Mybatis---01Mybatis动态代理过程分析

1.通过调试，session调用的getMapper是其实现类DefaultSQLSession中的

        //1.读取配置文件
        InputStream in = Resources.getResourceAsStream("mybatis-config.xml");
        //2.创建 SqlSessionFactory 的构建者对象
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        //3.使用构建者创建工厂对象 SqlSessionFactory
        SqlSessionFactory factory = builder.build(in);
        //4.使用 SqlSessionFactory 生产 SqlSession 对象
        SqlSession session = factory.openSession();
        //5.使用 SqlSession 创建 dao 接口的代理对象
        IBookDao bookDao = session.getMapper(IBookDao.class);
        //6.使用代理对象执行查询所有方法
        List<Book> books = bookDao.selectAll();
        for (Book book : books) {
            System.out.println(book);
        }
        //7.释放资源
        session.close();
        in.close();

2.在DefaultSQLSession类中getMapper方法如下，里面调用Configuration类的泛型方法<T> T getMapper(Class<T> type)

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

3.在Configuration类中查看getMapper(type, sqlSession)方法，里面调用的是MapperRegistry类里面的getMapper(type, sqlSession)方法。

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

4.进入MapperRegistry类里面查看getMapper(type, sqlSession)方法。里面通过属性knownMappers里面的方法get（Object key）获取MapperProxyFactory<T>泛型类，使用这个类里面的newInstance（SqlSession sqlSession）方法。

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

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);
    }
  }

5.进入MapperProxyFactory<T>泛型类，查看newInstance（SqlSession sqlSession）方法如下；这里面生成一个MapperProxy<T>对象mapperProxy，然后使用T newInstance(MapperProxy<T> mapperProxy)实现动态代理。其中MapperProxy<T>类实现了接口InvocationHandler。重写了Object invoke(Object proxy, Method method, Object[] args)方法

 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);
  }

6.查看MapperProxy<T>类里的invoke方法.

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {

      if (Object.class.equals(method.getDeclaringClass())) {//并不是每个方法都需要调用代理对象进行执行，如果这个方法是Object中通用的方法，则无需执行
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {//判断是不是接口默认方法，1.8之后接口就有默认方法了
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //从缓存中获取MapperMethod对象，如果缓存中没有，则创建一个，并添加到缓存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // 执行方法对应的 SQL 语句
    return mapperMethod.execute(sqlSession, args);
  }

7.查看MapperMethod 类的execute方法，这个方法是执行sql的主要方法，根据我写的程序这里是调用executeForMany方法，实际上这个方法DefaultSqlSession类里面的selectList方法。

 public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      //insert语句的处理逻辑
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      //update语句的处理逻辑
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      //delete语句的处理逻辑
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      //select语句的处理逻辑
      case SELECT:
        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 if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        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;
  }

  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      result = sqlSession.<E>selectList(command.getName(), param);
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }

在这里，动态代理过程就完成了。大致执行过程是  sqlSession.getMapper()【这个sqlSession实际上是 DefaultSqlSession】-----调用---->Configuration.getMapper()-----调用--->MapperRegistry.getMapper() ------调用-->MapperProxyFactory.newInstance() -------调用----> Proxy.newProxyInstance()【其中的InvocationHandler接口由实现类MapperProxy代替】------invoke----->MapperMethod.execute()-----调用----->DefaultSqlSession.相应方法