Mybatis提供了一个简单的逻辑分页类RowBounds，其原理类似于在内存中做了一个分页，不是数据库层面的分页，性能不算好，谨慎使用

一. RowBounds源码分析

1 RowBounds源码：

/***    Copyright 2009-2017 the original author or authors.**    Licensed under the Apache License, Version 2.0 (the "License");*    you may not use this file except in compliance with the License.*    You may obtain a copy of the License at**       http://www.apache.org/licenses/LICENSE-2.0**    Unless required by applicable law or agreed to in writing, software*    distributed under the License is distributed on an "AS IS" BASIS,*    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.*    See the License for the specific language governing permissions and*    limitations under the License.*/
package org.apache.ibatis.session;/*** @author Clinton Begin*/
public class RowBounds {public static final int NO_ROW_OFFSET = 0;public static final int NO_ROW_LIMIT = Integer.MAX_VALUE;public static final RowBounds DEFAULT = new RowBounds();private final int offset;private final 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;}}

2 SqlSession类

  /*** Retrieve a list of mapped objects from the statement key and parameter,* within the specified row bounds.* @param <E> the returned list element type* @param statement Unique identifier matching the statement to use.* @param parameter A parameter object to pass to the statement.* @param rowBounds  Bounds to limit object retrieval* @return List of mapped object*/<E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds);

我们发现SqlSession类中有一个重载的函数SelectList，其第三个参数是RowBounds
如下这样使用实现分页功能：

 SqlSession session = sqlSessionFactory.openSession();Map<String, String> sqlMap = new HashMap<>();sqlMap.put("sql", "select * from test_data");List<Map> result = null;String method = "com.iscas.biz.mp.mapper.DynamicMapper.dynamicSelect";RowBounds rowBounds = new RowBounds(600000, 20);result = session.selectList(method, sqlMap, rowBounds);

3 查看DefaultSqlSession中selectList的实现

 @Overridepublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {MappedStatement ms = configuration.getMappedStatement(statement);return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}}

4 查看BaseExecutor中query的实现

@Overridepublic <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {BoundSql boundSql = ms.getBoundSql(parameter);CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);return query(ms, parameter, rowBounds, resultHandler, key, boundSql);}

5 查看BaseExecutor中query重载实现

 @Overridepublic <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());if (closed) {throw new ExecutorException("Executor was closed.");}if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();}List<E> list;try {queryStack++;list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);}} finally {queryStack--;}if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();}// issue #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;}

6 查看BaseExecutor中queryFromDatabase实现

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;localCache.putObject(key, EXECUTION_PLACEHOLDER);try {list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {localCache.removeObject(key);}localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;}

7 查看BaseExecutor中doQuery的实现

@Overridepublic <E> List<E> doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)throws SQLException {Statement stmt = null;try {flushStatements();Configuration configuration = ms.getConfiguration();StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);Connection connection = getConnection(ms.getStatementLog());stmt = handler.prepare(connection, transaction.getTimeout());handler.parameterize(stmt);return handler.query(stmt, resultHandler);} finally {closeStatement(stmt);}}

8 查看Configuration中newStatementHandler的实现

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);return statementHandler;}

9 查看RoutingStatementHandler的构造器

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {switch (ms.getStatementType()) {case STATEMENT:delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;case PREPARED:delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;case CALLABLE:delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);break;default:throw new ExecutorException("Unknown statement type: " + ms.getStatementType());}}

10 查看SimpleStatementHandler的构造器

public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);}

11 查看BaseStatementHandler的构造器

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {this.configuration = mappedStatement.getConfiguration();this.executor = executor;this.mappedStatement = mappedStatement;this.rowBounds = rowBounds;this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();this.objectFactory = configuration.getObjectFactory();if (boundSql == null) { // issue #435, get the key before calculating the statementgenerateKeys(parameterObject);boundSql = mappedStatement.getBoundSql(parameterObject);}this.boundSql = boundSql;this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);}

12 查看Configuration中newResultSetHandler的实现

  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,ResultHandler resultHandler, BoundSql boundSql) {ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);return resultSetHandler;}

13 查看DefaultResultSetHandler构造器

  public DefaultResultSetHandler(Executor executor, MappedStatement mappedStatement, ParameterHandler parameterHandler, ResultHandler<?> resultHandler, BoundSql boundSql,RowBounds rowBounds) {this.executor = executor;this.configuration = mappedStatement.getConfiguration();this.mappedStatement = mappedStatement;this.rowBounds = rowBounds;this.parameterHandler = parameterHandler;this.boundSql = boundSql;this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();this.objectFactory = configuration.getObjectFactory();this.reflectorFactory = configuration.getReflectorFactory();this.resultHandler = resultHandler;}

重点来了，DefaultResultSetHandler是JDBC结果集的处理类，此类中有如下代码：

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {try {if (parentMapping != null) {handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);} else {if (resultHandler == null) {DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);multipleResults.add(defaultResultHandler.getResultList());} else {handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);}}} finally {// issue #228 (close resultsets)closeResultSet(rsw.getResultSet());}}//// HANDLE ROWS FOR SIMPLE RESULTMAP//public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {if (resultMap.hasNestedResultMaps()) {ensureNoRowBounds();checkResultHandler();handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);} else {handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);}}private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)throws SQLException {DefaultResultContext<Object> resultContext = new DefaultResultContext<>();ResultSet resultSet = rsw.getResultSet();skipRows(resultSet, rowBounds);while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);Object rowValue = getRowValue(rsw, discriminatedResultMap, null);storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);}}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 {for (int i = 0; i < rowBounds.getOffset(); i++) {if (!rs.next()) {break;}}}}

其中skipRows是此种分页方式的灵魂，根据RowBounds中的offSet偏移量，使ResultSet的游标向下移动若干，便实现了逻辑分页。