带有android.os.memoryfile的文件IO

Question

I am trying to write to a file using android.os.memoryfile and then read from the same file. I write to the file in one method and read from another method, all in the same class. I can write fine but When I read the file I get lines of a symbol instead of the content I had written to the file. It seems that the read method is not reading the file I had written to.

If I put code to write and read the file in same method, it seems to work fine. Reading file outputs the string I had written.

Here is my code:

    public class FileActivity extends Activity {

        MemoryFile memFile;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        try {
            memFile = new MemoryFile("MemoryFileTest", 1000000);
        } catch (IOException e) {
            e.printStackTrace();
        }
     }



      public void readFile () {
            StringBuffer strBuffer = new StringBuffer();

            try {

            InputStream in = memFile.getInputStream();
            BufferedReader reader = new BufferedReader(new  InputStreamReader(in));

           String inputString;
           while ((inputString = reader.readLine()) != null) {
                strBuffer.append(inputString + "\n");
           }

           in.close();
           reader.close();

             Log.d(TAG, strBuffer.toString());
          } catch (IOException e) {
              e.printStackTrace();
          }
    } 

    public void writeFile () {
         String example = "This is an example";
         byte[] bytes = example.getBytes();
         try {
              OutputStream out = memFile.getOutputStream();
              out.write(bytes);

              out.close();
         }  catch (IOException e) {
            e.printStackTrace();
         }
     }
}

In the above code, the MemoryFile is declared in top of class, and initialized in onCreate() method. I am then trying to write and read with that one MemoryFile.

What am I missing here?

Answer 1

I've not used MemoryFile and only starting learning about it for this question. I looked at the source code from within Android Studio. I couldn't find the raw source on the Web. The grepcode version is here . From my experiments and looking at the source code, it's clear that the end-of-file concepts we expect from file-based streams don't apply to a MemoryFile . The end-of-file is based on the length specified in the MemoryFile constructor, not the number of bytes that have been written to the file. Given this, the behavior of BufferedReader.readline() , which you are using in your code, is not well defined because it is expecting to find an end-of-file condition at the end of the character stream and it won't get that with a MemoryFile until it gets to the end of allocated memory buffer. This explains why you see all the diamond question marks in the logcat output.

To get a better understanding of the end-of-file behavior, you can modify readFile() like this. You will see that 'n' is the length you specify in the MemoryFile constructor.

public void readFile () {
    StringBuffer strBuffer = new StringBuffer();

    try {
        InputStream in = memFile.getInputStream();

        int b;
        int n = 0;
        while ((b = in.read()) != -1) {
            n++;
        }
        Log.d(TAG, String.format("Stream contains %d bytes", n));

        /***************
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));

        String inputString;
        while ((inputString = reader.readLine()) != null) {
            strBuffer.append(inputString + "\n");
        }
        in.close();
        reader.close();

        Log.i(TAG, strBuffer.toString());
        *******************/
    } catch (IOException e) {
        e.printStackTrace();
    }
}

I can't explain why you see output when the read and write are in the same method, but not when they are in separate methods. Separate methods worked for me (KitKat on Samsung S3). Now that it is understood that BufferReader can't be used on a MemoryFile, the question of why it worked one way and not the other is not significant.

You may find find these MemoryFile test cases helpful as examples of its use. I didn't find any other good examples.

Answer 2

I think the problem is the length of the MemoryFile is not set. memFile = new MemoryFile("MemoryFileTest", 1000000);

Try memFile = new MemoryFile("MemoryFileTest", LENGTH_OF_YOUR_STRING); Hope it helps.