Mapper接口的註冊過程

Mapper接口的註冊過程

CachedAuthorMapper cachedAuthorMapper = sqlSession.getMapper(CachedAuthorMapper.class);

使用Mybatis框架的時候，我們調用sqlSession的getMapper方法可以獲取到一個Mapper對象，但是我們定義的Mapper是接口，我們要調用方法必須是個實例對象，那麼getMapper方法返回的到底是個什麼呢？其實，getMapper方法返回的是一個我們定義的Mapper接口的動態代理對象。

Mapper接口的註冊時序

SqlSession的getMapper方法

實際上SqlSession將功能委託給了Configuration的getMapper對象。Configuration對象又將方法委託給了MapperRegistry的getMapper對象。

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

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

MapperRegistry的getMapper方法

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);
    }
  }

MapperRegistry的getMapper方法首先從knownMappers這個Map對象中獲取指定Mapper類型的MapperProxyFactory對象。那麼這個knownMappers中的元素是什麼時候註冊上去的呢？其實在構造Configuration對象，解析Mybatis主配置文件的時候，解析Mappers節點時會調用MapperRegistry的addMapper方法，這個時候會註冊Mapper類型和其對應的MapperProxyFactory對象。由MapperProxyFactory生成Mapper的動態代理對象。

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<>(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);
        }
      }
    }
  }

MapperProxyFactory

MapperProxyFactory是MapperProxy對象的工廠類，其封裝了MapperProxy的創建過程。在newInstance方法中，MapperProxyFactory使用jdk動態代理生成Mapper對象的動態代理對象。

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<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

MapperProxy

MapperProxy對象實現了InvocationHandler接口，根據動態代理的特性，在調用目標對象的方法時，方法執行會被攔截，轉而執行代理對象的invoke方法，在此方法中，調用SqlSession對象來執行實際的數據庫交互。

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 從Object類繼承的方法不做處理
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (method.isDefault()) {
        if (privateLookupInMethod == null) {
          return invokeDefaultMethodJava8(proxy, method, args);
        } else {
          return invokeDefaultMethodJava9(proxy, method, args);
        }
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }

    // 對Mapper接口中定義的方法進行封裝，生成MapperMethod對象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }