I'm glad to see that Matlab took a reasonable approach to this;  Calling the Java Objects own serialization method makes a lot of sense.

The function you probably want to modify is read_mat5_binary_element in ls-mat5.cc.  Near the top of the file there is an enum declaration with the various CLASSTYPES in a Matlab file.  In the function read_mat5_binary_element there is a case statement that keys off the CLASSTYPE.  My guess is that there is a special class for Java objects, although they may have used the MAT_FILE_OBJECT_CLASS type which is for any class object.

This is probably a huge coding effort :(

First, I'm not sure it will be possible to save java objects in Matlab and then re-load them in Octave.  It really depends on how Matlab did the Serialization/Deserialization and whether there is any documentation or clues to their approach.

But, before that can happen Octave itself needs to be able to load/save Java objects which it can't.  See the following code:

octave:1> x = javaObject ('java.lang.Double', 1)
x =

<Java object: java.lang.Double>

octave:2> save -binary tst.save x
error: octave_base_value::save_binary(): wrong type argument 'octave_java'
warning: save: no such variable 'x'

I think the first place to start would be to get load/save working with Octave's own formats.

I can give you a few pointers about how to do this.  The key is that a Java object is an octave_value.  This is an overloaded class where the underlying value might be a double, uint8, range (such as 1:3:10), Java object, etc.  If you look in the directory libinterp/octave-value you will find all of the different octave_value classes.  The overall octave_value class (ov.h) defines various virtual methods like save_ascii or save_binary.  It is up to the individual class to override these with a function that does whatever is necessary for that particular object to save itself.

I'll take the example of a range because it is particularly simple.

x = 1:3:10
x =

    1    4    7   10

save -text tst.txt x

And the resulting file is:

# Created by Octave 4.1.0+, Thu Apr 24 08:51:01 2014 PDT <rik@tesseract>
# name: x
# type: range
# base, limit, increment
1 10 3

Now if you look in ov-range.h you will see there is a prototype for save_ascii.  In ov-range.cc you can find the actual function:

octave_range::save_ascii (std::ostream& os)
{
  Range r = range_value ();
  double base = r.base ();
  double limit = r.limit ();
  double inc = r.inc ();
  octave_idx_type len = r.nelem ();

  if (inc != 0)
    os << "# base, limit, increment\n";
  else
    os << "# base, length, increment\n";

  octave_write_double (os, base);
  os << " ";
  if (inc != 0)
    octave_write_double (os, limit);
  else
    os << len;
  os << " ";
  octave_write_double (os, inc);
  os << "\n";

  return true;
}

The range class has a pretty simple way of saving itself.  For Java objects, something more complex will have to be designed.  But the approach is the same.  Add a prototype for save_ascii, save_binary, etc. in ov-java.h and then an implementation in ov-java.cc.

I have a file created in matlab as follows

> x = java.io.File('/usr/local/bin');
> save('/tmp/file.mat')

When I try to load this file into octave, I get the following results:

octave:1> load('/tmp/file.mat')
octave:2> x
x =

  scalar structure containing the fields:

    java =

error: octave_base_value::print (): wrong type argument `<unknown type>'

    java.io.File =

error: octave_base_value::print (): wrong type argument `<unknown type>'

It seems that octave doesn't quite understand matlab's file format for embedding java objects in .mat files.  I am able to create java objects directly in octave, so I think it's not just a problem with my octave/java install.

I'd be happy to look into fixing this, but I'd very much appreciate if someone could give me a pointer in the right direction - octave's codebase is very large and last time I looked for this I was having a lot of trouble tracking down the right place to work.