How to unit-test a class which has methods using database connection?

I am trying to run tests on a controller class and it has atleast a method which internally uses a DAO to retrieve information from the DB (MySQL).

The problem I have is that the dao method is giving null and I end up with NullPointerException error.

How do I tests a class which has methods that internally use a database connection?<br>
Haven't been able to find any useful post/answer.

Project structure:<br>
<pre>
src
main
java
[package]
RegisterController.java
Config.java // configuration class
test
java
[package]
RegisterControllerTest.java

</pre>

RegisterController.java

package com.webapp.controller;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

import com.webapp.Config;
import com.webapp.dao.AccountDao;

@Controller
public class RegisterController {

    @Autowired
	AccountDao accDao;


    public String validateUsername(String uname){
        List<String> errors = new ArrayList<String>();

        // ... unrelated code 

        // NullPointerException thrown here
        if(accDao.getAccountByUsername(uname) != null)
            errors.add("err#taken");

        return errors.toString();
    }
}

RegisterControllerTest.java

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.springframework.boot.test.context.SpringBootTest;

import com.webapp.controller.RegisterController;
import com.webapp.Config;

@SpringBootTest
public class RegisterControllerTest {
	
	@Mock
	private Config config;

	private RegisterController rc;
	
	@BeforeEach
	public void init() {
		config = Mockito.mock(Config.class);
		rc = new RegisterController();
	}
	

    @Test
    public void testValidateUsername() {
        assertEquals("[]", rc.validateUsername("Username123")); // N.P.E
        
    }
}

Config.java
package com.webapp;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.jdbc.datasource.DriverManagerDataSource;

import com.webapp.dao.AccountDao;
import com.webapp.dao.AccountDaoImpl;
import com.webapp.dao.Dao;

@Configuration
@ComponentScan(basePackages = { "com.webapp.controller", "com.webapp.dao", "com.webapp.test" })
public class Config {

	private static class Database {
		private static String host = "127.0.0.1";
		private static String user = "root";
		private static String pass = "root";
		private static String dbname = "memedb";
		private static int port = 3306;

		public static String getUrl() {
			return "jdbc:mysql://"+host+":"+port+"/"+dbname+"?serverTimezone=Europe/Stockholm";
		}
	}
    
	@Bean
	public DriverManagerDataSource getDataSource() {
		DriverManagerDataSource ds = new DriverManagerDataSource();
		ds.setDriverClassName("com.mysql.cj.jdbc.Driver");
		ds.setUrl(Database.getUrl());
		ds.setUsername(Database.user);
		ds.setPassword(Database.pass);
		return ds;
	}

	
    @Bean
    public AccountDao getAccDao() {
    	return new AccountDaoImpl(getDataSource());
    }

}

Instead of mocking config, mock the Dao. You are getting NPE, because the mocked config is not setting @Autowired accDao... so your accDao == null:

@Controller
public class RegisterController {

    AccountDao accDao;

    public RegisterController(AccountDao accDao) {
      this.accDao = accDao;
    }

    ...
}

@BeforeEach
public void init() {
  accDaoMock = Mockito.mock(AccountDao.class);
  rc = new RegisterController(accDaoMock);
}

@Test
public void testValidateUsername() {
  when(accDaoMock.getAccountByUsername("Username123")).thenReturn(null);
  assertEquals("[]", rc.validateUsername("Username123"));
}

Why are you configuring the connection DB programmatically? I advise you configure the connection DB with autoconfiguration of Spring Boot.

The DAO object have to mock in an unit test.
Here is a good article about the Junit test for a Spring Boot App.