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1. Overview

Spring JdbcTemplate is a powerful tool for developers to focus on writing SQL queries and extracting results. It connects to the back-end database and executes SQL queries directly.

Therefore, we can use integration tests to make sure that we can pull data from the database properly. Also, we can write unit tests to check the correctness of the related functionalities.

In this tutorial, we'll show how to unit test JdbcTemplate code.

2. JdbcTemplate and Running Queries

Firstly, let's start with a data access object (DAO) class that uses JdbcTemplate:

public class EmployeeDAO {
    private JdbcTemplate jdbcTemplate;

    public void setDataSource(DataSource dataSource) {
        jdbcTemplate = new JdbcTemplate(dataSource);
    }

    public int getCountOfEmployees() {
        return jdbcTemplate.queryForObject("SELECT COUNT(*) FROM EMPLOYEE", Integer.class);
    }
}

We dependency-inject a DataSource object into the EmployeeDAO class. Then, we create the JdbcTemplate object in the setter method. Also, we use JdbcTemplate in an example method getCountOfEmployees.

There are two ways to unit test methods that use JdbcTemplate.

We can use an in-memory database such as the H2 database as the data source for testing. However, in real-world applications, the SQL query could have complicated relationships, and we need to create complex setup scripts to test the SQL statements.

Alternatively, we can also mock the JdbcTemplate object to test the method functionality.

3. Unit Test With H2 Database

We can create a data source that connects to the H2 database and inject it into the EmployeeDAO class:

@Test
public void whenInjectInMemoryDataSource_thenReturnCorrectEmployeeCount() {
    DataSource dataSource = new EmbeddedDatabaseBuilder().setType(EmbeddedDatabaseType.H2)
      .addScript("classpath:jdbc/schema.sql")
      .addScript("classpath:jdbc/test-data.sql")
      .build();

    EmployeeDAO employeeDAO = new EmployeeDAO();
    employeeDAO.setDataSource(dataSource);

    assertEquals(4, employeeDAO.getCountOfEmployees());
}

In this test, we first construct a data source on the H2 database. During the construction, we execute schema.sql to create the EMPLOYEE table:

CREATE TABLE EMPLOYEE
(
    ID int NOT NULL PRIMARY KEY,
    FIRST_NAME varchar(255),
    LAST_NAME varchar(255),
    ADDRESS varchar(255)
);

Also, we run test-data.sql to add test data into the table:

INSERT INTO EMPLOYEE VALUES (1, 'James', 'Gosling', 'Canada');
INSERT INTO EMPLOYEE VALUES (2, 'Donald', 'Knuth', 'USA');
INSERT INTO EMPLOYEE VALUES (3, 'Linus', 'Torvalds', 'Finland');
INSERT INTO EMPLOYEE VALUES (4, 'Dennis', 'Ritchie', 'USA');

Then, we can inject this data source into the EmployeeDAO class and test the getCountOfEmployees method over the in-memory H2 database.

4. Unit Test With Mock Object

We can mock the JdbcTemplate object so that we don't need to run the SQL statement on a database:

public class EmployeeDAOUnitTest {
    @Mock
    JdbcTemplate jdbcTemplate;

    @Test
    public void whenMockJdbcTemplate_thenReturnCorrectEmployeeCount() {
        EmployeeDAO employeeDAO = new EmployeeDAO();
        ReflectionTestUtils.setField(employeeDAO, "jdbcTemplate", jdbcTemplate);
        Mockito.when(jdbcTemplate.queryForObject("SELECT COUNT(*) FROM EMPLOYEE", Integer.class))
          .thenReturn(4);

        assertEquals(4, employeeDAO.getCountOfEmployees());
    }
}

In this unit test, we first declare a mock JdbcTemplate object with the @Mock annotation. Then we inject it to the EmployeeDAO object using ReflectionTestUtilsAlso, we use the Mockito utility to mock the return result of the JdbcTemplate query. This allows us to test the functionality of the getCountOfEmployees method without connecting to a database.

We use an exact match on the SQL statement string when we mock the JdbcTemplate query. In real-world applications, we may create complex SQL strings, and it is hard to do an exact match. Therefore, we can also use the anyString() method to bypass the string check:

Mockito.when(jdbcTemplate.queryForObject(Mockito.anyString(), Mockito.eq(Integer.class)))
  .thenReturn(3);
assertEquals(3, employeeDAO.getCountOfEmployees());

5. Conclusion

In this tutorial, we showed two ways to unit test JdbcTemplate.

As always, the source code for the article is available over on GitHub.

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