Mybatis深入剖析

lanxi256

　　Statement在具体orm的过程总，主要靠这三个类进行参数装配，结果装配和相应的类型转换

• parameterHandler
  
• resultSetHandler
  
• typeHandlerRegistry
  

  protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
this.boundSql = mappedStatement.getBoundSql(parameterObject);
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}

typeHandlerRegistry


先介绍Typehandler,因为它是ParameterHandler和ResultsetHandler中，类型转化的工具，概括而说，typehandler就是把Java类型数据和JDBC类型的数据相互转换，mybatis给我们提供了一些常规类型的handler，我们可以自己实现一些特殊类型的handler



 
　　parameterHandler
　　    只有一个实现类，DefaultParameterHandler，它负责把代码传入的参数，装配到preparestatement中去，根据下面的代码可以看出，根据mapper中配置的解析出的parameterMapping规则从代码传入parameterObject的对象出反射取出属性值，在根据TypeHandler做java到jdbc类型的数据格式转换，最终装配到jdbc的statement中去

  MetaObject metaObject = parameterObject == null ? null : configuration.newMetaObject(parameterObject);
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
PropertyTokenizer prop = new PropertyTokenizer(propertyName);
if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else if (boundSql.hasAdditionalParameter(propertyName)) {
value = boundSql.getAdditionalParameter(propertyName);
} else if (propertyName.startsWith(ForEachSqlNode.ITEM_PREFIX)
&& boundSql.hasAdditionalParameter(prop.getName())) {
value = boundSql.getAdditionalParameter(prop.getName());
if (value != null) {
value = configuration.newMetaObject(value).getValue(propertyName.substring(prop.getName().length()));
}
} else {
value = metaObject == null ? null : metaObject.getValue(propertyName);
}
TypeHandler typeHandler = parameterMapping.getTypeHandler();
if (typeHandler == null) {
throw new ExecutorException("There was no TypeHandler found for parameter " + propertyName + " of statement " + mappedStatement.getId());
}
typeHandler.setParameter(ps, i + 1, value, parameterMapping.getJdbcType());
}
　　 
　　resultSetHandler
　　有两个实现，一般结果集处理由FastResultSetHandler完成，嵌套符合结果集的处理由NestedResultSetHandler完成，看下面的resultset的配置就能明白是啥意思了

ResultSetHandler resultSetHandler = mappedStatement.hasNestedResultMaps() ? new NestedResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql,
rowBounds) : new FastResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);

<resultMap id="processDefinitionResultMap"  
type="com.apusic.bpm.engine.definition.ProcessDefinition">  
<constructor>  
<idArg column="ID_" jdbcType="VARCHAR" javaType="java.lang.String" />  
<arg column="KEY_" jdbcType="VARCHAR" javaType="java.lang.String" />  
<arg column="NAME_" jdbcType="VARCHAR" javaType="java.lang.String" />  
<arg column="VERSION_" jdbcType="INTEGER" javaType="java.lang.Long" />  
<arg column="DESCRIPTION_" jdbcType="VARCHAR" javaType="java.lang.String" />  
<arg javaType="com.apusic.bpm.engine.definition.ProcessDefinitionResourceMapping" resultMap="resourceMappingResultMap" />  
</constructor>  
</resultMap>  
<resultMap id="resourceMappingResultMap"  
type="com.apusic.bpm.engine.definition.ProcessDefinitionResourceMapping">  
<constructor>  
<arg column="PROCESS_DEFINITION_PATH_" jdbcType="VARCHAR" javaType="java.lang.String"/>  
<arg column="RESOURCE_PATHS_" jdbcType="VARCHAR" javaType="java.util.Set" />  
</constructor>  
</resultMap>  
　　 FastResultSetHandler的核心代码如下，从jdbc resultset结果集中去除结果，根据TypeHandler将jdbc类型的数据转换成java类型的数据，塞回到结果集java对象中

public List handleResultSets(Statement stmt) throws SQLException {
final List multipleResults = new ArrayList();
final List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
int resultSetCount = 0;
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;
}
}
}
validateResultMapsCount(rs, resultMapCount);
while (rs != null && resultMapCount > resultSetCount) {
final ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rs, resultMap, multipleResults);
rs = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
return collapseSingleResultList(multipleResults);
}