Running a Spring Boot application in Spring Boot - Deep Dive

Overview - Running a Spring Boot application

What is it?

Running a Spring Boot application means starting a Java program built with the Spring Boot framework. This framework helps developers create web apps and services quickly by providing ready-made tools and settings. When you run the application, it sets up everything needed to handle web requests, connect to databases, and more. It works like turning on a machine that is already assembled and ready to do its job.

Why it matters

Without a simple way to run Spring Boot apps, developers would spend a lot of time setting up servers and configurations manually. This would slow down building and testing software, making it harder to deliver features fast. Running the app easily lets developers focus on writing code that solves real problems, not on technical setup. It also helps teams share and deploy their work consistently.

Where it fits

Before learning to run a Spring Boot app, you should know basic Java programming and understand what a web server does. After mastering running apps, you can learn how to package them for deployment, monitor their health, and scale them in cloud environments.

Mental Model

Core Idea

Running a Spring Boot application is like pressing the start button on a pre-built machine that sets up everything automatically to serve your app.

Think of it like...

Imagine buying a coffee machine that comes fully assembled and programmed. You just plug it in and press start to get coffee, instead of building the machine yourself. Running a Spring Boot app is like pressing that start button to get your app working immediately.

┌───────────────────────────────┐
│       Spring Boot App          │
│ ┌───────────────┐             │
│ │ Auto-Config   │             │
│ │ Web Server    │             │
│ │ Components   │             │
│ └───────────────┘             │
│           │                   │
│           ▼                   │
│   Application Starts Running  │
└───────────────────────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding Spring Boot Basics

Concept: Learn what Spring Boot is and how it simplifies Java app development.

Spring Boot is a framework that helps you build Java applications quickly by providing default settings and tools. It removes the need to write lots of setup code. Instead of configuring servers and libraries manually, Spring Boot does it for you.

Result

You know that Spring Boot is a tool that makes Java apps easier to create and run.

Understanding Spring Boot's purpose helps you appreciate why running its apps is simpler than traditional Java apps.

2

Foundation

Setting Up Your Development Environment

Concept: Prepare your computer with Java and build tools to run Spring Boot apps.

To run a Spring Boot app, you need Java installed (JDK 17 or newer recommended) and a build tool like Maven or Gradle. These tools compile your code and manage dependencies. You also need an IDE or text editor to write code.

Result

Your computer is ready to build and run Spring Boot applications.

Having the right tools installed is essential before you can run any Spring Boot app.

3

Intermediate

Running with the Main Method

🤔Before reading on: do you think running a Spring Boot app requires a special command or just running a Java main method? Commit to your answer.

Concept: Spring Boot apps start by running a main method that launches the whole application context.

Every Spring Boot app has a main class with a main() method. This method calls SpringApplication.run(), which boots up the app, starts the embedded web server, and loads all components automatically. You can run this main method from your IDE or command line.

Result

The app starts running, ready to handle web requests or other tasks.

Knowing that the main method is the entry point clarifies how Spring Boot apps start and lets you run them easily.

4

Intermediate

Using Build Tools to Run Apps

🤔Before reading on: do you think build tools like Maven or Gradle only compile code, or can they also run the app? Commit to your answer.

Concept: Build tools can compile and run Spring Boot apps with simple commands.

Maven and Gradle have plugins that let you run your Spring Boot app without manually running the Java command. For example, 'mvn spring-boot:run' or 'gradle bootRun' compiles and starts the app in one step. This is handy during development.

Result

You can start your app quickly using build tool commands.

Using build tools to run apps speeds up development and reduces manual steps.

5

Intermediate

Running Packaged Executable Jars

🤔Before reading on: do you think you can run a Spring Boot app without an IDE or build tool after packaging? Commit to your answer.

Concept: Spring Boot apps can be packaged as executable jar files that run anywhere with Java installed.

You can package your app into a single jar file using 'mvn package' or 'gradle build'. This jar includes everything needed to run the app. Then, you run it with 'java -jar yourapp.jar'. This is how apps are deployed to servers.

Result

Your app runs independently of your development environment.

Knowing how to run packaged jars is key for deploying apps outside development.

6

Advanced

Customizing Run Behavior with Profiles

🤔Before reading on: do you think Spring Boot runs the same way in all environments, or can it change behavior? Commit to your answer.

Concept: Spring Boot supports profiles to run the app with different settings for development, testing, or production.

Profiles let you define different configurations for your app. You can activate a profile when running the app, like 'java -jar yourapp.jar --spring.profiles.active=prod'. This changes database connections, logging, and other settings without changing code.

Result

Your app adapts its behavior based on the active profile.

Understanding profiles helps you run the same app safely in different environments.

7

Expert

Inside the Embedded Server Startup

🤔Before reading on: do you think Spring Boot starts a separate server process, or embeds it inside the app? Commit to your answer.

Concept: Spring Boot embeds a web server inside the app, starting it automatically during run time.

Spring Boot includes embedded servers like Tomcat or Jetty inside the app jar. When you run the app, it creates and starts this server in the same process. This removes the need to install and configure servers separately. The app listens on a port and handles requests directly.

Result

The app runs as a standalone server without external dependencies.

Knowing the embedded server mechanism explains why Spring Boot apps are easy to run and deploy.

Under the Hood

When you run a Spring Boot app, the main method calls SpringApplication.run(), which creates an application context. This context scans your code for components, configures beans, and sets up an embedded web server. The embedded server runs inside the same Java process, listening on a port for HTTP requests. Spring Boot uses auto-configuration to detect what libraries are present and configures them automatically. This process hides complex setup behind simple commands.

Why designed this way?

Spring Boot was designed to reduce the complexity of traditional Java web apps, which required manual server setup and configuration files. Embedding the server inside the app and using auto-configuration makes development faster and deployment simpler. Alternatives like deploying to external servers were slower and error-prone. This design trades off some flexibility for ease of use and speed.

┌───────────────────────────────┐
│        main() method          │
│           │                   │
│           ▼                   │
│  SpringApplication.run()      │
│           │                   │
│           ▼                   │
│  Create ApplicationContext    │
│           │                   │
│           ▼                   │
│  Auto-configure Beans & Server│
│           │                   │
│           ▼                   │
│  Start Embedded Web Server    │
│           │                   │
│           ▼                   │
│  App Ready to Serve Requests  │
└───────────────────────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do you think you must install a separate web server to run a Spring Boot app? Commit to yes or no.

Common Belief: You need to install and configure a web server like Tomcat separately before running a Spring Boot app.

Reality: Spring Boot embeds the web server inside the app, so no separate installation is needed.

Why it matters: Believing this causes unnecessary setup work and confusion, slowing down development.

Quick: Do you think running the app from an IDE and running the packaged jar are the same process? Commit to yes or no.

Common Belief: Running the app in an IDE and running the packaged jar are identical in behavior and environment.

Reality: They can differ because the packaged jar runs independently with its own classpath and settings, which may expose issues not seen in the IDE.

Why it matters: Ignoring this can cause bugs to appear only after deployment, making troubleshooting harder.

Quick: Do you think Spring Boot apps always run on port 8080? Commit to yes or no.

Common Belief: Spring Boot apps always use port 8080 unless you change code.

Reality: The default port is 8080, but it can be changed easily via configuration or command line without code changes.

Why it matters: Assuming the port is fixed can cause conflicts and deployment failures.

Quick: Do you think Spring Boot apps start instantly regardless of size? Commit to yes or no.

Common Belief: Spring Boot apps start instantly no matter how big or complex they are.

Reality: Startup time depends on app size, number of components, and auto-configuration complexity; large apps can take noticeable time to start.

Why it matters: Expecting instant startup can lead to impatience and misdiagnosis of performance issues.

Expert Zone

1

Spring Boot's embedded server lifecycle hooks allow fine control over startup and shutdown, which experts use to optimize resource management.

2

Profiles can be combined and layered, enabling complex environment setups that go beyond simple dev/prod distinctions.

3

Running the app with JVM arguments can tune performance and debugging, a subtle but powerful technique often overlooked.

When NOT to use

Spring Boot is not ideal when you need ultra-lightweight microservices with minimal startup time or when you require full control over server configuration. Alternatives like Micronaut or Quarkus may be better for such cases.

Production Patterns

In production, Spring Boot apps are often containerized with Docker, run behind reverse proxies, and managed with orchestration tools like Kubernetes. Experts use health checks, metrics, and logging integrated into the app to monitor and maintain uptime.

Connections

Containerization with Docker

Builds-on

Understanding how to run Spring Boot apps helps when packaging them into containers, as both involve managing app startup and environment configuration.

Operating System Services

Similar pattern

Running a Spring Boot app is like starting a service on an operating system, which manages lifecycle and resource allocation, helping understand app management.

Event-Driven Systems

Builds-on

Spring Boot's startup triggers many events internally; knowing this helps grasp event-driven programming concepts in other domains.

Common Pitfalls

#1 Trying to run the app without Java installed.

Wrong approach:

java -jar myapp.jar # but Java is not installed or not in PATH

Correct approach:

Install JDK 17 or newer and ensure 'java' command works, then run: java -jar myapp.jar

Root cause:

Not understanding that Java runtime is required to execute Spring Boot apps.

#2 Running the app with the wrong main class or missing main method.

Wrong approach:

java -cp myapp.jar com.example.WrongClass

Correct approach:

java -jar myapp.jar # uses the correct main class defined in the jar manifest

Root cause:

Confusing how Spring Boot packages the app and which class to run.

#3 Not activating the correct profile for the environment.

Wrong approach:

java -jar myapp.jar # runs with default profile in production

Correct approach:

java -jar myapp.jar --spring.profiles.active=prod

Root cause:

Ignoring environment-specific configurations leads to wrong behavior or security issues.

Key Takeaways

Running a Spring Boot application starts a pre-configured Java program that includes an embedded web server.

The main method is the entry point that triggers the app startup and auto-configuration.

Build tools like Maven and Gradle simplify running apps during development with single commands.

Packaging the app as an executable jar allows running it anywhere with Java installed, enabling easy deployment.

Profiles and embedded servers make Spring Boot flexible and easy to run in different environments without extra setup.