Spring Data JPA之Derived query

一、Derived Query

Derived query的前缀定义可以在PartTree类找到，

package org.springframework.data.repository.query.parser;

public class PartTree implements Streamable<OrPart> {
    // ...
    private static final String QUERY_PATTERN = "find|read|get|query|stream";
    private static final String COUNT_PATTERN = "count";
    private static final String EXISTS_PATTERN = "exists";
    private static final String DELETE_PATTERN = "delete|remove";
    // ...
}

1、exists前缀

// exists避免了代码中丑陋的ifn操作
boolean exists = repository.existsById(searchId)

2、count前缀

// 使用JPQL
@Query(" select count(t) from FollowerInfo t where investUserId = :invUserId")
Integer findFollowerNumberByInvUserId(@Param("invUserId") Long invUserId);

// 使用count前缀的命名查询
Long countByInvestUserId(Long investUserId);

注意：根据方法名解析的count查询也存在一些限制，首先是它的支持格式，

// 除了countDistinctByName其他4个方法对应的sql均为：
    select
        count(student0_.id) as col_0_0_ 
    from
        student student0_ 
    where
        student0_.name=?

// countDistinctByName对应的sql为：
    select
        distinct count(distinct student0_.id) as col_0_0_ 
    from
        student student0_ 
    where
        student0_.name=?

// 也就是无论By后面的是哪个字段，count的对象都是id。所以某些需求就无法通过方法名解析实现，例如，获取唯一的名字个数
    select count(distinct name) from student;

// 此时，你只能使用JPQL或者原生SQL实现你的查询需求。

2、stream和Pageable可以优化程序的内存占用(类似Python的生成器)

public interface UserRepository extends JpaRepository<User, Integer> {
    // ...
    Page<User> findAll(Pageable pageable);
    // ...
}

public interface UserRepository extends JpaRepository<User, Integer> {
    // ...
    Stream<User> findAllByName(String name);
    // ...
}

3、Optional可以有效解决Null值问题

public interface UserRepository extends JpaRepository<User, Integer> {
     
    Optional<User> findOneByName(String name);
     
}

4、相关链接：

https://www.baeldung.com/spring-data-exists-query
https://www.baeldung.com/spring-data-java-8
http://knes1.github.io/blog/2015/2015-10-19-streaming-mysql-results-using-java8-streams-and-spring-data.html

方法名中支持的关键字