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;
- }
- }
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();
- }
- }
- }