The Core Container is the heart of the Spring Framework, offering key features like dependency injection (DI) and inversion of control (IoC). These patterns help developers create loosely coupled, maintainable applications by managing object lifecycles and dependencies.

• Core: Provides fundamental components for dependency management.
• Beans: Manages beans and their dependencies.
• Context: Creates the ApplicationContext to configure and manage beans.
• SpEL (Spring Expression Language): Offers a powerful language to query and manipulate objects in Spring applications.

The AOP (Aspect-Oriented Programming) module allows developers to address concerns like logging, security, and transaction management without cluttering the core application logic. This modular approach leads to cleaner, more maintainable code.

Key Features:

• AOP: Helps apply cross-cutting concerns across different layers of the application.
• Instrumentation: Enables monitoring of JVM activities, method calls, and classloading

Spring simplifies data access and integration with various data sources and messaging systems. These modules help you interact with relational databases, ORM frameworks, and messaging systems like JMS.

Key Modules:

• Spring JDBC: Simplifies database interaction with Java Database Connectivity (JDBC).
• ORM (Object-Relational Mapping): Integrates with popular ORM frameworks like Hibernate and JPA (Java Persistence API).
• JMS (Java Message Service): Facilitates messaging for enterprise applications.
• OXM (Object XML Mapping): Supports conversion between objects and XML for seamless data integration.

The Web modules are designed to support the development of both traditional web applications and RESTful APIs. These modules provide the flexibility to create scalable, high-performance web applications, leveraging Spring’s powerful MVC framework, WebSocket support, and more.

Key Web Modules:

• Spring Web: The foundation for building web applications in Spring.
• Spring MVC (Model-View-Controller): A powerful framework for developing dynamic, scalable web applications.
• Spring WebSocket: Supports real-time communication for web applications.
• Spring Servlet and Portlet: Provides specialized functionality for servlet-based and portlet-based applications.

Testing is crucial in modern software development, and Spring offers comprehensive tools for both unit and integration testing. The Spring Testing module integrates seamlessly with frameworks like JUnit and TestNG.

Key Testing Features:

• TestContext Framework: Manages the setup and configuration of test environments.
• Mock Objects: Helps create mock beans for testing.
• Integration Testing: Allows testing components in a fully configured application context.

Spring offers two primary ways to configure beans and their dependencies:

• XML Configuration: Traditional method of defining beans and their relationships using XML files.
• Annotation-Based Configuration: A modern approach using annotations like @Component, @Autowired, and @Configuration to define and inject beans.

In Spring, dependency injection (DI) is used to inject the required dependencies into beans at runtime. There are two main types of dependency injection:

• Setter Injection: Dependencies are injected using setter methods. This approach is suitable when a dependency is optional or can be changed after object creation.

class Student {
    private String id, name, address;
    
    public void setId(String id) { this.id = id; }
    public void setName(String name) { this.name = name; }
    public void setAddress(String address) { this.address = address; }
}

class Address {
    private String street, city, state, country;

    public void setStreet(String street) { this.street = street; }
    public void setCity(String city) { this.city = city; }
    public void setState(String state) { this.state = state; }
    public void setCountry(String country) { this.country = country; }
}

• Constructor Injection: Dependencies are injected via the constructor, ensuring that all required dependencies are provided at the time of object creation.

class Student {
    private String id, name, address;

    public Student(String id, String name, String address) {
        this.id = id;
        this.name = name;
        this.address = address;
    }
}

class Address {
    private String street, city, state, country;

    public Address(String street, String city, String state, String country) {
        this.street = street;
        this.city = city;
        this.state = state;
        this.country = country;
    }
}

Spring supports multiple types of dependencies to suit various application needs:: Key Modules for Building Java Applications

• Primitive Data Types: Integers, booleans, strings, etc.
• Collection Types: Lists, Sets, Maps, and Properties.
• Reference Types: Custom objects or classes defined within the application.