Question

每次我发送传感器收集的数据时，我的应用都会崩溃。给出的错误如下：

06-20 14:50:00.784  22983-22983/com.example.adam.proj2 E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
        at com.example.adam.proj2.SensorActivity.onClick(SensorActivity.java:124)
        at android.view.View.performClick(View.java:3549)
        at android.view.View$PerformClick.run(View.java:14393)
        at android.os.Handler.handleCallback(Handler.java:605)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:4944)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
        at dalvik.system.NativeStart.main(Native Method)

以下是收集传感器数据的代码：

 public void onSensorChanged(SensorEvent event) {
    float x = event.values[0];
    ArrayList<Float> enrolAcc = new ArrayList<>();
    ArrayList<Float> authAcc = new ArrayList<>();
    TextView textEnrol = (TextView) findViewById(R.id.textView);
    if (choice == 1) {
        mPreviousAcc = mCurrentAcc;
        mCurrentAcc = (float) Math.sqrt((double) (x * x));
        float delta = mCurrentAcc - mPreviousAcc;
        mDiffAcc = mDiffAcc * 0.9f + delta;
        if (enrolAcc.size() < 100) {
            enrolAcc.add(x);

        } else {
            enrolAcc.remove(0);
            enrolAcc.add(x);
        }
        walkData = enrolAcc.toString();
        textEnrol.setText(walkData);
    }

这是写入文件的代码（这发生在单击按钮上）：

 public void onClick(View v) {
    switch (v.getId()) {
        case R.id.enrolBtn:
            choice = 1;
            Toast.makeText(this, "Enrolment Mode Selected", Toast.LENGTH_SHORT).show();
            break;
        case R.id.authBtn:
            choice = 2;
            Toast.makeText(this, "Authentication Service Starting", Toast.LENGTH_SHORT).show();
            break;
        case R.id.sendBtn:
            choice = 3;
            String baseDir = android.os.Environment.getExternalStorageDirectory().getAbsolutePath();
            String fileName = "Walk Data.csv";
            String filePath = baseDir + File.separator + fileName;
            File f = new File(filePath);
            FileOutputStream out = null;
            try {
                out = new FileOutputStream(f);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            try {
                out.write(walkData.getBytes());
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }

            android.net.Uri u1 = Uri.fromFile(f);
            Intent sendIntent = new Intent(Intent.ACTION_SEND);
            sendIntent.putExtra(Intent.EXTRA_STREAM, u1);
            sendIntent.setType("text/html");
            startActivity(sendIntent);
            break;
    }
}

从我看到的异常是由out.write方法生成的？保存传感器值的数组列表存储在walkData字符串中，以便可以将该字符串写入存储在外部设备存储器中的csv文件中。我希望数据采用CSV格式。

我很难过，无法弄清楚如何防止这种情况，任何帮助都会非常感激。

Answer 1

您收到错误是因为您尝试写入READ ONLY文件第out = new FileOutputStream(f)行引发异常： java.io.FileNotFoundException: /storage/sdcard/Walk Data.csv: open failed: EROFS (Read-only file system)，但你实际上忽略了它，所以out = NULL然后你得到了另一个例外将文件移动到可以写入文件的位置 -

    String fileName = "Walk Data.csv";
    String baseDir = getFilesDir() + "/" + fileName;
    File f = new File(baseDir);

Answer 2

查看代码：

        try {
            out = new FileOutputStream(f);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        try {
            out.write(walkData.getBytes());

在最后一行抛出异常。那么，那条线路可能出现什么问题呢？

out可能为null。如果找不到该文件，out 将为空，因为您捕获该异常并假装之前没有发生任何错误，将out保留为null。如果您未能初始化它，则不应尝试使用out。 try块应该使用out包含所有行，而不仅仅是初始化它的行。

walkData也可以为null。但由于我们不知道它来自哪里，我们不能说。使用调试器知道哪个为null。无论答案是什么，修复您的异常处理代码。它应该看起来像

        FileOutputStream out = null;
        try {
            out = new FileOutputStream(f);
            out.write(walkData.getBytes());

            android.net.Uri u1 = Uri.fromFile(f);
            Intent sendIntent = new Intent(Intent.ACTION_SEND);
            sendIntent.putExtra(Intent.EXTRA_STREAM, u1);
            sendIntent.setType("text/html");
            startActivity(sendIntent);
        } catch (IOException e) {
            // TODO signal the error to the user. Printing a stack trace is not enough
        }
        finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                    // TODO signal the error to the user. Printing a stack trace is not enough
                }
             }
        }

写入文件时的空指针异常 - Android Studio

2 个答案: