Do we need to use MappedByteBuffer.force() to flush data to disk?

Question

I am using MappedByteBuffer to speed up file read/write operations(). My questions as below:

1. I am not sure if I need to use .force() method to flush the content to disk or not. It seems like without .force(), the .getInt() can still work perfectly (well, since this is a memory-mapped buffer, i assume .getInt() fetches the data from disk, which means the data has been flushed into disk already.

2. Is the .force() method a blocking method?

3. Is a blocking method a synchronized block?

4. There is a huge performance difference with or without calling .force() method. What's the benefit of calling .force() manually? Under which situation should we use it? I assume without calling it, the data will still be written to disk behind the scene.

5. If we needs to call .force(), will calling it from another thread helps to improve performace? Will it corrupt the data because of synchronization problem?

   import java.io.FileNotFoundException; import java.io.IOException; import java.io.RandomAccessFile; import java.nio.MappedByteBuffer; import java.nio.channels.FileChannel; import java.nio.channels.FileChannel.MapMode;

public class Main {

public static void main(String[] args) throws IOException {
    System.out.println("start");

    RandomAccessFile raf = new RandomAccessFile("test.map", "rw");
    FileChannel fc = raf.getChannel();
    MappedByteBuffer mbb = fc.map(MapMode.READ_WRITE, 0, 2000000);

    int total = 0;
    long startTime = System.currentTimeMillis();
    for (int i = 0; i < 2000000; i += 4) {
        mbb.putInt(i, i);
        //mbb.force();
        total += mbb.getInt(i);
    }
    long stopTime = System.currentTimeMillis();

    System.out.println(total);
    System.out.println(stopTime - startTime);
    System.out.println("stop");
}

}

Answer 1

MappedByteBuffer.force() is not useless in Windows. I used "Process Monitor" tool from http://technet.microsoft.com/en-us/sysinternals/bb896645 to monitor the file access. According to the logging, MappedByteBuffer.force() will immediately call Windows API WriteFile() in Non-cached Synchronous mode. The reliability should be similar as FileChannel.force() which will call Windows API FlushFileBuffers() immediately to write file. Therefore, MappedByteBuffer.force() is reliable enough for most kinds of usages. Tested on Windows 7 64bit with Java 1.6.0_24.

Answer 2

1. You should call it only if you have extreme transactional requirements, i.e. you are implementing a database. getInt() reads from memory: the operating system pages the file into and out of that memory.

2. It is not specified.

3. Methods are synchronized if so specified. It has nothing to do with whether they block or not.

4. See (1). Data will still be written but at the operating system's whim rather than yours.

5. I doubt it, but see (2), and I doubt that you need to call it at all, see (1).