我正在构建一个android应用程序，我选择在外部存储器中使用flatfiles作为持久化方法。我有一个用一些静态实例方法扩展应用程序的类。在调试了几次之后，我注意到没有一个实例变量是null，并且NullPointerException来自Context.getExternalFilesDir(null)中。
我想在任何答案前加上它不返回空值。此外，我在清单中拥有正确的权限，并检查SD卡是否已安装到设备上。
这个类保存对getExternalFilesDir(null)的调用并抛出错误。
public class WhileImOut extends Application {
public static TaskManager taskManager;
private static Context appContext;
public WhileImOut() {
super();
appContext = this;
initialize();
}
public static void initialize() {
if (taskManager == null && Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
try{
File f = appContext.getExternalFilesDir(null); // Exception being thrown here
taskManager = new TaskManager(f.getAbsolutePath());
}catch(NullPointerException e){
Log.d("Bad","NPE");
}catch(Exception e){
}
}
}
}
这在我的androidmanifest.xml中：
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
任何帮助都将不胜感激。我已经挣扎了几个小时，我会接受任何建议。谢谢你
编辑：logcat信息
10-10 18:55:56.424: E/AndroidRuntime(964): FATAL EXCEPTION: main
10-10 18:55:56.424: E/AndroidRuntime(964): java.lang.RuntimeException: Unable to resume activity {zsmith.capstone.whileimout/zsmith.capstone.whileimout.TaskListActivity}: java.lang.NullPointerException
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2575)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2603)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2089)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.access$600(ActivityThread.java:130)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.os.Handler.dispatchMessage(Handler.java:99)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.os.Looper.loop(Looper.java:137)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.main(ActivityThread.java:4745)
10-10 18:55:56.424: E/AndroidRuntime(964): at java.lang.reflect.Method.invokeNative(Native Method)
10-10 18:55:56.424: E/AndroidRuntime(964): at java.lang.reflect.Method.invoke(Method.java:511)
10-10 18:55:56.424: E/AndroidRuntime(964): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
10-10 18:55:56.424: E/AndroidRuntime(964): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-10 18:55:56.424: E/AndroidRuntime(964): at dalvik.system.NativeStart.main(Native Method)
10-10 18:55:56.424: E/AndroidRuntime(964): Caused by: java.lang.NullPointerException
10-10 18:55:56.424: E/AndroidRuntime(964): at zsmith.capstone.whileimout.TaskListActivity.onResume(TaskListActivity.java:58)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1184)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.Activity.performResume(Activity.java:5082)
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2565)
最佳答案
您正在构造函数中进行初始化；这将不起作用。您应该在onCreate()中进行初始化。调用构造函数时，未正确初始化上下文。
public class WhileImOut extends Application {
public static TaskManager taskManager;
private static Context appContext;
public void onCreate() {
super();
appContext = this;
initialize();
}
public static void initialize() {
if (taskManager == null && Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
try{
File f = appContext.getExternalFilesDir(null); // Exception being thrown here
taskManager = new TaskManager(f.getAbsolutePath());
}catch(NullPointerException e){
Log.d("Bad","NPE");
}catch(Exception e){
}
}
}
}
（顺便问一下，有没有理由单独使用
initialize()方法？如果是的话，是否有任何原因使其public？关于java - getExternalFilesDir(null)引发NullPointerException，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/12830827/