Mybatis之ResultSetHandler
mybatis-3.4.6.release.
ResultSetHandler是个接口，如List-1
List-1
public interface ResultSetHandler {
//将结果转换为List
<E> List<E> handleResultSets(Statement stmt) throws SQLException;
//将结果转换为游标Cursor
<E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
void handleOutputParameters(CallableStatement cs) throws SQLException;
}实现类只有DefaultResultSetHandler，实现有点复杂，因为要考虑的情况很多。
结果封装原理
List-2
<resultMap type="Person" id="resultPersonMap">
<result property="id" column="id"/>
<result property="username" column="username"/>
<result property="password" column="password"/>
<result property="fltNum" column="flt_num"/>
</resultMap>如List-2, 定义这样一个ResultMap后，使用ObjectFactory创建一个Person对象，
person.setId(resultSet.getInt("id"))
person.setUsername(resultSet.getString("username"))
person.setPassword(resultSet.getString("password"))
person.setFltNum(resultSet.getString("flt_num"))
不过这个转换过程在实现上很复杂，其中就用到TypeHandler。
开始之前，来看下ResultSetWrapper，如下List-3，通过ResultSet获取ResultSetMetaData来获取列的属性，遍历列，获取列名称、列类型、对应的JdbcType。
List-3
public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {
super();
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.resultSet = rs;
final ResultSetMetaData metaData = rs.getMetaData();
final int columnCount = metaData.getColumnCount();
for (int i = 1; i <= columnCount; i++) {
columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));
jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));
classNames.add(metaData.getColumnClassName(i));
}
}来看DefaultResultSetHandler的handleResultSets方法，
List-4
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<Object>();
int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt);//1
List<ResultMap> resultMaps = mappedStatement.getResultMaps();//2
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);//3
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);//4
handleResultSet(rsw, resultMap, multipleResults, null);//5
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
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);
}1处通过ResultSetMetadata获取列的属性
2处获取我们定义的resultMap，如List-2
3处验证resultMap个数，如果小于1则会报错
4处获取resultMap，从List中
5处是核心，调用handleResultSet->handleRowValues->handleRowValuesForSimpleResultMap
List-5
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
Object rowValue = getRowValue(rsw, discriminatedResultMap);//1
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}handleRowValuesForSimpleResultMap方法中，不断调用resultSet.next()方法，会获取resultSet中的所有数据。
List-5的1处，调用getRowValue方法，该方法获取resultSet中的一行数据，并将数据封装位对象
List-6
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);//1
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, null) || foundValues;//2
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
return rowValue;
}List-6中1处，通过反射，创建Person对象(如List-7)，之后在List-6的2处
List-7
<select id="findByUsername" resultType="Person" parameterType="Person">
select * from person where username=#{username};
</select>List-8
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (!autoMapping.isEmpty()) {
for (UnMappedColumnAutoMapping mapping : autoMapping) {
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
// gcode issue #377, call setter on nulls (value is not 'found')
metaObject.setValue(mapping.property, value);
}
}
}
return foundValues;
}List-8中，createAutomaticMappings方法返还sql查询的列属性——从ResultSetWrapper中获取的，之后遍历这些列，对每一列，都调用typeHandler.getResult方法获取值，之后用metaObject.setValue，内部通过反射的方式设置值。这样createAutomaticMappings方法执行完成后，就获取了resultSet中的一行数据，且封装到对象中了。
回到List-5中，getRowValue方法返回值，storeObject方法中将值放入到List中。
描述较为泛华，建议个人阅读源码。此外，比如懒加载在ResultSetHandler中实现。
Reference
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