java - OpenFileOutput() 得到一个 nullPointerException

Question

我创建了一个静态处理程序来从监听传入蓝牙数据的线程接收数据。此处理程序调用:measure_finish2() 并且此方法调用一个名为:measure_finish() 的非静态方法，在最后一个方法中我需要调用 openFileOutput 方法但我有一个 nullPointerException，即使我调用一个简单的 Toast 我也有相同的方法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，永远不要自己创建一个Activity的实例。 Android 的框架创造了这些，而不是你。此 o 未正确初始化，因此任何尝试将其用于 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。