JavaHacker

14/06/2026

HashMap internal working flow

03/01/2026

01/01/2026

28/12/2025

How many people will fail this simple Quiz 👇

28/12/2025

27/12/2025

26/12/2025

Spring Security Architecture – Complete Flow Explained

Spring Security may look complex, but its architecture follows a very clear flow.
Understanding this flow is crucial for interviews and real-world projects.

Let’s break it down step by step 👇
1. Client Sends Request
A user sends an HTTP request (login or API access) from browser/mobile/Postman.
2. Security Filter Chain
Before reaching your controller, the request passes through Spring Security filters.
Example filters:
UsernamePasswordAuthenticationFilter
JwtAuthenticationFilter
👉 Filters intercept every request.
3. Authentication Manager
The filter delegates authentication to AuthenticationManager.
✔ Responsible for validating user identity
✔ Delegates work to AuthenticationProvider
4. Authentication Provider
This is where actual verification happens.
Calls UserDetailsService
Validates credentials using PasswordEncoder
Returns authenticated Authentication object
5. UserDetailsService
Loads user data from:
Database
LDAP
In-memory store
Returns a UserDetails object.
6. SecurityContextHolder
After successful authentication: ✔ User details
✔ Roles / Authorities
are stored in SecurityContextHolder.
👉 This context is available throughout the request lifecycle.
7. Authorization (Access Decision)
Spring Security checks:
User roles
Permissions
Method-level security
🔹 Components involved:
AccessDecisionManager
AccessDecisionVoter
,
8. Access Granted or Denied
✅ If authorized → request reaches controller
❌ If not → 403 Forbidden or 401 Unauthorized

26/12/2025

🔥 Difficult Java Technical Questions – Explained Simply

1. Why is String immutable in Java?
Because: ✔ Thread-safe by design
✔ Improves performance via String Pool
✔ Enhances security (class loading, DB URLs)
🧠 Key Insight:
Immutability allows JVM to safely reuse String objects.

2. Difference between == and .equals()?
== → compares memory referenceequals() → compares content
🧠 Interview Twist:
If .equals() is not overridden, it behaves like ==.

3. Why is HashMap not thread-safe?
Because multiple threads can: ✔ Modify structure simultaneously
✔ Cause infinite loops during resize (pre-Java 8)
🧠 Solution:
Use ConcurrentHashMap for concurrency.

4. How does volatile work internally?
✔ Ensures visibility across threads
✔ Prevents instruction reordering
❌ Does NOT guarantee atomicity
🧠 Example:
volatile ≠ thread-safe increment

5. Why doesn’t Java support multiple inheritance?
To avoid Diamond Problem 💎
🧠 Workaround:
Java supports multiple inheritance via interfaces.

6. Difference between Comparable and Comparator?
Comparable → natural ordering (inside class)
Comparator → custom ordering (external)
🧠 Real Use:
Sorting by name, salary, date, etc.

25/12/2025

Spring Boot Project Structure (STS) – Explained Simply
A clean project structure is the foundation of a scalable & maintainable Spring Boot application.
Here’s how a standard Spring Boot project is organized in Spring Tool Suite (STS) 👇
📁 src/main/java
➡️ Contains all Java source code
controller
Handles HTTP requests (, , )
service
Contains business logic
Keeps controller clean and reusable
repository
Used for database operations
Typically extends JpaRepository
model / entity
Represents database tables using
exception
Centralized exception handling
Example: GlobalExceptionHandler
📁 src/main/resources
➡️ Configuration & static files
application.properties / application.yml
static/ (css, js, images)
templates/ (Thymeleaf, if used)
📁 src/test/java
➡️ Unit & integration test cases
SpringBootProjectApplicationTests
📄 pom.xml
➡️ Manages dependencies & build configuration

25/12/2025