在 mybatis 中，使用 RowBounds 进行分页，非常方便，不需要在 sql 语句中写 limit，即可完成分页功能。但是由于它是在 sql 查询出所有结果的基础上截取数据的，所以在数据量大的sql中并不适用，它更适合在返回数据结果较少的查询中使用
最核心的是在 mapper 接口层，传参时传入 RowBounds(int offset, int limit) 对象，即可完成分页
注意：由于 java 允许的最大整数为 2147483647，所以 limit 能使用的最大整数也是 2147483647，一次性取出大量数据可能引起内存溢出，所以在大数据查询场合慎重使用
mapper 接口层代码如下
List<Book> selectBookByName(Map<String, Object> map, RowBounds rowBounds);调用如下
List<Book> list = bookMapper.selectBookByName(map, new RowBounds(0, 5));说明: new RowBounds(0, 5)，即第一页，每页取5条数据
数据库数据
mapper 接口层
@Mapper public interface BookMapper { //添加数据 int insert(Book book); //模糊查询 List<Book> selectBookByName(Map<String, Object> map, RowBounds rowBounds); }mapper.xml 文件
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" > <mapper namespace="com.demo.mapper.BookMapper"> <resultMap id="BaseResultMap" type="com.demo.bean.Book"> <id column="id" property="id" jdbcType="VARCHAR" /> <result column="book_name" property="bookName" jdbcType="VARCHAR" /> <result column="book_author" property="bookAuthor" jdbcType="VARCHAR" /> <result column="create_date" property="createDate" jdbcType="VARCHAR" /> <result column="update_date" property="updateDate" jdbcType="VARCHAR" /> </resultMap> <sql id="Base_Column_List"> book_name as bookName, book_author as bookAuthor, create_date as createDate, update_date as updateDate </sql> <insert id="insert" useGeneratedKeys="true" keyProperty="id" parameterType="com.demo.bean.Book"> insert into book(book_name, book_author, create_date, update_date) values(#{bookName}, #{bookAuthor}, #{createDate}, #{updateDate}) </insert> <select id="selectBookByName" resultMap="BaseResultMap"> <bind name="pattern_bookName" value="'%' + bookName + '%'" /> <bind name="pattern_bookAuthor" value="'%' + bookAuthor + '%'" /> select * from book where 1 = 1 <if test="bookName != null and bookName !=''"> and book_name LIKE #{pattern_bookName} </if> <if test="bookAuthor != null and bookAuthor !=''"> and book_author LIKE #{pattern_bookAuthor} </if> </select> </mapper>测试代码
@RunWith(SpringRunner.class) @SpringBootTest public class SpringbootJspApplicationTests { @Autowired private BookMapper bookMapper; @Test public void contextLoads() { Book book = new Book(); book.setBookName("隋唐演义"); book.setBookAuthor("褚人获"); SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); book.setCreateDate(sdf.format(new Date())); book.setUpdateDate(sdf.format(new Date())); bookMapper.insert(book); System.out.println("返回的主键: "+book.getId()); } @Test public void query() { Map<String, Object> map = new HashMap<String, Object>(); map.put("bookName", ""); map.put("bookAuthor", ""); List<Book> list = bookMapper.selectBookByName(map, new RowBounds(0, 5)); for(Book b : list) { System.out.println(b.getBookName()); } } }运行 query 查询第一页，5 条数据，效果如下
Mybatis提供了一个简单的逻辑分页使用类RowBounds（物理分页当然就是我们在sql语句中指定limit和offset值），在DefaultSqlSession提供的某些查询接口中我们可以看到RowBounds是作为参数用来进行分页的，如下接口：
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds)
RowBounds源码如下：
public class RowBounds {
/* 默认offset是0**/
public static final int NO_ROW_OFFSET = 0;
/* 默认Limit是int的最大值，因此它使用的是逻辑分页**/
public static final int NO_ROW_LIMIT = Integer.MAX_VALUE;
public static final RowBounds DEFAULT = new RowBounds();
private int offset;
private int limit;
public RowBounds() {
this.offset = NO_ROW_OFFSET;
this.limit = NO_ROW_LIMIT;
}
public RowBounds(int offset, int limit) {
this.offset = offset;
this.limit = limit;
}
public int getOffset() {
return offset;
}
public int getLimit() {
return limit;
}
}
逻辑分页的实现原理：
在DefaultResultSetHandler中，逻辑分页会将所有的结果都查询到，然后根据RowBounds中提供的offset和limit值来获取最后的结果，DefaultResultSetHandler实现如下：
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
//跳过RowBounds设置的offset值
skipRows(rsw.getResultSet(), rowBounds);
//判断数据是否小于limit，如果小于limit的话就不断的循环取值
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
Object rowValue = getRowValue(rsw, discriminatedResultMap);
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
private boolean shouldProcessMoreRows(ResultContext<?> context, RowBounds rowBounds) throws SQLException {
//判断数据是否小于limit，小于返回true
return !context.isStopped() && context.getResultCount() < rowBounds.getLimit();
}
//跳过不需要的行,应该就是rowbounds设置的limit和offset
private void skipRows(ResultSet rs, RowBounds rowBounds) throws SQLException {
if (rs.getType() != ResultSet.TYPE_FORWARD_ONLY) {
if (rowBounds.getOffset() != RowBounds.NO_ROW_OFFSET) {
rs.absolute(rowBounds.getOffset());
}
} else {
//跳过RowBounds中设置的offset条数据
for (int i = 0; i < rowBounds.getOffset(); i++) {
rs.next();
}
}
}
总结：Mybatis的逻辑分页比较简单，简单来说就是取出所有满足条件的数据，然后舍弃掉前面offset条数据，然后再取剩下的数据的limit条
