这是我参与2022首次更文挑战的第10天，活动详情查看：2022首次更文挑战
简介
上篇中解析了MyBatis3中参数是如何传递处理的，本篇接着看看在获取到查询结果后，MyBatis3是如何将SQL查询结果与我们接口函数定义的返回结果对应的
源码
获取结果后处理的入口
在：MyBatis3源码解析(3)查询语句执行中，我们大致知道了查询结果处理的想代码位置，如下：
public class PreparedStatementHandler extends BaseStatementHandler {
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
// 前面的语句已经执行，处理结果返回
return resultSetHandler.handleResultSets(ps);
}
}
SQL结果处理
下面是具体的处理逻辑，先获取结果集和Mapper中定义的返回结果的相关信息，然后对应进行处理
public class DefaultResultSetHandler implements ResultSetHandler {
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
// 获取结果集，并用自定义包装起来
ResultSetWrapper rsw = getFirstResultSet(stmt);
// 获取ResultMap对象，一般只有一个，其实就是需要返回的类信息之类的
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
// 在这里会循环获取SQL结果集并进行转换
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
// 这个不太请求，资料上说一般不会指定这个，暂时放过
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}
private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
ResultSet rs = stmt.getResultSet();
while (rs == null) {
// move forward to get the first resultset in case the driver
// doesn't return the resultset as the first result (HSQLDB 2.1)
if (stmt.getMoreResults()) {
rs = stmt.getResultSet();
} else {
if (stmt.getUpdateCount() == -1) {
// no more results. Must be no resultset
break;
}
}
}
return rs != null ? new ResultSetWrapper(rs, configuration) : null;
}
}
我们一直跟下去，中间步骤不是太关键，下发是SQL Result遍历和结果转换的相关代码
public class DefaultResultSetHandler implements ResultSetHandler {
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);
// SQLresult遍历
while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
// 将单条SQL结果转成对象
Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
// 存储
storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
}
}
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
// 根据返回的类型生成对象
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final MetaObject metaObject = configuration.newMetaObject(rowValue);
boolean foundValues = this.useConstructorMappings;
if (shouldApplyAutomaticMappings(resultMap, false)) {
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
// 结果转换
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
return rowValue;
}
private boolean applyPropertyMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
final List<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix);
boolean foundValues = false;
// 结果相关的属性
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
// 遍历，结果对应对应的值
for (ResultMapping propertyMapping : propertyMappings) {
// 列名称
String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
if (propertyMapping.getNestedResultMapId() != null) {
// the user added a column attribute to a nested result map, ignore it
column = null;
}
if (propertyMapping.isCompositeResult()
|| (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH)))
|| propertyMapping.getResultSet() != null) {
// 转换，获取值
Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);
// issue #541 make property optional
final String property = propertyMapping.getProperty();
if (property == null) {
continue;
} else if (value == DEFERRED) {
foundValues = true;
continue;
}
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !metaObject.getSetterType(property).isPrimitive())) {
// 更新结果
// gcode issue #377, call setter on nulls (value is not 'found')
metaObject.setValue(property, value);
}
}
}
return foundValues;
}
private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
if (propertyMapping.getNestedQueryId() != null) {
return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
} else if (propertyMapping.getResultSet() != null) {
addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
return DEFERRED;
} else {
// 将SQL值转成对应java对象值，MyBatis3已内容Int Str等转换器
final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
return typeHandler.getResult(rs, column);
}
}
}
在上面代码中，我们看到了：
- 循环遍历SQL结果：ResultSet
- 遍历SQL列，转成Java类型
MyBatis中已经内置很多的类型转换器，也可以自己自定义，比如内容Long如下：
public class LongTypeHandler extends BaseTypeHandler<Long> {
@Override
public Long getNullableResult(ResultSet rs, String columnName)
throws SQLException {
long result = rs.getLong(columnName);
return result == 0 && rs.wasNull() ? null : result;
}
}
总结
本篇文章中探索的MyBatis中如何将查询结果转换成我们想要的Java对象
- SQL执行
- 遍历ResultSet
- 遍历数据库Column，根据列名称，获取对应的值，转成Java对象
