源码
SimpleJpaRepository的定义如下：
/**
* Default implementation of the {@link org.springframework.data.repository.CrudRepository} interface. This will offer
* you a more sophisticated interface than the plain {@link EntityManager} .
*
* @author Oliver Gierke
* @author Eberhard Wolff
* @author Thomas Darimont
* @author Mark Paluch
* @author Christoph Strobl
* @author Stefan Fussenegger
* @author Jens Schauder
* @author David Madden
* @author Moritz Becker
* @param <T> the type of the entity to handle
* @param <ID> the type of the entity's identifier
*/
@Repository
@Transactional(readOnly = true)
public class SimpleJpaRepository<T, ID> implements JpaRepositoryImplementation<T, ID> {
解读：
SimpleJpaRepository实现了JpaRepositoryImplementation接口。
JpaRepositoryImplementation的定义如下：
/**
* SPI interface to be implemented by {@link JpaRepository} implementations.
*
* @author Oliver Gierke
* @author Stefan Fussenegger
* @author Jens Schauder
*/
@NoRepositoryBean
public interface JpaRepositoryImplementation<T, ID> extends JpaRepository<T, ID>, JpaSpecificationExecutor<T> { /**
* Configures the {@link CrudMethodMetadata} to be used with the repository.
*
* @param crudMethodMetadata must not be {@literal null}.
*/
void setRepositoryMethodMetadata(CrudMethodMetadata crudMethodMetadata); /**
* Configures the {@link EscapeCharacter} to be used with the repository.
*
* @param escapeCharacter Must not be {@literal null}.
*/
default void setEscapeCharacter(EscapeCharacter escapeCharacter) { }
}
解读：
JpaRepositoryImplementation接口继承了JpaSpecificationExecutor。
类图
调用链路
观察SimpleJpaRepository中findOne(Example<S> example)方法的实现，代码如下：
/*
* (non-Javadoc)
* @see org.springframework.data.repository.query.QueryByExampleExecutor#findOne(org.springframework.data.domain.Example)
*/
@Override
public <S extends T> Optional<S> findOne(Example<S> example) { try {
return Optional
.of(getQuery(new ExampleSpecification<S>(example, escapeCharacter), example.getProbeType(), Sort.unsorted())
.getSingleResult());
} catch (NoResultException e) {
return Optional.empty();
}
}
解读：
此方法由QueryByExampleExecutor接口定义。
观察SimpleJpaRepository中findOne(@Nullable Specification<T> spec)方法的实现，代码如下：
/*
* (non-Javadoc)
* @see org.springframework.data.jpa.repository.JpaSpecificationExecutor#findOne(org.springframework.data.jpa.domain.Specification)
*/
@Override
public Optional<T> findOne(@Nullable Specification<T> spec) { try {
return Optional.of(getQuery(spec, Sort.unsorted()).getSingleResult());
} catch (NoResultException e) {
return Optional.empty();
}
}
解读：
此方法由JpaSpecificationExecutor接口定义。
小结：
上述两个findOne方法最终都是调用了如下getQuery方法：
/**
* Creates a {@link TypedQuery} for the given {@link Specification} and {@link Sort}.
*
* @param spec can be {@literal null}.
* @param domainClass must not be {@literal null}.
* @param sort must not be {@literal null}.
* @return
*/
protected <S extends T> TypedQuery<S> getQuery(@Nullable Specification<S> spec, Class<S> domainClass, Sort sort) { CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<S> query = builder.createQuery(domainClass); Root<S> root = applySpecificationToCriteria(spec, domainClass, query);
query.select(root); if (sort.isSorted()) {
query.orderBy(toOrders(sort, root, builder));
} return applyRepositoryMethodMetadata(em.createQuery(query));
}
解读：
getQuery方法调用了applySpecificationToCriteria方法，该方法的实现如下：
/**
* Applies the given {@link Specification} to the given {@link CriteriaQuery}.
*
* @param spec can be {@literal null}.
* @param domainClass must not be {@literal null}.
* @param query must not be {@literal null}.
* @return
*/
private <S, U extends T> Root<U> applySpecificationToCriteria(@Nullable Specification<U> spec, Class<U> domainClass,
CriteriaQuery<S> query) { Assert.notNull(domainClass, "Domain class must not be null!");
Assert.notNull(query, "CriteriaQuery must not be null!"); Root<U> root = query.from(domainClass); if (spec == null) {
return root;
} CriteriaBuilder builder = em.getCriteriaBuilder();
Predicate predicate = spec.toPredicate(root, query, builder); if (predicate != null) {
query.where(predicate);
} return root;
}
解读：
applySpecificationToCriteria方法调用了Specification的toPredicate方法，该方法是一个抽象方法，其定义如下：
/**
* Creates a WHERE clause for a query of the referenced entity in form of a {@link Predicate} for the given
* {@link Root} and {@link CriteriaQuery}.
*
* @param root must not be {@literal null}.
* @param query must not be {@literal null}.
* @param criteriaBuilder must not be {@literal null}.
* @return a {@link Predicate}, may be {@literal null}.
*/
@Nullable
Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder);
解读：
纵观前述调用过程可知，此处Specification的toPredicate方法由findOne方法传递给getQuery方法的参数对应的类实现。
从findOne(Example<S> example)的实现可知，toPredicate方法由参数new ExampleSpecification<S>(example, escapeCharacter)对应ExampleSpecification类实现。
ExampleSpecification
由前面的类图可知，ExampleSpecification是SimpleJpaRepository的内部类，其定义如下：
/**
* {@link Specification} that gives access to the {@link Predicate} instance representing the values contained in the
* {@link Example}.
*
* @author Christoph Strobl
* @since 1.10
* @param <T>
*/
private static class ExampleSpecification<T> implements Specification<T> { private static final long serialVersionUID = 1L; private final Example<T> example;
private final EscapeCharacter escapeCharacter; /**
* Creates new {@link ExampleSpecification}.
*
* @param example
* @param escapeCharacter
*/
ExampleSpecification(Example<T> example, EscapeCharacter escapeCharacter) { Assert.notNull(example, "Example must not be null!");
Assert.notNull(escapeCharacter, "EscapeCharacter must not be null!"); this.example = example;
this.escapeCharacter = escapeCharacter;
} /*
* (non-Javadoc)
* @see org.springframework.data.jpa.domain.Specification#toPredicate(javax.persistence.criteria.Root, javax.persistence.criteria.CriteriaQuery, javax.persistence.criteria.CriteriaBuilder)
*/
@Override
public Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
return QueryByExamplePredicateBuilder.getPredicate(root, cb, example, escapeCharacter);
}
}
解读：
ExampleSpecification实现了Specification接口中的toPredicate方法。
小结：
如果需要调用SimpleJpaRepository的findOne方法，需要构造Specification或者Example对应的实例。
示例：
以匿名内部类的形式构造Specification对应的实例
Specification<User> specification = new Specification<>() {
@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
Path<Integer> path = root.get("id");
return cb.lt(path, id);
}
};
Note：
有如下一些方法调用了getQuery方法
