Git revert vs cherry pick

 I have a main branch and a development branch. I merged development into main, but later realized that the changes should not have gone into main. Even though several commits were added to main after the merge, the correct and safe way to roll back only the merged changes is to revert the merge commit, not reset the branch.

Using git revert -m 1 <merge_commit_hash> creates a new commit that undoes only the changes introduced by the merge while keeping all subsequent commits intact. This approach is safe for shared branches and preserves Git history.

git cherry-pick, on the other hand, is used to selectively copy specific commits from one branch to another. It is useful when only certain fixes or features are needed, but it should not be used for rolling back a merge. For rollback scenarios, git revert is the correct choice; for selectively applying commits, git cherry-pick is appropriate.

stream performance testing

 https://medium.com/javarevisited/high-performance-programming-with-java-streams-debeb374008f

helm chart cheatset

 https://helm.sh/docs/intro/CheatSheet

Kubernetes commands

 KUBERNETES COMMANDS CHEAT SHEET

1. Cluster Information

kubectl version

kubectl cluster-info

kubectl get nodes

kubectl describe node

2. Pods

kubectl get pods

kubectl get pods -A

kubectl get pod -o wide

kubectl describe pod

kubectl logs

kubectl logs -f

kubectl logs -c

kubectl exec -it -- /bin/bash

kubectl delete pod

kubectl delete pod --force --grace-period=0

3. Deployments

kubectl get deploy

kubectl describe deploy

kubectl apply -f deployment.yaml

kubectl create -f deployment.yaml

kubectl scale deploy --replicas=3

kubectl rollout status deploy/

kubectl rollout history deploy/

kubectl rollout undo deploy/

4. Serviceskubectl get svc

kubectl describe svc

kubectl apply -f service.yaml

kubectl port-forward svc/ 8080:80

kubectl port-forward pod/ 9090:9000

5. Namespaces

kubectl get ns

kubectl create ns dev

kubectl delete ns dev

kubectl config set-context --current --namespace=dev

6. ConfigMaps & Secrets

kubectl get configmap

kubectl describe configmap

kubectl apply -f configmap.yaml

kubectl get secret

kubectl describe secret

kubectl create secret generic my-secret --from-literal=username=pawan

7. Dry Run

kubectl apply -f app.yaml --dry-run=client -o yaml

8. Troubleshooting

kubectl get events --sort-by=.metadata.creationTimestamp

kubectl run debug --image=busybox -it -- sh

kubectl get pod -o wide

9. Context & Authentication

kubectl config get-contexts

kubectl config use-contextkubectl config view

10. Delete Everything

kubectl delete -f .

kubectl delete all --all -n dev

kubectl scale deployment hrms-deployment --replicas=0

 kubectl get deployments

kubectl delete deployment hrms-deployment

kubectl get svc

kubectl get pods


kubectl apply -f mysql-deployment.yaml

kubectl apply -f redis-deployment.yaml

kubectl apply -f hrms-deployment.yaml

 462  clear

  463  kubectl get deployments

  464  kubectl delete  deployments hrms-application-deployment

  465  kubectl delete  deployments mysql

  466  kubectl delete  deployments redis

  467  kubectl get deployments

  468  kubectl get svc

  469  kubectl delete svc hrms-service

  470  kubectl delete svc mysql

  471  kubectl delete svc redis

  472  clear

  473  kubectl get pods

  474  clear

  475  kubectl apply -f mysql-deployment.yaml

  476  kubectl apply -f redis-deployment.yaml

  477  kubectl apply -f hrms-service.yaml

  478  kubectl apply -f hrms-service.yaml

  479  kubectl apply -f hrms-service.yaml

  480  kubectl apply -f hrms-deployment.yaml

Spring security interview question.

 

🧩 1. Core Spring Security Architecture

Q1. What are the main components of Spring Security?
Q2. Explain the Spring Security filter chain and how it works internally.
Q3. What is the difference between FilterChainProxy and DelegatingFilterProxy?
Q4. How does Spring Security integrate with the Servlet container?
Q5. What is the role of SecurityContext and SecurityContextHolder?

---

🔐 2. Authentication & Authorization

Q6. Explain the authentication flow in Spring Security.
Q7. What is the difference between AuthenticationManager and AuthenticationProvider?
Q8. How does Spring Security handle authorization decisions?
Q9. What is the purpose of AccessDecisionManager and AccessDecisionVoter?
Q10. How do you customize authentication (e.g., using database, LDAP, JWT, or OAuth)?

---

💾 3. JWT (JSON Web Token) & Stateless Authentication

Q11. Explain how JWT-based authentication works in Spring Security.
Q12. What are the pros and cons of using JWT vs session-based authentication?
Q13. How do you implement token invalidation (logout) in JWT-based systems?
Q14. How can you refresh JWT tokens securely?
Q15. How would you prevent token replay attacks?

---

🧰 4. Configuration Approaches

Q16. What’s the difference between WebSecurityConfigurerAdapter and the new SecurityFilterChain approach (Spring Security 5.7+)?
Q17. How do you configure multiple HttpSecurity instances for different API paths?
Q18. How would you disable security for a specific endpoint (e.g., /health, /actuator)?
Q19. How to secure REST APIs using Spring Security annotations (@PreAuthorize, @Secured)?
Q20. Explain method-level vs URL-level security.

---

⚙️ 5. Customization & Extensibility

Q21. How do you create a custom authentication filter?
Q22. How do you plug in a custom UserDetailsService?
Q23. Explain how to add custom claims to JWT during login.
Q24. How do you handle multi-factor authentication (MFA) in Spring Security?
Q25. How can you secure microservices communicating over REST (e.g., internal JWT validation)?

---

🧠 6. Advanced Concepts

Q26. Explain SecurityContextPersistenceFilter and its purpose.
Q27. What is AnonymousAuthenticationFilter and when does it come into play?
Q28. How does Spring Security handle CSRF protection in REST APIs?
Q29. Explain how CORS and Spring Security interact.
Q30. What are stateless sessions, and how are they configured?

---

🧩 7. OAuth2 / OpenID Connect

Q31. Explain the OAuth2 authorization code flow.
Q32. What are the key differences between OAuth2 and OpenID Connect?
Q33. How would you secure a resource server and client application using Spring Security OAuth2?
Q34. What’s the difference between BearerTokenAuthenticationFilter and BasicAuthenticationFilter?
Q35. How do you refresh access tokens securely in OAuth2?

---

🧱 8. Security Best Practices

Q36. How do you prevent brute-force attacks in Spring Security?
Q37. How do you implement rate-limiting or IP-based blocking?
Q38. How do you store passwords securely in the database?
Q39. What are common security misconfigurations you’ve seen in Spring projects?
Q40. How do you secure WebSockets with Spring Security?

---

☁️ 9. Real-World & Integration

Q41. How would you design authentication for a microservices architecture (e.g., using JWT + Redis)?
Q42. How can you propagate authentication across multiple services (SSO)?
Q43. How to integrate Spring Security with AWS Cognito or Azure AD?
Q44. How do you secure actuator endpoints in production?
Q45. How do you test secured endpoints using JUnit and Mockito?

---

🧪 10. Troubleshooting & Debugging

Q46. How do you debug 403 Forbidden issues in Spring Security?
Q47. How do you log authentication and authorization events?
Q48. How do you verify your filter order in the Spring Security chain?
Q49. What’s the difference between @WithMockUser and @WithUserDetails in testing?
Q50. How would you profile or optimize security filters for performance?

---

✅ Bonus: Scenario-Based Questions

1. You need to allow multiple concurrent logins per user, but also support logout from all sessions — how do you implement it?

2. You want to support JWT authentication but also allow API key-based access for internal systems — how would you design it?

3. You want to cache user roles and permissions to avoid frequent DB hits — how would you do that securely?

4. You want to secure communication between two Spring Boot microservices — what’s your approach (TLS, JWT, OAuth2)?

5. How would you audit who accessed which endpoint and when?

---

Would you like me to create a PDF or a structured study sheet (with answers and diagrams for each topic like JWT, filter chain, etc.)?
It’s perfect for 10+ yrs Spring Security interview prep.

How to Create a Pure Java Service-Layer JAR from a Spring Boot Project

When working on a Spring Boot application, the service layer often contains the business logic that you may want to reuse in other projects. But what if you want to create a JAR of only the service layer and use it in a plain Java project without any Spring dependencies? This guide walks you through the process step by step.

Why Separate the Service Layer?

Separating the service layer into its own module or project has several advantages:

• Reusability: You can use it in multiple applications, including plain Java apps.

• Decoupling: Keeps business logic separate from controllers or web-specific code.

• Simpler Testing: Service logic can be tested independently of web or Spring Boot context.

Step 1: Remove Spring Dependencies

Spring annotations like @Service, @Component, @Autowired, or @Transactional won’t work in a plain Java application because they rely on Spring’s dependency injection and application context.

You need to:

• Remove all Spring annotations.

• Replace @Autowired dependencies with constructor or setter injection.

• Handle transactions manually if needed.

Before (Spring Boot Service):

@Service

public class MyService {

    @Autowired

    private MyRepository repo;

    @Transactional

    public void doSomething() {

        repo.saveData();

    }

}

After (Plain Java Service):

public class MyService {

    private final MyRepository repo;

    public MyService(MyRepository repo) {

        this.repo = repo;

    }

    public void doSomething() {

        repo.saveData();

    }

}

Step 2: Prepare the Service Layer Module

Organize your project into a separate module for the service layer. Example structure:

my-app

│

├─ service-layer       <-- This will become the JAR

│   └─ src/main/java/... (all service classes)

│   └─ pom.xml

│

├─ web-layer           <-- Spring Boot app (controllers)

│   └─ src/main/java/...

│   └─ pom.xml

│

└─ pom.xml             <-- parent pom

Step 3: Create a Minimal pom.xml

Since you don’t want any Spring dependency, the pom.xml is very simple:

<project>

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>

    <artifactId>service-layer</artifactId>

    <version>1.0.0</version>

    <packaging>jar</packaging>

    <dependencies>

        <!-- Include only libraries your service actually needs -->

    </dependencies>

</project>

Only include libraries like JDBC drivers, Redis client, or other third-party APIs if your service uses them.

Step 4: Build the JAR

From the service-layer module folder, run:

mvn clean install

service-layer/target/service-layer-1.0.0.jar

This JAR now contains only your service layer and no controllers or Spring Boot classes.

Step 5: Use the JAR in a Plain Java Application

Add the JAR to your Java project classpath (or as a Maven dependency if installed in the local repo) and use it like any normal Java library:

public class Main {

    public static void main(String[] args) {

        MyRepository repo = new MyRepository();

        MyService service = new MyService(repo);

        service.doSomething();

    }

}

No Spring required, everything works in pure Java.

Key Takeaways

1. Spring annotations do nothing in a plain Java app. To use your service layer outside Spring Boot, remove them.

2. Use constructor or setter injection to manage dependencies manually.

3. Keep only necessary dependencies in the service-layer pom.xml to make it lightweight.

4. Build as a JAR using Maven and reuse it anywhere, including plain Java projects.

This approach allows you to decouple your business logic from Spring, making your service layer reusable, lightweight, and independent of any specific framework.

 

Stream Concat example

 https://www.netjstech.com/2022/01/java-stream-concat-with-examples.html

//code for fetch user assigned Specific design group
Long[] designGroupAssignedToUser = this.getDesignGroupBuUserId(user);
designGroupIds = Stream.concat(
                Arrays.stream(Optional.ofNullable(designGroupIds).orElse(new Long[0])),
                Arrays.stream(Optional.ofNullable(designGroupAssignedToUser).orElse(new Long[0]))
        )
        .distinct()
        .toArray(Long[]::new);
//end of code