How to determine subclasses of a class

[Olli Plough]

Hello,

I have a situation where all classes that implement a certain
interface are candidates to have a specific annotation. Now I want to
obtain all classes that implement that interface and select those that
have the anotation of interest. Question is how this can be done.
There seems to be no way you can ask a class for its subclasses (as
f.e. in Smalltalk), which would make things realy easy. Any ideas how
this can be solved? Do I have to write my own class loader? Hopefully
not ...

Thanks for any hints.
Cheers, Oliver

 

[Are Nybakk]

Hello,

I have a situation where all classes that implement a certain
interface are candidates to have a specific annotation. Now I want to
obtain all classes that implement that interface and select those that
have the anotation of interest. Question is how this can be done.
There seems to be no way you can ask a class for its subclasses (as
f.e. in Smalltalk), which would make things realy easy. Any ideas how
this can be solved? Do I have to write my own class loader? Hopefully
not ...

Thanks for any hints.
Cheers, Oliver

What you ask is really out of my field, but I came to think of generics
wild cards the moment I read your post. Maybe that can be used somehow?

Good luck

 

[Silvio Bierman]

Hello,

I have a situation where all classes that implement a certain
interface are candidates to have a specific annotation. Now I want to
obtain all classes that implement that interface and select those that
have the anotation of interest. Question is how this can be done.
There seems to be no way you can ask a class for its subclasses (as
f.e. in Smalltalk), which would make things realy easy. Any ideas how
this can be solved? Do I have to write my own class loader? Hopefully
not ...

Thanks for any hints.
Cheers, Oliver

There is no predefined way of achieving this. Class loaders are supposed
to supply the definition of a class X as soon as it is needed. They are
not intended for such queries, simply because the scope of your query is
undefined.
You probably mean all classes Y that implement interface X that are in
the classpath or something. That classpath is an artifact of the default
system class loader and as such is not a part of the Java language but
of its current implementation. If you would be using a class loader that
implements completely different class lookup logic the results would be
completely different.

If you are using the standard class loader then there is nothing that
keeps you from scanning the classpath, looking for .class files on it
and looking in all .tar and .zip files to see if the have .class files
in them. Load the classes you find and you can check if the implement X.

Kind regards,

Silvio Bierman

 

[Stefan Ram]

obtain all classes that implement that interface and select those that

A similar operation exists in the library »ram.jar«.
This operation is based on Code by Ralf Ullrich.

One can find all classes of a jar that have any chosen
property, as long as this property can be obtained from the
class reference.

For example, to find all classes implementing »java.util.Map«,
one sets a filter accepting only classes which
»java.util.Map« is assignable from:

public boolean accepts( final java.lang.Class class_ )
{ return java.util.Map.class.isAssignableFrom( class_ ); }

One also needs to provide a starting class to find the jar, which
is done as follows.

public java.lang.String entryPath(){ return "java.lang.Object"; }

The jar containing this type will be chosen for exploration.

However, the client does not need to specify this »entryPath«,
as "java.lang.Object" is the default obtained by extending
»de.dclj.ram.java.lang.reflect.Finder.DefaultSpecification«.

The example client is:

public class Main
{ /* based on an idea and on code by Ralf Ullrich from 2006 */

public static void main( final java.lang.String[] args )
{ new de.dclj.ram.java.lang.reflect.Finder
( new de.dclj.ram.java.lang.reflect.Finder.DefaultSpecification()
{
public boolean isClassFinder(){ return true; }

public boolean accepts( final java.lang.Class class_ )
{ return java.util.Map.class.isAssignableFrom( class_ ); }

}).inspectJar(); }}

class java.lang.ProcessEnvironment
class java.rmi.server.RemoteObjectInvocationHandler$MethodToHash_Maps$1
class java.security.AuthProvider
(...)
class java.util.Properties
class java.util.Hashtable
interface java.util.Map

The library »ram.jar« is an early publication in alpha state,
it is experimental, changing, and mostly undocumented. See:

http://www.purl.org/stefan_ram/pub/ram-jar

 

[Lew]

In general, it cannot. The best you can hope for is to find all the classes
that *at the moment* implement the interface and are accessible in the classpath.

 

[Daniel Pitts]

In general, it cannot. The best you can hope for is to find all the
classes that *at the moment* implement the interface and are accessible
in the classpath.

Or, use the Service Provider pattern. This I believe pattern used in the
Java crypto API.

 

[Roedy Green]

I have a situation where all classes that implement a certain
interface are candidates to have a specific annotation. Now I want to
obtain all classes that implement that interface and select those that
have the anotation of interest. Question is how this can be done.
There seems to be no way you can ask a class for its subclasses (as
f.e. in Smalltalk), which would make things realy easy. Any ideas how
this can be solved? Do I have to write my own class loader? Hopefully
not ...

the catch is, many of the classes you want are probably not even
loaded yet. I think you will have to scan the byte codes of classes
on the classpath.

See http://mindprod.com/jgloss/javaclassformat.html

 

[Chris]

I have a situation where all classes that implement a certain

interface are candidates to have a specific annotation. Now I want to
obtain all classes that implement that interface and select those that
have the anotation of interest. Question is how this can be done.
There seems to be no way you can ask a class for its subclasses (as
f.e. in Smalltalk), which would make things realy easy. Any ideas how
this can be solved? Do I have to write my own class loader? Hopefully
not ...

I have the same problem. This is the best answer I have found:

http://weblogs.java.net/blog/enicholas/archive/2006/04/creating_a_serv.html

Unfortunately, it depends on putting a properties file in your jar that
references the classes the implement the interface. Not ideal.

JDK 1.6 has a service provide interface that's worth looking at.

http://java.sun.com/javase/6/docs/api/java/util/ServiceLoader.html

If you find a more direct way, please post it here.