mybatis的使用及源码分析(八) mybatis的rowbounds分析
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 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
@Override
public <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
@Override
public <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
@Override
public <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 #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
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
@Override
public <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 statement
generateKeys(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。