Spring

 

✅ API is like a method or interface that defines what can be done (like login(), searchProduct()), but not how it’s done.

✅ Web Service is the actual implementation of that API which runs over the internet or network using HTTP, providing real data/responses.

✅ Microservices is an architectural style where these services (API + implementation) are split into small, independent parts, each doing one specific task and running independently, often communicating via APIs.

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Spring 

It helps Java developers build large applications easily by using features like:

Dependency Injection (DI) – to reduce tight coupling between classes

Aspect-Oriented Programming (AOP) – to separate common tasks (like logging, security) from business logic

Spring Core provides the foundation that other Spring modules (like Spring MVC, Spring Boot, Spring Data) are built on.

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|       Spring Container      |

|   (ApplicationContext)      |

+-------------+---------------+

              |

              v

+-------------+---------------+

|         BeanFactory         |

|   (Creates & Manages Beans) |

+-------------+---------------+

              |

              v

+-------------+---------------+

|       Dependency Injection   |

|    (Wires dependencies)      |

+-------------+---------------+

              |

              v

+-------------+---------------+

|     Aspect-Oriented Prog.    |

|  (AOP: Logging, Security...) |

+-----------------------------+

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Spring MVC is a part of the Spring Framework designed for creating web applications using the Model-View-Controller (MVC) design pattern.

Model: Represents the application data.

View: Handles the user interface (like JSP, Thymeleaf, etc.).

Controller: Processes user requests and returns the appropriate response.

🧩 Key Components of Spring MVC:

DispatcherServlet

Front controller that handles all incoming HTTP requests.

It is configured in web.xml or using annotations in Spring Boot.

Routes the request to the appropriate controller.

Controller (@Controller)

Handles user requests.

Calls the service layer to process data.

Returns view name or data.

Service Layer (@Service)

Contains business logic.

Communicates with the DAO/repository layer.

DAO / Repository Layer (@Repository)

Handles interaction with the database.

Uses JPA, JDBC, Hibernate, etc.

Model

Carries data between the controller and the view.

View (JSP, Thymeleaf, etc.)

Displays the data to the user.

🔄 Request Flow in Spring MVC:

User sends a request (e.g., clicking a button on a web page).

DispatcherServlet receives it and forwards it to the correct Controller.

The Controller calls the Service, and the service may call the DAO.

The DAO interacts with the database and returns data.

The Controller sends data to the View (e.g., a JSP page).

View renders the data and sends the response back to the user.

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Difference 

Spring Boot makes it easy and fast to create entire Spring-based applications, while Spring MVC is a part of the Spring framework used mainly for building web-based UI following the MVC design.

JPA 

JPA is a Java specification that helps you store, update, delete, and retrieve data from a database using Java objects instead of writing SQL queries manually.

In Spring Boot, we often use Spring Data JPA, which uses JPA + Hibernate to handle database access without writing SQL queries.

JPA defines what should be done, like operations for saving, deleting, or updating data — it’s just a specification (interface).

Hibernate provides how it is done, meaning it’s the actual implementation of JPA that knows how to interact with the database.

✅ Using JPA (defines what should be done)

import org.springframework.data.jpa.repository.JpaRepository;

// JPA interface — defines what should be done

public interface StudentRepository extends JpaRepository<Student, Integer> {

    // No method body, just defines that we can save, delete, update Student

}

✅ Hibernate (provides how it is done)

You don’t write Hibernate code directly when using Spring Boot — it works behind the scenes. Hibernate will provide the actual implementation of how the save(), findById(), etc. are performed.

But if we write it manually (without Spring Data JPA):

// This is how Hibernate provides the implementation (low-level manual code)

Session session = sessionFactory.openSession();

Transaction tx = session.beginTransaction();

Student student = new Student();

student.setId(1);

student.setName("Arman");

session.save(student);  // Hibernate handles the DB operations here

tx.commit();

session.close();

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“AOP helps us write code for things like logging, security, or transactions separately from our main business logic.”

“For example, if we want to log every method call, instead of writing log statements in every method, we can use AOP to do it automatically.”

“Spring AOP lets us use annotations like @Before, @After, or @Around to run extra code before or after a method runs.”

“This makes our code cleaner and easier to manage.”

Basically, AOP (Aspect-Oriented Programming) is used to avoid writing the same code again and again in multiple places.

✅ Main AOP Annotations in Spring

Annotation Description

@Aspect Marks a class as an Aspect (where your AOP logic lives).

@Before Runs before the target method executes.

@After Runs after the target method completes (whether success or exception).

@AfterReturning Runs after the target method completes successfully.

@AfterThrowing Runs if the target method throws an exception.

@Around Runs before and after the method; gives complete control.

@Pointcut Defines reusable expressions for matching methods (called pointcuts).

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questions from spring 

@Component, @Service, @Repository, @Controller, @Autowired

@ComponentScan 

@Configuration, @Bean

@Cacheable

@Conditional

@Requestparam

@PathVariable

@Pathparam