引用 Spring Data JPA

JPA (Java Persistence API)，直译为 Java 持久层 API，是 Java EE 的一套标准。

Spring Data JPA 旨在抽象各种数据库的数据访问层，以减少样板式代码（boilerplate code），这也正是 Spring 的精髓。

引用 Spring Data JPA 的方式如下（本文基于 spring-data 2.5.3）：
maven

<dependencies>
  <dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-jpa</artifactId>
  </dependency>
<dependencies>

gradle

dependencies {
    // ...
    implementation 'org.springframework.data:spring-data-jpa'
    // ...
}

Repository

JPA 的核心概念是Repository，它本身是一个空的接口。

package org.springframework.data.repository;

import org.springframework.stereotype.Indexed;

/**
 * Central repository marker interface. Captures the domain type to manage as well as the domain type's id type. General
 * purpose is to hold type information as well as being able to discover interfaces that extend this one during
 * classpath scanning for easy Spring bean creation.
 * <p>
 * Domain repositories extending this interface can selectively expose CRUD methods by simply declaring methods of the
 * same signature as those declared in {@link CrudRepository}.
 * 
 * @see CrudRepository
 * @param <T> the domain type the repository manages
 * @param <ID> the type of the id of the entity the repository manages
 * @author Oliver Gierke
 */
@Indexed
public interface Repository<T, ID> {

}

CrudRepository

CrudRepository接口继承Repository，提供基本的增删改查（CRUD）功能。

import java.util.Optional;

@NoRepositoryBean
public interface CrudRepository<T, ID> extends Repository<T, ID> {

    <S extends T> S save(S entity);

    <S extends T> Iterable<S> saveAll(Iterable<S> entities);

    Optional<T> findById(ID id);

    boolean existsById(ID id);

    Iterable<T> findAll();

    Iterable<T> findAllById(Iterable<ID> ids);

    long count();

    void deleteById(ID id);

    void delete(T entity);

    void deleteAllById(Iterable<? extends ID> ids);

    void deleteAll(Iterable<? extends T> entities);

    void deleteAll();
}

<T>为Entity，<ID>为Entity的ID类型，例如：

public interface DemoRepository extends CrudRepository<DemoEntity, Long> {

}

PagingAndSortingRepository

PagingAndSortingRepository继承CrudRepository，提供了分页查询的能力，它的定义如下：

package org.springframework.data.repository;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;

/**
 * Extension of {@link CrudRepository} to provide additional methods to retrieve entities using the pagination and
 * sorting abstraction.
 *
 * @author Oliver Gierke
 * @see Sort
 * @see Pageable
 * @see Page
 */
@NoRepositoryBean
public interface PagingAndSortingRepository<T, ID> extends CrudRepository<T, ID> {

	/**
	 * Returns all entities sorted by the given options.
	 *
	 * @param sort
	 * @return all entities sorted by the given options
	 */
	Iterable<T> findAll(Sort sort);

	/**
	 * Returns a {@link Page} of entities meeting the paging restriction provided in the {@code Pageable} object.
	 *
	 * @param pageable
	 * @return a page of entities
	 */
	Page<T> findAll(Pageable pageable);
}

分页查询，假设查询第一页（page 0），每页10条数据，写法如下：

Page<DemoEntity> demoEntityPage = testPagingRepository.findAll(PageRequest.of(0, 10));

其中PageRequest继承自Pageable。

当然在Controller中支持基于url的写法如下：

GET https://demo.ffish.net/demo/entities?page=0&size=10

查询语句写法

声明自定义interface，继承Repository或其子类（CrudRepository, PagingAndSortingRepository, JpaRepository），然后添加查询函数。

interface PersonRepository extends PagingAndSortingRepository<Person, Long>
{
    List<Person> findByLastname(String lastname);
    Page<Person> findByAge(int age, Pageable pageable);
    List<Person> findByLastnameOrderByFirstnameAsc(String lastname);
}

这样就可以了，不需要写查询语句。关于Repository方法的编写规则，请参考下面的文章。

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