Mybatis源码分析_struts源码

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发布于 2022-11-08 19:56:29

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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

 @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。

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原始发表：2022年9月22日，如有侵权请联系 cloudcommunity@tencent.com 删除

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