mybatis启动流程

作者: 93张先生 | 来源:发表于2020-09-17 10:14 被阅读0次
    SqlSessionFactoryBuilder

    SqlSessionFactoryBuilder使用了Builder模式用来创建SqlSessionFactory

    创建 SqlSessionFactory 对象两种方式:

    1. 使用 Configuration 对象(mybaits全局配置对象),创建一个默认的DefaultSqlSessionFactory对象;
    2. 通过配置文件*.xml文件流、environmentproperties 等解析为Configuration对象,然后通过Configuration对象,创建默认的DefaultSqlSessionFactory对象;
      public SqlSessionFactory build(InputStream inputStream, Properties properties) {
        return build(inputStream, null, properties);
      }
    
      public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        try {
          XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
          return build(parser.parse());
        } catch (Exception e) {
          throw ExceptionFactory.wrapException("Error building SqlSession.", e);
        } finally {
          ErrorContext.instance().reset();
          try {
            reader.close();
          } catch (IOException e) {
            // Intentionally ignore. Prefer previous error.
          }
        }
      }
    public SqlSessionFactory build(Configuration config) {
        return new DefaultSqlSessionFactory(config);
    }
    
    SqlSessionManager
    • SqlSessionManager 实现了SqlSessionFactorySqlSession 接口,重写了这两个接口的所有方法;
    public class SqlSessionManager implements SqlSessionFactory, SqlSession 
    
    image.png
    • 包含两个成员变量 sqlSessionFactorysqlSessionProxy,通过newInstance()方法调用构造方法进行赋值;
    public static SqlSessionManager newInstance(Reader reader) {
        return new SqlSessionManager(new SqlSessionFactoryBuilder().build(reader, null, null));
    }
    private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
        this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(
            SqlSessionFactory.class.getClassLoader(),
            new Class[]{SqlSession.class},
            new SqlSessionInterceptor());
    }
    
    • sqlSessionProxy 为一个代理对象,通过 Java 反射技术生成代理对象。
    private class SqlSessionInterceptor implements InvocationHandler {
        public SqlSessionInterceptor() {
            // Prevent Synthetic Access
        }
    
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
          final SqlSession sqlSession = SqlSessionManager.this.localSqlSession.get();
          if (sqlSession != null) {
            try {
              return method.invoke(sqlSession, args);
            } catch (Throwable t) {
              throw ExceptionUtil.unwrapThrowable(t);
            }
          } else {
            try (SqlSession autoSqlSession = openSession()) {
              try {
                final Object result = method.invoke(autoSqlSession, args);
                autoSqlSession.commit();
                return result;
              } catch (Throwable t) {
                autoSqlSession.rollback();
                throw ExceptionUtil.unwrapThrowable(t);
              }
            }
          }
        }
    }
    
    • SqlSeesion 接口定义了 close()commit()rollback()clearCache() 等方法,为了操作同一个SqlSeesion对象,引入了 ThreadLocal 变量,通过localSqlSession确保操作同一个SqlSession,保证了线程安全。
    private final ThreadLocal<SqlSession> localSqlSession = new ThreadLocal<>();
    //通过此方法,对localSqlSession赋值
    public void startManagedSession() {
        this.localSqlSession.set(openSession());
    }
    
    BaseBuilder

    BaseBuilder 是一个基础抽象类,定义了一些通用方法,有多个具体实现类;成员变量包括:解决数据类型别名的注册器 typeAliasRegistry,及处理 JDBC 类型和 Java 数据类型对应的、类型处理转换器注册中心 typeHandlerRegistry,;还有一些简单的工具方法;

    包含3个成员变量:

    • Configuration mybatis 全局配置对象;
    • TypeAliasRegistry 类型别名注册中心,处理各种类型别名;
    • TypeHandlerRegistry 类型处理器注册中心,处理各种TypeHandler ;
    多个具体实现类图,介绍其中的XMLConfigBuilder
    image.png
    XMLConfigBuilder

    XMLConfigBuilder 是一个读取并解析 mybatis-config.xml 配置的文件的类;
    s
    成员变量:

      private boolean parsed;  // 标识是否已经解析 mybats-config.xml 配置丈件
      private final XPathParser parser;  //用于解析 mybatis config xml 配置文件的 XPathParser 对象
      private String environment;   //标识<environment>配置的名称,默认读取<environment> 标签的 default 属性
      //ReflectorFactory 负责创建和缓存 Reflector 对象
      private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();
    
    使用

    在 SqlSessionFactoryBuilder 中创建 XMLConfigBuilder 对象,并解析 mybatis-config.xml 文件,返回全局的 Configuration 对象;

    public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        try {
          // 解析配置文件
          XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
          // 构建 SqlSessionFactory 对象
          return build(parser.parse());
        } catch (Exception e) {
          throw ExceptionFactory.wrapException("Error building SqlSession.", e);
        } finally {
          ErrorContext.instance().reset();
          try {
            reader.close();
          } catch (IOException e) {
            // Intentionally ignore. Prefer previous error.
          }
        }
      }
    
    方法
    • parse() 解析 mybatis-config 文件,并返回全局 Configuration 对象
      // 解析 mybatis-config 文件,并返回全局 Configuration 对象
      public Configuration parse() {
        if (parsed) {
          throw new BuilderException("Each XMLConfigBuilder can only be used once.");
        }
        parsed = true;
        //在 mybatis-config xml 配置文件中查找<configuration>节点,并开始解析
        parseConfiguration(parser.evalNode("/configuration"));
        return configuration;
      }
    
    • parseConfiguration(XNode root) 解析 mybatis-config.xml 文件中的各个节点
      private void parseConfiguration(XNode root) {
        try {
          //issue #117 read properties first
          //<properties resource="org/apache/ibatis/databases/blog/blog-derby.properties"/>
          //解析<properties>节
          propertiesElement(root.evalNode("properties"));
          //  <settings>
          //    <setting name="cacheEnabled" value="true"/>
          //    <setting name="lazyLoadingEnabled" value="false"/>
          //  </settings>
          //解析<settings>节点
          Properties settings = settingsAsProperties(root.evalNode("settings"));
          // TODO: 2020/8/4  vfs 虚拟文件系统
          loadCustomVfs(settings);    // 设置 vfsimpl 字段
          loadCustomLogImpl(settings);
          //解析<typeAliases>节点
          typeAliasesElement(root.evalNode("typeAliases"));
          //解析<plugins>节
          pluginElement(root.evalNode("plugins"));
          //解析<objectFactory>节点
          objectFactoryElement(root.evalNode("objectFactory"));
          //解析 <objectWrapperFactory> 节点
          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
          //解析<reflectorFactory>节点
          reflectorFactoryElement(root.evalNode("reflectorFactory"));
          // 解析各种设置,在 mybatis 环境中生效
          settingsElement(settings);
          // read it after objectFactory and objectWrapperFactory issue #631
          //解析<environments>节点
          environmentsElement(root.evalNode("environments"));
          //解析<databaseidProvider>节点
          databaseIdProviderElement(root.evalNode("databaseIdProvider"));
          //解析<typeHandlers>节点
          typeHandlerElement(root.evalNode("typeHandlers"));
          //解析<mappers>节点
          mapperElement(root.evalNode("mappers"));
        } catch (Exception e) {
          throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
        }
      }
    
    • 解析中具体使用到的方法和注解说明
      // 检测 setting 属性是否是 Configuration 中指定的,包含 setter 方法, 就是指定的 Configuration 属性
      // 使用 MetaClass 检测 key 定的属性在 configuration 类中是否有对应 setter 方法的步骤。
      private Properties settingsAsProperties(XNode context) {
        if (context == null) {
          return new Properties();
        }
        Properties props = context.getChildrenAsProperties();
        // Check that all settings are known to the configuration class
        // 检查 setting 设置的参数,是否是指定的,并可被解析的
        // 创建 Configuration 类的元信息包装类,localReflectorFactory 为 DefaultReflectorFactory 新建的对象;
        // metaConfig 为短暂的方法生命周期对象,近在此方法中临时创建,并近存活于此方法中;
        // 检测 Configuration 是否定义了 key 指定属性相应的 setter 方法
        MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
        for (Object key : props.keySet()) {
          if (!metaConfig.hasSetter(String.valueOf(key))) {
            throw new BuilderException("The setting " + key + " is not known.  Make sure you spelled it correctly (case sensitive).");
          }
        }
        return props;
      }
    
      private void loadCustomVfs(Properties props) throws ClassNotFoundException {
        String value = props.getProperty("vfsImpl");
        if (value != null) {
          String[] clazzes = value.split(",");
          for (String clazz : clazzes) {
            if (!clazz.isEmpty()) {
              @SuppressWarnings("unchecked")
              Class<? extends VFS> vfsImpl = (Class<? extends VFS>)Resources.classForName(clazz);
              configuration.setVfsImpl(vfsImpl);
            }
          }
        }
      }
    
      private void loadCustomLogImpl(Properties props) {
        Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
        configuration.setLogImpl(logImpl);
      }
     /**
      * 解析类的别名
       <typeAliases>
        <typeAlias alias="BlogAuthor" type="org.apache.ibatis.domain.blog.Author"/>
        <typeAlias type="org.apache.ibatis.domain.blog.Blog"/>
        <typeAlias type="org.apache.ibatis.domain.blog.Post"/>
        <package name="org.apache.ibatis.domain.jpetstore"/>
      </typeAliases>
      */
      private void typeAliasesElement(XNode parent) {
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            // 如果包含 package,解析 package 下类,进行别名注册
            // 处理<package >节点
            if ("package".equals(child.getName())) {
              String typeAliasPackage = child.getStringAttribute("name");
              // 通过 TypeAliasRegistry 扫描指定包中所有的类,并解析@Alias 注解,完成别名注册
              configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
            } else {
              String alias = child.getStringAttribute("alias");  // 获取指定的别名
              String type = child.getStringAttribute("type");    // 获取别名对应的类型
              try {
                Class<?> clazz = Resources.classForName(type);
                if (alias == null) {
                  typeAliasRegistry.registerAlias(clazz); // 扫描 @Alias 完成注册 ,完成注册
                } else {
                  typeAliasRegistry.registerAlias(alias, clazz);
                }
              } catch (ClassNotFoundException e) {
                throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
              }
            }
          }
        }
      }
    
      // 以通过添加自定义插件在 SQL 语句执行过程中的某点进行拦截。 MyBatis 中的自定义插件只需实现 Interceptor 接口,并通过注解指定想
      // 要拦截的方法签名即可
    
      /**
       * <plugins>
       *   <plugin interceptor="org.apache.ibatis.builder.ExamplePlugin">
       *      <property name="pluginProperty" value="100"/>
       *   </plugin>
       *  </plugins>
       * @param parent
       * @throws Exception
       */
      private void pluginElement(XNode parent) throws Exception {
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            String interceptor = child.getStringAttribute("interceptor");
            Properties properties = child.getChildrenAsProperties();
            // 创建插件实例,插件是 Interceptor 的具体实现
            Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
            // 设置插件的属性
            interceptorInstance.setProperties(properties);
            // 设置插件到拦截器链中
            configuration.addInterceptor(interceptorInstance);
          }
        }
      }
    
      /**
       *  <objectFactory type="org.apache.ibatis.builder.ExampleObjectFactory">
       *    <property name="objectFactoryProperty" value="100"/>
       *  </objectFactory>
       * @param context
       * @throws Exception
       */
      // 解析 objectFactory 节点, objectFactory 创建对象的工厂
      // 可以通过添加自定义 ObjectFactory 实现类、ObjectWrapperFactory 实现类、ReflectorFactory 实现类对 Mybatis 进行扩展
      private void objectFactoryElement(XNode context) throws Exception {
        if (context != null) {
          String type = context.getStringAttribute("type");
          Properties properties = context.getChildrenAsProperties();
          ObjectFactory factory = (ObjectFactory) resolveClass(type).getDeclaredConstructor().newInstance(); // 创建 objectFactory 对象
          factory.setProperties(properties);  // 设置对象属性
          configuration.setObjectFactory(factory); // 反写 configuration 对象中的 objectFactory 属性
        }
      }
      /**
       * 解析 <objectWrapperFactory> 节点
       * @param context
       * @throws Exception
       */
      private void objectWrapperFactoryElement(XNode context) throws Exception {
        if (context != null) {
          String type = context.getStringAttribute("type");
          ObjectWrapperFactory factory = (ObjectWrapperFactory) resolveClass(type).getDeclaredConstructor().newInstance();
          configuration.setObjectWrapperFactory(factory);
        }
      }
      /**
       * 解析<reflectorFactory>节点
       * @param context
       * @throws Exception
       */
      private void reflectorFactoryElement(XNode context) throws Exception {
        if (context != null) {
          String type = context.getStringAttribute("type");
          ReflectorFactory factory = (ReflectorFactory) resolveClass(type).getDeclaredConstructor().newInstance();
          configuration.setReflectorFactory(factory);
        }
      }
      //解析properties节点
      private void propertiesElement(XNode context) throws Exception {
        if (context != null) {
          //得到所有的孩子属性;
          Properties defaults = context.getChildrenAsProperties();
          //parse only one resource element : resource or url
          String resource = context.getStringAttribute("resource");
          String url = context.getStringAttribute("url");
          if (resource != null && url != null) {
            throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
          }
          if (resource != null) {
            defaults.putAll(Resources.getResourceAsProperties(resource));
          } else if (url != null) {
            defaults.putAll(Resources.getUrlAsProperties(url));
          }
    
          // ...与 Configurat on 对象中 variables 集合合并(略)
          Properties vars = configuration.getVariables();
          if (vars != null) {
            defaults.putAll(vars);
          }
          //设置 解析器 variables; 在后面的解析过程中,会使用该 Properties 对象中的信息替换占位符。
          parser.setVariables(defaults);
          //更新 configuration 的 variables 变量
          configuration.setVariables(defaults);
        }
      }
    
      /**
       * 解析全局配置,并应用到 mybatis 全局变量,使配置生效
       * @param props
       */
      private void settingsElement(Properties props) {
        configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
        configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
        configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
        configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
        configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
        configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
        configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
        configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
        configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
        configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
        configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
        configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
        configuration.setDefaultResultSetType(resolveResultSetType(props.getProperty("defaultResultSetType")));
        configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
        configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
        configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
        configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
        configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
        configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
        configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
        configuration.setDefaultEnumTypeHandler(resolveClass(props.getProperty("defaultEnumTypeHandler")));
        configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
        configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
        configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
        configuration.setLogPrefix(props.getProperty("logPrefix"));
        configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
      }
    
      /**
       * <environments default="development">
       *     <environment id="development">
       *       <transactionManager type="JDBC">
       *         <property name="" value="" />
       *       </transactionManager>
       *       <dataSource type="UNPOOLED">
       *         <property name="driver" value="org.hsqldb.jdbcDriver" />
       *         <property name="url" value="jdbc:hsqldb:mem:localtime" />
       *         <property name="username" value="sa" />
       *       </dataSource>
       *     </environment>
       *   </environments>
       *
       * @param context
       * @throws Exception
       */
      private void environmentsElement(XNode context) throws Exception {
        if (context != null) {
          //未指定 XMLConfigBuilder environment 字段,则使用 default 性指定的 <environment>
          if (environment == null) {
            environment = context.getStringAttribute("default");
          }
          for (XNode child : context.getChildren()) {
            String id = child.getStringAttribute("id");
            if (isSpecifiedEnvironment(id)) {
              TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); // 创建 TransactionFactory
              DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); // 创建 DataSourceFactory
              DataSource dataSource = dsFactory.getDataSource();        // 创建 DataSource
              Environment.Builder environmentBuilder = new Environment.Builder(id)
                  .transactionFactory(txFactory)
                  .dataSource(dataSource);
              configuration.setEnvironment(environmentBuilder.build()); // 创建 Environment,并反写 configuration 对象
            }
          }
        }
      }
    
      /**
       *   <databaseIdProvider type="DB_VENDOR">
       *     <property name="Apache Derby" value="derby"/>
       *   </databaseIdProvider>
       * @param context
       * @throws Exception
       */
      private void databaseIdProviderElement(XNode context) throws Exception {
        DatabaseIdProvider databaseIdProvider = null;
        if (context != null) {
          String type = context.getStringAttribute("type");
          // awful patch to keep backward compatibility
          if ("VENDOR".equals(type)) {  // 为了保证兼容性,修改 type 取值
            type = "DB_VENDOR";
          }
          Properties properties = context.getChildrenAsProperties();
          databaseIdProvider = (DatabaseIdProvider) resolveClass(type).getDeclaredConstructor().newInstance();
          databaseIdProvider.setProperties(properties);
        }
        Environment environment = configuration.getEnvironment();
        if (environment != null && databaseIdProvider != null) {
          // 根据前面确定的 DataSource 确定当前使用的数据库产品,得到不同厂商数据库Id
          String databaseId = databaseIdProvider.getDatabaseId(environment.getDataSource());
          configuration.setDatabaseId(databaseId);
        }
      }
    
      /**
       * 根据 配置文件中的节点,创建 TransactionFactory 对象
       * @param context
       * @return
       * @throws Exception
       */
      private TransactionFactory transactionManagerElement(XNode context) throws Exception {
        if (context != null) {
          String type = context.getStringAttribute("type");
          Properties props = context.getChildrenAsProperties();
          TransactionFactory factory = (TransactionFactory) resolveClass(type).getDeclaredConstructor().newInstance();
          factory.setProperties(props);
          return factory;
        }
        throw new BuilderException("Environment declaration requires a TransactionFactory.");
      }
    
      /**
       * 根据 配置文件中的节点,创建 DataSourceFactory 对象
       * @param context
       * @return
       * @throws Exception
       */
      private DataSourceFactory dataSourceElement(XNode context) throws Exception {
        if (context != null) {
          String type = context.getStringAttribute("type");
          Properties props = context.getChildrenAsProperties();
          DataSourceFactory factory = (DataSourceFactory) resolveClass(type).getDeclaredConstructor().newInstance();
          factory.setProperties(props);
          return factory;
        }
        throw new BuilderException("Environment declaration requires a DataSourceFactory.");
      }
      /**
       * 解析<typeHandlers>节点
       * @param parent
       */
      private void typeHandlerElement(XNode parent) {
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            if ("package".equals(child.getName())) {
              String typeHandlerPackage = child.getStringAttribute("name");
              typeHandlerRegistry.register(typeHandlerPackage);
            } else {
              String javaTypeName = child.getStringAttribute("javaType");
              String jdbcTypeName = child.getStringAttribute("jdbcType");
              String handlerTypeName = child.getStringAttribute("handler");
              Class<?> javaTypeClass = resolveClass(javaTypeName);
              JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
              Class<?> typeHandlerClass = resolveClass(handlerTypeName);
              if (javaTypeClass != null) {
                if (jdbcType == null) {
                  typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
                } else {
                  typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
                }
              } else {
                typeHandlerRegistry.register(typeHandlerClass);
              }
            }
          }
        }
      }
    
      /**
       * 解析实体 mapper 文件;
       *   <mappers>
       *     <mapper resource="org/apache/ibatis/builder/BlogMapper.xml"/>
       *     <mapper url="file:./src/test/java/org/apache/ibatis/builder/NestedBlogMapper.xml"/>
       *     <mapper class="org.apache.ibatis.builder.CachedAuthorMapper"/>
       *     <package name="org.apache.ibatis.builder.mapper"/>
       *   </mappers>
       * @param parent
       * @throws Exception
       */
      private void mapperElement(XNode parent) throws Exception {
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            if ("package".equals(child.getName())) { // <package >子节点
              String mapperPackage = child.getStringAttribute("name");
              configuration.addMappers(mapperPackage);
            } else {
              // 获取<mapper >节点的 resource url class 属性,这三个属性互斥
              String resource = child.getStringAttribute("resource");
              String url = child.getStringAttribute("url");
              String mapperClass = child.getStringAttribute("class");
              // 如果<mapper >节点指定了 resource 或是 url 属性,则创建 XMLMapperBuilder 对象
              // 并通过该对象解析 resource 或是 url 属性指定的 Mapper 配置文件
              if (resource != null && url == null && mapperClass == null) {
                ErrorContext.instance().resource(resource);
                InputStream inputStream = Resources.getResourceAsStream(resource);
                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());
                // 解析 mapper 节点
                mapperParser.parse();
              } else if (resource == null && url == null && mapperClass != null) {
                // 如果<mapper>节点指定了 class 属性,则 MapperRegistry 注册该 Mapper 接口
                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.");
              }
            }
          }
        }
      }
    
      /**
       * 检测是否是,指定的环境
       * @param id
       * @return
       */
      private boolean isSpecifiedEnvironment(String id) {
        if (environment == null) {
          throw new BuilderException("No environment specified.");x
        } else if (id == null) {
          throw new BuilderException("Environment requires an id attribute.");
        } else if (environment.equals(id)) {
          return true;
        }
        return false;
      }
    

    创建SqlSessionFactory

    经过上面的解析和加载过程,已经创建了 SqlSessionFactory 对象,并初始化了 Configuration 全局配置对象,最后返回一个 SqlSessionFactory 供 SqlSessionManager 使用。然后通过构造方法创建了 SqlSessionManager 对象。

      // 通过 newInstance() 方法创建 SqlSessionManager 对象
      public static SqlSessionManager newInstance(Reader reader) {
        // 创建 SqlSessionFactory 对象,作为成员变量
        return new SqlSessionManager(new SqlSessionFactoryBuilder().build(reader, null, null));
    // SqlSessionManager 构造方法
    private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
        // 使用动态代理的方式创建 SqlSession 的代理对象
        this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(
            SqlSessionFactory.class.getClassLoader(),
            new Class[]{SqlSession.class},
            new SqlSessionInterceptor());
      }
      }
    

    启动流程

    • 解析 mybatis-congfig.xml 配置文件
    • 解析各个实体 mapper.xml 文件
    • 解析配置文件创建全局配置对象 Configuration 对象
    • 通过 Configuration 成员变量,构建 SqlSessionFactory 对象
    • 通过 SqlSessionFactory.openSession() 创建 SqlSession 对象
    • 创建 SqlSession 对象同时,也创建了 Transaction、Executor 等对象
      private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          // 获取 mybatis-config.xml 配置文件中配置的 Environment 对象
          final Environment environment = configuration.getEnvironment();
          // 获取 TransactionFactory
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          // 创建 Transaction 对象
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          // 创建 Executor 对象
          final Executor executor = configuration.newExecutor(tx, execType);
          // 创建 DefaultSqlSession 对象
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          // 关闭 Transaction
          closeTransaction(tx); // may have fetched a connection so lets call close()
          throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }
    
    • 还会通过 Configuration 对象创建 StatementHandler、parameterHandler、resultSetHandler 对象
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
    ParameterHandler parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    ResultSetHandler resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
    

    至此为止 MyBatis 已经启动完毕,创建了 SqlSession、Executor 、StatementHandler、parameterHandler、resultSetHandler等关键对象,为后续数据查询奠定了基础。

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