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2.1. Defining constraints Constraints in Bean Validation are expressed via Java annotations. In this section we show how to annotate an object model with these annotations. We have to differentiate between three different type of constraint annotations - field-, property-, and class-level annotations. Note Not all constraints can be placed on all of these levels. In fact, none of the default constraints defined by Bean Validation can be placed at class level. The java.lang.annotation.Target annotation in the constraint annotation itself determines on which elements a constraint can be placed. See Chapter 3, Creating custom constraints for more information.