Hibernate XML Mapping: Lazy False or Fetch Select?

问题

I have a simple question. I found this Hibernate config on our project:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge"
    lazy="false"
    fetch="select">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

Doesn't fetch="select" mean "Lazy load all the collections and entities" based on Fetching Strategies. But by writing lazy="false" mean do not lazy load. So the config above says: "Disable lazy loading. Enable lazy loading." In effect, this means the property is lazy loaded?

So I could shorten that config as:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge"
    fetch="select">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

But isn't fetch="select" the default mode? So in effect, I can declare the same config as:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

Am I correct? Wrong? Ideas? Thanks

If I want to enable lazy loading, I must add lazy="true" and remove lazy="false"?

回答1:

I think Fetch Mode and time of fetch are concepts that don't totally overlap.

Lazy="true|false" controls whether an association is loaded eagerly or on demand.

fetch="select|subselect|join|batch" controls how is that entity or collection loaded, when it's required to be loaded.

So, to answer your question, having fetch="select" means:

"a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you access the association." (http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html#performance-fetching)

This doesn't mean that lazy loading is disabled! That's controlled by the lazy="true|false" flag. With lazy="true" and fetch="select" Hibernate will lazy load the collection and will load it with a select. If you set lazy="false", the same select will be executed, the difference will be that it will get executed eagerly. Hope this makes sense.

Have a look here as well: http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies

回答2:

If you set it to lazy, then the outer map won't select the employee if it is not necessary. The fetch shows how would it query the employee if it necessary.

来源：https://stackoverflow.com/questions/4374528/hibernate-xml-mapping-lazy-false-or-fetch-select

标签

hibernate

mapping

lazy-loading