In a rule flow, when the action is in MVEL, it is not possible to import classes

example :
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Long montant = (Long)context.getVariable("surimposition");
System.out.println ("Surimposition : '" + montant + "'");
filter = new org.drools.runtime.ClassObjectFilter (fr.sopra.impot.Impot.class);
java.util.Collection impots = context.getKnowledgeRuntime().getObjects(filter);
if (impots.size()==1) {
Impot impot = (Impot)impots.toArray()[0];
impot.setMontantRedressement(montant);
}
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it says at design time :
Process 'ImpotProcess' [fr.sopra.impot.ImpotProcess]: Action node 'Traitement des resultats' [13] has invalid action: was expecting type: java.lang.Object; but found type: null.