How to use H2 databases with a custom file extension?

Question

I'm building an application which stores its data in H2 database files, so my save files have the extension .h2.db.

When opening the application, the user has to select which save file should be loaded. To make it easier to recognize these files, is it possible to tell H2 to use custom file extensions?

Looking at the Database URL overview of H2, I can only specify the name of the database. I would prefer a extension like .save over the default .h2.db. Is there a reasonable way to achieve this?

A workaround would be to link the *.save-file to a temporary folder, renaming it to the correct suffix. If this is the only solution, I guess I would go with the default extension.

Answer 1

H2 database supports pluggable file system so with a bit of extra code you can use any extension you want. You just need to create a wrapper, register it and use your own database URL. The wrapper could look like this:

package my.test;

import org.h2.store.fs.FilePath;
import org.h2.store.fs.FilePathWrapper;

public class FilePathTestWrapper extends FilePathWrapper {
  private static final String[][] MAPPING = {
    {".h2.db", ".save"}, 
    {".lock.db", ".save.lock"}
  };

  @Override
  public String getScheme() {
    return "save";
  }

  @Override
  public FilePathWrapper wrap(FilePath base) {
    // base.toString() returns base.name
    FilePathTestWrapper wrapper = (FilePathTestWrapper) super.wrap(base);
    wrapper.name = getPrefix() + wrapExtension(base.toString());
    return wrapper;
  }

  @Override
  protected FilePath unwrap(String path) {
    String newName = path.substring(getScheme().length() + 1);
    newName = unwrapExtension(newName);
    return FilePath.get(newName);
  }

  protected static String wrapExtension(String fileName) {
    for (String[] pair : MAPPING) {
      if (fileName.endsWith(pair[1])) {
        fileName = fileName.substring(0, fileName.length() - pair[1].length()) + pair[0];
        break;
      }
    }
    return fileName;
  }

  protected static String unwrapExtension(String fileName) {
    for (String[] pair : MAPPING) {
      if (fileName.endsWith(pair[0])) {
        fileName = fileName.substring(0, fileName.length() - pair[0].length()) + pair[1];
        break;
      }
    }
    return fileName;
  }
}

Then you'll need to register it:

FilePathTestWrapper wrapper = new FilePathTestWrapper();
FilePath.register(wrapper);

And use database URL like this:

"jdbc:h2:save:./mydatabase"

Note the "save:" prefix, it should match string returned by getScheme() method. I put a bit more details here: http://shuvikov.net/blog/renaming-h2-database-files