Java How To ...: Java Annotations with No Target

7/07/2006

Java Annotations with No Target

Perhaps you would expect every Java annotation to have one or multiple targets, indicating where it can be applied, either on a class, method, field, or some combination. It's correct for the most part, but it also turns out some java annotations have no target. Their target() field is an empty array. For example,

javax.ejb.ActivationConfigProperty:

@Target(value={})
@Retention(value=RUNTIME)
public @interface ActivationConfigProperty

javax.persistence.EntityResult

@Target(value={})
@Retention(value=RUNTIME)
public @interface EntityResult

Where can we use these targetless annotations? They are to be used as nested annotations inside other enclosing annotations. Maybe we need a new target value NESTED? For example, @ActivationConfigProperty can only be used inside @javax.ejb.MessageDriven to specify properties for the Message-Driven Bean (MDB):

@MessageDriven(name="MessageBean",
activationConfig = {
@ActivationConfigProperty(propertyName="destinationType",                   
 propertyValue="javax.jms.Topic"),
@ActivationConfigProperty(propertyName="acknowledgeMode",                     
 propertyValue="Auto-acknowledge"),
@ActivationConfigProperty(propertyName="subscriptionDurability",                     
 propertyValue="Durable"),
@ActivationConfigProperty(propertyName="messageSelector",
 propertyValue="name='java' AND bugs < 1")
})
public class MessageBean implements MessageListener {
 ...
}

@javax.persistence.EntityResult can only be used as nested annotation inside @javax.persistence.SqlResultSetMapping. Here is a more complex example from its java doc, with 3 nested annotations at 3 different levels:

@SqlResultSetMapping(name="OrderResults",
 entities={
     @EntityResult(entityClass=com.acme.Order.class, fields={
         @FieldResult(name="id", column="order_id"),
         @FieldResult(name="quantity", column="order_quantity"),
         @FieldResult(name="item", column="order_item")})},
 columns={
     @ColumnResult(name="item_name")}
)