Mapping a JDBC ResultSet to an object

问题:

I have a user class that has 16 attributes, things such as firstname, lastname, dob, username, password etc... These are all stored in a MySQL database and when I want to retrieve users I use a ResultSet. I want to map each of the columns back to the user attributes but the way I am doing it seems terribly inefficient. For example I am doing:

//ResultSet rs;
while(rs.next()) {
   String uid = rs.getString("UserId");
   String fname = rs.getString("FirstName");
   ...
   ...
   ...
   User u = new User(uid,fname,...);
   //ArrayList<User> users 
   users.add(u);
} 

i.e I retrieve all the columns and then create user objects by inserting all the column values into the User constructor.

Does anyone know of a faster, neater, way of doing this?

回答1:

No need of storing resultSet values into String and again setting into POJO class. Instead set at the time you are retrieving.

Or best way switch to ORM tools like hibernate instead of JDBC which maps your POJO object direct to database.

But as of now use this:

List<User> users=new ArrayList<User>();

while(rs.next()) {
   User user = new User();      
   user.setUserId(rs.getString("UserId"));
   user.setFName(rs.getString("FirstName"));
  ...
  ...
  ...


  users.add(user);
} 

回答2:

If you don't want to use any JPA provider such as openJPA or hibernate, you can just give apache dbutils a try.

http://commons.apache.org/proper/commons-dbutils/examples.html

then your code will look like this

QueryRunner run = new QueryRunner(dataSource);

// Use the BeanListHandler implementation to convert all
// ResultSet rows into a List of Person JavaBeans.
ResultSetHandler<List<Person>> h = new BeanListHandler<Person>(Person.class);

// Execute the SQL statement and return the results in a List of
// Person objects generated by the BeanListHandler.
List<Person> persons = run.query("SELECT * FROM Person", h);

回答3:

Let's assume you want to use core Java, w/o any strategic frameworks. If you can guarantee, that field name of an entity will be equal to the column in database, you can use Reflection API (otherwise create annotation and define mapping name there)

By FieldName

/**

Class<T> clazz - a list of object types you want to be fetched
ResultSet resultSet - pointer to your retrieved results 

*/

    List<Field> fields = Arrays.asList(clazz.getDeclaredFields());
    for(Field field: fields) {
        field.setAccessible(true);
    }

    List<T> list = new ArrayList<>(); 
    while(resultSet.next()) {

        T dto = clazz.getConstructor().newInstance();

        for(Field field: fields) {
            String name = field.getName();

            try{
                String value = resultSet.getString(name);
                field.set(dto, field.getType().getConstructor(String.class).newInstance(value));
            } catch (Exception e) {
                e.printStackTrace();
            }

        }

        list.add(dto);

    }

By annotation

@Retention(RetentionPolicy.RUNTIME)
public @interface Col {

    String name();
}

---

DTO:

class SomeClass {

   @Col(name = "column_in_db_name")
   private String columnInDbName;

   public SomeClass() {}

   // ..

}

---

Same, but

    while(resultSet.next()) {

        T dto = clazz.getConstructor().newInstance();

        for(Field field: fields) {
            Col col = field.getAnnotation(Col.class);
            if(col!=null) {
                String name = col.name();
                try{
                    String value = resultSet.getString(name);
                    field.set(dto, field.getType().getConstructor(String.class).newInstance(value));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }

        list.add(dto);

    }

Thoughts

In fact, iterating over all Fields might seem ineffective, so I would store mapping somewhere, rather than iterating each time. However, if our T is a DTO with only purpose of transferring data and won't contain loads of unnecessary fields, that's ok. In the end it's much better than using boilerplate methods all the way.

Hope this helps someone.

回答4:

Complete solution using @TEH-EMPRAH ideas and Generic casting from Cast Object to Generic Type for returning

import annotations.Column;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.sql.SQLException;
import java.util.*;

public class ObjectMapper<T> {

    private Class clazz;
    private Map<String, Field> fields = new HashMap<>();
    Map<String, String> errors = new HashMap<>();

    public DataMapper(Class clazz) {
        this.clazz = clazz;

        List<Field> fieldList = Arrays.asList(clazz.getDeclaredFields());
        for (Field field : fieldList) {
            Column col = field.getAnnotation(Column.class);
            if (col != null) {
                field.setAccessible(true);
                fields.put(col.name(), field);
            }
        }
    }

    public T map(Map<String, Object> row) throws SQLException {
        try {
            T dto = (T) clazz.getConstructor().newInstance();
            for (Map.Entry<String, Object> entity : row.entrySet()) {
                if (entity.getValue() == null) {
                    continue;  // Don't set DBNULL
                }
                String column = entity.getKey();
                Field field = fields.get(column);
                if (field != null) {
                    field.set(dto, convertInstanceOfObject(entity.getValue()));
                }
            }
            return dto;
        } catch (IllegalAccessException | InstantiationException | NoSuchMethodException | InvocationTargetException e) {
            e.printStackTrace();
            throw new SQLException("Problem with data Mapping. See logs.");
        }
    }

    public List<T> map(List<Map<String, Object>> rows) throws SQLException {
        List<T> list = new LinkedList<>();

        for (Map<String, Object> row : rows) {
            list.add(map(row));
        }

        return list;
    }

    private T convertInstanceOfObject(Object o) {
        try {
            return (T) o;
        } catch (ClassCastException e) {
            return null;
        }
    }
}

and then in terms of how it ties in with the database, I have the following:

// connect to database (autocloses)
try (DataConnection conn = ds1.getConnection()) {

    // fetch rows
    List<Map<String, Object>> rows = conn.nativeSelect("SELECT * FROM products");

    // map rows to class
    ObjectMapper<Product> objectMapper = new ObjectMapper<>(Product.class);
    List<Product> products = objectMapper.map(rows);

    // display the rows
    System.out.println(rows);

    // display it as products
    for (Product prod : products) {
        System.out.println(prod);
    }

} catch (Exception e) {
    e.printStackTrace();
}

回答5:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.json.simple.JSONObject;
import com.google.gson.Gson;

public class ObjectMapper {

//generic method to convert JDBC resultSet into respective DTo class
@SuppressWarnings("unchecked")
public static Object mapValue(List<Map<String, Object>> rows,Class<?> className) throws Exception
{

        List<Object> response=new ArrayList<>(); 
        Gson gson=new Gson();

        for(Map<String, Object> row:rows){
        org.json.simple.JSONObject jsonObject = new JSONObject();
        jsonObject.putAll(row);
        String json=jsonObject.toJSONString();
        Object actualObject=gson.fromJson(json, className);
        response.add(actualObject);
        }
        return response;

    }

    public static void main(String args[]) throws Exception{

        List<Map<String, Object>> rows=new ArrayList<Map<String, Object>>(); 

        //Hardcoded data for testing
        Map<String, Object> row1=new HashMap<String, Object>();
        row1.put("name", "Raja");
        row1.put("age", 22);
        row1.put("location", "India");


        Map<String, Object> row2=new HashMap<String, Object>();
        row2.put("name", "Rani");
        row2.put("age", 20);
        row2.put("location", "India");

        rows.add(row1);
        rows.add(row2);


        @SuppressWarnings("unchecked")
        List<Dto> res=(List<Dto>) mapValue(rows, Dto.class);


    }

    }

    public class Dto {

    private String name;
    private Integer age;
    private String location;

    //getters and setters

    }

Try the above code .This can be used as a generic method to map JDBC result to respective DTO class.

回答6:

I would like to hint on q2o. It is a JPA based Java object mapper which helps with many of the tedious SQL and JDBC ResultSet related tasks, but without all the complexity an ORM framework comes with. With its help mapping a ResultSet to an object is as easy as this:

while(rs.next()) {
    users.add(Q2Obj.fromResultSet(rs, User.class));
}

More about q2o can be found here.

回答7:

Use Statement Fetch Size , if you are retrieving more number of records. like this.

Statement statement = connection.createStatement();
statement.setFetchSize(1000); 

Apart from that i dont see an issue with the way you are doing in terms of performance

In terms of Neat. Always use seperate method delegate to map the resultset to POJO object. which can be reused later in the same class

like

private User mapResultSet(ResultSet rs){
     User user = new User();
     // Map Results
     return user;
}

If you have the same name for both columnName and object's fieldName , you could also write reflection utility to load the records back to POJO. and use MetaData to read the columnNames . but for small scale projects using reflection is not an problem. but as i said before there is nothing wrong with the way you are doing.

回答8:

using DbUtils...

The only problem I had with that lib was that sometimes you have relationships in your bean classes, DBUtils does not map that. It only maps the properties in the class of the bean, if you have other complex properties (refering other beans due to DB relationship) you'd have to create "indirect setters" as I call, which are setters that put values into those complex properties's properties.