Introduction to the Spring Framework

Overview of Spring Framework

Released in 2002, Spring is an open source Java platform developed by Rod Johnson and colleagues. It is licensed under the Apache License 2.0.

Spring makes Java application development easier by providing a comprehensive infrastructure support for the Java platform. The features include:

• Inversion of Control (IoC) container
• Data access framework with JDBC support
• REST web services framework
• Security framework
• and many more integrations

These enable rapid application development, with clean architecture and code. The lightweight container model also makes testing easier.

Today Spring has become the de facto standard for building enterprise Java applications for web and cloud. Let’s look at some of its major advantages:

Lightweight and Non-invasive

Spring does not force you to inherit framework classes. Its non-invasive architecture integrates with existing code nicely.

Dependency Injection

Spring promotes loose coupling by injecting dependencies declaratively through configuration instead of hard-coded.

Aspect-Oriented Programming

Spring AOP helps implement cross-cutting concerns like logging, transactions separately from core business logic.

Backed by Major Companies

Spring is backed by Pivotal, VMware and has strong community adoption. It has a bright future.

Web Capabilities

Spring MVC provides a robust model-view-controller framework for Java web apps.

Next, we’ll explore the core concept of inversion of control.

Inversion of Control and Dependency Injection

Inversion of Control (IoC) is a key concept in Spring. Traditionally Java code relies on frameworks by calling them explicitly. But with IoC, the framework handles instantiating and injecting dependencies instead. This inverts the control.

For example, say class A depends on an interface B, traditionally:

public class A {
  B b = new BImpl(); //A controls instantiation
  //...
}

With IoC, an external class injects the dependency B:

public class A {
  B b; //Declare reference
  
  //B is injected
  public void setB(B b) {
    this.b = b;
  }
  
  //...
}

This loose coupling enables swapping implementations easily. IoC also facilitates unit testing without mock classes.

Spring IoC container handles injecting dependencies defined through configuration like XML. This is Dependency Injection (DI).

Now let’s explore Spring’s container and wiring further.

Spring Container and Configuration

The Spring IoC container manages instantiating, configuring and assembling objects called beans. The container relies on dependency injection based configuration.

There are two types of container configuration:

XML Configuration

Beans are declared in bean configuration XML files. For example:

<!-- Bean declaration -->
<bean id="car" class="com.example.Car">
</bean>

<!-- Injecting dependency --> 
<bean id="engine" class="com.example.Engine"/>

<bean id="car" class="com.example.Car">
  <property name="engine" ref="engine"/> 
</bean>

Annotation Configuration

Beans can be configured via annotations in Java code without XML.

@Component
public class Engine {
  //...
}

@Component 
public class Car {
  
  @Autowired
  Engine engine;
  
  //...
}

Annotations like @Component and @Autowired handle instantiation and wiring.

In most cases today annotation-based configuration is preferred for its elegance.

Let’s look at some key annotations for Spring wiring:

• @Component – Generic component annotation to instantiate a bean
• @Service – For service classes
• @Repository – For data access objects like DAOs
• @Controller – For web controllers
• @Autowired – Injects a dependency by type

Together the container and wiring handle nearly all creation and injection of dependencies. This keeps code clean and decoupled.

Now let’s shift gears and explore Spring MVC.

Spring MVC for Web Development

The Spring Web MVC framework provides a clean Model-View-Controller architecture for Java web applications.

Spring MVC is highly customizable but also provides default configurations for simplicity.

Some key components of Spring MVC:

Front Controller – DispatcherServlet handles all incoming requests and routes them to controllers

Model – Stores and retrieves data, typically implemented via POJOs

Views – Renders model data, usually JSP templates

Controllers – Handles requests and controls flow to models and views

Here is a typical MVC flow:

1. DispatcherServlet receives a request
2. Maps it to the appropriate controller
3. Controller executes business logic
4. Interacts with model to save or retrieve data
5. Returns view name back to DispatcherServlet
6. View is rendered with model data

This clean separation of concerns keeps code well-organized as the application grows and promotes easier testing and maintenance.

For example, here is a simple controller:

@Controller
public class UserController {

  @Autowired
  UserService userService;  

  @GetMapping("/users")
  public String listUsers(Model model) {    
    model.addAttribute("users", userService.listUsers());
    return "users"; //view
  }
}

Spring MVC is a very popular choice for Java web applications due to its flexibility and ease of use.

Next let’s look at handling data access with Spring.

Spring Data Access and ORM

Accessing and persisting data is a key part of most applications. Spring provides a convenient abstraction for data access through the JDBC and ORM integration modules.

Spring JDBC

Spring JDBC provides a layer of abstraction over raw JDBC code. It handles low-level tasks like:

• Establishing connections
• Statement preparation
• Result set handling
• Exception translation

This frees developers to focus on SQL and mapping application objects.

For example, using Spring’s JdbcTemplate:

JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); 

List<User> users = jdbcTemplate.query("SELECT * FROM users", 
  new BeanPropertyRowMapper(User.class));

Spring JDBC makes database connectivity code clean and simple to write.

Spring ORM

Spring supports popular ORM frameworks like Hibernate, JPA and JDO. It provides a consistency layer over different implementations.

For example, with Hibernate the database entities can be configured as Spring beans:

@Entity
@Table(name="users")
public class User {
  
  @Id
  private int id;

  private String name;
  
  //...
}

And HibernateSessionFactory configured:

<bean id="sessionFactory" class="org.springframework.orm.hibernate5.LocalSessionFactoryBean">
  <property name="dataSource" ref="dataSource"/>
  <property name="annotatedClasses">
    <list>
      <value>com.example.User</value> 
    </list>
  </property>
</bean>

Spring’s consistent data access interfaces minimize vendor lock-in and integrate nicely with the application architecture.

Now let’s shift gears and explore Spring Security.

Securing Apps with Spring Security

Spring Security provides powerful, customizable authentication and authorization for Spring-based applications.

It supports both declarative and programmatic security configurations. Some key features are:

• Comprehensive authentication mechanisms (LDAP, OAuth, SAML, etc)
• Authorization based on roles, permissions, etc
• Protection against attacks like session fixation, clickjacking, etc
• Integration with popular web authentication frameworks
• Easy extension points for customization

For example, declarative method security:

@EnableGlobalMethodSecurity(securedEnabled = true)
public class SecurityConfig { }

@Secured("ROLE_ADMIN")
@GetMapping("/admin")
public String adminPage() {
  // ...
}

And custom authentication provider:

public class CustomAuthProvider implements AuthenticationProvider {

  @Override
  public Authentication authenticate(Authentication auth) {
    // Custom authentication logic
    return new Authentication(true);
  }
  
  //...
}

This enables extremely flexible security management tailored to application needs.

Up next is building REST APIs with Spring.

Building REST APIs with Spring REST

Spring provides first-class support for building RESTful web services through the Spring REST annotations:

@RestController public class UserController {

@GetMapping("/api/users") public List<User> getUsers() { // ... }

@PostMapping("/api/users") public void addUser(@RequestBody User user) { // ... } }

Spring REST simplifies exposing CRUD endpoints for resources. It handles mapping requests to handler methods through annotations. Useful features include:

• Content negotiation for JSON, XML etc
• Type conversion for requests
• Validation support
• Exception handling

This enables quickly building production-grade REST APIs integrated with the Spring environment.

Additionally, Spring provides client REST capabilities for calling external services through the RestTemplate class.

Now let’s look at how Spring enables microservices.

Spring and Microservices

The lightweight nature of Spring makes it an ideal choice for building microservices. Key advantages of using Spring for microservices:

• Spring Boot simplifies creating standalone, production-ready services.
• Distributed configuration management with Spring Cloud Config.
• Dynamic scaling using container deployments.
• Multiple service registration and discovery options like Eureka and ZooKeeper.
• APIs gateways like Spring Cloud Gateway.
• Circuit breakers with Spring Cloud Hystrix.
• Distributed tracing solutions.

With these features, Spring makes implementing robust microservices easier.

For example, a microservice configuration with Spring Cloud:

@EnableDiscoveryClient
@SpringBootApplication
public class UserService {

  public static void main(String[] args) {
    SpringApplication.run(UserService.class, args);
  }

}

The @EnableDiscoveryClient integrates with service discovery. The service can be dynamically deployed and configured.

Spring Cloud provides an abundance of tools to build cloud-native, distributed systems.

Summary

In this comprehensive beginner’s guide we explored the key capabilities of the Spring framework:

• Dependency injection and inversion of control
• Declarative bean configuration
• Web development with Spring MVC
• Database access using Spring JDBC and ORM
• Security with Spring Security
• Building RESTful APIs
• Microservice support with Spring Cloud

Spring provides a tremendously useful set of tools for enterprise Java. It promotes good coding practices and is highly customizable.

The extensive documentation, wide adoption, and active community also make it an excellent framework to build on.

I hope this overview has provided a great introduction to Spring and given you the foundation to start building your own Spring-based applications!