Spring Boot — Mocking Dependencies with Mockito

package com.ourcompiler.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.*;

// Example demonstration for Mocking Dependencies with Mockito
when(repository.findById(1L)).thenReturn(Optional.of(course));
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

  .   ____          _            __ _ _
 /\\\\ / ___'_ __ _ _(_)_ __  __ _ \\ \\ \\ \\
( ( )\\___ | '_ | '_| | '_ \\/ _` | \\ \\ \\ \\
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\\__, | / / / /
 =========|_|==============|___/=/_/ /_/
 :: Spring Boot ::                (v3.2.3)

2026-08-21T14:30:00.123Z  INFO 12345 --- [main] c.o.app.Application : Starting Application using Java 21...
2026-08-21T14:30:01.890Z  INFO 12345 --- [main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port 8080 (http)
2026-08-21T14:30:02.100Z  INFO 12345 --- [main] c.o.app.Application : Started Application in 2.345 seconds (process running for 2.8)

// Production Service implementation for Mocking Dependencies with Mockito
package com.ourcompiler.app.service;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Service
@Transactional(readOnly = true)
public class CourseService {

    private static final Logger log = LoggerFactory.getLogger(CourseService.class);

    public String getCourseDetails(Long id) {
        log.info("Fetching course details for ID: {}", id);
        return "Spring Boot Masterclass — Chapter 32: Mocking Dependencies with Mockito";
    }
}