[josm-dev] How do I add a new projection, EPSG:2924?

[Petr Nejedlý]

Frederik Ramm wrote:
> Dirk,

[...]
> 
> java.lang.ClassNotFoundException: 
> org.openstreetmap.josm.plugins.fredprojection.FredProjection
>         at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
>         at java.security.AccessController.doPrivileged(Native Method)
>         at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
>         at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
>         at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
>         at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
>         at java.lang.Class.forName0(Native Method)
>         at java.lang.Class.forName(Class.java:169)
>         at 
> org.openstreetmap.josm.gui.preferences.ProjectionPreference.setProjection(ProjectionPreference.java:206) 

[...]

> 
> Class.forName(name).newInstance();
> 
> for the name of my projection (which is derived from that very 
> instance!) fails. I'm afraid that my Java skills stop there - somehow 
> the default class loader does not know about classes added by plugins, 
> and would need to be made aware of them.

There are two approaches possible for this problem.
The simpler one would be to change ProjectionPreference to not call 
Class.forName(), but instead getProjections(), and iterate the 
registered _instances_ until getClass().getName() matches.

The more complex, but generally preferable (as it would solve many 
similar problems once for good) would be to code a delegating class 
loader that would try to fulfill the load request by trying each 
plugin's loader in sequence. This overall loader would be then set as 
the context class loader of every thread. Similar to 
http://hg.netbeans.org/main/file/tip/o.n.bootstrap/src/org/netbeans/ProxyClassLoader.java
but many many revisions ago (before the packageCoverage optimization was 
implemented) and simplified...

Regards,
   Nenik