背景

專案中我們經常用到分頁查詢，一般使用的場景有兩個

1. 前端查詢大批量資料時指定page、size分頁查詢

2. 後臺定時處理一批資料時，從資料庫獲取待處理資料，從而避免一次查詢太多資料到記憶體，處理完一批後再查詢下一頁的資料處理

對於場景2也有兩種實現方式，Page和Slice，下面就分別介紹

Page

public interface UserRepository extends JpaRepository<User,Long> {
    Page<User> findByAge(int age, Pageable pageable)
 
;
}

@Slf4j
@RunWith(SpringRunner.class)
@SpringBootTest
public class JpaTests {
	@Autowired
    private UserRepository userRepository;
    
	@Before
    public void before(){

        userRepository.saveAll(Arrays.asList(
                new User(null,"劉","一", 20),
                new User(null,"陳",
 
"二", 20),
                new User(null,"張","三", 20),
                new User(null,"李","四", 20),
                new User(null,"王","五", 20),
                new User(null,"趙","六", 20),
                new User(null,"孫","七", 20),
                new User(null,"周","八", 20)
        ));
    }
    
	@Test
    public
 
 void queryByPage(){
        int pageNum = 0;
        while(true){
            Pageable pageable = PageRequest.of(pageNum, 3);
            Page<User> page = userRepository.findByAge(20, pageable);
            List<User> users = page.getContent();
            users.forEach(System.out::println);

            if (users.size() == 0){
                break;
            }

            pageNum++;
        }
    }
}

控制檯輸出

Hibernate: select user0_.id as id1_0_, user0_.age as age2_0_, user0_.first_name as first_na3_0_, user0_.last_name as last_nam4_0_ from t_user user0_ where user0_.age=? limit ?
Hibernate: select count(user0_.id) as col_0_0_ from t_user user0_ where user0_.age=?
User(id=1, firstName=劉, lastName=一, age=20)
User(id=2, firstName=陳, lastName=二, age=20)
User(id=3, firstName=張, lastName=三, age=20)
Hibernate: select user0_.id as id1_0_, user0_.age as age2_0_, user0_.first_name as first_na3_0_, user0_.last_name as last_nam4_0_ from t_user user0_ where user0_.age=? limit ? offset ?
Hibernate: select count(user0_.id) as col_0_0_ from t_user user0_ where user0_.age=?
User(id=4, firstName=李, lastName=四, age=20)
User(id=5, firstName=王, lastName=五, age=20)
User(id=6, firstName=趙, lastName=六, age=20)
Hibernate: select user0_.id as id1_0_, user0_.age as age2_0_, user0_.first_name as first_na3_0_, user0_.last_name as last_nam4_0_ from t_user user0_ where user0_.age=? limit ? offset ?
User(id=7, firstName=孫, lastName=七, age=20)
User(id=8, firstName=周, lastName=八, age=20)

程式碼比較簡單，就是迴圈增加pageNum，只到查詢為空就終止

Slice

Slice是一個數據塊，表明是否有更多可用資料，能夠迴圈的獲取資料。

@Test
    public void queryBySlice(){
        Pageable pageable = PageRequest.of(0,3);
        while(true){
           Slice<User> slice = userRepository.findByAge(20, pageable);
           List<User> users = slice.getContent();
           users.forEach(System.out::println);

           if (!slice.hasNext()){
               break;
           }

           pageable = slice.nextPageable();
        }
    }

可以看到跟Page的區別是，不需要自定義的判斷是否有資料，也不用自己累加頁數了。使用slice.hasNext()就能判斷是否還有下一個Slice，這裡要注意需要呼叫slice.nextPageable()獲取下一個pageable，怎麼樣看起來是不是比Page更高階。