java.lang.NullPointerException :ava. io.File android.content.Context.getCacheDir()' on null

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我正在尝试使用Retrofit & OKHttp来缓存 HTTP 响应。但我得到java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getCacheDir()在空对象引用上。在这里我将放置我的 API Java 类。请帮助我，我们将非常感谢您的帮助。

这是我的 ApiManager 类，其中包含拦截器，getAPI() 具有改造构建部分:

public class ApiManager {


    private static Context context;

    public static final String API_URL = "https://www.alot.com/webservice/"; //Production

    private static ProductionAPIService service;

    public ProductionAPIService getService() {
        return service;
    }


    private static OkHttpClient okClient;
    private static ApiManager apiManager = null;

    private ApiManager() {
    }

    public static ApiManager getApi() {
        if (apiManager == null) {
            apiManager = new ApiManager();





            File httpCacheDirectory = new File(context.getCacheDir(), "responses");
            int cacheSize = 10 * 1024 * 1024; // 10 MiB
            Cache cache = new Cache(httpCacheDirectory, cacheSize);


            OkHttpClient client = new OkHttpClient.Builder()
                    .addNetworkInterceptor(REWRITE_CACHE_CONTROL_INTERCEPTOR)
                    .cache(cache)
                    .build();

            Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl(API_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    //    .client(SelfSigningClientBuilder.createClient(MyApplication.context))
                    .client(client)
                    .build();


            service = retrofit.create(ProductionAPIService.class);
        }
        return apiManager;
    }

    private static final Interceptor REWRITE_CACHE_CONTROL_INTERCEPTOR = new Interceptor() {
        @Override
        public Response intercept(Chain chain) throws IOException {
            Response originalResponse = chain.proceed(chain.request());
            if (AppUtils.isConnectedToInternet(context,true)) {
                int maxAge = 60; // read from cache for 1 minute
                return originalResponse.newBuilder()
                        .header("Cache-Control", "public, max-age=" + maxAge)
                        .build();
            } else {
                int maxStale = 60 * 60 * 24 * 28; // tolerate 4-weeks stale
                return originalResponse.newBuilder()
                        .header("Cache-Control", "public, only-if-cached, max-stale=" + maxStale)
                        .build();
            }
        }


    };
}

Answer 1

当分配给变量的内存中没有对象时，会抛出

NullPointerException。换句话说，您没有将任何 Context 分配给您的 context 变量。您应该将其传递到静态 getApi() 方法中。最终结果应该是这样的

 public static ApiManager getApi(@NonNull Context context) {
    if (apiManager == null) {
        apiManager = new ApiManager();

        this.context = context;

        File httpCacheDirectory = new File(context.getCacheDir(), "responses");
        int cacheSize = 10 * 1024 * 1024; // 10 MiB
        Cache cache = new Cache(httpCacheDirectory, cacheSize);


        OkHttpClient client = new OkHttpClient.Builder()
                .addNetworkInterceptor(REWRITE_CACHE_CONTROL_INTERCEPTOR)
                .cache(cache)
                .build();

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(API_URL)
                .addConverterFactory(GsonConverterFactory.create())
                //    .client(SelfSigningClientBuilder.createClient(MyApplication.context))
                .client(client)
                .build();


        service = retrofit.create(ProductionAPIService.class);
    }
    return apiManager;
}