MyBatis工作原理源码分析（一）——SqlSessionFactory的初始化

阳光穿透心脏的1/2处 2022-03-19 03:46 255阅读 0赞

## 一、引言 ##

在Spring项目配置中，配置Mybatis时会有如下配置，这样，Spring容器会自动将SqlSessionFactory注入到容器中。

但手动使用Mybatis时，通常有如下四个步骤：

获取SqlSessionFactory对象;
    获取sqlSession对象;
    获取接口的代理对象（MapperProxy）;
    执行增删改查sql方法;

对应的代码如下：

@Test
    public void test() throws IOException{ 
     	   String resource = "mybatis-config.xml";
           InputStream inputStream = Resources.getResourceAsStream(resource);
           
           // 1、获取sqlSessionFactory对象
           SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
           // 2、获取sqlSession对象
           SqlSession openSession = sqlSessionFactory.openSession();
           try { 
               // 3、获取接口的实现类对象，会为接口自动的创建一个代理对象，代理对象去执行增删改查方法
               DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
               // 4、执行增删改查方法
               Department dept = mapper.getDeptById(1);
               System.out.println(dept);
           } finally { 
               openSession.close();
           }
        }

## 二、源码分析 ##

下面来分析第一步，SqlSessionFactory 是如何创建的？

1、首先，SqlSessionFactoryBuilder会去读取mybatis的配置文件，然后build一个DefaultSqlSessionFactory。源码如下：

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { 
    
        try { 
          //创建XMLConfigBuilder解析器，用于解析mybatis的配置文件
          XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
          
          //第一步：调用解析器的parse()方法进行mybatis配置文件的解析，返回一个Configuration！！！！
          //第二步：再将Configuration参数用build()方法创建一个DefaultSqlSessionFactory对象
          return build(parser.parse());
          
        } catch (Exception e) { 
          throw ExceptionFactory.wrapException("Error building SqlSession.", e);
        } finally { 
          ErrorContext.instance().reset();
          try { 
            inputStream.close();
          } catch (IOException e) { 
          }
        }
      }
     
    //用解析完成后返回的Configuration，创建一个DefaultSqlSessionFactory对象
    public SqlSessionFactory build(Configuration config) { 
       return new DefaultSqlSessionFactory(config);
     }

2、XMLConfigBuilder解析器的parse()解析方法，返回Configuration

public Configuration parse() { 
        if (parsed) { 
          throw new BuilderException("Each XMLConfigBuilder can only be used once.");
        }
        parsed = true;
        
        //parser.evalNode(“/configuration”) 用XPathParser类的evalNode方法，获取配置文件的根节点<configuration>
        //返回的是XNode对象，XNode对象保存了解析后的对应节点下的所有信息。
        parseConfiguration(parser.evalNode("/configuration"));
        return configuration;
      }

3、解析每一个标签，并把详细信息保存在Configuration对象中

private void parseConfiguration(XNode root) { 
        try { 
    
          // 解析properties节点并设置到Configuration
          propertiesElement(root.evalNode("properties"));
          
          //获取了<configuration>下的<settings>节点
          Properties settings = settingsAsProperties(root.evalNode("settings"));
          loadCustomVfs(settings);
          
          //解析typeAliases节点并设置到Configuration
          typeAliasesElement(root.evalNode("typeAliases"));
          //解析plugin节点并设置到Configuration
          pluginElement(root.evalNode("plugins"));
          //解析objectFactory节点并设置到Configuration
          objectFactoryElement(root.evalNode("objectFactory"));
          //解析objectWrapperFactory节点并设置到Configuration 
          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
          reflectorFactoryElement(root.evalNode("reflectorFactory"));
          //解析settings节点并设置到Configuration 
          settingsElement(settings);
          //解析environments节点并设置到Configuration 即Environments对象
          environmentsElement(root.evalNode("environments"));
          //解析databaseIdProvider节点并设置到Configuration
          databaseIdProviderElement(root.evalNode("databaseIdProvider"));
          //解析typeHandlers节点并设置到Configuration 
          typeHandlerElement(root.evalNode("typeHandlers"));
          //解析mappers节点并设置到Configuration
          mapperElement(root.evalNode("mappers"));
        } catch (Exception e) { 
          throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
        }
      }

4、解析配置文件中节点（如下）下的mappers节点并设置到Configuration

private void mapperElement(XNode parent) throws Exception { 
        if (parent != null) { 
          for (XNode child : parent.getChildren()) { 
            if ("package".equals(child.getName())) { 
              String mapperPackage = child.getStringAttribute("name");
              configuration.addMappers(mapperPackage);
            } else { 
           	  //获取mapper标签resource属性
              String resource = child.getStringAttribute("resource");
              //获取mapper标签url属性
              String url = child.getStringAttribute("url");
              //获取mapper标签class属性
              String mapperClass = child.getStringAttribute("class");
              
              if (resource != null && url == null && mapperClass == null) { 
              
                ErrorContext.instance().resource(resource);
                InputStream inputStream = Resources.getResourceAsStream(resource);
                //用XMLMapperBuilder 解析器，解析mapper文件
                XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
                mapperParser.parse();
                
              } else if (resource == null && url != null && mapperClass == null) { 
              
                ErrorContext.instance().resource(url);
                InputStream inputStream = Resources.getUrlAsStream(url);
                XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
                mapperParser.parse();
                
              } else if (resource == null && url == null && mapperClass != null) { 
              
                Class<?> mapperInterface = Resources.classForName(mapperClass);
                configuration.addMapper(mapperInterface);
                
              } else { 
                throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
              }
            }
          }
        }
      }
    
    //XMLMapperBuilder解析器中的parse()方法用来解析mapper文件
    public void parse() { 
        if (!configuration.isResourceLoaded(resource)) { 
          //解析mapper.xml文件中的内容
          configurationElement(parser.evalNode("/mapper"));
          configuration.addLoadedResource(resource);
          bindMapperForNamespace();
        }
    
        parsePendingResultMaps();
        parsePendingCacheRefs();
        parsePendingStatements();
      }
    
    //解析mapper.xml文件中的内容，主要是增删改查标签的解析
    private void configurationElement(XNode context) { 
        try { 
          //mapper文件中的命名空间属性
          String namespace = context.getStringAttribute("namespace");
          if (namespace == null || namespace.equals("")) { 
            throw new BuilderException("Mapper's namespace cannot be empty");
          }
          builderAssistant.setCurrentNamespace(namespace);
          //缓存
          cacheRefElement(context.evalNode("cache-ref"));
          cacheElement(context.evalNode("cache"));
          parameterMapElement(context.evalNodes("/mapper/parameterMap"));
          //自定义结果集
          resultMapElements(context.evalNodes("/mapper/resultMap"));
          //可重用的sql
          sqlElement(context.evalNodes("/mapper/sql"));
          
          //解析mapper.xml文件中的增删改查标签！！！！！！
          buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
        } catch (Exception e) { 
          throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
        }
      }
    
      //解析mapper.xml文件中的增删改查标签
      private void buildStatementFromContext(List<XNode> list) { 
        if (configuration.getDatabaseId() != null) { 
          buildStatementFromContext(list, configuration.getDatabaseId());
        }
        buildStatementFromContext(list, null);
      }
    
    private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { 
      for (XNode context : list) { 
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try { 
        	//解析增删改查标签的解析器
          statementParser.parseStatementNode();
        } catch (IncompleteElementException e) { 
          configuration.addIncompleteStatement(statementParser);
        }
      }
    }
    
    public void parseStatementNode() { 
    	//拿到每一个增删改查的id值
       String id = context.getStringAttribute("id");
       String databaseId = context.getStringAttribute("databaseId");
    
       if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { 
         return;
       }
    
       //标签能写的所有属性，都可以拿到
       Integer fetchSize = context.getIntAttribute("fetchSize");
       Integer timeout = context.getIntAttribute("timeout");
       String parameterMap = context.getStringAttribute("parameterMap");
       String parameterType = context.getStringAttribute("parameterType");
       Class<?> parameterTypeClass = resolveClass(parameterType);
       String resultMap = context.getStringAttribute("resultMap");
       String resultType = context.getStringAttribute("resultType");
       String lang = context.getStringAttribute("lang");
       LanguageDriver langDriver = getLanguageDriver(lang);
    
       Class<?> resultTypeClass = resolveClass(resultType);
       String resultSetType = context.getStringAttribute("resultSetType");
       StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
       ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    
       String nodeName = context.getNode().getNodeName();
       SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
       boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
       boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
       boolean useCache = context.getBooleanAttribute("useCache", isSelect);
       boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
    
       // Include Fragments before parsing
       XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
       includeParser.applyIncludes(context.getNode());
    
       // Parse selectKey after includes and remove them.
       processSelectKeyNodes(id, parameterTypeClass, langDriver);
       
       // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
       SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
       String resultSets = context.getStringAttribute("resultSets");
       String keyProperty = context.getStringAttribute("keyProperty");
       String keyColumn = context.getStringAttribute("keyColumn");
       KeyGenerator keyGenerator;
       String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
       keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
       if (configuration.hasKeyGenerator(keyStatementId)) { 
         keyGenerator = configuration.getKeyGenerator(keyStatementId);
       } else { 
         keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
             configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
             ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
       }
    
    	//将标签中的所有信息拿出来，调用addMappedStatement()方法，该方法会返回一个MappedStatement对象。
        builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
            fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
            resultSetTypeEnum, flushCache, useCache, resultOrdered, 
            keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
      }

此处可以知道，一个MappedStatement对象，就代表一个增删改查标签的详细信息。然后将MappedStatement存放到Configuration中

public MappedStatement addMappedStatement(
        String id,
        SqlSource sqlSource,
        StatementType statementType,
        SqlCommandType sqlCommandType,
        Integer fetchSize,
        Integer timeout,
        String parameterMap,
        Class<?> parameterType,
        String resultMap,
        Class<?> resultType,
        ResultSetType resultSetType,
        boolean flushCache,
        boolean useCache,
        boolean resultOrdered,
        KeyGenerator keyGenerator,
        String keyProperty,
        String keyColumn,
        String databaseId,
        LanguageDriver lang,
        String resultSets) { 
    
      if (unresolvedCacheRef) { 
        throw new IncompleteElementException("Cache-ref not yet resolved");
      }
    
      id = applyCurrentNamespace(id, false);
      boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    
      MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
          .resource(resource)
          .fetchSize(fetchSize)
          .timeout(timeout)
          .statementType(statementType)
          .keyGenerator(keyGenerator)
          .keyProperty(keyProperty)
          .keyColumn(keyColumn)
          .databaseId(databaseId)
          .lang(lang)
          .resultOrdered(resultOrdered)
          .resultSets(resultSets)
          .resultMaps(getStatementResultMaps(resultMap, resultType, id))
          .resultSetType(resultSetType)
          .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
          .useCache(valueOrDefault(useCache, isSelect))
          .cache(currentCache);
    
      ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
      if (statementParameterMap != null) { 
        statementBuilder.parameterMap(statementParameterMap);
      }
    
      MappedStatement statement = statementBuilder.build();
      //将MappedStatement存放到Configuration中
      configuration.addMappedStatement(statement);
      return statement;
    }

## 三、小结 ##

上面就是根据配置文件创建SQLSessionFactory的详细过程，下面是对应的时序图：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70]  
  
总结：  
（1）、一个MappedStatement代表一个增删改查的详细信息  
（2）、把Mybatis配置文件的信息解析并保存在Configuration对象中，返回包含了Configuration的DefaultSqlSession对象；  
（3）、Configuration封装了所有配置文件的详细信息，如下图所示  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70 1]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70 2]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70 1]: /images/20220319/f3ad76ec817c4345baffbf5cff35c99e.png
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