JPA List转成Page方法

桃扇骨 2021-09-11 02:48 476阅读 0赞

## List转成Page ##

当对非直接查询数据库得到的List，想转成通过JPA查询的Page分页列表，可以使用这个方法：

代码实现：

import org.fluttercode.datafactory.impl.DataFactory;
    import org.springframework.data.domain.Page;
    import org.springframework.data.domain.PageImpl;
    import org.springframework.data.domain.PageRequest;
    import org.springframework.data.domain.Pageable;
    import org.springframework.data.repository.support.PageableExecutionUtils;
    
    import java.lang.reflect.Modifier;
    import java.util.*;
    
    public class DataUtils { 
    /** * pageSize 每页数量以pageable中为准 * page 从0开始 * * @param list * @param pageable * @param <T> * @return */
        public static <T> Page<T> listToPage(List<T> list, Pageable pageable) { 
            return getPage(list, pageable);
        }
        
        /** * pageSize 每页数量为15 * page 从0开始 * * @param list * @param page * @param <T> * @return */
        public static <T> Page<T> listToPage(List<T> list, int page) { 
            PageRequest pageable = PageRequest.of(page, 15);
            return getPage(list, pageable);
        }
    
        private static <T> Page<T> getPage(List<T> list, Pageable pageable) { 
            //第n页起始值
            int pageStart = pageable.getPageNumber() * pageable.getPageSize();
            //第n页期望结尾值
            int expectPageEnd = pageStart + pageable.getPageSize() - 1;
            return Optional.of(list.size())
                    .filter(size -> size >= pageStart)
                    .map(size -> getRealSubList(pageStart, expectPageEnd, list, pageable))
                    .orElseGet(() -> getEmptySubList(list, pageable));
        }
    
        private static <T> PageImpl<T> getRealSubList(int pageStart, int expectPageEnd, List<T> list, Pageable pageable) { 
            int realPageEnd = (list.size() > expectPageEnd || list.size() == expectPageEnd) ? expectPageEnd + 1 : list.size();
            return new PageImpl<>(list.subList(pageStart, realPageEnd), pageable, list.size());
        }
    
        private static <T> PageImpl<T> getEmptySubList(List<T> list, Pageable pageable) { 
            return new PageImpl<>(new ArrayList(), pageable, list.size());
        }
    }

单元测试：

@Test
        public void listToPage() { 
            List<Account> integers = new ArrayList<>();
            for (int i = 0; i < 40; i++) { 
                Account account = new Account();
                account.setId(Long.valueOf(i));
                integers.add(account);
            }
            PageRequest page1 = PageRequest.of(0, 15);
            Page<Account> integersPage1 = DataUtils.listToPage(integers, page1);
            TestCase.assertTrue(14 == integersPage1.getContent().get(14).getId());
    
            PageRequest page2 = PageRequest.of(1, 15);
            Page<Account> integersPage2 = DataUtils.listToPage(integers, page2);
            TestCase.assertTrue(29 == integersPage2.getContent().get(14).getId());
    
            PageRequest page3 = PageRequest.of(2, 15);
            Page<Account> integersPage3 = DataUtils.listToPage(integers, page3);
            TestCase.assertTrue(39 == integersPage3.getContent().get(9).getId());
    
            PageRequest page4 = PageRequest.of(3, 15);
            Page<Account> integersPage4 = DataUtils.listToPage(integers, page4);
            TestCase.assertTrue(0 == integersPage4.getContent().size());
        }