Spring boot Interview Questions

Poojitha — Fri, 22 May 2026 18:18:21 +0000

1. What is CORS and why is it required?
CORS (Cross-Origin Resource Sharing) is a browser security mechanism that allows/restricts APIs from being accessed by another domain.
Example:
Frontend: http://localhost:3000
Backend API: http://localhost:8080
These are different origins because ports are different.
Without CORS configuration, browser blocks the request.
Why required?
To securely allow frontend applications to call backend APIs hosted on different domains/ports.
Interview Answer:
CORS is a browser security feature that controls cross-origin HTTP requests. It is required when frontend and backend run on different domains, ports, or protocols. In Spring Boot, we configure CORS to allow trusted origins to access APIs securely.
2. How do you configure CORS in Spring Boot?
Using @CrossOrigin
Java
@RestController
@CrossOrigin(origins = "http://localhost:3000")
public class UserController {
}
Global Configuration
Java
@Configuration
public class CorsConfig {

@Bean
public WebMvcConfigurer corsConfigurer() {
    return new WebMvcConfigurer() {
        @Override
        public void addCorsMappings(CorsRegistry registry) {
            registry.addMapping("/**")
                    .allowedOrigins("http://localhost:3000")           .allowedMethods("GET", "POST", "PUT", "DELETE");
        }
    };
}

}
Real-time usage:
In production, React/Angular frontend calls Spring Boot APIs from another domain.

3. What is CSRF attack?
CSRF = Cross Site Request Forgery
It tricks a logged-in user into performing unwanted actions.
Example:
User logged into banking site
Malicious website sends transfer request automatically
Browser sends session cookie
Server thinks request is genuine
Interview Answer:
CSRF attack occurs when a malicious site tricks an authenticated user into sending unauthorized requests to another application where the user is already logged in.

application.properties

Poojitha — Fri, 22 May 2026 11:11:17 +0000

1.spring.datasource.hikari.connectionTimeout
Meaning:
How long an application waits to get a DB connection from the pool before throwing an exception.
Ex:
spring.datasource.hikari.connectionTimeout=30000
= Wait for 30 seconds
Real-time Scenario
Suppose:
All DB connections are busy
New request comes
Application asks pool for connection
If no connection becomes available within 30 seconds:
Plain text
SQLTransientConnectionException:
Connection is not available, request timed out

2.spring.datasource.hikari.idleTimeout
Meaning:
How long an unused/idle connection stays in pool before being removed.
Unit
Milliseconds
Example
Properties
spring.datasource.hikari.idleTimeout=600000
= 10 minutes
Real-time Understanding
Suppose:
Your app created 20 DB connections
Only 5 are being used
Remaining 15 are idle
After idleTimeout duration:
Extra unused connections may be closed
Helps save DB resources
Important Point
This works mainly when:
Plain text
minimumIdle < maximumPoolSize
Otherwise pool keeps all connections alive.

3.spring.datasource.hikari.maximumPoolSize
Meaning
Maximum number of DB connections allowed in pool.
Example
Properties
spring.datasource.hikari.maximumPoolSize=20
Maximum 20 DB connections can exist.
Real-time Scenario
Suppose:
20 requests are using DB connections
21st request comes
Then:
It waits for available connection
Until connectionTimeout occurs

4.spring.datasource.hikari.minimumIdle
Meaning
Minimum number of idle connections always maintained in pool.
Example
Properties
spring.datasource.hikari.minimumIdle=5
Even if connections are unused:
Hikari keeps at least 5 ready connections
Real-time Benefit
When traffic suddenly increases:
App gets DB connections immediately
No delay creating new connection

DEV Community: Poojitha

Spring boot Interview Questions

application.properties