I created a little proof of concept using Javassist
The class inheriting the trait isn't much more advanced than this. The trick
is to make the class abstract and have the library handle instantiation and
implementation.
public abstract class HelloWorld implements Swedish
{
public void sayHello()
{
System.out.println(hello());
}
}
For a library API I guess this is ok.
public class HelloWorldTest {
public static void main(String[] args) {
final HelloWorld hw = Trait4J.createInstance(HelloWorld.class);
hw.sayHello();
}
}
The actual trait leaves something to be desired. But I guess it's the best
we can do.
@Implementation(Swedish.Impl.class)
public interface Swedish
{
public static abstract class Impl implements Swedish
{
public String hello()
{
return "Hej";
}
}
public String hello();
}
The (vary naive) library just creates a subclass and then spawns instances
from this.
public static <T> Class<? extends T> implementTraits(Class<T> clazz)
{
try
{
final ClassPool cp = ClassPool.getDefault();
final CtClass target = cp.get(clazz.getName());
final CtClass impl = cp.makeClass(target.getName() + "Impl");
for(Class<?> iface : clazz.getInterfaces())
{
final Implementation implSrc = iface.getAnnotation(Implementation.class);
if(implSrc != null)
{
for(CtMethod sm : cp.get(implSrc.value().getName()).getDeclaredMethods())
{
impl.addMethod(CtNewMethod.copy(sm, impl, null));
}
}
}
impl.setSuperclass(target);
return impl.toClass();
}
catch (Exception e) {
e.printStackTrace();
}
return null;
}
BR,
John
On Thu, Nov 6, 2008 at 10:35 PM, Jess Holle <[EMAIL PROTECTED]> wrote:
> There are big limitations imposed by dynamic proxies here as I see it.
> Dynamic proxies are beautiful interceptors for one or more interfaces, but
> they themselves are of a synthetic class.
>
> If you have a class A and you want to mix-in/apply an interface to it,
> dynamic proxies could help wire a default implementation around an
> interface, but you'd still have to manually delegate to the proxy from your
> class. Overall it still leaves a gaping "can't get there from here" hole in
> the middle of things.
>
>
> John Nilsson wrote:
>
> I haven't done any benchmarking. But I can't imagine the overhead being
> "terrible", mesurable: yes, but I hear the reflection stuff in Java is
> rather quick.
> If performance is a problem you could probably use some runtime class
> generation instead. But I imagine it would take a little more effort to
> assemble such a system.
>
> BR,
> John
>
> On Thu, Nov 6, 2008 at 8:20 PM, Casper Bang <[EMAIL PROTECTED]> wrote:
>
>>
>> But aren't dynproxies terribly slow? I thought that's why people
>> dabble on invokedynamic.
>>
>> /Casper
>>
>> On Nov 6, 8:16 pm, "John Nilsson" <[EMAIL PROTECTED]> wrote:
>> > It shouldn't be to hard to implement traits as a library in current Java
>> > using a dynamic proxy.
>> > Should make for an interesting project.
>> >
>> > BR,
>> > Jphn
>> >
>> > On Wed, Nov 5, 2008 at 3:00 AM, Mark Derricutt <[EMAIL PROTECTED]>
>> wrote:
>> > > Please please please bring on traits! I'm somewhat on the fence of
>> rather
>> > > seeing traits than closures in java sooner than the other.
>> >
>> > > I'm finding LOTS of places in my code where traits would just make
>> things
>> > > cleaner.
>> >
>> > > More and more I think I just want scala :)
>> >
>> > > On Wed, Nov 5, 2008 at 12:15 PM, hlovatt <[EMAIL PROTECTED]>
>> wrote:
>> >
>> > >> I thinks that Traits are a great idea for Java and judging by #215
>> the
>> > >> posse, particularly Dick, like them. I wrote about them for Java 7
>> in:
>> >
>> > >>http://www.artima.com/weblogs/viewpost.jsp?thread=220916
>> >
>> > >> What do you think?
>> >
>> > > --
>> > > "It is easier to optimize correct code than to correct optimized
>> code." --
>> > > Bill Harlan
>>
>>
>
>
>
>
> >
>
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