Extending database access

RowHandler - returning custom data structures

When convenience methods don't work and you need to customize the data structure that is returned, you can roll your own RowHandler. This interface defines the handle method that takes a ResultSet and a parameterized type T. It maps data in a single row to a class. In fact, all of the query convenience methods described in this documentation use out of the box implementations of RowHandler function. For instance, MapRowHandler and BeanRowHandler are such implementations.

To use RowHandler, first define an implementation. If you plan to reuse the implementation, implement it normally as a public class. If it is a one off implementation, you can define it anonymously. The following example shows RowHandler being implemented as an anonymous class:

public class CustomClass {
    public String customProp1;
    public Date customProp2;
}

import java.sql.ResultSet;
import com.ibm.pdq.runtime.handlers.RowHandler;
...
CustomClass myCustCls = data.queryFirst( "SELECT column1,column2 FROM sometable WHERE id=?", new RowHandler() {
    public CustomClass handle(ResultSet resultSet, CustomClass obj) {
        CustomClass custom = new CustomClass();
        custom.customProp1 = resultSet.getString(1);
        custom.customProp2 = resultSet.getDate(2);
        return custom;
    }
}, 100 /* don't forget the parameters! */);

System.out.print(myCustCls.toString()); // do something interesting with myCustCls

Using ResultHandler to handle the entire ResultSet

Using the ResultHandler is especially useful in translating ResultSet directly into customized Java classes. Many other frameworks would require mapping the ResultSet to a Java bean and then mapping the bean to another class or data structure. Using the pluggable ResultHandler you can completely customize the data access functionality. The previous example showed how implementing a custom RowHandler can change how Manager processes individual ResultSet rows. If, however, more fine grained control over ResultSet handling is required, implement ResultHandler. Like RowHandler, this can be a one off as an anonymous class or a reused pattern as its own class. ResultHandler is responsible for looping over the ResultSet and producing something useful. It might or might not use a RowHandler. In fact, depending on needs, it can use more than one RowHandler.