4、Mybatis的SQL执行原理

 

 

1、Mybatis整体框架图

4、Mybatis的SQL执行原理

上图为Mybatis的框架图,本文重点说明数据处理层中的参数映射、SQL解析、SQL执行、结果映射。

2、配置使用

2.1、pom.xml

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.0</version>
</dependency>

2.2、获取mapper并操作数据库代码如下

InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");  
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().
    build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
LiveCourseMapper mapper = sqlSession.getMapper(LiveCourseMapper.class);
List<LiveCourse> liveCourseList = mapper.getLiveCourseList();

3、配置文件加载

配置文件加载最终还是通过 ClassLoader.getResourceAsStream 来加载文件,关键代码如下:

public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
        InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
        if (in == null) {
          throw new IOException("Could not find resource " + resource);
        }
        return in;
  }

  InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
      for (ClassLoader cl : classLoader) {
        if (null != cl) {

          // try to find the resource as passed
          InputStream returnValue = cl.getResourceAsStream(resource);

          // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
          if (null == returnValue) {
            returnValue = cl.getResourceAsStream("/" + resource);
          }

          if (null != returnValue) {
            return returnValue;
          }
        }
  }
  return null;
}

4、配置文件解析

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

我们以 SqlSessionFactoryBuilder 为入口,看下 MyBatis 是如何解析配置文件,并创建 SqlSessionFactory 的,SqlSessionFactoryBuilder.build 方法实现如下:

 XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
 //解析出configuration对象,并创建SqlSessionFactory
 return build(parser.parse());

重点为解析 configuration 对象,然后根据 configuration 创建 DefualtSqlSessionFactory。

4、解析configuration

 private void parseConfiguration(XNode root) {
    try {
      Properties settings = settingsAsPropertiess(root.evalNode("settings"));
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      loadCustomVfs(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectionFactoryElement(root.evalNode("reflectionFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

通过 XPathParser 解析 configuration 节点下的 properties,settings,typeAliases,plugins,objectFactory,objectWrapperFactory,reflectionFactory,environments,databaseIdProvider,typeHandlers,mappers 这些节点,解析过程大体相同,都是通过 XPathParser 解析相关属性、子节点,然后创建相关对象,并保存到 configuration 对象中。

4.1、解析properties

解析properties,并设置到configuration对象下的variables属性,protected Properties variables = new Properties();

4.2、解析settings

解析settings配置,如lazyLoadingEnabled(默认false),defaultExecutorType(默认SIMPLE),jdbcTypeForNull(默认OTHER),callSettersOnNulls(默认false)

4.3、解析typeAliases

通过typeAliasRegistry来注册别名,别名通过key,value的方式来进行存储,mybatis默认会创建一些基础类型的别名,如string->String.class,int->Integer.class,map->Map.class,hashmap->HashMap.class,list->List.class。别名和class关系通过HashMap来存储

private final Map<String, Class<?>> TYPE_ALIASES = new HashMap<String, Class<?>>();

4.4、解析plugins

解析插件,然后设置Configuration的InterceptorChain。

protected final InterceptorChain interceptorChain = new InterceptorChain();

InterceptorChain:

private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
}

在创建的时候构造了拦截器链,在执行的时候也会经过拦截器链,此处为典型的责任链模式

4.5、解析objectFactory

可以自定义ObjectFactory,对象工厂,默认为DefaultObjectFactory

4.6、解析objectWrapperFactory

默认为DefaultObjectWrapperFactory

4.7、reflectionFactory

反射工厂,在通过反射创建对象时(如结果集对象),可以通过自定义的反射工厂来创建对象。objectFactory,objectWrapperFactory,reflectionFactory这又是典型的工厂模式,将对象的创建交由相应的工厂来创建

4.8、databaseIdProvider

用来支持不同的数据库,很少在项目中用到

4.9、解析typeHandlers

解析TypeHandler并通过typeHandlerRegistry注册到configuration中,通过TYPEHANDLERMAP保存typeHandler:

private final Map<Type, Map<JdbcType, TypeHandler<?>>> TYPE_HANDLER_MAP = new HashMap<Type, Map<JdbcType, TypeHandler<?>>>();

4.10、解析mappers

读取通过url指定的配置文件,然后通过XmlMapperBuilder进行解析

4.11、解析mapper

解析mapper的入口为XmlMapperBuilder.parse方法,在解析的时候会解析cache-ref,cache,parameterMap,resultMap,sql,select|insert|update|delete。cache-ref,cache和缓存相关,parameterMap目前已很少使用,这里就不再说明了。

4.11.1、解析resultMap

入口方法为XmlMapperBuilder.resultMapElement,解析resultMap主要包含如下步骤:

(1)解析resultMap属性

解析id,type,autoMapping属性,type取值的优先级为 type -> ofType -> resultType -> javaType

String type = resultMapNode.getStringAttribute("type",
    resultMapNode.getStringAttribute("ofType",
        resultMapNode.getStringAttribute("resultType",
            resultMapNode.getStringAttribute("javaType"))));

(2)解析resultMap下的result子节点,创建ResultMapping对象

resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));

解析result节点的property,column,javaType,jdbcType,select,resultMap,notNullColumn,typeHandler,resultSet,foreignColumn,lazy属性。

此处需要注意的点为:解析select属性与resultMap属性,因为这块涉及嵌套查询与嵌套映射(后面在结果集映射时会讲下这块)。如果result节点中存在select属性则认为是嵌套查询,而嵌套映射的判断条件如下:

String nestedResultMap = context.getStringAttribute("resultMap",
processNestedResultMappings(context, Collections.<ResultMapping> emptyList()));

如果result节点存在resultMap则肯定是嵌套映射:

  private String processNestedResultMappings(XNode context, List<ResultMapping> resultMappings) throws Exception {
    if ("association".equals(context.getName())
        || "collection".equals(context.getName())
        || "case".equals(context.getName())) {
      if (context.getStringAttribute("select") == null) {
        ResultMap resultMap = resultMapElement(context, resultMappings);
        return resultMap.getId();
      }
    }
    return null;
  }

如果是association,collection,case这些节点,并且select属性为空的话,则认为是嵌套映射

(3)注册ResultMap

通过resultMapResolver.resolve()来解析resultMap属性,然后创建ResultMap对象,并保存到resultMaps 属性中。

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

4.11.2、解析SQL

sql解析相对简单,主要是解析sql节点,然后保存到sqlFragments。

4.11.3、 解析select|insert|update|delete

入口方法为XMLStatementBuilder.parseStatementNode,解析statementNode主要包含如下步骤:

(1)解析statementNode属性

属性主要包括id,parameterMap,parameterType,resultMap,resultType,statementType(默认为PREPARED,预处理的statement)

(2)解析include

将include替换为sql片段,然后移除include节点

(3)解析selectKey

Parse selectKey after includes and remove them.

(4)创建sqlSource

SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);

langDriver默认为XMLLanguageDriver,此处很重要,请允许我多列点代码:

public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
    XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType);
    return builder.parseScriptNode();
  }

XMLScriptBuilder.parseScriptNode:

 public SqlSource parseScriptNode() {
    List<SqlNode> contents = parseDynamicTags(context);
    MixedSqlNode rootSqlNode = new MixedSqlNode(contents);
    SqlSource sqlSource = null;
    if (isDynamic) {
      sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
    } else {
      sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
    }
    return sqlSource;
  }

解析动态节点:

 List<SqlNode> parseDynamicTags(XNode node) {
    List<SqlNode> contents = new ArrayList<SqlNode>();
    NodeList children = node.getNode().getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
      XNode child = node.newXNode(children.item(i));
      if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
        String data = child.getStringBody("");
        TextSqlNode textSqlNode = new TextSqlNode(data);
        //如果包含${}的话则认为是动态节点
        if (textSqlNode.isDynamic()) {
          contents.add(textSqlNode);
          isDynamic = true;
        } else {
          contents.add(new StaticTextSqlNode(data));
        }
      } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
        String nodeName = child.getNode().getNodeName();
        NodeHandler handler = nodeHandlers(nodeName);
        if (handler == null) {
          throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
        }
        handler.handleNode(child, contents);
        isDynamic = true;
      }
    }
    return contents;
  }

如果statement节点下存在子节点,如trim,if,where,那么statement肯定是动态节点;如果statement节点下不存在子节点,但是文本中包含${},那么也认为是动态节点。

创建SqlSource:如果包含动态节点创建DynamicSqlSource,否则创建RawSqlSource。

(5)创建MappedStatement并注册

根据解析出的属性创建MappedStatement对象,然后注册到configuration对象中:

protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");

5、SQL执行

在配置文件解析这一节,我们解析了configuration,mapper等节点,并创建了SqlSessionFactory,下面我们就来分析下SQL执行的过程。

(1)创建SqlSession

SqlSession sqlSession = sqlSessionFactory.openSession();

final Environment environment = configuration.getEnvironment();
  final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
  tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
  final Executor executor = configuration.newExecutor(tx, execType);
  return new DefaultSqlSession(configuration, executor, autoCommit);

因为没有和spring进行整合,事务为JdbcTransaction,executor为默认的SimpleExecutor,autoCommit为false

(2)创建mapper代理类

我们顺着DefaultSqlSession.getMapper方法来看下mybatis是如何创建mapper代理类的。

 public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

可以看到最终是会通过mapperProxyFactory来创建MapperProxy代理类,实现代码如下:

 public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
}

protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

通过jdk动态代理来创建最终的Proxy代理类,最终类结构如下图所示:

4、Mybatis的SQL执行原理

(3)调用mapper方法

MapperProxy.invoke方法实现如下:

   public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

如果执行的是Object类的方法,那么直接执行方法即可;其它方法的话通过MapperMethod来执行。实现如下:

  1. 如果是insert命令,则调用sqlSession.insert方法;
  2. 如果是update命令,则调用sqlSession.update方法;
  3. 如果是delete命令,则调用sqlSession.delete方法;
  4. 如果是select命令,相对insert,update,delete命令来说稍微复杂些,要区分方法的返回值,如果返回List集合的话则调用executeForMany,如果返回单个对象的话则调用selectOne,返回map的话则调用executeForMap

insert,update,delete,select命令它们实现原理都差不多,select只是比它们多了结果集映射这一步,我们就以select命令的executeForMany方法为例来说明sql的执行过程。

MapperMethod.executeMany会调用DefaultSqlSession.selectList,而selectList方法实现如下:

 //获取MappedStatement,在mapper解析的时候注册到configuration对象中的
MappedStatement ms = configuration.getMappedStatement(statement);
//默认为SimpleExecutor,sql的执行类
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);

Executor.query:

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //获取BoundSql,在此处处理if,where,choose动态节点,很重要
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

 5.1 getBoundSql

public class BoundSql {
  private String sql;
  private List<ParameterMapping> parameterMappings;
  private Object parameterObject;
  private Map<String, Object> additionalParameters;
  private MetaObject metaParameters;

BoundSql为最终执行的sql,为处理完动态节点后的sql。通过SqlSource来获取BoundSql,通过前面我们了解到存在两种SqlSource:DynamicSqlSource,RawSqlSource。

5.1.1 DynamicSqlSource.getBoundSql

public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    rootSqlNode.apply(context);
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
      boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
    }
    return boundSql;

在getBoundSql时主要包含如下几个步骤:

(1)SqlNode.apply

  public boolean apply(DynamicContext context) {
    for (SqlNode sqlNode : contents) {
      sqlNode.apply(context);
    }
    return true;
  }

在此处处理IfSqlNode,MixedSqlNode,ForEachSqlNode,TrimSqlNode这些动态节点

(2)sqlSourceParser.parse

  public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) {
ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters);
GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);
//将#{}替换为?,解析出ParameterMappings
String sql = parser.parse(originalSql);
return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
}解析SqlSource,将#{}替换为?,解析出ParameterMappings,最终生成静态的StaticSqlSource。

public String handleToken(String content) {
  parameterMappings.add(buildParameterMapping(content));
  return "?";
}

ParameterMapping主要包括property名称,jdbcType,javaType,typeHandler。如果未指定javaType的话默认取得是传递的参数对象中属性的类型。

StaticSqlSource.getBoundSql最终返回结果如下:

public BoundSql getBoundSql(Object parameterObject) {
  return new BoundSql(configuration, sql, parameterMappings, parameterObject);
}

5.1.2 RawSqlSource.getBoundSql

RawSqlSource相比DynamicSqlSource就简单多了,在创建RawSqlSource时直接就将sql解析了,getBoundSql时直接创建BoundSql返回即可:

 public RawSqlSource(Configuration configuration, SqlNode rootSqlNode, Class<?> parameterType) {
    this(configuration, getSql(configuration, rootSqlNode), parameterType);
  }
  public RawSqlSource(Configuration configuration, String sql, Class<?> parameterType) {
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> clazz = parameterType == null ? Object.class : parameterType;
    sqlSource = sqlSourceParser.parse(sql, clazz, new HashMap<String, Object>());
  }
  private static String getSql(Configuration configuration, SqlNode rootSqlNode) {
    DynamicContext context = new DynamicContext(configuration, null);
    rootSqlNode.apply(context);
    return context.getSql();
  }
  public BoundSql getBoundSql(Object parameterObject) {
    return sqlSource.getBoundSql(parameterObject);
  }

5.2 query

在上面的小节中生成了最终的sql,下面就可以执行sql了。我们以SimpleExecutor为例来看下sql的执行过程:

  Configuration configuration = ms.getConfiguration();
  //创建StatementHandler,默认为PreparedStatementHandler
  StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
  stmt = prepareStatement(handler, ms.getStatementLog());
  return handler.<E>query(stmt, resultHandler);

(1)prepareStatement

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    //设置fetchSize,timeout
    stmt = handler.prepare(connection, transaction.getTimeout());
    //statement.setParameter sql实际执行参数设置
    handler.parameterize(stmt);
    return stmt;
  }


public void parameterize(Statement statement) throws SQLException {
   parameterHandler.setParameters((PreparedStatement) statement);
}

最终通过typeHandler.setParameter(ps, i + 1, value, jdbcType);来设置参数

(2)query

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  ps.execute();  //sql执行
  return resultSetHandler.<E> handleResultSets(ps); //处理结果集
}

处理结果集也块相对也比较重要,我们单独来讲下。

6、结果集映射

方法入口为DefaultResultSetHandler.handleResultSets,关键代码如下:

  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);
    }
  }

在处理结果集行值时分为两部分,处理简单resultMap对应的行值和处理嵌套resultMap对应的行值,是否嵌套映射在解析mapper resultMap的时候已经解释过了,这里不再重复。处理简单resultMap对应的行值稍微简单些,我们先从简单的看起吧。

6.1 简单映射

     private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
    DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
    //处理分页,跳过指定的行,如果rs类型不是TYPE_FORWARD_ONLY,直接absolute,否则的话循环rs.next
    skipRows(rsw.getResultSet(), rowBounds);
    //循环处理结果集,获取下一行值
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
      //处理行值,重点分析
      Object rowValue = getRowValue(rsw, discriminatedResultMap);
      //保存对象,通过list保存生成的对象Object
      storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
  }

6.1.1、getRowValue

      private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
    if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      final MetaObject metaObject = configuration.newMetaObject(resultObject);
      boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
      if (shouldApplyAutomaticMappings(resultMap, false)) {
        foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
      }
      foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
      foundValues = lazyLoader.size() > 0 || foundValues;
      resultObject = foundValues ? resultObject : null;
      return resultObject;
    }
    return resultObject;
  }

获取行值主要包含如下3个步骤:

(1)createResultObject创建结果集对象

根据resultType,通过ObjectFactory.create来创建对象,其实现原理还是通过反射来创建对象。在创建对象时如果resultMap未配置constructor,通过默认构造方法来创建对象,否则通过有参的构造方法来创建对象

(2)自动映射属性

如果ResultMap配置了autoMapping="true",或者AutoMappingBehavior为PARTIAL会自动映射在resultSet查询列中存在但是未在resultMap中配置的列。

(3)人工映射属性

映射在resultMap中配置的列,主要包括两步:获取属性的值和设置属性的值。

//获取属性的值
Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);
//设置属性的值,通过反射来设置
metaObject.setValue(property, value);

获取属性的值:

     private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
      throws SQLException {
    //获取嵌套查询对应的属性值,最终还是通过Executor.query来获取属性值
    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 DEFERED;
    } else {
      final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();
      final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);
      //通过typeHandler来获取属性的值,如StringTypeHandler获取属性值:rs.getString(columnName)
      return typeHandler.getResult(rs, column);
    }
  }

6.2 嵌套映射

嵌套resultMap主要用来处理collection,association属性,并且select属性为空,如:

<resultMap id="liveCourseMap" type="com.jd.mybatis.entity.LiveCourse">
        <result column="id" property="id"></result>
        <result column="course_name" property="courseName"></result>
        <!-- 通过嵌套映射来获取关联属性的值 -->
        <collection property="users" ofType="com.jd.mybatis.entity.LiveCourseUser">
            <result column="uid" property="id"></result>
            <result column="user_name" property="userName"></result>
            <result column="id" property="liveCourseId"></result>
        </collection>
</resultMap>

处理嵌套映射主要包括如下几个步骤:

(1)skipRows(rsw.getResultSet(), rowBounds); 同简单映射

(2)createRowKey,根据resultMap下的列创建rowKey,很有用。在如上liveCourseMap配置中,mybatis将会根据id列和course_name列的值来创建rowKey,类似于类似于

 -1421739516:769980325:com.jd.mybatis.mapper.LiveCourseMapper.liveCourseMap:id:121:course_name:j2ee

(3)getRowValue

这块代码稍微有点绕,我通过例子来说明吧。

我的sql很简单,查询出课程和参加课程的用户,结果集如下:

4、Mybatis的SQL执行原理

  1. 处理第一行的值,创建LiveCourse对象(id=121,courseName=j2ee),同时创建User对象(id=1,userName=张三),并放到List中,然后设置LiveCourse的users属性

  2. 处理第二行的值,因为在第一行已经创建了LiveCourse对象,所以这一次不会再创建LiveCourse对象,根据rowKey来判断创没创建LiveCourse对象(创建完对象会保存)

  3. 创建User对象(id=2,userName=李四),然后放到List中

 大体过程如上所述,我们再来看下相应的源码:

//创建rowKey,根据rowKey判断对应创建没创建
final CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);
//创建的LiveCourse对象会保存到nestedResultObjects
Object partialObject = nestedResultObjects.get(rowKey);

 rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
 //只有是第一次创建LiveCourse时才会进行保存
 if (partialObject == null) {
     storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }

getRowValue:

Object resultObject = partialObject;
//如果已经创建LiveCoure对象
if (resultObject != null) {
  final MetaObject metaObject = configuration.newMetaObject(resultObject);
  putAncestor(resultObject, resultMapId, columnPrefix);
  //不用创建LiveCouse对象,直接处理嵌套映射即可
  applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
  ancestorObjects.remove(resultMapId);
} else {
  final ResultLoaderMap lazyLoader = new ResultLoaderMap();
  //创建LiveCoure对象,同简单映射
  resultObject = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
  if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
    final MetaObject metaObject = configuration.newMetaObject(resultObject);
    boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
    //自动映射,同简单映射
    if (shouldApplyAutomaticMappings(resultMap, true)) {
      foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
    }
    //人工映射,同简单映射
    foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
    putAncestor(resultObject, resultMapId, columnPrefix);
    //处理嵌套映射
    foundValues = applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
    ancestorObjects.remove(resultMapId);
    foundValues = lazyLoader.size() > 0 || foundValues;
    resultObject = foundValues ? resultObject : null;
  }
  if (combinedKey != CacheKey.NULL_CACHE_KEY) {
    nestedResultObjects.put(combinedKey, resultObject);
  }
}
在处理嵌套映射属性时,主要是创建对象,设置属性值,然后添加到外层对象的colletion属性中。

      private void linkObjects(MetaObject metaObject, ResultMapping resultMapping, Object rowValue) {
    final Object collectionProperty = instantiateCollectionPropertyIfAppropriate(resultMapping, metaObject);
    //如果外层对象已经有集合属性值时,直接将创建的对象添加到集合中
    if (collectionProperty != null) {
      final MetaObject targetMetaObject = configuration.newMetaObject(collectionProperty);
      targetMetaObject.add(rowValue);
    } else {
      //创建集合,然后设置属性值
      metaObject.setValue(resultMapping.getProperty(), rowValue);
    }
  }

(4)storeObject

保存创建的对象,同简单映射。

 7、Mybatis中的设计模式

(1)工厂模式

SqlSessionFactory,MapperProxyFactory

(2)动态代理

MapperProxy

(3)组合模式

在解析SqlNode时使用了组合模式,MixSqlNode相当于树枝节点,具体的SqlNode,像IfSqlNode,WhereSqlNode为叶子节点

(4)模板方法

Executor,BaseExecutor封装了update,select的骨架,具体的实现交由不同的子类来实现,如SimpleExecutor,ResumeExecutor,BatchExecutor

(5)建造者模式

将一个复杂对象的构建与它的表示分离,使得同样的构建过程可以创建不同的表示,建造者模式又可以称为生成器模式。XMLConfigBuilder,XMLMapperBuilder,XMLStatementBuilder

(6)责任链模式

plugins插件处理

上面这几个设计模式只是mybatis中比较常见的设计模式

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