mybatis如何根据mapper接口生成其实现类

怼烎@ 2023-06-15 15:29 12阅读 0赞

### 目录 ###

*   *  序
     *  问题
     *   *  mapper的实现类如何生成
         *  mybatis-spring帮忙做了什么
     *  一、mapper的实现类如何生成
     *  二、mybatis-spring

## 序 ##

mybatis里，sqlSession执行sql的时候，有两种方式，一种是写mapper的xml的namespace+statementId，如下：

public Student findStudentById(Integer studId) { 
            logger.debug("Select Student By ID :{}", studId);
            SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
            try { 
                return sqlSession.selectOne("com.mybatis3.StudentMapper.findStudentById", studId);
            } finally { 
                sqlSession.close();
            }
        }

另外一种方法是指定mapper的接口：

public Student findStudentById(Integer studId) { 
            logger.debug("Select Student By ID :{}", studId);
            SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
            try { 
                StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
                return studentMapper.findStudentById(studId);
            } finally { 
                sqlSession.close();
            }
        }

一般的话，推荐第二种写法，手写namespace和statementId极大增加了出现错误的概率，降低了开发效率。

## 问题 ##

--------------------

### mapper的实现类如何生成 ###

如果使用mapper接口的方式，问题来了，这个是个接口，通过sqlSession对象get出来的一定是个实现类，问题是，我们并没有手工去写实现类，那么谁去干了这件事情呢？答案是mybatis通过JDK的动态代理方式，在启动加载配置文件时，解析mapper xml生成。

### mybatis-spring帮忙做了什么 ###

构建mybatis configuration，自动open和close session，以spring bean的方式加载Mapper等等。

--------------------

## 一、mapper的实现类如何生成 ##

如果不是集成spring的，会去读取configuration文件中的mappers节点，加载mapper的xml配置

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
    <configuration>
        <settings>
            <setting name="cacheEnabled" value="true"/>
            <setting name="lazyLoadingEnabled" value="true"/>
            <setting name="multipleResultSetsEnabled" value="true"/>
            <setting name="useColumnLabel" value="true"/>
            <setting name="useGeneratedKeys" value="false"/>
            <setting name="defaultExecutorType" value="SIMPLE"/>
            <setting name="defaultStatementTimeout" value="2"/>
        </settings>
        <typeAliases>
            <typeAlias alias="CommentInfo" type="com.xixicat.domain.CommentInfo"/>
        </typeAliases>
        <environments default="development">
            <environment id="development">
                <transactionManager type="JDBC"/>
                <dataSource type="POOLED">
                    <property name="driver" value="com.mysql.jdbc.Driver"/>
                    <property name="url" value="jdbc:mysql://localhost:3306/demo"/>
                    <property name="username" value="root"/>
                    <property name="password" value=""/>
                </dataSource>
            </environment>
        </environments>
        <mappers>
            <mapper resource="com/xixicat/dao/CommentMapper.xml"/>
        </mappers>
    </configuration>

如果是**集成spring**的，会读取spring的xml配置中sqlSessionFactory bean的mapperLocations属性，然后去解析该路径下的mapper xml文件。

然后绑定namespace(XMLMapperBuilder)

private void bindMapperForNamespace() { 
        String namespace = builderAssistant.getCurrentNamespace();
        if (namespace != null) { 
          Class<?> boundType = null;
          try { 
            boundType = Resources.classForName(namespace);
          } catch (ClassNotFoundException e) { 
            //ignore, bound type is not required
          }
          if (boundType != null) { 
            if (!configuration.hasMapper(boundType)) { 
              // Spring may not know the real resource name so we set a flag
              // to prevent loading again this resource from the mapper interface
              // look at MapperAnnotationBuilder#loadXmlResource
              configuration.addLoadedResource("namespace:" + namespace);
              configuration.addMapper(boundType);
            }
          }
        }
      }

这里先去判断该namespace能不能找到对应的class，若可以则调用

configuration.addMapper(boundType);

configuration对象委托给MapperRegistry去addMapper:

public <T> void addMapper(Class<T> type) { 
        mapperRegistry.addMapper(type);
    }

#### 生成该mapper的代理工厂(`MapperProxyFactory`) ####

public <T> void addMapper(Class<T> type) { 
        if (type.isInterface()) { 
          if (hasMapper(type)) { 
            throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
          }
          boolean loadCompleted = false;
          try { 
            knownMappers.put(type, new MapperProxyFactory<T>(type));
            // It's important that the type is added before the parser is run
            // otherwise the binding may automatically be attempted by the
            // mapper parser. If the type is already known, it won't try.
            MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
            parser.parse();
            loadCompleted = true;
          } finally { 
            if (!loadCompleted) { 
              knownMappers.remove(type);
            }
          }
        }
      }

这里的重点就是MapperProxyFactory类：

public class MapperProxyFactory<T> { 
    
      private final Class<T> mapperInterface;
      private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
    
      public MapperProxyFactory(Class<T> mapperInterface) { 
        this.mapperInterface = mapperInterface;
      }
    
      public Class<T> getMapperInterface() { 
        return mapperInterface;
      }
    
      public Map<Method, MapperMethod> getMethodCache() { 
        return methodCache;
      }
    
      @SuppressWarnings("unchecked")
      protected T newInstance(MapperProxy<T> mapperProxy) { 
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] {  mapperInterface }, mapperProxy);
      }
    
      public T newInstance(SqlSession sqlSession) { 
        final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
      }
    }

MapperProxyFactory在getMapper的时候被调用，生成mapper代理类

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);
       }
     }

new出来MapperProxy代理对象

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);
    }

MapperProxy实现InvocationHandler接口方法进行代理

public class MapperProxy<T> implements InvocationHandler, Serializable { 
    
      private static final long serialVersionUID = -6424540398559729838L;
      private final SqlSession sqlSession;
      private final Class<T> mapperInterface;
      private final Map<Method, MapperMethod> methodCache;
    
      public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) { 
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
        this.methodCache = methodCache;
      }
    
      @Override
      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);
      }
    
      private MapperMethod cachedMapperMethod(Method method) { 
        MapperMethod mapperMethod = methodCache.get(method);
        if (mapperMethod == null) { 
          mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
          methodCache.put(method, mapperMethod);
        }
        return mapperMethod;
      }
    }

这里的代理拦截，根据调用的method构建MapperMethod，通过MapperMethod去执行SQL。

public Object execute(SqlSession sqlSession, Object[] args) { 
        Object result;
        if (SqlCommandType.INSERT == command.getType()) { 
          Object param = method.convertArgsToSqlCommandParam(args);
          result = rowCountResult(sqlSession.insert(command.getName(), param));
        } else if (SqlCommandType.UPDATE == command.getType()) { 
          Object param = method.convertArgsToSqlCommandParam(args);
          result = rowCountResult(sqlSession.update(command.getName(), param));
        } else if (SqlCommandType.DELETE == command.getType()) { 
          Object param = method.convertArgsToSqlCommandParam(args);
          result = rowCountResult(sqlSession.delete(command.getName(), param));
        } else if (SqlCommandType.SELECT == command.getType()) { 
          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);
          }
        } else if (SqlCommandType.FLUSH == command.getType()) { 
            result = sqlSession.flushStatements();
        } else { 
          throw new BindingException("Unknown execution method for: " + command.getName());
        }
        if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) { 
          throw new BindingException("Mapper method '" + command.getName() 
              + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
        }
        return result;
      }

SqlSession只是mybatis对外公布的一个门面，它会将sql委托给了executor去执行。

#### 什么时候触发getMapper ####

*  手动getMapper

public void createStudent(Student student) { 
         SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession();
         try { 
             StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
             studentMapper.insertStudent(student);
             sqlSession.commit();
         } finally { 
             sqlSession.close();
         }
     }

*  集成spring的话，在spring容器给指定的bean注入mapper的时候触发(见MapperFactoryBean的getObject方法)

--------------------

## 二、mybatis-spring ##

### mybatis-spring是什么 ###

\[mybatis-spring\](http://mybatis.org/spring/zh/index.html) 会帮助你将 mybatis 代码无缝地整合到 Spring 中。它将允许 mybatis 参与到 Spring 的事务管理之中，创建映射器 mapper 和 SqlSession 并注入到 bean 中，以及将 Mybatis 的异常转换为 Spring 的 DataAccessException。

**具体分析：**  
通过MapperScannerConfigurer将mapper适配成spring bean。

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
         <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
         <property name="basePackage" value="com.xixicat.dao"/>
     </bean>

这里使用 MapperFactoryBean将Mapper接口配置成 Spring bean 实体同时注入sqlSessionFactory。

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException { 
        if (this.processPropertyPlaceHolders) { 
          processPropertyPlaceHolders();
        }
    
        ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
        scanner.setAddToConfig(this.addToConfig);
        scanner.setAnnotationClass(this.annotationClass);
        scanner.setMarkerInterface(this.markerInterface);
        scanner.setSqlSessionFactory(this.sqlSessionFactory);
        scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
        scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
        scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
        scanner.setResourceLoader(this.applicationContext);
        scanner.setBeanNameGenerator(this.nameGenerator);
        scanner.registerFilters();
        scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
      }

委托给ClassPathMapperScanner去scan

public Set<BeanDefinitionHolder> doScan(String... basePackages) { 
        Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
    
        if (beanDefinitions.isEmpty()) { 
          logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
        } else { 
          for (BeanDefinitionHolder holder : beanDefinitions) { 
            GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
    
            if (logger.isDebugEnabled()) { 
              logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() 
                  + "' and '" + definition.getBeanClassName() + "' mapperInterface");
            }
    
            // the mapper interface is the original class of the bean
            // but, the actual class of the bean is MapperFactoryBean
            definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
            definition.setBeanClass(MapperFactoryBean.class);
    
            definition.getPropertyValues().add("addToConfig", this.addToConfig);
    
            boolean explicitFactoryUsed = false;
            if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) { 
              definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
              explicitFactoryUsed = true;
            } else if (this.sqlSessionFactory != null) { 
              definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
              explicitFactoryUsed = true;
            }
    
            if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) { 
              if (explicitFactoryUsed) { 
                logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
              }
              definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
              explicitFactoryUsed = true;
            } else if (this.sqlSessionTemplate != null) { 
              if (explicitFactoryUsed) { 
                logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
              }
              definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
              explicitFactoryUsed = true;
            }
    
            if (!explicitFactoryUsed) { 
              if (logger.isDebugEnabled()) { 
                logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
              }
              definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
            }
          }
        }
    
        return beanDefinitions;
      }

这里出现了MapperFactoryBean的身影

### 关于MapperFactoryBean ###

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> { 
    
      private Class<T> mapperInterface;
    
      private boolean addToConfig = true;
    
      /** * Sets the mapper interface of the MyBatis mapper * * @param mapperInterface class of the interface */
      public void setMapperInterface(Class<T> mapperInterface) { 
        this.mapperInterface = mapperInterface;
      }
    
      /** * If addToConfig is false the mapper will not be added to MyBatis. This means * it must have been included in mybatis-config.xml. * <p> * If it is true, the mapper will be added to MyBatis in the case it is not already * registered. * <p> * By default addToCofig is true. * * @param addToConfig */
      public void setAddToConfig(boolean addToConfig) { 
        this.addToConfig = addToConfig;
      }
    
      /** * {@inheritDoc} */
      @Override
      protected void checkDaoConfig() { 
        super.checkDaoConfig();
    
        notNull(this.mapperInterface, "Property 'mapperInterface' is required");
    
        Configuration configuration = getSqlSession().getConfiguration();
        if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) { 
          try { 
            configuration.addMapper(this.mapperInterface);
          } catch (Throwable t) { 
            logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", t);
            throw new IllegalArgumentException(t);
          } finally { 
            ErrorContext.instance().reset();
          }
        }
      }
    
      /** * {@inheritDoc} */
      public T getObject() throws Exception { 
        return getSqlSession().getMapper(this.mapperInterface);
      }
    
      /** * {@inheritDoc} */
      public Class<T> getObjectType() { 
        return this.mapperInterface;
      }
    
      /** * {@inheritDoc} */
      public boolean isSingleton() { 
        return true;
      }
    
    }

注意这里继承了SqlSessionDaoSupport，在spring把sqlSessionFactory创建出来后，会去把之前注入的引用改为真的实例，调用SqlSessionDaoSupport的setSqlSessionFactory方法。

public abstract class SqlSessionDaoSupport extends DaoSupport { 
    
      private SqlSession sqlSession;
    
      private boolean externalSqlSession;
    
      public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) { 
        if (!this.externalSqlSession) { 
          this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
        }
      }
    
      public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) { 
        this.sqlSession = sqlSessionTemplate;
        this.externalSqlSession = true;
      }
    
      /** * Users should use this method to get a SqlSession to call its statement methods * This is SqlSession is managed by spring. Users should not commit/rollback/close it * because it will be automatically done. * * @return Spring managed thread safe SqlSession */
      public SqlSession getSqlSession() { 
        return this.sqlSession;
      }
    
      /** * {@inheritDoc} */
      protected void checkDaoConfig() { 
        notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
      }
    
    }

这里值得注意的是setSqlSessionFactory方法new了一个SqlSessionTemplate。

### SqlSessionTemplate ###

它的一个重要的构造器为

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
          PersistenceExceptionTranslator exceptionTranslator) { 
    
        notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
        notNull(executorType, "Property 'executorType' is required");
    
        this.sqlSessionFactory = sqlSessionFactory;
        this.executorType = executorType;
        this.exceptionTranslator = exceptionTranslator;
        this.sqlSessionProxy = (SqlSession) newProxyInstance(
            SqlSessionFactory.class.getClassLoader(),
            new Class[] {  SqlSession.class },
            new SqlSessionInterceptor());
      }

mybatis-srping比传统mybatis方法多做的事情就在于此，生成了一个sqlSessionProxy。这里static import了java.lang.reflect.Proxy.newProxyInstance;也就是使用使用jdk代理进行了SqlSessionInterceptor拦截。

### SqlSessionInterceptor ###

rivate class SqlSessionInterceptor implements InvocationHandler { 
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
          SqlSession sqlSession = getSqlSession(
              SqlSessionTemplate.this.sqlSessionFactory,
              SqlSessionTemplate.this.executorType,
              SqlSessionTemplate.this.exceptionTranslator);
          try { 
            Object result = method.invoke(sqlSession, args);
            if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) { 
              // force commit even on non-dirty sessions because some databases require
              // a commit/rollback before calling close()
              sqlSession.commit(true);
            }
            return result;
          } catch (Throwable t) { 
            Throwable unwrapped = unwrapThrowable(t);
            if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) { 
              // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
              closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
              sqlSession = null;
              Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
              if (translated != null) { 
                unwrapped = translated;
              }
            }
            throw unwrapped;
          } finally { 
            if (sqlSession != null) { 
              closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
            }
          }
        }
      }

到了这里就明白了mybatis-spring帮忙做了session的open和close。