對於Mybatis物理分頁個人理解
阿新 • • 發佈:2018-11-17
Mybatis提供了一個簡單的邏輯分頁使用類RowBounds(物理分頁當然就是我們在sql語句中指定limit和offset值),在DefaultSqlSession提供的某些查詢介面中我們可以看到RowBounds是作為引數用來進行分頁的,如下介面:
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds)
RowBounds原始碼如下:
public class RowBounds {
/* 預設offset是0**/
public static final int NO_ROW_OFFSET = 0;
/* 預設Limit是int的最大值,因此它使用的是邏輯分頁**/
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;
}
}
邏輯分頁的實現原理:
在DefaultResultSetHandler中,邏輯分頁會將所有的結果都查詢到,然後根據RowBounds中提供的offset和limit值來獲取最後的結果,DefaultResultSetHandler實現如下:
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
//跳過RowBounds設定的offset值
skipRows(rsw.getResultSet(), rowBounds);
//判斷資料是否小於limit,如果小於limit的話就不斷的迴圈取值
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 {
//判斷資料是否小於limit,小於返回true
return !context.isStopped() && context.getResultCount() < rowBounds.getLimit();
}
//跳過不需要的行,應該就是rowbounds設定的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 {
//跳過RowBounds中設定的offset條資料
for (int i = 0; i < rowBounds.getOffset(); i++) {
rs.next();
}
}
}
總結:Mybatis的邏輯分頁比較簡單,簡單來說就是取出所有滿足條件的資料,然後捨棄掉前面offset條資料,然後再取剩下的資料的limit條