Android文件编写

Question

Having a problem writing out to a file, this code is taken directly from the android developer page and then tweaked a bit by me. Is there something i am missing? Quite new to Android development so sorry if it's something blatantly obvious.

     send.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            FileOutputStream outputStream;
            String data = "hello";
            File fileDir = new File("data.txt");
            if (!fileDir.exists())
                fileDir.mkdirs();

            try {
                outputStream = openFileOutput("data.txt",Context.MODE_PRIVATE);
                outputStream.write(data.getBytes());
                outputStream.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }


    });

Answer 1

Basically, your problem is that you are trying to do it twice, once in a way that won't work, and once in a way that will, but hides the result.

        File fileDir = new File("data.txt");
        if (!fileDir.exists())
            fileDir.mkdirs();

This would create a Java File object connected to a hypothetical file called "data.txt" located in the current working directory, which for an Android app is the root directory of the device - a place you most definitely are not allowed to write to. However, this may not obviously cause any errors, as the root directory exists so mkdirs() will do nothing, and you only create a File object, you don't actually try to create a file on "disk". Effectively this code does nothing for you - get rid of it.

Next you try something basically workable:

        try {
            outputStream = openFileOutput("data.txt",Context.MODE_PRIVATE);
            outputStream.write(data.getBytes());
            outputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

openFileOutput() is a method of a Context (Activity or Service) which creates an output stream to write to an actual file located in the private internal storage area of your app. This is all fine and good, and normally a good choice for storing typical data. However, it is not a place that you will be able to examine when running a release app on a secured device, as neither ADB based tools nor Mass Storage or MTP access over USB have rights to it. So it's entirely possible that this code worked, but you had no way to discover that fact. If you are on an emulator, you can access this location with ADB or the DDMS browser, and if your apk is a debug one, you can use the run-as command line tool in the shell.

If you want to share the data, you might consider putting it on the External Storage instead.