Unmap MMapIndexInput's in a delayed way, and avoid WeakReference usage.

public class ByteBufferUnmapper {
  /**
   * <code>true</code>, if this platform supports unmapping mmapped files.
   */
  public static final boolean UNMAP_SUPPORTED;
  private static final ScheduledExecutorService SCHEDULED_EXECUTOR_SERVICE = Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("unmapper-"));

  static {
    boolean v = false;
    try {
      Class.forName("sun.misc.Cleaner");
      Class.forName("java.nio.DirectByteBuffer")
        .getMethod("cleaner");
      v = true;
    } catch (Exception e) {
      // Do nothing
    } finally {
      UNMAP_SUPPORTED = v;
    }
  }


  public static void unmap(final ByteBuffer buffer) throws IOException {
    try {
      AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
        public Object run() throws Exception {
          Method getCleanerMethod = buffer.getClass().getMethod("cleaner");
          getCleanerMethod.setAccessible(true);
          final Object cleaner = getCleanerMethod.invoke(buffer);
          if (cleaner != null) {
            cleaner.getClass().getMethod("clean").invoke(cleaner);
          }
          return null;
        }
      });
    } catch (PrivilegedActionException e) {
      final IOException ioe = new IOException("unable to unmap the mapped buffer");
      ioe.initCause(e.getCause());
      throw ioe;
    }
  }

  public static void unmapLater(final ByteBuffer buffer, long delay, TimeUnit unit) {
    SCHEDULED_EXECUTOR_SERVICE.schedule(new Runnable() {
      public void run() {
        try {
          unmap(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }, delay, unit);
  }
}

// MMapDirectory
  final void cleanMapping(final ByteBuffer buffer) throws IOException {
    if (useUnmapHack) {
      ByteBufferUnmapper.unmapLater(buffer, 10, TimeUnit.SECONDS);
    }
  }

// MMapIndexInput
    @Override
    public short readShort() throws IOException {
      if (closed[0]) {
        throw new AlreadyClosedException("MMapIndexInput already closed: " + this);
      }

    @Override
    public void close() throws IOException {
      try {
        if (isClone || buffers == null) return;
        closed[0] = true;
        
        // make local copy, then un-set early
        final ByteBuffer[] bufs = buffers;
        
        for (final ByteBuffer b : bufs) {
          cleanMapping(b);
        }
      } finally {
        unsetBuffers();
      }
    }