转载自 mybatis源码阅读(七) ---ResultSetHandler了解一下
1、MetaObject
MetaObject用于反射创建对象、反射从对象中获取属性值、反射给对象设置属性值，参数设置和结果封装，用的都是这个MetaObject提供的功能。
public static MetaObject forObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory, ReflectorFactory reflectorFactory) {
if (object == null) {
return SystemMetaObject.NULL_META_OBJECT;
} else {
return new MetaObject(object, objectFactory, objectWrapperFactory, reflectorFactory);
}
}
public Object getValue(String name) {
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
return null;
} else {
return metaValue.getValue(prop.getChildren());
}
} else {
return objectWrapper.get(prop);
}
}
public void setValue(String name, Object value) {
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
if (value == null && prop.getChildren() != null) {
// don't instantiate child path if value is null
return;
} else {
metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
}
}
metaValue.setValue(prop.getChildren(), value);
} else {
objectWrapper.set(prop, value);
}
}
Object object：要反射的对象，比如Student。
ObjectFactory objectFactory：通过Class对象反射创建对象实例的工厂类，比如创建一个User对象。
ObjectWrapperFactory ：对目标对象进行包装，比如可以将Properties对象包装成为一个Map并返回Map对象。
ReflectorFactory ：为了避免多次反射同一个Class对象，ReflectorFactory提供了Class对象的反射结果缓存。
getValue(String name)：属性取值。
setValue(String name, Object value)：属性赋值。
2、结果封装实现原理
Mybatis的结果封装，分为两种，一种是有ResultMap映射表，明确定义了结果集列名与对象属性名的配对关系，另外一种是对象类型，没有明确定义结果集列名与对象属性名的配对关系，如resultType是User对象。
<resultMap type="userT" id="userResult">
<id property="id" column="t_id"/>
<result property="name" column="name" />
</resultMap>
<select id="findUserById" resultMap="userResult">
select * from m_user where id = #{id}
</select>
原理非常简单：使用ObjectFactory ，创建一个userT对象实例。
userT.setId(resultSet.getInt("t_id"))；
userT.setName(resultSet.getString("name"));
如果是对象类型，如User对象类似，原理也非常简单。
<select id="findUserById" resultType="com.lpf.entity.User">
select * from m_user where id = #{id}
</select>
原理：使用ObjectFactory ，创建一个User对象实例。
user.setId(resultSet.getInt("id"));
user.setName(resultSet.getString("name"));
3、DefaultResultSetHandler
DefaultResultSetHandler继承了，ResultSetHandler是一个接口，提供了两个函数分别用来处理普通操作和存储过程的结果，
源码如下：
/**
* @author Clinton Begin
*/
public interface ResultSetHandler {
<E> List<E> handleResultSets(Statement stmt) throws SQLException;
<E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
void handleOutputParameters(CallableStatement cs) throws SQLException;
}
DefaultResultSetHandler，其实现的步骤就是将Statement执行后的结果集，按照Mapper文件中配置的ResultType或ResultMap来封装成对应的对象，最后将封装的对象返回即可，
结果封装实现原理
DefaultResultSetHandler部分源码如下：
public class DefaultResultSetHandler implements ResultSetHandler {
private static final Object DEFERED = new Object();
// 此Map用来保存当前层级内部的结果对象（一对多关系的多方对象），key为combinedKey
private final Map<CacheKey, Object> nestedResultObjects = new HashMap<CacheKey, Object>();
// 此Map用来保存当前层级的根对象（一对多关系中的一方对象），key为absoluteKey
private final Map<String, Object> ancestorObjects = new HashMap<String, Object>();
private Object previousRowValue;
// multiple resultsets
private final Map<String, ResultMapping> nextResultMaps = new HashMap<String, ResultMapping>();
private final Map<CacheKey, List<PendingRelation>> pendingRelations = new HashMap<CacheKey, List<PendingRelation>>();
// Cached Automappings
private final Map<String, List<UnMappedColumnAutoMapping>> autoMappingsCache = new HashMap<String, List<UnMappedColumnAutoMapping>>();
// temporary marking flag that indicate using constructor mapping (use field to reduce memory usage)
private boolean useConstructorMappings;
private final PrimitiveTypes primitiveTypes;
private static class PendingRelation {
public MetaObject metaObject;
public ResultMapping propertyMapping;
}
/**
* 调用存储过程返回结果，将结果值放在参数中
* @param cs
* @throws SQLException
*/
@Override
public void handleOutputParameters(CallableStatement cs) throws SQLException {
final Object parameterObject = parameterHandler.getParameterObject();
final MetaObject metaParam = configuration.newMetaObject(parameterObject);
final List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
for (int i = 0; i < parameterMappings.size(); i++) {
final ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() == ParameterMode.OUT || parameterMapping.getMode() == ParameterMode.INOUT) {
if (ResultSet.class.equals(parameterMapping.getJavaType())) {
handleRefCursorOutputParameter((ResultSet) cs.getObject(i + 1), parameterMapping, metaParam);
} else {
final TypeHandler<?> typeHandler = parameterMapping.getTypeHandler();
metaParam.setValue(parameterMapping.getProperty(), typeHandler.getResult(cs, i + 1));
}
}
}
}
/**
* 对普通查询到的结果转换
* @param stmt
* @return
* @throws SQLException
*/
@Override
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;
//获取第一个结果值
ResultSetWrapper rsw = getFirstResultSet(stmt);
//获得resultMap
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
//这边应该为1吧，一般resultMap为一个
int resultMapCount = resultMaps.size();
//判断是否有resultMap,没有的话抛出异常
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
//获得resultMap，实体类和表中数据字段的对应关系
ResultMap resultMap = resultMaps.get(resultSetCount);
//将值设置成对应的resultmap对象
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
//获得resultSets
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);
}
//获得第一个值，并将值打包
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;
}
//获取下一个值，将值打包
private ResultSetWrapper getNextResultSet(Statement stmt) throws SQLException {
// Making this method tolerant of bad JDBC drivers
try {
if (stmt.getConnection().getMetaData().supportsMultipleResultSets()) {
// Crazy Standard JDBC way of determining if there are more results
if (!(!stmt.getMoreResults() && stmt.getUpdateCount() == -1)) {
ResultSet rs = stmt.getResultSet();
if (rs == null) {
return getNextResultSet(stmt);
} else {
return new ResultSetWrapper(rs, configuration);
}
}
}
} catch (Exception e) {
// Intentionally ignored.
}
return null;
}
//关闭resultSet
private void closeResultSet(ResultSet rs) {
try {
if (rs != null) {
rs.close();
}
} catch (SQLException e) {
// ignore
}
}
//校验结果的resultMap，如果没有的话就抛出异常
private void validateResultMapsCount(ResultSetWrapper rsw, int resultMapCount) {
if (rsw != null && resultMapCount < 1) {
throw new ExecutorException("A query was run and no Result Maps were found for the Mapped Statement '" + mappedStatement.getId()
+ "'. It's likely that neither a Result Type nor a Result Map was specified.");
}
}
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);
//将查询到的每个字段和Bean实体中的属性对应起来，生成一个Result对象
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
//结果映射对象及值添加到multipleResults中
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}
//操作列值
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);
}
}
//判断记录行数
private void ensureNoRowBounds() {
if (configuration.isSafeRowBoundsEnabled() && rowBounds != null && (rowBounds.getLimit() < RowBounds.NO_ROW_LIMIT || rowBounds.getOffset() > RowBounds.NO_ROW_OFFSET)) {
throw new ExecutorException("Mapped Statements with nested result mappings cannot be safely constrained by RowBounds. "
+ "Use safeRowBoundsEnabled=false setting to bypass this check.");
}
}
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);
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, null);
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;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
return rowValue;
}
private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
if (propertyMapping.getNestedQueryId() != null) {//关联查询
// 执行另外一个select查询，把查询结果赋值给属性值，比如Student对象的teacher属性
return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
} else if (propertyMapping.getResultSet() != null) {
addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
return DEFERED;
} else {
final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
return typeHandler.getResult(rs, column);
}
}
//创建结果对象
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
this.useConstructorMappings = false; // reset previous mapping result
final List<Class<?>> constructorArgTypes = new ArrayList<Class<?>>();
final List<Object> constructorArgs = new ArrayList<Object>();
//结果对象
Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
//获取配置的resultMap的字段与表中数据的映射关系
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
for (ResultMapping propertyMapping : propertyMappings) {
// issue gcode #109 && issue #149
if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
break;
}
}
}
this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
return resultObject;
}
//得到嵌套查询值
private Object getNestedQueryMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
final String nestedQueryId = propertyMapping.getNestedQueryId();
final String property = propertyMapping.getProperty();
final MappedStatement nestedQuery = configuration.getMappedStatement(nestedQueryId);
final Class<?> nestedQueryParameterType = nestedQuery.getParameterMap().getType();
final Object nestedQueryParameterObject = prepareParameterForNestedQuery(rs, propertyMapping, nestedQueryParameterType, columnPrefix);
Object value = null;
if (nestedQueryParameterObject != null) {
final BoundSql nestedBoundSql = nestedQuery.getBoundSql(nestedQueryParameterObject);
final CacheKey key = executor.createCacheKey(nestedQuery, nestedQueryParameterObject, RowBounds.DEFAULT, nestedBoundSql);
final Class<?> targetType = propertyMapping.getJavaType();
if (executor.isCached(nestedQuery, key)) {
//如果已经有一级缓存了，则延迟加载(实际上deferLoad方法中可以看到则是立即加载)
executor.deferLoad(nestedQuery, metaResultObject, property, key, targetType);
value = DEFERED;
} else {
// ResultLoader保存了关联查询所需要的所有信息
final ResultLoader resultLoader = new ResultLoader(configuration, executor, nestedQuery, nestedQueryParameterObject, targetType, key, nestedBoundSql);
if (propertyMapping.isLazy()) {
// 执行延迟加载
// 语意：resultLoader的查询结果将赋值给metaResultObject源对象的property属性,resultLoader的查询参数值来自于metaResultObject源对象属性中。
// 举例：查询Teacher，赋值给Student的teacher属性，参数来自于查询Student的ResultSet的teacher_id列的值。
// 由于需要执行延迟加载，将查询相关信息放入缓存，但不执行查询，使用该属性时，自动触发加载操作。
lazyLoader.addLoader(property, metaResultObject, resultLoader);
value = DEFERED;
} else {
// 不执行延迟加载，立即查询并赋值
value = resultLoader.loadResult();
}
}
}
return value;
}
//生成嵌套对象值 一对多查询
private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
final DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
Object rowValue = previousRowValue;
// 解决鉴别过的结果映射，逻辑如下：
// 获取结果映射中的鉴别器，通过鉴别指定字段通过配置对象获取对应的另一个结果映射，循环往复，
// 直到找不到鉴别器为止，返回最终的结果映射
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
final CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);
Object partialObject = nestedResultObjects.get(rowKey);
// issue #577 && #542
if (mappedStatement.isResultOrdered()) {
if (partialObject == null && rowValue != null) {
nestedResultObjects.clear();
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
} else {
rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
if (partialObject == null) {
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
}
if (rowValue != null && mappedStatement.isResultOrdered() && shouldProcessMoreRows(resultContext, rowBounds)) {
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
previousRowValue = null;
} else if (rowValue != null) {
previousRowValue = rowValue;
}
}
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, CacheKey combinedKey, String columnPrefix, Object partialObject) throws SQLException {
final String resultMapId = resultMap.getId();
Object rowValue = partialObject;
if (rowValue != null) {
final MetaObject metaObject = configuration.newMetaObject(rowValue);
putAncestor(rowValue, resultMapId);
applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
ancestorObjects.remove(resultMapId);
} else {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
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, true)) {
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
putAncestor(rowValue, resultMapId);
// 解析NestedResultMappings并封装结果，赋值给源对象的关联查询属性上
foundValues = applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
ancestorObjects.remove(resultMapId);
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
if (combinedKey != CacheKey.NULL_CACHE_KEY) {
nestedResultObjects.put(combinedKey, rowValue);
}
}
return rowValue;
}
private boolean applyNestedResultMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String parentPrefix, CacheKey parentRowKey, boolean newObject) {
boolean foundValues = false;
for (ResultMapping resultMapping : resultMap.getPropertyResultMappings()) {
final String nestedResultMapId = resultMapping.getNestedResultMapId();
if (nestedResultMapId != null && resultMapping.getResultSet() == null) {
try {
final String columnPrefix = getColumnPrefix(parentPrefix, resultMapping);
final ResultMap nestedResultMap = getNestedResultMap(rsw.getResultSet(), nestedResultMapId, columnPrefix);
if (resultMapping.getColumnPrefix() == null) {
// try to fill circular reference only when columnPrefix
// is not specified for the nested result map (issue #215)
Object ancestorObject = ancestorObjects.get(nestedResultMapId);
if (ancestorObject != null) {
if (newObject) {
linkObjects(metaObject, resultMapping, ancestorObject); // issue #385
}
continue;
}
}
final CacheKey rowKey = createRowKey(nestedResultMap, rsw, columnPrefix);
final CacheKey combinedKey = combineKeys(rowKey, parentRowKey);
Object rowValue = nestedResultObjects.get(combinedKey);
boolean knownValue = rowValue != null;
instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject); // mandatory
if (anyNotNullColumnHasValue(resultMapping, columnPrefix, rsw)) {
rowValue = getRowValue(rsw, nestedResultMap, combinedKey, columnPrefix, rowValue);
if (rowValue != null && !knownValue) {
linkObjects(metaObject, resultMapping, rowValue);
foundValues = true;
}
}
} catch (SQLException e) {
throw new ExecutorException("Error getting nested result map values for '" + resultMapping.getProperty() + "'. Cause: " + e, e);
}
}
}
return foundValues;
}
}