Add classLoader to mapper options builder

[jonahseguin]

This is necessary for some use cases (for example, when working with Spigot/Bukkit JavaPlugins) (see https://spigotmc.org) when the PluginClassLoader is used. In order to prevent NoClassDefFoundError for entity classes, the JavaPlugin's ClassLoader needs to be used, and I'm sure this is applicable to other applications as well.

Example of my use in the previous version (when it was possible to override the DefaultCreator's getClassLoaderForClass() method):

MapperOptions options = morphia.getMapper().getOptions();
        morphia.getMapper().setOptions(MapperOptions.builder(options)
                .objectFactory(new DefaultCreator(options) {
                    @Override
                    protected ClassLoader getClassLoaderForClass() {
                        return myClassLoader;
                    }
                })
                .build());

Example usage with this commit:

morphia.getMapper().setOptions(MapperOptions.builder(morphia.getMapper().getOptions())
                .classLoader(myClassLoader)
                .build());

[evanchooly]

Have you tried this with 1.5.7? It was just published in the last day or two and I've done a fair bit of work in this exact area.

[jonahseguin]

@evanchooly Yeah, just tried with 1.5.7 -- the classLoader variable is read-only in the MapperOptions class and there is no option for it in the builder

[evanchooly]

and it's still necessary? That's disappointing...

Not the end of the world, though, i guess. It's a nice escape hatch, I suppose, for when morphia can't quite figure it all out. I'll take a look in the next day or two then.

[jonahseguin]

@evanchooly Yeah, just for some use cases where we need to provide our own class loader.
Thanks

[evanchooly]

I just pushed 1.5.8 so you should see this on central in the next hour or two.