当我们执行批量操作时，比如从数据库中查找“Person”的所有实例或者根据国家查找每个人，我们经常进行分页，以便我们可以向最终用户提供一个小数据块，并在下一个请求中，我们获取下一个数据块。
Spring Data为分页提供支持。它创建了实现分页的所有逻辑，例如所有页面的行计数等等。
在Spring Data中实现分页非常简单。我们只需要按照以下步骤操作：
- 在自定义存储库中，扩展 PagingAndSortingRepository。
- 创建PageRequest对象，该对象是Pageable接口的实现。 此PageRequest对象获取页码，页面大小以及排序方向和排序字段。
- 通过传递请求的页码和页面限制，您可以获取此页面的数据。如果您传递错误的页码，Spring Data将负责处理并且不返回任何数据。
1.创建扩展PagingAndSortingRepository的存储库。
@Repository
public interface PersonRepositary extends PagingAndSortingRepository<Person, Long>,QueryDslPredicateExecutor<Person> {
@Query("select p from Person p where p.country like ?1 order by country")
List<Person> findByCountryContains(String country);
List<Person> findPersonByHobbyName(String name);
@Query("select p from Person p where p.id = ?1 and country='America'")
Person findOne(Long id);
}
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2. 创建域对象。
@Entity
public class Person {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Long id;
private String name;
private String country;
private String gender;
@OneToMany(mappedBy="person",targetEntity=Hobby.class,
fetch=FetchType.EAGER,cascade=CascadeType.ALL)
List<Hobby> hobby;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public List<Hobby> getHobby() {
return hobby;
}
public void setHobby(List<Hobby> hobby) {
this.hobby = hobby;
}
public void addHobby(Hobby ihobby)
{
if(hobby == null)
{
hobby = new ArrayList<Hobby>();
}
hobby.add(ihobby);
}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + ", country=" + country + ", gender=" + gender + "]";
}
}
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3.获取所有人员。创建一个限制为1的PageRequest对象并请求第一页。
@SpringBootApplication
@EnableJpaRepositories("com.example.repo")
public class PersonApplication {
@Autowired
HobbyRepository hRepo;
private static final Logger log = LoggerFactory.getLogger(PersonApplication.class);
@Bean
public CommandLineRunner demo(PersonRepositary repository) {
findAll(repository);
return null;
}
private PageRequest gotoPage(int page)
{
PageRequest request = new PageRequest(page,1)
return request;
}
private void findAll(PersonRepositary repository)
{
Iterable<Person> pList = repository.findAll(gotoPage(0));
for(Person p : pList)
log.info("Person " + p);
}
public static void main(String[] args) {
SpringApplication.run(PersonApplication.class, args);
}
}
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运行时SQL输出：
Hibernate:
select
count(person0_.id) as col_0_0_
from
person person0_
Hibernate:
select
person0_.id as id1_1_,
person0_.country as country2_1_,
person0_.gender as gender3_1_,
person0_.name as name4_1_
from
person person0_ limit ?
Person Person [id=13, name=Samir mitra, country=America, gender=male]
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分页和排序代码实现
要进行排序，我们必须传递排序方向和排序字段以及页码和限制。假设我们想按国家名称按升序排序 - 我们修改 goto 方法如下：
private PageRequest gotoPage(int page)
{
PageRequest request = new PageRequest(page,1,Sort.Direction.ASC,"country");
return request;
}
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SQL输出：
select
count(person0_.id) as col_0_0_
from
person person0_
Hibernate:
select
person0_.id as id1_1_,
person0_.country as country2_1_,
person0_.gender as gender3_1_,
person0_.name as name4_1_
from
person person0_
order by
person0_.country asc limit ?
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