How do you implement custom row mapping in JdbcTemplate?

Table of Contents

• Introduction
• Understanding RowMapper in JdbcTemplate
  • What is RowMapper?
• Implementing Custom Row Mapping
  • Step 1: Define Your Custom Java Object
  • Step 2: Create a Custom RowMapper
  • Step 3: Use JdbcTemplate with Custom RowMapper
  • Step 4: Calling the Method to Retrieve Data
  • Output Example:
• Practical Example: Mapping Nested Data
• Conclusion

Introduction

In Spring Framework, JdbcTemplate provides a convenient way to interact with a relational database by simplifying database operations like querying, updating, and deleting data. One of the core features of JdbcTemplate is its ability to map SQL result sets to Java objects. By default, Spring uses simple row mappings, but often, you may need custom row mapping for more complex mappings between SQL results and custom Java objects. In this guide, we will explore how to implement custom row mapping using the RowMapper interface.

Understanding RowMapper in JdbcTemplate

What is RowMapper?

RowMapper is an interface provided by Spring's JdbcTemplate to map each row of a ResultSet to a Java object. The RowMapper interface has a method called mapRow(), which is responsible for extracting values from the result set and converting them into an object.

While Spring provides a default implementation of RowMapper for basic types (like Integer, String, etc.), custom row mapping is often necessary when working with custom objects.

Implementing Custom Row Mapping

Step 1: Define Your Custom Java Object

Before implementing custom row mapping, you need a custom Java object to map the data from the SQL result set. Let’s say we have a Person class with properties like id, name, and age.

Step 2: Create a Custom RowMapper

Now, you can implement the RowMapper interface to define how to map each row of the SQL result set to an instance of the Person class. The mapRow() method extracts the data from the ResultSet and sets it into the Java object.

In the mapRow() method:

• We retrieve data from the ResultSet using the column names (e.g., "id", "name", and "age").
• We set the retrieved values into the Person object and return it.

Step 3: Use JdbcTemplate with Custom RowMapper

After defining your custom row mapper, you can now use JdbcTemplate to query the database and map the result set to your Person objects using the custom RowMapper.

In this example:

• The query() method of JdbcTemplate is used to execute the SQL query.
• We pass the PersonRowMapper to the query() method to map the result set to Person objects.

Step 4: Calling the Method to Retrieve Data

Finally, you can call the method getAllPersons() to retrieve a list of Person objects from the database.

Output Example:

Practical Example: Mapping Nested Data

If you need to map more complex SQL queries, such as those that involve joining multiple tables, you can extend the RowMapper to handle nested objects or collections.

For example, if we have an Address object related to the Person:

You can extend the RowMapper to handle this more complex mapping:

Conclusion

Implementing custom row mapping in JdbcTemplate using the RowMapper interface allows you to map SQL result sets to Java objects effectively. Whether dealing with simple mappings or more complex, nested data, Spring provides a flexible mechanism to handle various database interaction scenarios. By defining a custom RowMapper, you ensure that the data returned from your SQL queries is properly transformed into Java objects that your application can use. This enhances the readability, maintainability, and flexibility of your database access code.