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/* * Some other comments about the Event class go here. */ package com.manning.hq.ch09; /** * This is the right place. XDoclet looks for * class-level tags here. * @hibernate.class */ public class Event implements Serializable { }
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9.2.2 The @hibernate.id tag
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Table 9.2 list the common properties of the @hibernate.id tag.
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Table 9.2 Common @hibernate.id attributes Attribute generator-class Description Contains the key generator that Hibernate will use to insert new instances. Specifies the Hibernate type for this field. Default None. It s mandatory so you have to pick one. When in doubt, using native will work for most databases. The return type of the field; as primary keys tend to be Longs or Strings, it usually isn t necessary to specify this. The property name. Null. Generally, if you use a String or Long as the primary key, you don t need to specify this. The default size for the field type. For a Long, it s a given, but for a String key, you might need to specify it.
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column unsaved-value
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Contains the name of the column. Contains a value that will distinguish transient instances from persistent ones. Specifies the size of the database column.
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That s actually all of the properties you can specify for the id field. Generally, unless you are doing something fairly complex, you just need the generator-class and possibly the column field.2 The @hibernate.id tag is a property-level tag, so it only goes along with a property, not with a class. You need to keep in mind a few tricky things about property-based tags. For example, you have to mark the getter method, not the field itself or the setter methods. The following is wrong:
/** * Wrong! Don't put on the field. * @hibernate.id generator-class="native" column="uid" */ private Long id; /** * Wrong! Don't put on the setter method either. * @hibernate.id generator-class="native" column="uid" */ public void setId(Long id) { this.id = id; }
This is the correct place to put it: on the getId() method.
/** * The right place for the tag, on getter method. * @hibernate.id generator-class="native" column="uid" */ public Long getId() { return id; }
Most property-based tags (including both property and id) will have a column attribute, which is usually optional. We think it s good practice to specify it, though, since it makes columns obvious in the class documentation.
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Another aspect that might be confusing is unsaved-value.
unsaved-value
The issue we ll discuss next isn t related just to XDoclet, but is a more general Hibernate one. We discussed this issue earlier in chapter 3, but a quick review might be in order. When Hibernate goes to save an object, it looks at the value of the identifier field to determine whether the object is new (transient) and needs to be inserted or whether it is persistent. If a transient object is passed to saveOrUpdate(), a new row is created. If the identifier is anything else, an update for the matching row is performed. For most persistent classes, if the identifier is an object, such as a Long or a String, the default unsaved-value of null is fine. This is the case with your Event, which has an id field that s a Long, so you don t need to specify a value for unsaved-value. So the following would create a new Event row in the database:
Event event = new Event(); session.saveOrUpdate(event);
So when you create a new Event, its id is null to start and thus will be inserted. If you are using a primitive for a key (like long or int), you should set the unsaved-value attribute to 0. So if you changed your Event.id field to be a long, you d have to specify the unsaved-value field like this:
/** * Using a primitive long instead of a Long to * demonstrate unsaved-value. * @hibernate.id generator-class="native" * column="uid" unsaved-value="0" */ public long getId() { return id; }
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