JDBC学习笔记(四)

　　减少各个Dao类间的重复代码，有以下几种方式：

写一个DBConnectionManager，将公共的查询逻辑做成方法，将sql语句作为参数传递给方法。

public class DBConnectionManager{

  static{
   Class.forName("com.mysql.jdbc.Driver");
  }

 //读操作
 public static List<Map<String,Object>> selectObject(String sql, String[] params) throws Exception {
  Connection conn = null;
  PreparedStatement pstmt = null;
  ResultSet rs = null;
  List<Map<String,Object>>> result  = new ArrayList<Map<String,Object>>>();
  try {
   conn = DBConnectionManager.getConnection();
   pstmt = conn.prepareStatement(sql);

   for (int i = 0; params != null && i < params.length; i++) {
    pstmt.setString(i + 1, params[i]);
   }
   rs = pstmt.executeQuery();
   ResultSetMetaData meta = rs.getMetaData();
   while (rs.next()) {
    Map<String,Object> columnValue = new HashMap<String,Object>
    int size = meta.getColumnCount();
    for (int i = 1; i <= size; i++) {
     String columnName = meta.getColumnLabel(i);  //getColumnName返回的是数据库列名，getColumnLabel如有别名将返回列的别名，否则和getColumnName相同
     columnValue.add(columnName,rs.getObject(columnName));
    }
    result.add(columnValue);
   }
   return result;
  } catch (Exception e) {
   //logger.info("Execute sql : " + sql + " fail!!!");
   throw e;
  } finally {
   DBConnectionManager.free(conn, pstmt, rs);
  }
 }

 //增删改操作
 public static void updateObject(String sql, String[] params) throws Exception {
  Connection conn = null;
  PreparedStatement pstmt = null;
  ResultSet rs = null;
  try {
   conn = DBConnectionManager.getConnection();
   pstmt = conn.prepareStatement(sql);

   for (int i = 0; params != null && i < params.length; i++) {
    pstmt.setObject(i + 1, params[i]);
   }
   rs = pstmt.executeUpdate();

  } catch (Exception e) {
   //logger.info("Execute sql : " + sql + " fail!!!");
   throw e;
  } finally {
   DBConnectionManager.free(conn, pstmt, rs);
  }
 }

 //更好的做法是从数据库连接池中取链接
 public static Connection getConnection(){
      String dbName = "nnm5";
      String passwrod = "OSSDB123";
      String userName = "root";
      String url = "jdbc:mysql://localhost:13306/" + dbName; 

      Connection conn = DriverManager.getConnection(url, userName,passwrod);
      return conn;
 }

 public static void free(Connection conn,PreparedStatement pstmt,ResultSet rs){
    if (rs != null) {
              try {
                  rs.close();
              } finally{
　　　　　　　　　if(ps != null){
　　　　　　　　　　 try{
　　　　　　　　　　　　 ps.close();
                        }finally{
　　　　　　　　　　　　　if(conn != null){
　　　　　　　　　　　　　　 conn.close();
                         }
                      }
　　　　　　　　  }
          }
     }

}

　　上述方法不好的地方在于返回的结果是 List<Map<String,Object>>，如果希望能像ORM框架那样的返回对象，就可以实现一个RowMapper，类似于Spring中提供的SimpleJdbcTemplate。

代码中加上RowMapper也有两种方式，第一种是使用模板方式，第二种是使用策略方式。

使用模板方式的例子如下,修改上面的模板类中的方法:

public static List selectObject(String sql, String[] params) throws Exception {
  Connection conn = null;
  PreparedStatement pstmt = null;
  ResultSet rs = null;
  List<Map<String,Object>>> result  = new ArrayList<Map<String,Object>>>();
  try {
   conn = DBConnectionManager.getConnection();
   pstmt = conn.prepareStatement(sql);

   for (int i = 0; params != null && i < params.length; i++) {
    pstmt.setString(i + 1, params[i]);
   }
   rs = pstmt.executeQuery();
   List list = new ArrayList();
   int index=0;
   while (rs.next()) {
      list.add(objectMapper(rs));  //传递的是每次减去一行后的结果集
   index++;
   }
   return list ;
  } catch (Exception e) {
   //logger.info("Execute sql : " + sql + " fail!!!");
   throw e;
  } finally {
   DBConnectionManager.free(conn, pstmt, rs);
  }
 }

public abstract  Object objectMapper(ResultSet rs);

　　具体的Dao中的代码如下：

public List getPerson(Integer id){
     String sql="select id,name from person where id<?";
     Object [] objs=new Object[]{id};
     return super.getObject(sql,objs);  //实际调用的还是该类中的objectMapper。
}

public Object objectMapper(ResultSet rs){
     Person person=new Person;
   try{
              person.setId((Integer)rs.getObject(1));
              person.setName((String)rs.getObject(2));
      }catch(Exception e){
           logger.log(e.printStackTrace());
     }
     return person;
}

使用策略模式修改模板类中的代码如下：

首先需要一个接口：

public interface RowMapper {
    public Object objectMapper(ResultSet rs);
}

再修改模板类：

public static List selectObject(String sql, String[] params, RowMapper mapper) throws Exception {
  Connection conn = null;
  PreparedStatement pstmt = null;
  ResultSet rs = null;
  List<Map<String,Object>>> result  = new ArrayList<Map<String,Object>>>();
  try {
   conn = DBConnectionManager.getConnection();
   pstmt = conn.prepareStatement(sql);

   for (int i = 0; params != null && i < params.length; i++) {
    pstmt.setString(i + 1, params[i]);
   }
   rs = pstmt.executeQuery();
   List list = new ArrayList();
   int index=0;
   while (rs.next()) {
      list.add(mapper.objectMapper(rs));  //传递的是每次减去一行后的结果集
   index++;
   }
   return list ;
  } catch (Exception e) {
   //logger.info("Execute sql : " + sql + " fail!!!");
   throw e;
  } finally {
   DBConnectionManager.free(conn, pstmt, rs);
  }
 }

　　具体的Dao中的代码：

public List getPerson(Integer id) {
         String sql="select id,name from person where id<?";
         Object [] objs=new Object[]{id};
         return mu.getObject(sql,objs,new MyRowMapper1());
}

class MyRowMapper1 implements RowMapper{
         public Object objectMapper(ResultSet rs) {
         Person person=new Person();
         try{
                             person.setId((Integer)rs.getObject(1));
                             person.setName((String)rs.getObject(2));
         }catch(Exception e){
              logger.log(e.printStackTrace());
         }
         return person;
      }
}