Handling exceptions generated by invkoe

Question

hi!
i have the next problem, i use invoke to call a method in the class of a certain object, but i don't know how handle the exepctions dispatched by this method
my code is like thisClass[] tiposParametros = {Clase.getClass()};
try {
     java.lang.reflect.Method metodo = objeto.getClass().getMethod("metodo", tiposParametros);
     Object[] parametros = Clase.getParametros();
     regreso = (Object[])metodo.invoke(objeto, parametros); // this method has its own exceptions
} catch (Exception e) {
     // exceptions like IllegalAccesException, IllegalArgumentException,..
} catch (OtherException ex) {
     // Here i have an "unreachable catch"
}any advice on as I can you handle my own exceptions?
regards

Answer 1

Okay, it sounds like you need a little firmer background on exceptions in Java. Here's as brief a rundown as I can give you and still cover the major points.
First, there is a class hierarchy.
Throwable
    |
    +--Error
    |    | ...  (subclasses of Error)
    |
    +--Exception
          |
          +--RuntimeException
          |      +
          |      |
          |      +--ClassCastException
          |      |
          |      +--NullPointerException
          |      | ...
          |
          +--InvocationTargetException
          |
          +--YourExceptions
          | ...
Error and its subclasses, and RuntimeException and its subclasses are called "unchecked exceptions." Everything is a "checked exception".
Whenever a checked exception can thrown during the execution of a method, that method must either catch the checked exception, or declare "throws ExceptionName" in the method declaration. This is done so that callers of the method no that the method cannot throw any checked excpetions other than those listed. If I'm calling a method, that means I know exactly which kind of exceptions I need to deal with. As long as I handle those, nothing else will get by me. If you don't follow this rule, you get a "must be caught or declared to be thrown" error at compile time.
For unchecked excpetions (Error, RuntimeException) there is no such requirement. You can throw an unchecked exception without explicitly stating so. The reason is this: Errors are internal VM problems that you can't do anything about anyway (OutOfMemory, etc.), and if things get bad enough to throw an Error, chances are that the state of the VM is such that just exiting immediately is the best course of action. RuntimeExceptions, on the other hand, occur because of a programmer error, and could have been prevented. For example, you can always prevent NullPointerException by doing an explicit check for null anywhere  that it could occur and cause a problem.
The checked exceptions are the ones that you can't really prevent just with good coding practices, so you have to find a way to deal with them: Disk filling up, socket closing unexpectedly, etc.
One final point: when you say "throws SomeException", that means you're allowed to throw that exception and any of its descendant classes. When you say "catch (SomeException e)", you'll catch that class and any of its descendant classes (unless a prior catch block named a more specific class that ends up matching).
Armed with that knowledge, lets look at your problems:
hi again jeff!
i try your suggestions(1,2,3), but i have the next
error messages:
1. unreported exception: java.lang.Throwable; must be
caught or declared to be thrown} catch (java.lang.reflect.InvocationTargetException
ex) {
try {
throw ex.getTargetException();
} catch (ExceptionCon e1) {
System.out.println("con");
} catch (ExceptionVac es) {
System.out.println("vac");
This is the "catch it or declare it" rule. Since ex.getTargetException returns a Throwable, it could be anything in the entire exception hierarchy--checked or unchecked--as far as the compiler knows. We've only handled two checked exceptions. We need to either:
1) change our throw statement to throw something no higher up the heirarchy than what we're catching
or
2) change or add to our catch block to make sure that if what we've just thrown isn't one of the two excptions we named, we still handle it. Kind of like a "default" in a case statement.
2. i cant't get the class from my Exception, i don't
know why; identifier required, but package found
I have no idea what you're saying here.
3. i recibe this message; method
procesaExcepcion(java.lang.Throwable) not found; in i
have declared that method like: void procesaExcepcion(ExcepcionCon e) {
Remember, getTargetException gives us a Throwable, which is an ancestor class of ExcepcionCon. Even if we know that at runtime it can only be ExcepcionCon, the compiler doesn't know that. As far as the compiler knows, it could be anything. So we're calling processExcepcion with a Throwable, but we don't have prcoessException(Throwable).
Actually, I apologize. Now that I think about it, this way won't work. I got confused and tried to do "reverse polymorphism (TM)" again. Forget I even mentioned this way. A similar approach is doable with reflection, but it's fairly complex, and you end up possibly throwing another InvocationTargetException.
Anyway, why don't you chew on that first problem for a while, and if you get stuck or have more specific questions, post again.