dynamic proxy and method annotation

Question

I have the following classes:

interface Ivisitor{ @deduceStrategy("...") void visit(Icosmos c); }

Visitor implements this interface:

class Visitor implements Ivisitor{ @deduceStrategy("...") public void visit(Icosmos c) { .... } }

The dynamic proxy:

public class strategyLoader{ public static <T> T create(Class<T> clazz,Object wrap) { T object = (T) Proxy.newProxyInstance(strategyLoader.class.getClassLoader(), new Class[] { clazz },new Handler(wrap)); return object; } }

Relevant portion of the handler class:

public class Handler implements InvocationHandler { Object obj; public Handler(Object obj) { this.obj = obj; } public Object invoke(Object proxy, Method m, Object[] args) throws Throwable { if (m.isAnnotationPresent(deduceStrategy.class)) { Class[] parameterTypes = m.getParameterTypes(); if((parameterTypes.length==1)&&(**Icosmos.class.isInstance(parameterTypes[0])**)) { ........ }

I need to load the appropriate strategy based on the exact type of Icosmos passed into Visitor.visit. However,the parameter[0] is never resolving to an instance of Icosmos. Would appreciate if someone showed me the way out. The client invokes visitor as:

Ivisitor visitor = strategyLoader.create(Ivisitor.class,Visitor.class.newInstance());

Ian Roberts · Accepted Answer · 2013-01-03 13:41:06Z

Icosmos.class.isInstance(parameterTypes[0])

is exactly equivalent to parameterTypes[0] instanceof Icosmos, and checks whether the object parameterTypes[0] is an instance of Icosmos (which it isn't - it's a java.lang.Class).

There are two possibilities for what you really need. One would be

Icosmos.class.isAssignableFrom(parameterTypes[0])

which checks whether the class parameterTypes[0] is assignment-compatible with Icosmos, i.e. given an x which is an instance of the class represented by parameterTypes[0], would

Icosmos foo = x;

be legal without a cast. This would check that the declared type of the first formal parameter is compatible with Icosmos.

The second alternative would be

args[0] instanceof Icosmos

which checks the runtime type of the actual argument value rather than the declared type of the formal parameter.

instanceof did not compile: inconvertible types found : java.lang.Class required: com.patterns.visitor.Icosmos isassignablefrom did work. What causes this difference? When would a type be assignable but not an instance of a base interface or class?
@IUnknown I'm not quite sure what you mean here - isAssignableFrom tests for a relationship between two java.lang.Class objects, the instanceof operator tests an object reference against a class or interface name. The dynamic version of instanceof to test an object reference against a java.lang.Class is the isInstance() method you started with.

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