mybatis的启动及代理过程

傷城~ 2023-10-05 16:10 48阅读 0赞

## 1、spring-boot项目中启动mybatis ##

mybatis在提供了mybatis-spring-boot-autoconfigure，用于spring-boot项目中自动加载注入mybatis类，这里采用了springboot指定SPI规范，SPI规范可参考https://blog.csdn.net/u013978512/article/details/111088250

在mybatis-spring-boot-autoconfigure包的META-INF目录的spring-factories文件中，配置了mybatis的自动配置的类，boot项目在启动的时候会扫描所有jar包下的/META-INF/spring-factories文件，所以可以扫描到该文件资源。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM5Nzg1MTI_size_16_color_FFFFFF_t_70]  
![在这里插入图片描述][20201219170709808.png]  
key值为org.springframework.boot.autoconfigure.EnableAutoConfiguration，下图是spring-boot jar包下的spring-factories文件，可以看到该文件中也有一个key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的键值对（其中值有很多类名以逗号隔开）。也就是说配置了该key，项目在启动的时候，boot就会进行自动配置

在MybatisAutoConfiguration配置类中有mybatis的核心Bean如SqlSessionFactory等，整个串起来就可以知道，为什么spring-boot项目启动的时候就会注入mybatis各种bean了。

## 2、mybatis启动过程 ##

以下是MybatisAutoConfiguration配置类中sqlSessionFactory对象的创建过程

@Bean
        @ConditionalOnMissingBean
        public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
            SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
            factory.setDataSource(dataSource);
            factory.setVfs(SpringBootVFS.class);
            if (StringUtils.hasText(this.properties.getConfigLocation())) {
                factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
            }
    
            org.apache.ibatis.session.Configuration configuration = this.properties.getConfiguration();
            if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
                configuration = new org.apache.ibatis.session.Configuration();
            }
    
            if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
                Iterator var4 = this.configurationCustomizers.iterator();
    
                while(var4.hasNext()) {
                    ConfigurationCustomizer customizer = (ConfigurationCustomizer)var4.next();
                    customizer.customize(configuration);
                }
            }
    
            factory.setConfiguration(configuration);
            if (this.properties.getConfigurationProperties() != null) {
                factory.setConfigurationProperties(this.properties.getConfigurationProperties());
            }
    
            if (!ObjectUtils.isEmpty(this.interceptors)) {
                factory.setPlugins(this.interceptors);
            }
    
            if (this.databaseIdProvider != null) {
                factory.setDatabaseIdProvider(this.databaseIdProvider);
            }
    
            if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
                factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
            }
    
            if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
                factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
            }
    
            if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
                factory.setMapperLocations(this.properties.resolveMapperLocations());
            }
    
            return factory.getObject();
        }

可以看到，先new出了SqlSessionFactoryBean，该类实现了FactoryBean，继承关系结构图如下：最后是调用了factory.getObject()

public SqlSessionFactory getObject() throws Exception {
            if (this.sqlSessionFactory == null) {
                this.afterPropertiesSet();
            }
    
            return this.sqlSessionFactory;
        }

getObject方法中，先判断sqlSessionFactory是否为null，如果为null，会调用afterPropertiesSet创建sqlSessionFactory.

sqlSessionFactory方法总结如下：

1.  dataSource 注入SqlSessionFactoryBean
2.  configLocation注入SqlSessionFactoryBean
3.  创建configuration， 并注入SqlSessionFactoryBean
4.  MybatisProperties中的configurationProperties注入SqlSessionFactoryBean
5.  拦截器注入SqlSessionFactoryBean
6.  解析出properties的mapperLocations注入SqlSessionFactoryBean
7.  buildSqlSessionFactory
8.  xmlConfigBuilder 解析 mybatis-config.xml 配置
9.  xmlMapperBuilder解析mapper配置
10. 前两步解析的数据都放在了configuration对象中

每个mapper接口会构建成一个个BeanDefinition对象，以备后续spring创建对象使用，我们知道mapper接口在项目启动后都会有代理对象，而代理对象就是通过BeanDefinition生成的。mapper的BeanDefinition创建过程可参考https://www.cnblogs.com/bug9/p/11793728.htm

## 3 mapperInterface的代理过程分析 ##

每一个mapper接口对应着一个BeanDefinition，BeanDefinition中的class是MapperFactoryBean  
在spring创建对象的时候，遇到BeanDefinition的class是FactoryBean的实现的情况下，就会调用实现类的getObject()，这里针对mapper的话，就是调用MapperFactoryBean的getObject()获取mapper对象。

public T getObject() throws Exception {
        return getSqlSession().getMapper(this.mapperInterface);
      }

在MapperFactoryBean中有一个属性sqlSession，sqlSession对象实际是SqlSessionTemplate

public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
        if (!this.externalSqlSession) {
          this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
        }
      }

而在SqlSessionTemplate类中有一个属性sqlSessionProxy

public class SqlSessionTemplate implements SqlSession, DisposableBean {
    
      private final SqlSessionFactory sqlSessionFactory;
    
      private final ExecutorType executorType;
    
      private final SqlSession sqlSessionProxy;
    
      private final PersistenceExceptionTranslator exceptionTranslator;

在SqlSessionTemplate的构造函数中，对sqlSessionProxy进行了初始化，而sqlSessionProxy是通过动态代理实现的初始化，对应的InvocationHandler实现类是SqlSessionInterceptor

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());
      }

而SqlSessionTemplate的getMapper方法实现了mapperInterface的代理对象的实现过程

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

这里我们可以看到，调用了configure的getMapper，并将SqlSessionTemplate自己的引用传了进去

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
            MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
            if (mapperProxyFactory == null) {
                throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
            } else {
                try {
                    return mapperProxyFactory.newInstance(sqlSession);
                } catch (Exception var5) {
                    throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
                }
            }
        }

再进去之后，是MapperRegistry的getMapper接口，这里获得了该mapperInterface对应的mapperProxyFactory，并调用了mapperProxyFactory的newInstance方法

public T newInstance(SqlSession sqlSession) {
            MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
            return this.newInstance(mapperProxy);
        }
    
    protected T newInstance(MapperProxy<T> mapperProxy) {
            return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
        }

在mapperProxyFactory的newInstance方法中，创建了mapperProxy对象，并通过动态代理创建了该mapperInterface的代理对象。

这里不难猜到，MapperProxy是InvocationHandler的实现类，而在调用mapperInterface方法时，都会调用MapperProxy的invoke方法（动态代理的规范），那么我们来看MapperProxy的invoke方法的代码逻辑

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            try {
    //若是Object的方法，那么直接调用该方法执行即可
                if (Object.class.equals(method.getDeclaringClass())) {
                    return method.invoke(this, args);
                }
    
                if (this.isDefaultMethod(method)) {
                    return this.invokeDefaultMethod(proxy, method, args);
                }
            } catch (Throwable var5) {
                throw ExceptionUtil.unwrapThrowable(var5);
            }
    //这里说明了，mapper的每一个自定义函数都对应一个MapperMethod，调用该方法时，就是调用该方法对应的MapperMethod的execute方法
            MapperMethod mapperMethod = this.cachedMapperMethod(method);
            return mapperMethod.execute(this.sqlSession, args);
        }

调用mapperInterface的方法时，会调用mapperMethod.execute(this.sqlSession, args); 这里的sqlSession就是我们上面分析的SqlSessionTemplate，我们看execute内部流程

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

在该方法中，我们看到了分INSERT、UPDATE、DELETE、SELECT等不清的情况进行不同的调用，这里我们以INSERT为例分析一下，在INSERT分支中，其实是通过sqlSession的insert实现的，我们再看sqlSession的insert实现（这里其实就是SqlSessionTemplate类的insert方法）

public int insert(String statement, Object parameter) {
        return this.sqlSessionProxy.insert(statement, parameter);
      }

这里，sqlSession的insert其实是委托给了sqlSessionProxy对象的insert方法，前面我们分析过，sqlSessionProxy对象的创建其实是通过动态代理生成的，其InvocationHandler实现类是SqlSessionInterceptor，所以在调用sqlSessionProxy都会走到SqlSessionInterceptor的invoke方法中。

private class SqlSessionInterceptor implements InvocationHandler {
        @Override
        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);
            }
          }
        }
      }

在SqlSessionInterceptor的invoke方法中，又重新生成了一个sqlSession，通过这个新生成的sqlSession进行了数据库的操作。

从上面的分析，我们可以总结出，

各对象的引用关系如下（实线箭头代表引用的关系，虚线箭头代表的代理的关系）

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service中调用某个mapper接口中的方法（如insert），其内部走的大概流程如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM5Nzg1MTI_size_16_color_FFFFFF_t_70 2]

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[20201219170709808.png]: https://img-blog.csdnimg.cn/20201219170709808.png
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