Mybatis-DatabaseIdProvider源码解析

Mybatis3.5.1源码分析

1. Mybatis-SqlSessionFactoryBuilder，XMLConfigBuilder，XPathParser源码解析
2. Mybatis-Configuration源码解析
3. Mybatis-事务对象源码解析
4. Mybatis-数据源源码解析
5. Mybatis缓存策略源码解析
6. Mybatis-DatabaseIdProvider源码解析
7. Mybatis-TypeHandler源码解析
8. Mybatis-Reflector源码解析
9. Mybatis-ObjectFactory，ObjectWrapperFactory源码分析
10. Mybatis-Mapper各类标签封装类源码解析
11. Mybatis-XMLMapperBuilder,XMLStatmentBuilder源码分析
12. Mybatis-MapperAnnotationBuilder源码分析
13. [Mybatis-MetaObject,MetaClass源码解析]https://www.jianshu.com/p/f51fa552f30a)
14. Mybatis-LanguageDriver源码解析
15. Mybatis-SqlSource源码解析
16. Mybatis-SqlNode源码解析
17. Mybatis-KeyGenerator源码解析
18. Mybatis-Executor源码解析
19. Mybatis-ParameterHandler源码解析
20. Mybatis-StatementHandler源码解析
21. Mybatis-DefaultResultSetHandler(一)源码解析
22. Mybatis-DefaultResultSetHandler(二)源码解析
23. Mybatis-ResultHandler,Cursor,RowBounds 源码分析
24. Mybatis-MapperProxy源码解析
25. Mybatis-SqlSession源码解析
26. Mybatis-Interceptor源码解析

DatabaseIdProvider

/**
 *    Copyright 2009-2019 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.mapping;

import java.sql.SQLException;
import java.util.Properties;

import javax.sql.DataSource;

/**
 * Should return an id to identify the type of this database.
 * That id can be used later on to build different queries for each database type
 * This mechanism enables supporting multiple vendors or versions
 *  应该返回一个id来识别这个数据库的类型。
 *  以后可以使用该id为每个数据库类型建立不同的查询,
 *  这个机制使得能够支持多个供应商或者版本
 * @author Eduardo Macarron
 */
public interface DatabaseIdProvider {

  void setProperties(Properties p);

  String getDatabaseId(DataSource dataSource) throws SQLException;
}

VendorDatabaseIdProvider

/**
 *    Copyright 2009-2019 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.mapping;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.SQLException;
import java.util.Map;
import java.util.Properties;

import javax.sql.DataSource;

import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;

/**
 * Vendor DatabaseId provider.
 * 供应商的数据库Id提供者
 * 

 * It returns database product name as a databaseId.
 * If the user provides a properties it uses it to translate database product name
 * key="Microsoft SQL Server", value="ms" will return "ms".
 * It can return null, if no database product name or
 * a properties was specified and no translation was found.
 * 

 *     它会返回数据库产品名称作为数据库ID.
 *     如果使用用户提供一个属性列表，它会使用这个属性列表去转化数据库产品名
 *     如：key='Microsoft SQL Server',value='ms',就会返回'ms'.
 *     如果没有数据库产品名或者指定了属性列表当但是没有找到译文，就会返回null.
 * 
 * @author Eduardo Macarron
 */
public class VendorDatabaseIdProvider implements DatabaseIdProvider {

  private Properties properties;

  /**
   * 获取数据库ID
   * 

   *     如果 {@link @dataSoure} 为null会抛出异常。实际直接调用 {@link VendorDatabaseIdProvider#getDatabaseName(DataSource)}
   * 
   */
  @Override
  public String getDatabaseId(DataSource dataSource) {
    if (dataSource == null) {
      throw new NullPointerException("dataSource cannot be null");
    }
    try {
      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;
  }

  /**
   * 获取数据库名
   * 

   *     调用 {@link VendorDatabaseIdProvider#getDatabaseName(DataSource)} 获取到数据库产品名赋值给{@link @productName}，
   *     然后遍历 {@link VendorDatabaseIdProvider#properties} 找出第一个能与 {@link @productName} 匹配的 key ,
   *     最后返回其 value ( p:主要是第一个匹配的key，而不是最匹配的key ).如果{@link VendorDatabaseIdProvider#properties}
   *     没有找到，就直接返回{@link @productName}
   * 
   */
  private String getDatabaseName(DataSource dataSource) throws SQLException {
    String productName = getDatabaseProductName(dataSource);
    if (this.properties != null) {
      for (Map.Entry property : properties.entrySet()) {
        /**
         * 用contains能够校验的容错率
         * 这种方式对应出现 productName=’Oracle11’,property的key有'oracle'和'oracle11' 的情况时，
         * 会返回key='oracle'的值，而不是key='oracle11'的值，（取第一个能匹配的key)
         */
        if (productName.contains((String) property.getKey())) {
          return (String) property.getValue();
        }
      }
      // no match, return null
      return null;
    }
    return productName;
  }

  /**
   * 获取数据库产品名
   * 

   *     通过获取数据库来连接得到数据库的描述信息类，从中获取数据库产品名称
   * 
   */
  private String getDatabaseProductName(DataSource dataSource) throws SQLException {
    Connection con = null;
    try {
      con = dataSource.getConnection();
      //DatabaseMetaData 其实就是数据库的描述信息。其实'Meta'的翻译过来是'元',当其实在英文中是表示描述信息的意思。
      DatabaseMetaData metaData = con.getMetaData();
      return metaData.getDatabaseProductName();
    } finally {
      if (con != null) {
        try {
          con.close();
        } catch (SQLException e) {
          // ignored
        }
      }
    }
  }

  /**
   * 只是封装了一个 {@link Log} 并赋值给 {@link LogHolder#log}
   */
  private static class LogHolder {
    private static final Log log = LogFactory.getLog(VendorDatabaseIdProvider.class);
  }

}