MyBatis源码解析（六）：执行SQL

不管是单独使用还是和Spring集成，我们都是使用接口定义的方式声明数据库的增删改查方法。那么我们只声明一个接口，MyBatis是如何帮我们来实现SQL？

通过前面MyBatis源码解析（五）：获取SqlSeesion，

调用sqlSession.getMapper

UserMapper mapper = session.getMapper(UserMapper.class);

SqlSession直接把任务给到Configuration。

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

Configuration又把任务给到MapperRegistry

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

MapperRegistry.getMapper

public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
    

    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {

    //调用mapperProxyFactory.newInstance()。
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

mapperProxyFactory.newInstance(sqlSession);

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

MapperProxy这个类实现了jdk动态代理接口InvocationHandler。在invoke方法中实现代理方法调用的细节。

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (method.isDefault()) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    
    //mapper代理，调用方法时，将方法封装进MapperMethod
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

mapperMethod.execute(sqlSession, args);

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);

        //sqlSession执行insert 并将影响行数返回
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        
        Object param = method.convertArgsToSqlCommandParam(args);

        //sqlSession执行UPDATE 并将影响行数返回
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);

        //sqlSession执行delete 并将影响行数返回
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      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);
          if (method.returnsOptional()
              && (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.ofNullable(result);
          }
        }
        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;
  }

sqlSession.update(command.getName(), param) 看sqlSession是怎么执行sql的

@Override
  public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      
      //调用对应执行器的update
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

//以执行器SimpleExecutor.doUpdate为例
  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      //获取连接
      Configuration configuration = ms.getConfiguration();

      //handler
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);

      //获取Statement
      stmt = prepareStatement(handler, ms.getStatementLog());

      //调用handler的update
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

 handler的update就回到了MyBatis源码解析（一）：MyBatis使用示例中JDBC的操作了

 /**
   * 这里调用JDBC的操作
   * @param statement
   * @return
   * @throws SQLException
   */
  @Override
  public int update(Statement statement) throws SQLException {
    //获取sql
    String sql = boundSql.getSql();
    
    //获取参数
    Object parameterObject = boundSql.getParameterObject();
    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    int rows;
    if (keyGenerator instanceof Jdbc3KeyGenerator) {
      
    //调用statement.execute
      statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
      rows = statement.getUpdateCount();
      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
    } else if (keyGenerator instanceof SelectKeyGenerator) {
      statement.execute(sql);
      rows = statement.getUpdateCount();
      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
    } else {
      statement.execute(sql);
      rows = statement.getUpdateCount();
    }
    return rows;
  }