全面解析JPA 仓库repository中的findAll()方法

Page<T> findAll(@Nullable Specification<T> spec, Pageable pageable);

public Page<T> findAll(@Nullable Specification<T> spec, Pageable pageable) {
		//参数spec只是一个实例，没有查询条件
		TypedQuery<T> query = getQuery(spec, pageable);
		return isUnpaged(pageable) ? new PageImpl<T>(query.getResultList())
				: readPage(query, getDomainClass(), pageable, spec);
	}

protected TypedQuery<T> getQuery(@Nullable Specification<T> spec, Pageable pageable) {
		Sort sort = pageable.isPaged() ? pageable.getSort() : Sort.unsorted();
		return getQuery(spec, getDomainClass(), sort);
	}

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));
	}

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;
	}

query.select(root);
		if (sort.isSorted()) {
			query.orderBy(toOrders(sort, root, builder));
		}

return applyRepositoryMethodMetadata(em.createQuery(query));

return isUnpaged(pageable) ? new PageImpl<T>(query.getResultList())
				: readPage(query, getDomainClass(), pageable, spec);

select generatedAlias0 from Students as generatedAlias0 where 1=1 order by generatedAlias0.studentAge desc

@GetMapping("/user/{id}")
public User getUser(@PathVariable("id") Integer id) {
    User user = new User();
    user.setId(id);
    Example<User> example = Example.of(user);
    Optional<User> one = userRepository.findOne(example);
    return one.get();
}
@GetMapping("/user/all")
public List<User> getAll() {
    List<User> all = userRepository.findAll();
    System.out.println(all);
    return  all;
}