Question

I tried to replace our Factory class that returns classes by string name using reflection with a CDI-based method.

So I put an @Inject @Any Instance<IData> possibleCandidates as a class variable. Then in the getIDataInstanceByClassname(String className) I’m creating an instance via Class.forName and compare it to every instance in the possibleCandidates list to then return a matching “injectable” instance.

However accessing possibleCandidates ends up in a NullPointerException. Whether I start a for loop or use possibleCandidates.iterator(). However, inspecting the variable does not return null but InstanceImpl<T> filled with various values.

How come accessing it produces a NPE?

Some code:

@Instance
@Any
private Instance<IData> possibleCandidates;

public IData getClassByClassname( String className ) {

    try {
        Class<?> clazz = Class.forName( className );
        Iterator<IData> it = possibleCandidates.iterator(); // NPE gets thrown here
        while ( it.hasNext() ) {
            IData dataInstance = it.next();
            if ( dataInstance.getClass().equals( clazz ) ) {
                return dataInstance;
            }
        }
    }
    catch( Exception e ) { ... }
}

Answer 1

There was one class in that list, that produced a NullPointerException at construction time which caused any access to the Instance list to fail. I could not see the cause of the NPE in the Debugging mode, but when I added some logging, the underlying reason was clear.

So, remember kids: Always rethrow exceptions in the catch block or dome something with it. But never throw a different exception without putting the original exception as the cause into it.