Navigating the Inner Workings of Spring Boot

Eleftheria DrosopoulouFebruary 26th, 2024Last Updated: February 15th, 2024

0 949 4 minutes read

Hello everyone! Today, we’ll delve into the mechanics of Spring Boot, unraveling the mysteries of its workings and shedding light on the essence of Spring Boot auto-configuration. Join me on this journey of understanding the intricacies behind this powerful framework.

Below is a simple example of a main class for a Spring Boot application. This example demonstrates the basic structure of a Spring Boot application and how the execution starts from the main method:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

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

In this example:

@SpringBootApplication annotation is used to mark the class as the main application class and also includes additional annotations like @Configuration, @EnableAutoConfiguration, and @ComponentScan.
The main method is the starting point of the application. It uses SpringApplication.run to bootstrap the Spring Boot application.

This is a minimalistic example, and a real-world Spring Boot application would typically include additional components, services, and configurations. However, this simple setup gives you a starting point to understand the basic structure of a Spring Boot application.

1. @SpringBootApplication annotation

Now lets understand the @SpringBootApplication annotation.

The @SpringBootApplication annotation is a powerful annotation in the Spring Boot framework, designed to simplify the configuration and bootstrapping of Spring applications. It combines three commonly used annotations: @Configuration, @EnableAutoConfiguration, and @ComponentScan.

Let’s break down what each part does:

@Configuration:
- The annotation signals that the class holds configuration methods intended for processing by the Spring container.
- It serves as a substitute for XML-based configuration, offering a programmatic approach to define and configure beans in Spring applications.
@EnableAutoConfiguration:
- This annotation enables Spring Boot’s auto-configuration mechanism.
- Auto-configuration automatically configures the Spring application based on the libraries on the classpath. It simplifies the setup by providing sensible defaults.
@ComponentScan:
- This annotation tells Spring to scan the specified packages for components, such as controllers, services, and repositories.
- It ensures that Spring discovers and registers beans within the specified packages.

By combining these three annotations into @SpringBootApplication, developers can create a concise and powerful main application class. It not only marks the class as the main configuration class but also enables auto-configuration and component scanning. This helps in reducing boilerplate code and makes the Spring Boot application highly customizable.

In summary, @SpringBootApplication serves as a convenient, all-encompassing annotation that kickstarts the configuration, auto-configuration, and component scanning processes, providing a solid foundation for building and running Spring Boot applications.

2. @Conditional annotation

The @Conditional annotation in Spring is a powerful mechanism that allows developers to conditionally enable or disable beans and configurations based on certain conditions. It helps tailor the application context dynamically, depending on the specified conditions.

Here’s how it works:

Conditional Annotations:
- @Conditional functions as a meta-annotation, operating on other annotations to establish conditions for either bean creation or configuration.
- Developers can define conditions based on factors like the presence of specific classes on the classpath, the value of properties, or the existence of particular beans.
ConditionalOnClass:
- One commonly used condition is @ConditionalOnClass, which checks whether a specified class is present on the classpath.
- If the specified class is present, the annotated bean or configuration is enabled; otherwise, it is skipped.

Let’s look at an example:

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyConfiguration {

    @Bean
    @Conditional(ConditionalOnClassPresentCondition.class)
    public MyBean myBean() {
        return new MyBean();
    }
}

In this example:

@ConditionalOnClassPresentCondition is a custom condition class that implements the Condition interface. It checks whether a specific class is present on the classpath.
The myBean method is annotated with @Conditional(ConditionalOnClassPresentCondition.class), indicating that the MyBean bean will only be created if the condition specified in ConditionalOnClassPresentCondition is met.

Here is a simplified version of the ConditionalOnClassPresentCondition class:

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class ConditionalOnClassPresentCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        try {
            // Check if the specified class is present on the classpath
            context.getClassLoader().loadClass("com.example.RequiredClass");
            return true; // If present, condition is met
        } catch (ClassNotFoundException e) {
            return false; // If not present, condition is not met
        }
    }
}

In this example, MyBean will only be created if the class com.example.RequiredClass is present on the classpath. The @Conditional annotation provides a flexible way to control the creation of beans or configurations based on runtime conditions.

3. SpringApplication.run() method

The SpringApplication.run() method is a crucial part of Spring Boot applications, responsible for bootstrapping and launching the Spring application. It is typically found in the main method of the main application class.

Here’s an overview of what this method does:

Bootstrap the Application:
- SpringApplication.run() is the entry point that bootstraps the Spring application. It sets up the Spring application context and initializes various components required for the application to run.
Start the Embedded Web Server:
- If the application is a web application, SpringApplication.run() also starts the embedded web server. Spring Boot includes an embedded web server (like Tomcat or Jetty) to simplify deployment and configuration.
Application Context Initialization:
- The method triggers the initialization of the Spring application context, which involves scanning for components, configuring beans, and setting up the environment.
Auto-Configuration:
- Spring Boot’s auto-configuration feature comes into play during the SpringApplication.run() process. It automatically configures the application based on the dependencies present in the classpath. This is a key feature that simplifies setup and reduces boilerplate code.
Application Events:
- SpringApplication.run() also manages the firing of various application events. These events can be used for custom initialization, monitoring, or logging purposes. Examples include ApplicationStartedEvent and ApplicationReadyEvent.

Here’s a simple example of using SpringApplication.run() in a main class:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MyApplication {

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

In this example:

@SpringBootApplication is used to indicate that this class is the main configuration class for the Spring Boot application.
SpringApplication.run(MyApplication.class, args) initializes and starts the Spring Boot application. The class argument specifies the configuration class, and args are command-line arguments that can be passed to the application.

This single line of code encapsulates the intricate process of setting up a Spring Boot application, making it easy for developers to launch their applications with minimal configuration.

4. Wrapping Up

In wrapping up our exploration of Spring Boot and its SpringApplication.run() method, we’ve unveiled the magic behind launching Spring applications. It’s like pressing the start button for your Java-powered creation. With just a single line of code, Spring Boot takes care of the heavy lifting, setting up the stage for your application to shine. So, remember, when you see SpringApplication.run(), you’re not just starting a program – you’re launching a Spring-powered journey into the world of efficient and streamlined Java development. Happy coding!