JMRI: Portable File Access
Author: Bob Jacobsen
Keywords: JMRI technical code portable file location

# JMRI Code: Portable File Access

JMRI uses files for icons, images, panel files, scripts, and lots of other things. References to those are routinely used by various code, and stored in XML files. Some of the files are distributed with JMRI, and some are created outside the JMRI distribution directory by the local user.

In addition, JMRI is a cross-platform application, and we want to be able to move JMRI XML files from one computer to another, even of a different type, and have them still work.

To do this, we've created a system of pseudo-URLs for accessing file locations. Filenames are used internally and stored in XML files with prefixes that identify where the file is to be found in a system independent manner.

program:
  Treats the path as a relative path below the JMRI installation directory. In JMRI 2.13.1 and earlier, this was the `resources:` prefix.
preference:
  Treats the path as a relative path below the user files (or preferences) directory. In JMRI 2.13.1 and earlier, thie was the `file:` prefix.
profile:
  Treats the path as a relative path below the configuration profile directory.
settings:
  Treats the path as a relative path below the settings directory (the directory where per-computer preferences are stored).
home:
  Treats the path as a relative path below the user's home directory.

Otherwise, treat the name as a relative path below the program directory to provide backward compatibility. In any case, absolute pathnames will work, but won't be cross-platform compatible.

## Implementation

The jmri.util.FileUtil class provides translation routines to make this automatic:

  static public String getExternalFilename(String pName)
Convert an internal name (with prefixes as above) into an absolute pathname for use on the local computer. There's no guarantee that the file or any enclosing directory is actually present, but the pathname will be syntactically valid on the present computer.
  static public String getPortableFilename(File file)
  static public String getPortableFilename(String filename)
Convert a File or filename string into a string name with the appropriate prefix.

You should call getExternalFilename() before passing the filename to any Java common, and getPortableFilename() whenever you get a file or filename from a Java class. It's OK to call either one multiple times, as they'll just pass through in that case. In particular, call getPortableFilename() before storing to an XML file.