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Spring - RowMapper Interface with Example

In the Spring JDBC framework, the RowMapper interface is used to map rows of a ResultSet to a specific object, so that each row of the ResultSet corresponds to an instance of the object. It's typically used when executing queries that return multiple rows.

Here's the primary method from the RowMapper interface:

T mapRow(ResultSet rs, int rowNum) throws SQLException;

This method should map the current row of the given ResultSet to an instance of the target class (T). The rowNum argument is the number of the current row, starting from 0.

Let's illustrate its usage with an example.

Given the same User domain object from the previous example:

public class User {
    private int id;
    private String name;
    private String email;
    
    // getters, setters, and other methods
}

We can create a RowMapper for the User class:

public class UserRowMapper implements RowMapper<User> {

    @Override
    public User mapRow(ResultSet rs, int rowNum) throws SQLException {
        User user = new User();
        user.setId(rs.getInt("id"));
        user.setName(rs.getString("name"));
        user.setEmail(rs.getString("email"));
        return user;
    }
}

Now, we can use the JdbcTemplate along with our RowMapper to fetch the list of users:

@Autowired
private JdbcTemplate jdbcTemplate;

public List<User> fetchAllUsers() {
    String sql = "SELECT * FROM users";
    return jdbcTemplate.query(sql, new UserRowMapper());
}

When the query is executed, for each row in the ResultSet, the mapRow method of the UserRowMapper will be called, mapping each row to a User object. The list of User objects is then returned.

Compared to ResultSetExtractor, RowMapper is more suited for straightforward row-to-object mapping scenarios, while ResultSetExtractor offers more flexibility for complex or non-standard transformations.

RowMapper interface in Spring example:

Description: The RowMapper interface in Spring is used to map a row from a ResultSet to a domain object. It provides a clean way to separate database mapping logic from data access code.

Code Example:

public class EmployeeRowMapper implements RowMapper<Employee> {
    @Override
    public Employee mapRow(ResultSet rs, int rowNum) throws SQLException {
        Employee employee = new Employee();
        employee.setId(rs.getLong("id"));
        employee.setName(rs.getString("name"));
        // Populate other fields
        return employee;
    }
}

How to use RowMapper in Spring JDBC:
- Description: Use RowMapper in Spring JDBC to map rows from a ResultSet to domain objects when querying the database.
- Code Example:
```
String sql = "SELECT * FROM employees";
List<Employee> employees = jdbcTemplate.query(sql, new EmployeeRowMapper());
```
Spring RowMapper vs ResultSetExtractor:
- Description: Compare Spring RowMapper and ResultSetExtractor. RowMapper maps a row to an object, while ResultSetExtractor processes the entire ResultSet.
- Code Example (ResultSetExtractor):
```
String sql = "SELECT * FROM employees";
List<Employee> employees = jdbcTemplate.query(sql, new CustomResultSetExtractor());
```

Custom RowMapper implementation in Spring:

Description: Implement a custom RowMapper in Spring for specialized mapping logic according to your domain object structure.

Code Example:

public class CustomObjectRowMapper implements RowMapper<CustomObject> {
    @Override
    public CustomObject mapRow(ResultSet rs, int rowNum) throws SQLException {
        // Custom mapping logic
    }
}

Mapping database rows to Java objects with RowMapper:
- Description: Use RowMapper to map database rows to Java objects, providing a clean separation of concerns in data access code.
- Code Example:
```
String sql = "SELECT * FROM my_table";
List<MyEntity> entities = jdbcTemplate.query(sql, new MyEntityRowMapper());
```
Spring JdbcTemplate RowMapper example:
- Description: Utilize RowMapper with Spring JdbcTemplate for mapping database rows to Java objects in a concise and efficient manner.
- Code Example:
```
String sql = "SELECT * FROM employees";
List<Employee> employees = jdbcTemplate.query(sql, new EmployeeRowMapper());
```

RowMapper in Spring NamedParameterJdbcTemplate:

Description: Apply RowMapper with Spring's NamedParameterJdbcTemplate for named parameter support in your SQL queries.

Code Example:

String sql = "SELECT * FROM employees WHERE department = :dept";
MapSqlParameterSource parameters = new MapSqlParameterSource().addValue("dept", "IT");
List<Employee> employees = namedParameterJdbcTemplate.query(sql, parameters, new EmployeeRowMapper());

Implementing RowMapper in Spring DAO:

Description: Implement RowMapper in a Spring DAO (Data Access Object) to encapsulate the mapping logic within your data access layer.

Code Example:

public class EmployeeDAO {
    public List<Employee> getAllEmployees() {
        String sql = "SELECT * FROM employees";
        return jdbcTemplate.query(sql, new EmployeeRowMapper());
    }
}

Working with RowMapper and JdbcTemplate:
- Description: Work seamlessly with RowMapper and JdbcTemplate to handle database result set mapping efficiently in a Spring application.
- Code Example:
```
String sql = "SELECT * FROM my_table";
List<MyEntity> entities = jdbcTemplate.query(sql, new MyEntityRowMapper());
```

Spring RowMapper for complex object mapping:

Description: Leverage Spring RowMapper for complex object mapping scenarios, allowing you to map database rows to complex Java objects.

Code Example:

public class ComplexEntityRowMapper implements RowMapper<ComplexEntity> {
    @Override
    public ComplexEntity mapRow(ResultSet rs, int rowNum) throws SQLException {
        // Custom logic for mapping complex entity
    }
}

RowMapper with Spring Data JDBC:
- Description: Use RowMapper in conjunction with Spring Data JDBC for seamless integration of custom result set mapping in Spring Data projects.
- Code Example:
```
public interface MyEntityRepository extends CrudRepository<MyEntity, Long> {
    @Query("SELECT * FROM my_entity")
    List<MyEntity> findAllEntitiesWithCustomMapping();
}
```

Handling NULL values with RowMapper in Spring:

Description: Handle NULL values gracefully while mapping rows with RowMapper in Spring, ensuring robustness in your data access layer.

Code Example:

public class MyEntityRowMapper implements RowMapper<MyEntity> {
    @Override
    public MyEntity mapRow(ResultSet rs, int rowNum) throws SQLException {
        String nullableField = rs.getString("nullable_field");
        // Check for NULL and handle accordingly
    }
}

RowMapper vs JPA in Spring:
- Description: Compare RowMapper and JPA (Java Persistence API) in Spring. RowMapper is a lightweight option for mapping results, while JPA provides a more comprehensive ORM solution.
- Code Example (JPA):
```
@Entity
@Table(name = "employees")
public class Employee {
    // JPA annotations and fields
}
```

Custom result set mapping with RowMapper:

Description: Implement custom result set mapping logic using RowMapper to accommodate specific requirements in your Spring application.

Code Example:

public class CustomMappingRowMapper implements RowMapper<CustomObject> {
    @Override
    public CustomObject mapRow(ResultSet rs, int rowNum) throws SQLException {
        // Custom mapping logic
    }
}

RowMapper for handling date fields in Spring:

Description: Use RowMapper to handle date fields efficiently during the mapping process in a Spring application.

Code Example:

public class DateEntityRowMapper implements RowMapper<DateEntity> {
    @Override
    public DateEntity mapRow(ResultSet rs, int rowNum) throws SQLException {
        // Handle date fields appropriately
    }
}

Using RowMapper with Spring Boot:

Description: Integrate RowMapper seamlessly with Spring Boot for efficient result set mapping in a Spring Boot application.

Code Example:

@Repository
public class MyEntityRepository {
    public List<MyEntity> getAllEntities() {
        String sql = "SELECT * FROM my_entity";
        return jdbcTemplate.query(sql, new MyEntityRowMapper());
    }
}