Is it possible to map basic JDBC SQL types to a Ja

问题:

When retrieving a column from a ResultSet object using the getObject method, the class is chosen by the JDBC driver. I am looking for a way to choose which class the column is retrieved as at runtime.

In the example below type Class1 is created with variable T as an Integer.

Class Class1<T>
{
  public T getColumn(DataSource ds)
  {
    T value = null;
    Connection con = null;

    try
    {
      con = ds.getConnection();
      Statement st = con.createStatement();
      ResultSet rs = st.executeQuery("select 1 from dual");
      rs.next();
      Object o = rs.getObject(1); // I want an Integer but a BigDecimal is created!
      value = (T) o; // ClassCastException here!
      rs.close();
      st.close();
    }
    finally
    {
      if (con != null) { con.close(); }
    }

    return i;
  }
}

The ResultSet class provides a method called getObject that accepts a column argument and a type Map argument. This type Map is only used for UDT (user defined types). I need this functionality applied to basic SQL types as well. I know I can implement a solution to this using a switch/case statement but I'm trying to avoid that. Is this possible?

EDIT

I should've mentioned I am on Java 1.6. It looks like this may have been solved in 1.7. Thanks JB Nizet for the reference.

回答1:

You may have to write your own code. You may have to perform casting of the data you receive. To avoid switch/case, you can use an exact ORM tools like Hibernate, which will allow your Pojo to have exact names like that of table columns and they automatically populate your Pojo with the data from columns. After this you can write your data conversion code for each column. An ORM tool like Hibernate, MyBatis will do the mapping for you.

回答2:

JB Nizet has provided the answer for Java 1.7: http://docs.oracle.com/javase/8/docs/api/java/sql/ResultSet.html#getObject-int-java.lang.Class-

For earlier versions of Java, DolphinJava is correct. I will have to write my own code.

How I Solved It

Keeping with my example above using Class1, I created sub-classes that extend Class1 with T being the type needed and then made type-specific methods that call the getColumn method in Class1:

public Class1<T>
{
  public Object getColumn(DataSource ds)
  {
    Object o        = null;
    Connection con  = null;

    try
    {
      con           = ds.getConnection();
      Statement st  = con.createStatement();
      ResultSet rs  = st.executeQuery("select 1 from dual");
      rs.next();
      Object o      = rs.getObject(1);
      rs.close();
      st.close();
    }
    finally
    {
      if (con != null) { con.close(); }
    }

    return o;
  }
}

Class1Integer extends Class1<Integer>
{
  public Integer getIntegerColumn()
  {
    Object o  = getColumn(datasource);
    Integer i = new Integer(o);
    return i;
  }
}