Mybatis源码分析之--浅析ResultSetHandler

　　本文浅析ResultSetHandler，说道浅析就是ResultSetHandler本身其实很复杂的，甚至可以说是mybatis四大组件最复杂的，因为resultMap本身还可以嵌套assosiation，collection的标签，一个mapper空间还可以有多个resultMap彼此之间还能嵌套。本文只讨论最一般的情况，就是一个ResultMap，或者一个ResultType的情况。

一 知识铺垫

　　在Mybatis解析配置文件时，会按照id进行缓存保存在 Configuration里

　　 protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection"); 

  <resultMap id="BaseResultMap" type="me.gacl.domain.User" >
    <id column="user_id" property="userId" jdbcType="CHAR" />
    <result column="user_name" property="userName" jdbcType="VARCHAR" />
    <result column="user_birthday" property="userBirthday" jdbcType="VARCHAR" />
    <result column="user_salary" property="userSalary" jdbcType="DOUBLE" />
  </resultMap>

　　一个ResultMap大概张这个样子

　　 org.apache.ibatis.mapping.ResultMap 

　　

public class ResultMap {
  private String id; //me.gacl.dao.UserMapper.BaseResultMap
  private Class<?> type;
  private List<ResultMapping> resultMappings;
  //[ResultMapping{property='userId', column='user_id', javaType=class java.lang.String, jdbcType=CHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[ID], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userName', column='user_name', javaType=class java.lang.String, jdbcType=VARCHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userBirthday', column='user_birthday', javaType=class java.lang.String, jdbcType=VARCHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userSalary', column='user_salary', javaType=class java.lang.Double, jdbcType=DOUBLE, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}]
  private List<ResultMapping> idResultMappings;
  private List<ResultMapping> constructorResultMappings;
  private List<ResultMapping> propertyResultMappings;
　//[ResultMapping{property='userId', column='user_id', javaType=class java.lang.String, jdbcType=CHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[ID], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userName', column='user_name', javaType=class java.lang.String, jdbcType=VARCHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userBirthday', column='user_birthday', javaType=class java.lang.String, jdbcType=VARCHAR, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}, ResultMapping{property='userSalary', column='user_salary', javaType=class java.lang.Double, jdbcType=DOUBLE, nestedResultMapId='null', nestedQueryId='null', notNullColumns=[], columnPrefix='null', flags=[], composites=[], resultSet='null', foreignColumn='null', lazy=false}]
  private Set<String> mappedColumns;//[USER_SALARY, USER_ID, USER_NAME, USER_BIRTHDAY]
  private Discriminator discriminator;//null
  private boolean hasNestedResultMaps;
  private boolean hasNestedQueries;
  private Boolean autoMapping;

二 代码入口

　　一个ResultSetHandler是随着一个StatementHandler的创建而创建的，这点其实好ParameterHandler是一样的

　　在StatementHandler执行之后对结果进行处理

　　 PreparedStatementHandler.query 

@Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }

　　整个Mybatis对接口 ResultSetHandler的实现类只有一个  DefaultResultSetHandler 

三 关键代码分析

 @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;
    ResultSetWrapper rsw = getFirstResultSet(stmt);

    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
　　//我们不分析resultMap嵌套的情况
    String[] resultSets = mappedStatement.getResulSets();
    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 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());
    }

private 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<Object>();
    skipRows(rsw.getResultSet(), rowBounds);
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      Object rowValue = getRowValue(rsw, discriminatedResultMap);//经过这一步就是POJO对象了
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }

private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
      throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
    final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
    if (typeHandlerRegistry.hasTypeHandler(resultType)) {
      return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
    } else if (!constructorMappings.isEmpty()) {
      return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
    } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
      return objectFactory.create(resultType);//走这个分支里，构造一个对象出来
    } else if (shouldApplyAutomaticMappings(resultMap, false)) {
      return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix);
    }
    throw new ExecutorException("Do not know how to create an instance of " + resultType);
  }

　　最终会调用    DefaultResultSetHandler.applyPropertyMappings 

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;
    final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
    for (ResultMapping propertyMapping : propertyMappings) {
      String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);// 表中的类名 user_id
      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) {
        Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);//这里就是jdbc的接口
        // issue #541 make property optional
        final String property = propertyMapping.getProperty();//POJO里的属性 这里是 userId
        // issue #377, call setter on nulls
        if (value != DEFERED
            && property != null
            && (value != null || (configuration.isCallSettersOnNulls() && !metaObject.getSetterType(property).isPrimitive()))) {
          metaObject.setValue(property, value);//通过反射对POJO赋值
        }
        if (value != null || value == DEFERED) {
          foundValues = true;
        }
      }
    }
    return foundValues;
  }

getPropertyMappingValue方法中调用的方法就是下面的原理

StringTypeHandler.getNullableResult    
public String getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    return rs.getString(columnName);
  }
  

======================2021-12-05===========================

　　我们再看 metaObject.setValue(property, value);//通过反射对POJO赋值 原理

　　MetaObject#setValue

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);//简单对象只需要看这个
    }
  }

　　调用的是BeanWrapper#set方法

private void setBeanProperty(PropertyTokenizer prop, Object object, Object value) {
    try {
      Invoker method = metaClass.getSetInvoker(prop.getName());
      Object[] params = {value};
      try {
        method.invoke(object, params);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }

　　

可以看得出来，有set方法就调用set方法，没有就调用Field的方式，Field也是java反射的一种。

四 总结

　　1 如果用户用的不是ResultMap，而是ResultType怎么办？

public class ResultMap {
  private String id; //me.gacl.dao.UserMapper.selectName-Inline
  private Class<?> type; //class java.lang.String
  private List<ResultMapping> resultMappings; //[]都是空的

  private List<ResultMapping> idResultMappings;
  private List<ResultMapping> constructorResultMappings;
  private List<ResultMapping> propertyResultMappings;//[]
  private Set<String> mappedColumns;//[]
  private Discriminator discriminator;//null
  private boolean hasNestedResultMaps;
  private boolean hasNestedQueries;
  private Boolean autoMapping;

　　当使用resultType时，Mybatis在解析xml的时候还是构造成ResultMap只不过属性和真实的ResultMap是不一样的

　　

private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
      throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
    final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
    if (typeHandlerRegistry.hasTypeHandler(resultType)) {
      return createPrimitiveResultObject(rsw, resultMap, columnPrefix);//如果是ResultType走这个分支
    } else if (!constructorMappings.isEmpty()) {
      return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
    } else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
      return objectFactory.create(resultType);
    } else if (shouldApplyAutomaticMappings(resultMap, false)) {
      return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs, columnPrefix);
    }
    throw new ExecutorException("Do not know how to create an instance of " + resultType);
  }

  private Object createPrimitiveResultObject(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
    final Class<?> resultType = resultMap.getType();
    final String columnName;
    if (!resultMap.getResultMappings().isEmpty()) {
      final List<ResultMapping> resultMappingList = resultMap.getResultMappings();
      final ResultMapping mapping = resultMappingList.get(0);
      columnName = prependPrefix(mapping.getColumn(), columnPrefix);
    } else {
      columnName = rsw.getColumnNames().get(0);//user_name
    }
    final TypeHandler<?> typeHandler = rsw.getTypeHandler(resultType, columnName);
    return typeHandler.getResult(rsw.getResultSet(), columnName);//还是jdbc的东西
  }

　　2 返回值是List 还是单个值 是怎么判断的

　　首先要说的是ResultSetHandler的返回值一定是一个List

　　解析xml并不能确定是返回list还是一个值，而是靠业务写的mapper接口也就是dao接口，这样在MapperMethod执行的时候是能够判断返回值类型是单值还是List，如果是单值则调用selectOne

if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }