Review

class MMapDirectory {
  /**
   * This method enables the workaround for unmapping the buffers
   * from address space after closing {@link IndexInput}, that is
   * mentioned in the bug report. This hack may fail on non-Sun JVMs.
   * It forcefully unmaps the buffer on close by using
   * an undocumented internal cleanup functionality.
   * <p><b>NOTE:</b> Enabling this is completely unsupported
   * by Java and may lead to JVM crashes if <code>IndexInput</code>
   * is closed while another thread is still accessing it (SIGSEGV).
   * @throws IllegalArgumentException if {@link #UNMAP_SUPPORTED}
   * is <code>false</code> and the workaround cannot be enabled.
   */
  public void setUseUnmap(final boolean useUnmapHack) {
    if (useUnmapHack && !UNMAP_SUPPORTED)
      throw new IllegalArgumentException("Unmap hack not supported on this platform!");
    this.useUnmapHack=useUnmapHack;
  }

}