OpenFileOutput() 得到一个 nullPointerException

Question

我创建了一个静态处理程序来从一个监听传入蓝牙数据的线程接收数据。 这个处理程序调用：measure_finish2()，这个方法调用一个非静态方法：measure_finish()，在最后一个里面我需要调用 openFileOutput 方法但是我有一个 nullPointerException，即使我调用一个简单的 Toast 我也有同样的nullPointrException，这是我的代码的一部分：

    public class MainActivity Extends Activity{
    ...
    public void sending_tubes_to_clean(){
        stream_cleaning_tubes = "15";
        for(int i = 0;i<4;i++){
            if(itemsChecked[i] == true){
                stream_cleaning_tubes += "," + Integer.toString(i+1);
            }
        }
        if(conectToServerThread != null){
            conectToServerThread.commsThread.write(stream_cleaning_tubes);
            Toast.makeText(getBaseContext(), "Cleaning the selected Tubes!", Toast.LENGTH_SHORT).show();
            cleaning_status();
        }else{
            Toast.makeText(getBaseContext(), "You must be connected to the Viscometer!...", Toast.LENGTH_SHORT).show();
        }
        Log.d(tag,stream_cleaning_tubes);
    }        
public static Handler MainReceiver = new Handler(){
            @Override
            public void handleMessage(Message msg){
                int numOfBytesReceived = msg.arg1;
                byte[] buffer = (byte[]) msg.obj;
                String strReceived = new String(buffer);
                String comand;
                strReceived = strReceived.substring(3,numOfBytesReceived);
                comand = strReceived.substring(0,2);

                if(comand.toString().equals("03")){
                    //MainActivity o = new MainActivity();
                    measure_finish2(strReceived.substring(3));
                    //measure_finish2(strReceived.substring(3));
                }

            }
        };
        public static String get_data(String aux){
            String aux2 = null;
            aux2 = aux.substring(0,aux.indexOf(","));
            return aux2;
        }

        public static void measure_finish2(String data){
            MainActivity o = new MainActivity();
            o.measure_finish(data);
        }
        public void measure_finish(String data){
            String data_to_save,strdate,tube,temp,time,cte_tube,visco;
            String file_name = "results.txt";
            Calendar c = Calendar.getInstance();
            SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy HH:mm");
            strdate = sdf.format(c.getTime());
            tube = get_data(data);
            data = data.substring(data.indexOf(",")+1);
            temp = get_data(data);
            data = data.substring(data.indexOf(",")+1);
            time = get_data(data);
            data = data.substring(data.indexOf(",")+1);
            cte_tube = get_data(data);
            data = data.substring(data.indexOf(",")+1);
            visco = get_data(data);
            data_to_save = sample_id_var + user_id_var + strdate + "Tube# " + tube + temp + "°C"+ cte_tube + time + "s" + visco + "cSt" + "\n";
            try{
                FileOutputStream fos = openFileOutput(file_name,MODE_WORLD_READABLE);
                OutputStreamWriter osw = new OutputStreamWriter(fos);
                try {
                    osw.write(data_to_save);
                    osw.flush();
                    osw.close();
                }catch (IOException e) {
                    e.printStackTrace();
                }

            //Toast.makeText(getBaseContext(), "F saved", Toast.LENGTH_SHORT).show();
                }catch(FileNotFoundException e)
                {
                    e.printStackTrace();
                }

            samp_id_view.setText(data_to_save);
            cleaning_status();
        }

请任何人帮助我，这是我遇到的错误：

 04-17 17:09:18.926: E/AndroidRuntime(1685): FATAL EXCEPTION: main
04-17 17:09:18.926: E/AndroidRuntime(1685): Process: com.callirgos.viscopi, PID: 1685
04-17 17:09:18.926: E/AndroidRuntime(1685): java.lang.IllegalStateException: Could not execute method of the activity
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.view.View$1.onClick(View.java:4007)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.view.View.performClick(View.java:4756)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.view.View$PerformClick.run(View.java:19749)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.os.Handler.handleCallback(Handler.java:739)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.os.Handler.dispatchMessage(Handler.java:95)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.os.Looper.loop(Looper.java:135)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.app.ActivityThread.main(ActivityThread.java:5221)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at java.lang.reflect.Method.invoke(Native Method)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at java.lang.reflect.Method.invoke(Method.java:372)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
04-17 17:09:18.926: E/AndroidRuntime(1685): Caused by: java.lang.reflect.InvocationTargetException
04-17 17:09:18.926: E/AndroidRuntime(1685):     at java.lang.reflect.Method.invoke(Native Method)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at java.lang.reflect.Method.invoke(Method.java:372)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.view.View$1.onClick(View.java:4002)
04-17 17:09:18.926: E/AndroidRuntime(1685):     ... 10 more
04-17 17:09:18.926: E/AndroidRuntime(1685): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.FileOutputStream android.content.Context.openFileOutput(java.lang.String, int)' on a null object reference
04-17 17:09:18.926: E/AndroidRuntime(1685):     at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:181)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at com.callirgos.viscopi.MainActivity.measure_finish(MainActivity.java:434)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at com.callirgos.viscopi.MainActivity.measure_finish2(MainActivity.java:415)
04-17 17:09:18.926: E/AndroidRuntime(1685):     at com.callirgos.viscopi.MainActivity.test_files(MainActivity.java:478)
04-17 17:09:18.926: E/AndroidRuntime(1685):     ... 13 more

Answer 1

MainActivity o = new MainActivity();

首先，从不自己创建一个Activity 的实例。 Android 的框架创建了这些，而不是您。此o 未正确初始化，因此任何尝试使用它（例如openFileOutput()、Toast 等）通常都会崩溃。

其次，不要在主应用程序线程上进行磁盘 I/O。 measure_finish() 中显示的代码应该由蓝牙线程或另一个后台线程完成，而不是主应用程序线程。