Question

我有一个扩展Application的类，名为MyApplicaton，用于存储全局变量，即我希望能够在我的Android应用程序中随处访问的变量。

在活动中，我可以通过调用

来访问MyApplication

myapp = (MyApplication) this.getApplication();

我想将一些变量存储到文件中，并能够从文件中读取它们。现在，如果我从MyApplication构造函数中调用readFromStorage（），我会得到一个NullPointerException（下面的日志）。但是，如果我不从构造函数中调用readFromStorage（），但稍后说，例如在调用writeToStorage之后（而不是从构造函数中调用），则执行正常。

如果文件不存在，我希望看到FileNotFoundException。但这并没有被抛出。

这是MyApplication，它扩展了Application。滚动到底部查看我的writeToStorage和readFromStorage（）方法。

public class MyApplication extends Application {    
private static String FILENAME = "globs";
private static int CONTEXT = Context.MODE_PRIVATE;
private String value1 = new String();
private String value2 = new String();

public MyApplication(){
    //check for and load stored globals in internal storage
    Log.v("Here", "In MyApplication constructor");
    readFromStorage();
}

//Took out unnecessary getter and setters for this question

private void writeToStorage(){
    //Create JSON object
    Log.v("Here", "In MyApplication writeToStorage");
    JSONObject obj = new JSONObject();
    File f = new File(FILENAME);
    OutputStream out;
    try {
        obj.put("value1", value1);
        obj.put("value2", value2);
        FileOutputStream fos = openFileOutput(FILENAME, CONTEXT);
        fos.write(obj.toString().getBytes());
        fos.close();
    } catch (JSONException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }  
    //We've just written to the file. Lets see if we can read from it...
    this.readFromStorage();
}

public void readFromStorage(){
    Log.v("Here", "In MyApplication readFromStorage");
    try {
        Log.v("Here", "FILENAME: "+ FILENAME);
        FileInputStream out = openFileInput(FILENAME);
        BufferedReader inputReader = new BufferedReader(new InputStreamReader(out));
        String inputString;
        StringBuffer sb = new StringBuffer();                
        while ((inputString = inputReader.readLine()) != null) {
            sb.append(inputString + "\n");
        }
        JSONObject jsonObject = new JSONObject(sb.toString());     
        this.value1 = (String) jsonObject.get("value1");
        this.value2 = (String) jsonObject.get("value2");
        out.close();
        inputReader.close();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
}

导致nullpointer异常的行是：

FileInputStream out = openFileInput(FILENAME);

FILENAME不为null，我在调用之前用Log语句检查它。

这是日志输出：

03-26 13:08:05.462: V/Here(26212): In MyApplication constructor
03-26 13:08:05.462: V/Here(26212): In MyApplication readFromStorage
03-26 13:08:05.462: V/Here(26212): FILENAME: globs
03-26 13:08:05.462: W/dalvikvm(26212): threadid=1: thread exiting with uncaught exception (group=0x41fb62d0)
03-26 13:08:05.462: E/AndroidRuntime(26212): FATAL EXCEPTION: main
03-26 13:08:05.462: E/AndroidRuntime(26212): java.lang.RuntimeException: Unable to instantiate application com.example.tetherly.MyApplication: java.lang.NullPointerException
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.LoadedApk.makeApplication(LoadedApk.java:553)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4795)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.ActivityThread.access$1300(ActivityThread.java:151)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.os.Looper.loop(Looper.java:155)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.ActivityThread.main(ActivityThread.java:5485)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at java.lang.reflect.Method.invokeNative(Native Method)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at java.lang.reflect.Method.invoke(Method.java:511)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:795)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at dalvik.system.NativeStart.main(Native Method)
03-26 13:08:05.462: E/AndroidRuntime(26212): Caused by: java.lang.NullPointerException
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.content.ContextWrapper.openFileInput(ContextWrapper.java:159)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at com.example.tetherly.MyApplication.readFromStorage(MyApplication.java:106)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at com.example.package.MyApplication.<init>(MyApplication.java:32)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at java.lang.Class.newInstanceImpl(Native Method)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at java.lang.Class.newInstance(Class.java:1319)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.Instrumentation.newApplication(Instrumentation.java:988)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.Instrumentation.newApplication(Instrumentation.java:973)
03-26 13:08:05.462: E/AndroidRuntime(26212):    at android.app.LoadedApk.makeApplication(LoadedApk.java:544)
03-26 13:08:05.462: E/AndroidRuntime(26212):    ... 11 more

Answer 1

您在构造函数中过早使用Application作为Context。它未在应用生命周期Context之前设置为onCreate()使用。

基本上，初始化有三个与此问题相关的步骤：

实例化对象。
在对象中设置资源和上下文。
在对象上调用onCreate()。

在第1步实例化（构造函数，成员变量），该对象尚不可用作Context。

这同样适用于活动和服务：您只能在Context或更晚的时候将其用作onCreate()。

Answer 2

问题是您的应用程序上下文为空。

在MyApplication构造函数中，您没有调用super（），因此未正确创建应用程序。

public MyApplication(){
    super();
    //check for and load stored globals in internal storage
    Log.v("Here", "In MyApplication constructor");
    readFromStorage();
}

openFileInput（FILENAME）nullPointerException：FILENAME not null

2 个答案: