Toolkit - Tricks with Memory Byte Buffer

This post if part of a series about the File System Toolkit - a custom content delivery API for SDL Tridion.

In previous post Writing My Own Database Engine, I quickly mentioned the use of Memory Byte Buffer from Jana NIO that provides fast access to a file by mapping its content to memory. This post goes into more detail over some tricks that occurred with that implementation.

There is an issue with Memory Byte Buffer. Namely, once it is created by calling FileChannel.map method, it cannot be unmapped, closed or discarded. The byte buffer will exist until it is garbage collected.

From the JavaDoc:

A mapping, once established, is not dependent upon the file channel that was used to create it. Closing the channel, in particular, has no effect upon the validity of the mapping.

A mapped byte buffer and the file mapping that it represents remain valid until the buffer itself is garbage-collected.

The issue is affects Windows OS Java implementations, in the sense that it keep portions of the mapped file open and unavailable to modifications. The index file must be modifiable at any time because of publish/unpublish activities that must be able to modify the index.

Attempting to modify the index file on Windows OS will result in a FileNotFoundException (The requested operation cannot be per formed on a file with a user-mapped section open)

In order to work around this issue, I created a big hack. I created my own factory that creates and destroys MemoryByteBuffers.

public enum MemoryBufferFactory {

    INSTANCE;
    MemoryBufferFactory() {
    }

    public MemoryBuffer getBuffer(FileChannel fileChannel, FileChannel.MapMode mode) throws IOException {
        return new MappedMemoryBuffer(fileChannel, mode);
    }
}

The MemoryBuffer is a interface that defines the operations on a wrapped byte buffer:

public interface MemoryBuffer {
    int capacity();
    byte get(int position);
    void close();
}

The MappedMemoryBuffer implements the MemoryBuffer interface and wraps a java.nio.MappedByteBuffer object. The only trick it does is that in its close() method, it calls an unsupported, private unmap method of the MappedByteBuffer.

public class MappedMemoryBuffer implements MemoryBuffer {

    private final MappedByteBuffer buffer;
    private final FileChannel fileChannel;

    public MappedMemoryBuffer(FileChannel fileChannel, FileChannel.MapMode mode) {
        this.fileChannel = fileChannel;
        buffer = fileChannel.map(mode, 0, fileChannel.size());
    }

    public int capacity() {
        return buffer.capacity();
    }

    public byte get(int position) {
        return buffer.get(position);
    }

    public void close() {
        Class<?> clazz = fileChannel.getClass();
        Method method = clazz.getDeclaredMethod("unmap",
            new Class[]{MappedByteBuffer.class});
        method.setAccessible(true);
        method.invoke(null, new Object[]{buffer});
    }
}