Java newbie

Hibernate Lucene Integration

http://www.hibernate.org/hib_docs/annotations/reference/en/html/lucene.html

Chapter 4. Hibernate Lucene Integration
Lucene is a high-performance Java search engine library available from the Apache Software Foundation. Hibernate Annotations includes a package of annotations that allows you to mark any domain model object as indexable and have Hibernate maintain a Lucene index of any instances persisted via Hibernate.

4.1. Using Lucene to index your entities
4.1.1. Annotating your domain model
First, we must declare a persistent class as @Indexed:

@Entity
@Indexed(index="indexes/essays")
public class Essay {
...
}
The index attribute tells Hibernate where the Lucene index is located (a directory on your file system).

Lucene indexes contain four kinds of fields: keyword fields, text fields, unstored fields and unindexed fields. Hibernate Annotations provides annotations to mark a property of an entity as one of the first three kinds of indexed fields.

@Entity
@Indexed(index="indexes/essays")
public class Essay {
...

@Id
@Keyword(id=true)
public Long getId() { return id; }

@Text(name="Abstract")
public String getSummary() { return summary; }

@Lob
@Unstored
public String getText() { return text; }

}
These annotations define an index with three fields: Id, Abstract and Text.

Note: you must specify @Keyword(id=true) on the identifier property of your entity class.

4.1.2. Enabling automatic indexing
Finally, we enable the LuceneEventListener for the three Hibernate events that occur after changes are committed to the database.

<hibernate-configuration>
...
<event type="post-commit-update"
<listener
class="org.hibernate.lucene.event.LuceneEventListener"/>
</event>
<event type="post-commit-insert"
<listener
class="org.hibernate.lucene.event.LuceneEventListener"/>
</event>
<event type="post-commit-delete"
<listener
class="org.hibernate.lucene.event.LuceneEventListener"/>
</event>
</hibernate-configuration>

Hibernate，left join fetch，重复对象（记录）的问题

Hibernate 3.0中，mapping文件缺省配置的one-to-many的子集合是lazy declarations的，fetch的模式是join，当应用需要访问子表（1:M）中的数据时，hibernate框架会主动发起一条查询的sql。当我们需要在获取主记录的同时得到子记录的数据时，我们可以使用HSQL，“left join fetch”。但是当子记录存在时，主对象记录会有重复。

解决方法：http://forum.hibernate.org/viewtopic.php?t=938705&highlight=onetomany+query

fetch return distinct root instances

join has the same meaning as SQL join, so just use
Set results = new HashSet(query.list()) if you don't want duplicate results.