MyBatis的DatabaseIdProvider解析

深碍√TFBOYSˉ_ 2023-07-14 05:47 21阅读 0赞

# MyBatis 的 DatabaseIdProvider 解析 #

## 数据库厂商标识 ##

MyBatis 可以根据不同的数据库厂商执行不同的语句，这种多厂商的支持是基于映射语句中的 `databaseId` 属性。 MyBatis 会加载不带 `databaseId` 属性和带有匹配当前数据库 `databaseId` 属性的所有语句。 如果同时找到带有 `databaseId` 和不带 `databaseId` 的相同语句，则后者会被舍弃

配置方法

### DatabaseIdProvider 接口 ###

public interface DatabaseIdProvider { 
    
        default void setProperties(Properties p) { 
            // NOP
        }
    	// 返回数据源的产品名称
        String getDatabaseId(DataSource dataSource) throws SQLException;
    }

其继承关系如下

*  `org.apache.ibatis.mapping.DatabaseIdProvider`
    
     *  `org.apache.ibatis.mapping.VendorDatabaseIdProvider`

### VendorDatabaseIdProvider 实现类 ###

public class VendorDatabaseIdProvider implements DatabaseIdProvider { 
    
        private Properties properties;
    
        @Override
        public String getDatabaseId(DataSource dataSource) { 
            // 空值判定
            if (dataSource == null) { 
                throw new NullPointerException("dataSource cannot be null");
            }
            try { 
                // 获取数据源的产品名称
                return getDatabaseName(dataSource);
            } catch (Exception e) { 
                LogHolder.log.error("Could not get a databaseId from dataSource", e);
            }
            return null;
        }
    
        @Override
        public void setProperties(Properties p) { 
            this.properties = p;
        }
    
        private String getDatabaseName(DataSource dataSource) throws SQLException { 
            String productName = getDatabaseProductName(dataSource);
            if (this.properties != null) { 
                for (Map.Entry<Object, Object> property : properties.entrySet()) { 
                    if (productName.contains((String) property.getKey())) { 
                        return (String) property.getValue();
                    }
                }
                // no match, return null
                return null;
            }
            return productName;
        }
    
        private String getDatabaseProductName(DataSource dataSource) throws SQLException { 
            Connection con = null;
            try { 
                con = dataSource.getConnection();
                DatabaseMetaData metaData = con.getMetaData();
                return metaData.getDatabaseProductName();
            } finally { 
                if (con != null) { 
                    try { 
                        con.close();
                    } catch (SQLException e) { 
                        // ignored
                    }
                }
            }
        }
    
        private static class LogHolder { 
            private static final Log log = LogFactory.getLog(VendorDatabaseIdProvider.class);
        }
    
    }

查询数据源产品名称底层调用的是 `java.sql.DatabaseMetaData#getDatabaseProductName` 方法，在 mapper.xml 中选择对应的 `databaseId` 就会将信息保存在`org.apache.ibatis.mapping.MappedStatement` 对象中并保存在 Configuration 的 `Map<String, MappedStatement> mappedStatements` 中保存

## 配置读取 ##

private void databaseIdProviderElement(XNode context) throws Exception { 
        DatabaseIdProvider databaseIdProvider = null;
        if (context != null) { 
            String type = context.getStringAttribute("type");
            if ("VENDOR".equals(type)) { 
                type = "DB_VENDOR";
            }
            Properties properties = context.getChildrenAsProperties();
            // DB_VENDOR 类型创建 VendorDatabaseIdProvider 对象
            databaseIdProvider = (DatabaseIdProvider) resolveClass(type).getDeclaredConstructor().newInstance();
            databaseIdProvider.setProperties(properties);
        }
        Environment environment = configuration.getEnvironment();
        if (environment != null && databaseIdProvider != null) { 
            String databaseId = databaseIdProvider.getDatabaseId(environment.getDataSource());
            configuration.setDatabaseId(databaseId);
        }
    }

## 自定义 ##

public class MyDatabaseIdProvider implements DatabaseIdProvider { 
        private static final Log log = LogFactory.getLog(MyDatabaseIdProvider.class);
        private Properties properties;
    
        @Override
        public void setProperties(Properties p) { 
            this.properties = p;
        }
    
        @Override
        public String getDatabaseId(DataSource dataSource) throws SQLException { 
            Connection connection = dataSource.getConnection();
            DatabaseMetaData metaData = connection.getMetaData();
            String databaseProductName = metaData.getDatabaseProductName();
            log.debug("Current DataBase Product Name is: " + databaseProductName);
            for (Object key : properties.keySet()) { 
                if (key.equals(databaseProductName)) { 
                    log.debug("Find a matched property value: " + properties.get(key));
                    return (String) properties.get(key);
                }
            }
            return null;
        }
    }

在配置中引入