Spring Boot makes backend development faster and cleaner, but when something breaks, the error logs can feel overwhelming—especially if you’re new to the framework.
In this post, I’ll walk through some common Spring Boot errors I’ve personally encountered, show realistic error logs, explain why they happen, and share practical fixes that actually work. This guide is aimed at beginner to intermediate developers who want to debug Spring Boot applications with confidence.
1️⃣ Port Already in Use
🔴 Error Log
`Web server failed to start. Port 8080 was already in use.`
🤔 Why this happens
- Another application is already running on port 8080
- A previous Spring Boot instance didn’t shut down properly
✅ How to fix it
Option 1: Stop the running process
- Find the process using port 8080 and stop it
Option 2: Change the application port
server.port=8081
🧠 Tip
Always make sure old instances of your application are stopped before restarting.
2️⃣ BeanCreationException
🔴 Error Log
`Error creating bean with name 'userService':
Unsatisfied dependency expressed through field 'userRepository'`
🤔 Why this happens
- Missing Spring annotations like @Service, @Repository, or @Component
- Circular dependencies between beans
- Package scanning issues
✅ How to fix it
- Ensure correct annotations are present
- Verify your package structure
- Avoid circular dependencies
@Service
public class UserService {
}
3️⃣ NullPointerException
🔴 Error Log
java.lang.NullPointerException: Cannot invoke "User.getId()" because "user" is null
🤔 Why this happens
- Object was never initialized
- Repository returned null
- Missing validation or null checks
✅ How to fix it
- Validate inputs
- Use Optional when fetching data
Optional<User> user = userRepository.findById(id);
🧠 Tip
- Never assume data exists—always handle the null case explicitly.
4️⃣ Failed to Configure a DataSource
🔴 Error Log
Failed to configure a DataSource: 'url' attribute is not specified
🤔 Why this happens
- Database credentials are missing
- Database server is not running
- Incorrect driver dependency
✅ How to fix it
- Check your application.properties or application.yml:
spring.datasource.url=jdbc:mysql://localhost:3306/testdb
spring.datasource.username=root
spring.datasource.password=password
5️⃣ 404 Error for Controller APIs
🔴 Error Log
No mapping found for HTTP request
🤔 Why this happens
- Incorrect API URL
- Missing @RestController
- Wrong HTTP method used
✅ How to fix it
- Make sure your controller is properly annotated:
@RestController
@RequestMapping("/api/users")
public class UserController {
}
6️⃣ How to Read Spring Boot Logs Effectively
- Spring Boot logs can be noisy, but you don’t need to read everything.
✅ Focus on:
- The first Caused by message
- The exception name
- The line number in your code
🚫 Ignore:
- Repeated stack trace lines
- Framework internals unless required
- Learning how to read logs properly will save you hours of debugging time.
🏁 Conclusion
Errors are not a sign that you’re bad at Spring Boot—they’re part of the learning process.
The key is understanding why an error occurs, knowing where to look in the logs, and applying a systematic fix. Over time, debugging becomes less frustrating and more intuitive.
If this post helped you, feel free to share the most confusing Spring Boot error you’ve faced in the comments 👇
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