Global GeoTools Hints

Currently GeoTools opperates around a number of Singletons ...

• FactoryRegistries (in the varous FactoryFinder and CommonFactoryFinder classes)
• Lots of cached Factories in utility classes such as CRS

And so on.

There is a problem with this - we are suppoed to control the use of Factories with Hints ... but all these implementations do something like the following:

return CommonFactoryFinder.getFilterFactory( null );

Where "null" seems to mean - I don't care do something smart for me.

This change request is the obvious one - lets do something smart.

interface GeoTools {
    static Hints hints;
    static {
         hints = createHintsFromSystemProperties();
    }
    public static void init( Hints hints ){
         if( hints == null ){
             hints = createHintsFromSystemProperties();
         }
         else {
             this.init = hints;
         }
    }
    public Hints getDefaultHints(){
         return hints;
    }
}

We can then change the above code example to:

return CommonFactoryFinder.getFilterFactory( GeoTools.getDefaultHints() );

A couple of "non obvious" consequences:

• Creating the hints based on System propreties will not work in an applet, they will need to explicitly call the init method
• FactoryRegistry "partitions" the factory instances based on the Hints provided
  • This means the GeoTools.getDefaultHints() acts as a single singleton for the entire library
• We can change the GeoTools hints returned by getDefaultHints() based on ThreadGroup
  • Tthis is how you should do a singleton anyways
  • The GeoTools library would then become more safe for use in environments that care about ThreadGroup (like Java EE and Applets)
• We can use the GeoTools class to hold Keys for the library (rather then have them scattered)
• We can use the GeoTools class to hold getVersion() - something that has been requested previously

This change will restore the previous functionality of FactoryFinder (where you could control the library a bit with System properties).