getInstance() method in VolleySingleton.java is not supposed to

VolleySingleton.java

public class VolleySingleton {
    private static VolleySingleton mInstance = null;
    private RequestQueue mRequestQueue;
    private ImageLoader mImageLoader;

    private VolleySingleton(){
            mRequestQueue = Volley.newRequestQueue(HelperApplication.getAppContext());
            mImageLoader = new ImageLoader(this.mRequestQueue, new ImageLoader.ImageCache() {
            private final LruCache<String, Bitmap> mCache = new LruCache<String, Bitmap>(10);
            public void putBitmap(String url, Bitmap bitmap) {
                mCache.put(url, bitmap);
            }
            public Bitmap getBitmap(String url) {
                return mCache.get(url);
            }
        });
    }

    public static VolleySingleton getInstance(){
        if(mInstance == null){
            mInstance = new VolleySingleton();
        }
        return mInstance;
    }

    public RequestQueue getRequestQueue(){
        return this.mRequestQueue;
    }

    public ImageLoader getImageLoader(){
        return this.mImageLoader;
    }
}

HelperApplication.java

public class HelperApplication extends Application{
    private static HelperApplication mInstance;
    private static Context mAppContext;

    @Override
    public void onCreate() {
        super.onCreate();
        mInstance = this;

        this.setAppContext(getApplicationContext());
    }

    public static HelperApplication getInstance(){
        return mInstance;
    }
    public static Context getAppContext() {
        return mAppContext;
    }
    public void setAppContext(Context mAppContext) {
        this.mAppContext = mAppContext;
    }
}

This is the line of code I used from Volley before implementing the Singlton which worked perfectly but do not meet my needs:

RequestQueue queue = Volley.newRequestQueue(getActivity());

I am debugging on Genymotion. Fruthermore, row of code from Volley.java mentioned in the exception:

File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

I need help getting passed this error.

Answer 1

If you read the README of the Github repo that code links to, it specifically mentions, you need to add this to your manifest XML.

<application android:name="com.company.MyApplication">

</application>

In your case, change com.company.MyApplication to xxx.yyy.HelperApplication , though.