Classloading, Reflection, and Method Handles - Java in a Nutshell - page 325

Dynamic Proxies

One last piece of the Java Reflection story is the creation of dynamic proxies. These
are classes (which extend java.lang.reflect.Proxy ) that implement a number of
interfaces. The implementing class is constructed dynamically at runtime, and for‐
wards all calls to an invocation handler object:

g

R e l e c t i o n

InvocationHandler h = new InvocationHandler () {
@Override
public Object invoke ( Object proxy , Method method , Object [] args )
throws Throwable {
String name = method . getName ();
System . out . println ( "Called as: " + name );
switch ( name ) {
case "isOpen" :
return false ;
case "close" :
return null ;
}

d

return null ;
}
};

Channel c =
( Channel ) Proxy . newProxyInstance ( Channel . class . getClassLoader (),
new Class [] { Channel . class }, h );
c . isOpen ();
c . close ();
Proxies can be used as stand-in objects for testing (especially in test mocking
approaches).
Another use case is to provide partial implementations of interfaces, or to decorate
or otherwise control some aspect of delegation:

public class RememberingList implements InvocationHandler {
private final List < String > proxied = new ArrayList <>();

@Override
public Object invoke ( Object proxy , Method method , Object [] args )
throws Throwable {
String name = method . getName ();
switch ( name ) {
case "clear" :
return null ;
case "remove" :
case "removeAll" :
return false ;
}

return method . invoke ( proxied , args );