使用Hibernate3作为JDBC框架
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Some people even suggest that JDBC frameworks are a suitable alternative to ORM, even for those systems to which ORM is best suited: object-oriented applications with clean relational schemas. They argue that you are always better off with hand-written SQL than generated SQL. Well, I don't think this is true, not only because the overwhelming bulk of SQL code needed by most applications is of the tedious kind, and simply does not require human intervention, but also because a JDBC framework operates at a different semantic level to ORM. A solution like iBATIS knows a lot less about the semantics of the SQL it is issuing, and of the resulting datasets. This means that there is much less opportunity for performance optimizations such as effficent caching. (By "efficient", I am referring mainly to efficient cache invalidation strategies, which are crucial to the usefulness of the cache.) Furthermore, whenever we have seen handwritten SQL, we have seen N+1 selects problems. It is extremely tedious to write a new SQL query for each combination of associations I might need to fetch together. HQL helps significantly here, since HQL is much less verbose than SQL. For a JDBC framework to be able to make the kind of optimizations that an ORM can make, it would have to evolve to a similar level of sophistication. Essentially, it would need to become an ORM, minus SQL generation. In fact, we already start to see this evolution taking place in existing JDBC frameworks. This begins to erode one of the stated benefits: the claimed simplicity.
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It also raises the following interesting thought: if, by gradually adding stuff, a JDBC framework will eventually end up as ORM, minus SQL generation, why not just take an existing ORM solution like, ooh, um ... Hibernate, maybe ... and subtract the SQL generation?
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The Hibernate team has long recognized the need to mix and match generated SQL with the occasional handwritten query. In older versions of Hibernate, our solution was simply to expose the JDBC connection Hibernate is using, so you can execute your own prepared statement. This started to change a while ago, and Max Andersen has recently done a lot of work on this. Now, in Hibernate3, it is possible to write an entire application with no generated SQL, while still taking advantage of all of Hibernate's other features.
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Do we really expect or intend people to use Hibernate in this way? Well, not really - I doubt there are many people out there who really enjoy writing tedious INSERT, UPDATE, DELETE statements all day. On the other hand, we do think that quite a few people need to customize the occasional query. But to prove a point, I'll show you how you can do it, if you really want to.
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Let's take a simple Person-Employment-Organization domain model. (You can find the code in the org.hibernate.test.sql package, so I'm not going to reproduce it here.) The simplest class is Person; here's the mapping:
<class name="Person" lazy="true"><id name="id" unsaved-value="0">
<generator not-null="true"/>
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<loader query-ref="person"/>
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<sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-insert>
<sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE ID=?</sql-update>
<sql-delete>DELETE FROM PERSON WHERE ID=?</sql-delete>
</class>
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The first thing to notice is the handwritten INSERT, UPDATE and DELETE statements. The ? order of the parameters matches to the order in which properties are listed above (we'll have to eventually support named parameters, I suppose). I guess there is nothing especially interesting there.
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More interesting is the <loader> tag: it defines a reference to a named query which is to be used anytime we load a person using get(), load(), or lazy association fetching. In particular, the named query might be a native SQL query, which it is, in this case:
<sql-query name="person"><return alias="p" lock-mode="upgrade"/>
SELECT NAME AS {p.name}, ID AS {p.id} FROM PERSON WHERE ID=? FOR UPDATE
</sql-query>
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(A native SQL query may return multiple "columns" of entities; this is the simplest case, where just one entity is returned.)
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Employment is a bit more complex, in particular, not all properties are included in the INSERT and UPDATE statements:
<class name="Employment" lazy="true"><id name="id" unsaved-value="0">
<generator not-null="true" update="false"/>
<many-to-one name="employer" not-null="true" update="false"/>
<property name="startDate" not-null="true" update="false"
insert="false"/>
<property name="endDate" insert="false"/>
<property name="regionCode" update="false"/>
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<loader query-ref="employment"/>
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<sql-insert>
INSERT INTO EMPLOYMENT
(EMPLOYEE, EMPLOYER, STARTDATE, REGIONCODE, ID)
VALUES (?, ?, CURRENT_DATE, UPPER(?), ?)
</sql-insert>
<sql-update>UPDATE EMPLOYMENT SET ENDDATE=? WHERE ID=?</sql-update>
<sql-delete>DELETE FROM EMPLOYMENT WHERE ID=?</sql-delete>
</class>
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<sql-query name="employment"><return alias="emp" lazy="true">
<id name="id" unsaved-value="0">
<generator not-null="true"/>
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<set name="employments"
lazy="true"
inverse="true">
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<key column="employer"/> <!-- only needed for DDL generation -->
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<one-to-many collection="Organization.employments"/>
<return alias="emp" class="Organization"/>
SELECT DISTINCT NAME AS {org.name}, ID AS {org.id}
FROM ORGANIZATION org
INNER JOIN EMPLOYMENT e ON e.EMPLOYER = org.ID
</sql-query>
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Personally, I prefer to program in Java than in XML, so all this stuff is much too XML-heavy for my liking. I think I'll stick with SQL generation, wherever I can, which is almost everywhere. It's not that I don't like SQL. In fact, I am a great fan of SQL, and just love watching the queries scroll past when I turn Hibernate's logging on. It's just that Hibernate is much better at writing SQL than I am.
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About the author?
Gavin King (gavin@hibernate.org)
Blog: http://blog.hibernate.org/
Gavin King is the founder of the Hibernate project. He is co-author of the book Hibernate in Action, to be published by Manning Publications. He is currently involved in the JDO expert group and is employed by JBoss Inc, where he is redesigning the JBoss CMP engine.
