Question

我在使用AsyncTask填充ListView时遇到问题。通常它工作正常，但它非常慢。所以我实现了AsyncTask，它应该返回值，然后Adapter可以使用它们在ListView中显示。

但是如果我运行它，它会对值返回nullPointerException。

这两个值和数据源都是全局变量，所以它应该在那里造成任何伤害。它们都在没有使用线程时工作。

这可能是因为它是异步的线程没有创建的值，因此它转到nullPointer，我何时应该加载值呢？

        MyAsynch myAs = new MyAsynch();
        myAs.execute();



        MySimpleArrayAdapter adapter = new MySimpleArrayAdapter(this, values);

   private class MyAsynch extends AsyncTask<Void, Void, Void>{
        @Override
        protected Void doInBackground(Void...strings) { // run time intensive task in separate thread

            datasource = new ReadingsDataSource(getApplicationContext());
            datasource.open();
            values = datasource.getAllReadings();
            datasource.close();
            return null;
        }
    };

这是错误：

08-20 12:21:32.107: E/AndroidRuntime(6896): FATAL EXCEPTION: main
08-20 12:21:32.107: E/AndroidRuntime(6896): java.lang.RuntimeException: Unable to instantiate application android.app.Application: java.lang.NullPointerException
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.LoadedApk.makeApplication(LoadedApk.java:482)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.ActivityThread.handleBindApplication(ActivityThread.java:3952)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.ActivityThread.access$1300(ActivityThread.java:128)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1199)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at   android.os.Handler.dispatchMessage(Handler.java:99)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.os.Looper.loop(Looper.java:137)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.ActivityThread.main(ActivityThread.java:4514)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at java.lang.reflect.Method.invokeNative(Native Method)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at java.lang.reflect.Method.invoke(Method.java:511)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at dalvik.system.NativeStart.main(Native Method)
08-20 12:21:32.107: E/AndroidRuntime(6896): Caused by: java.lang.NullPointerException
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.LoadedApk.initializeJavaContextClassLoader(LoadedApk.java:362)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.LoadedApk.getClassLoader(LoadedApk.java:305)
08-20 12:21:32.107: E/AndroidRuntime(6896):     at android.app.LoadedApk.makeApplication(LoadedApk.java:474)
08-20 12:21:32.107: E/AndroidRuntime(6896):     ... 11 more

Answer 1

原因是因为你试图在空的时候访问你的值，当你使用AsyncTask时，doInBackground中的代码是在后台执行的，你会得到回调。所以你的代码应该是这样的

    private class MyAsynch extends AsyncTask<Void, Void, Void>{

    ProgressDialog pd;
    @Override
    protected void onPreExecute() {
     super.onPreExecute();
    pd=ProgressDialog.show(/*you context variable*/,/*Your Title*/,/* Your message while data   is loaded*/);
    }



            @Override
            protected Void doInBackground(Void...strings) { // run time intensive task in separate thread

                datasource = new ReadingsDataSource(getApplicationContext());
                datasource.open();
                values = datasource.getAllReadings();
                datasource.close();
                return null;
            }
        };


    @Override 

    protected void onPostExecute(String result) {
   super.onPostExecute(result);
    pd.dismiss();
    // Give the data to you adapter from here,instead of the place where you gave it earlier
      MySimpleArrayAdapter adapter = new MySimpleArrayAdapter(this, values);
    }
  }

Android Sqlite AsyncTask填充ListView Null指针

1 个答案: