MyBatis入门系列(14) -MyBatis运行原理之获取Mapper代理对象源码分析

怼烎@ 2023-10-10 13:02 63阅读 0赞

### 获取Mapper代理对象 ###

在获取了SqlSession对象后，调用getMapper方法传入一个Mapper接口，就会返回一个实例对象，众所周知，接口是不能实例化的，那么返回的肯定是接口的代理对象。

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

#### 大致流程 ####

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#### 源码分析 ####

##### 1. DefaultSqlSession.getMapper(type, this) #####

SqlSession对象为DefaultSqlSession，那么实际调用的是DefaultSqlSession.getMapper(type, this)方法，这个方式实际调用的又是configuration对象的getMapper方法。

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

##### 2. Configuration.getMapper(Class type, SqlSession sqlSession) #####

configuration.getMapper()方法实际调用的是configuration对象中mapperRegistry的getMapper方法。

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

##### 3. MapperRegistry.getMapper(Class type, SqlSession sqlSession) #####

mapperRegistry是Configuration的一个成员变量，其类为MapperRegistry，可以理解为Mapper注册器。

mapper注册器用于将所有的mapper接口添加到内存中,Mapper注册器自身维护着两个属性，config和knownMappers，其中knownMappers是一个Map集合。

private final Configuration config;
        private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

SqlSessionFactory创建之后，mapperRegistry会完成初始化，knownMapper中存放各种键值对，键为每个每个mapper接口，值为MapperProxyFactory对象，MapperProxyFactory是一个工厂类，用于创建MapperProxy。

knownMappers.put(type, new MapperProxyFactory<T>(type));

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MapperRegistry获取Mapper对象的源码如下：

/**
         * 获取Mapper 实例对象
         *
         * @param type       接口
         * @param sqlSession SqlSession
         * @return MapperProxy
         */
        public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        
            // 在knownMappers中获取Mapper代理对象工厂
            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);
            }
        }

##### 4. mapperProxyFactory.newInstance(sqlSession) #####

mapper代理对象工厂通过newInstance()方法创建实例mapper接口对应的代理对象MapperProxy。

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

##### 5. new MapperProxy(sqlSession, mapperInterface, methodCache) #####

通过MapperProxy构造方法创建MapperProxy对象。

final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);

MapperProxy类实现了JDK中的InvocationHandler接口，InvocationHandler接口是proxy代理实例的调用处理程序实现的一个接口，每一个proxy代理实例都有一个关联的调用处理程序；在代理实例调用方法时，方法调用被编码分派到调用处理程序的invoke方法。

/**
         *  构造方法
         * @param sqlSession SqlSession
         * @param mapperInterface mapper接口
         * @param methodCache 方法缓存
         */
        public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
        
            this.sqlSession = sqlSession;
            this.mapperInterface = mapperInterface;
            this.methodCache = methodCache;
        }

##### 6. Proxy.newProxyInstance() #####

获取了基础的MapperProxy对象后，调用Proxy.newProxyInstance()方法通过反射技术创建mapper接口正真的JAVA动态代理对象。

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

#### 总结 ####

获取Mapper代理对象的流程很简单，大致就是通过Configuration对象中加载好的mapper接口信息，然后使用JDK动态代理，返回MapperProxy代理对象。  
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