使用Singleton模式的Android Volley错误

我正在尝试按照本指南使用Singleton如何使用Volley.目标是使用此Singleton将我的RequestQueue放在Application上下文中,这样它就不会受到从landscape到portrait等的影响.

但是我收到以下错误:

java.lang.NullPointerException:尝试在com.android.volley.toolbox.Volley.newRequestQueue(Volley.java:45)上的空对象引用上调用虚方法'java.io.File android.content.Context.getCacheDir()' )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)在se.ccconsulting.arenaui3.VolleySingleton.getInstance(VolleySingleton.java:34)at se.ccconsulting.arenaui3.AdventureFragment.onCreateView(AdventureFragment.java:62)...

它指向这一行:

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

我不确定这里有什么问题,因为我在代码中可以看到VolleySingleton.java中的getInstance()方法不应该

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 mCache = new LruCache(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;
    }
}

这是我在实施Singlton之前从Volley使用的代码行,它完美地工作但不满足我的需求:

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

我在Genymotion上调试.Fruthermore,Volley.java中的一行代码在异常中提到:

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

我需要帮助才能传递此错误.

如果您阅读了代码链接到的Github repo的README ,它特别提到,您需要将它添加到清单XML中.

在你的情况下,更改com.company.MyApplication到xxx.yyy.HelperApplication,虽然.