Tweak Hibernate Lazy Loading via @BatchSize

September 07, 2012

Hibernate @BatchSize annotation is a great way to increase performance when dealing with large result sets:

@Entity
class Foo {
    @ManyToMany
    @BatchSize(size = 100)
    Set<Bar> bars;
    ...
}

The annotation is often misunderstood. @BatchSize on collections is not related to the collections size. Instead it’s related to the number of uninitialized Foos contained in the current session.

List<Foo> foos = session.createQuery("from Foo").list();
for (Foo foo : foos) {
    // select bars for 100 foos at once:
    Hibernate.initialize(foo.getBars());
}

So, when foos has a size of 100, all bars will be fetched with 1 SELECT instead of 100 SELECTs.

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