Your First Spring Boot Application

Q: Can properties and YAML coexist?

Yes, but in the case of duplicate properties, the properties definition takes precedence. It is recommended to use only one to avoid confusion. We recommend YAML, as it offers a clearer hierarchy.

Q: How do I disable the banner?

Set spring.main.banner-mode=off in the configuration file, or app.setBannerMode(Banner.Mode.OFF) in the code.

Q: What should I do if I get the "Port 8080 already in use" error when starting the app?

Modify server.port=8081 , or locate and terminate the process using the port: lsof -i :8080 (Linux/Mac) or netstat -ano | findstr 8080 (Windows).

Q: How do I specify the main entry class?

Configure it using the <mainClass> in the Maven plugin; run the class containing the main method directly in the IDE; the Spring Boot Maven Plugin will automatically detect it during packaging.

A single @SpringBootApplication annotation is all it takes to launch the entire application—the magic of Spring Boot lies in the ingenious combination of three annotations.

1. What You'll Learn

@SpringBootApplication Principles of Annotation Decomposition and Combination
Main Startup Class main() Methods and SpringApplication.run() Execution Flow
application.properties vs application.yml Configuration File Formats
Customizing the Spring Boot Banner and Interpreting Startup Logs
Run the first "Hello OrderFlow" interface

2. A True Story About a New Project

(1) Pain Point: It takes half a day to set up a project

Alice recalled her experience using traditional Spring MVC when she first started out: creating a project required configuring web.xml, applicationContext.xml, and spring-mvc.xml, as well as installing Tomcat and configuring the JNDI data source. Just getting the project up and running took half a day, not to mention debugging startup order and bean loading issues.

(2) The Spring Boot Solution

Spring Boot requires just one annotation and one main method:

JAVA

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

(3) Revenue

Alice used Spring Boot to create the OrderFlow project. It took her just 5 minutes to go from scratch to her first API response—no XML required, no external container needed, and the startup logs were clear and easy to read.

3. Breaking Down the @SpringBootApplication Annotation

(1) A combination of three annotations

@SpringBootApplication is a composite annotation equivalent to using the following three annotations simultaneously:

graph TB
    A["@SpringBootApplication"] --> B["@SpringBootConfiguration"]
    A --> C["@EnableAutoConfiguration"]
    A --> D["@ComponentScan"]
    B --> B1["Marks class as<br/>Configuration bean source"]
    C --> C1["Triggers auto-configuration<br/>based on classpath"]
    D --> D1["Scans components<br/>in same package tree"]

Annotation	Function	Equivalent Traditional Notation
`@SpringBootConfiguration`	Mark the current class as a configuration class	`@Configuration`
`@EnableAutoConfiguration`	Enable Auto-Configuration	`@EnableAutoConfiguration`
`@ComponentScan`	Component Scan	`<context:component-scan>`

(2) @ComponentScan scanning rules

@ComponentScan By default, the scan covers the package containing the main launch class and all its subpackages.

TEXT

com.orderflow ← The package containing the main launch class
├── OrderFlowApplication.java           ← @SpringBootApplication
├── controller/ ← Scanned by @ComponentScan
│   └── OrderController.java
├── service/ ← Scanned by @ComponentScan
│   └── OrderService.java
└── repository/ ← Scanned by @ComponentScan
    └── OrderRepository.java
com.other ← ⚠️ Not included in the scan!

🔥 Common Mistake: If you place the Controller in a package that is either one level above or at the same level as the package containing the main launch class, Spring Boot will not scan for it, resulting in a 404 error.

4. SpringApplication.run() Startup Process

(1) Startup Sequence

The execution process of SpringApplication.run() is a carefully designed pipeline:

sequenceDiagram
    participant Main as main()
    participant SA as SpringApplication
    participant Ctx as ApplicationContext
    participant Bean as Beans

    Main->>SA: new SpringApplication()
    SA->>SA: infer Primary Sources
    SA->>SA: check Web Application Type
    SA->>SA: load Initializers & Listeners
    Main->>SA: run(args)
    SA->>SA: create Bootstrap Context
    SA->>SA: prepare Environment
    SA->>SA: print Banner
    SA->>Ctx: create ApplicationContext
    SA->>Ctx: prepare Context (register sources)
    SA->>Ctx: refresh Context
    Ctx->>Bean: instantiate Beans
    Ctx->>Bean: auto-configure
    SA->>SA: call Runners
    SA->>Main: return ApplicationContext

(2) Determining the Web Application Type

Spring Boot Automatically Detects Application Type:

Condition	Type	Container Used
classpath contains `spring-webmvc`	SERVLET	Tomcat / Jetty / Undertow
classpath contains `spring-webflux` but not `spring-webmvc`	REACTIVE	Netty
None	NONE	No embedded containers

(1) ▶ Example: Customizing SpringApplication Startup

JAVA

@SpringBootApplication
public class OrderFlowApplication {
    public static void main(String[] args) {
        SpringApplication app = new SpringApplication(OrderFlowApplication.class);
        app.setBannerMode(Banner.Mode.CONSOLE);
        app.setWebApplicationType(WebApplicationType.SERVLET);
        app.run(args);
    }
}

Output:

TEXT

// Execution Successful

5. Configuration File Format

(1) Properties vs. YAML

Spring Boot supports two configuration file formats that are functionally equivalent but differ in syntax.

(1) ▶ Example: How to write application.properties

PROPERTIES

server.port=8080
spring.application.name=orderflow-service
spring.datasource.url=jdbc:mysql://localhost:3306/orderflow
spring.datasource.username=root
spring.datasource.password=secret
spring.jpa.hibernate.ddl-auto=update

Output:

TEXT

// Execution Successful

(2) ▶ Example: How to write application.yml

YAML

server:
  port: 8080

spring:
  application:
    name: orderflow-service
  datasource:
    url: jdbc:mysql://localhost:3306/orderflow
    username: root
    password: secret
  jpa:
    hibernate:
      ddl-auto: update

Output:

TEXT

The configuration has taken effect.

Dimension	Properties	YAML
Hierarchical Structure	Separated by `.`	Indentation Indicates Hierarchy
Readability	Simple key-value pairs, flat	Clear hierarchy, suitable for nesting
List Support	`list[0]=a`	`- a` More Intuitive
Repeated prefix	Must be repeated	Shared prefix at the same level
Parsing Performance	Faster	Slightly slower (requires parsing hierarchy)
Priority	Same	Same (properties take precedence when both exist)

⚠️ Note: YAML files are very sensitive to indentation; you must use spaces, not tabs, and indentation must be consistent within the same level.

(2) Quick Reference for Common Configuration Options

Configuration Option	Default Value	Description
`server.port`	8080	Application Listening Port
`spring.application.name`	—	App Name
`server.servlet.context-path`	/	Contextual path
`spring.main.banner-mode`	console	Banner Display Mode
`spring.jpa.show-sql`	false	Print SQL
`logging.level.root`	INFO	Global Log Level

Place a custom banner in src/main/resources/banner.txt so that it automatically displays when Spring Boot starts.

TEXT

  ____              _         ____  __
 / ___| _ __   __ _| |_ ___  / ___||  |  ___  _ __
| |    | '_ \ / _` | __/ _ \ \___ \|  |/ _ \| '_ \
| |___ | | | | (_| | ||  __/  ___) |  | (_) | | | |
 \____||_| |_|\__,_|\__\___| |____/|_|\___/|_| |_|

:: OrderFlow Service ::  v${application.version:1.0.0}
:: Spring Boot ${spring-boot.version} ::

Output:

TEXT

Execution Successful

(2) Interpreting the Startup Log

TEXT

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_  __ _   \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` |  \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  / / / /
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v3.2.5)

2024-01-15 10:00:01.123  INFO 12345 --- [main] c.o.OrderFlowApplication : Starting OrderFlowApplication
2024-01-15 10:00:03.456  INFO 12345 --- [main] o.s.b.w.e.t.TomcatWebServer : Tomcat initialized with port 8080 (http)
2024-01-15 10:00:04.789  INFO 12345 --- [main] o.s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data JPA repositories
2024-01-15 10:00:06.012  INFO 12345 --- [main] c.o.OrderFlowApplication : Started in 5.123 seconds

Log Keywords	Meaning
`Starting OrderFlowApplication`	App launching
`Tomcat initialized with port`	Starting Embedded Tomcat
`Bootstrapping Spring Data JPA`	Automatically Configure the JPA Repository
`Started in X seconds`	Startup complete, took

7. Run Your First "Hello OrderFlow" Interface

(1) ▶ Example: Hello OrderFlow Controller

JAVA

package com.orderflow.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.Map;

@RestController
public class HelloController {

    @GetMapping("/api/hello")
    public Map<String, String> hello() {
        return Map.of(
            "message", "Hello, OrderFlow!",
            "timestamp", java.time.Instant.now().toString()
        );
    }
}

Output:

TEXT

HTTP 200 OK
Content-Type: application/json

{"status":"success","data":{}}

💻 Output:

TEXT

$ curl http://localhost:8080/api/hello
{"message":"Hello, OrderFlow!","timestamp":"2024-01-15T10:00:00Z"}

(2) ▶ Example: Using @Value for configuration injection

JAVA

@RestController
public class HelloController {

    @Value("${spring.application.name}")
    private String appName;

    @GetMapping("/api/info")
    public Map<String, String> info() {
        return Map.of(
            "application", appName,
            "javaVersion", System.getProperty("java.version")
        );
    }
}

Output:

TEXT

// Execution Successful

8. Comprehensive Example: Complete OrderFlow Startup Configuration

JAVA

// src/main/java/com/orderflow/OrderFlowApplication.java
package com.orderflow;

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

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

// src/main/java/com/orderflow/controller/OrderFlowController.java
package com.orderflow.controller;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import java.time.Instant;
import java.util.Map;

@RestController
public class OrderFlowController {

    @Value("${spring.application.name:orderflow}")
    private String appName;

    @GetMapping("/api/health")
    public Map<String, Object> health() {
        return Map.of(
            "status", "UP",
            "application", appName,
            "timestamp", Instant.now()
        );
    }
}

YAML

# src/main/resources/application.yml
server:
  port: 8080

spring:
  application:
    name: orderflow-service

💻 Output:

TEXT

$ curl http://localhost:8080/api/health
{"status":"UP","application":"orderflow-service","timestamp":"2024-01-15T10:00:00Z"}

❓ FAQ

Q Can @SpringBootApplication be applied to any class?

A Technically, yes, but it is strongly recommended to place it in the root package. This is because @ComponentScan scans the package containing the main class and its subpackages by default; placing it in the wrong location may prevent other beans from being discovered.

Q Can properties and YAML coexist?

A Yes, but in the case of duplicate properties, the properties definition takes precedence. It is recommended to use only one to avoid confusion. We recommend YAML, as it offers a clearer hierarchy.

Q How do I disable the banner?

A Set spring.main.banner-mode=off in the configuration file, or app.setBannerMode(Banner.Mode.OFF) in the code.

Q What should I do if I get the "Port 8080 already in use" error when starting the app?

A Modify server.port=8081, or locate and terminate the process using the port: lsof -i :8080 (Linux/Mac) or netstat -ano | findstr 8080 (Windows).

Q What causes slow startup?

A Common causes: 1) The classpath is too large, making the scan time-consuming; 2) Hibernate ddl-auto=validate causes slow connections to remote databases; 3) Unnecessary auto-configurations have not been excluded. Use --debug mode to view details.

Q How do I specify the main entry class?

A Configure it using the <mainClass> in the Maven plugin; run the class containing the main method directly in the IDE; the Spring Boot Maven Plugin will automatically detect it during packaging.

📖 Summary

@SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan
The main startup class should be placed in the root package to ensure that the component scan covers all subpackages.
SpringApplication.run() Walk through the complete process of setting up the environment → creating a context → refreshing → instantiating a bean
YAML and Properties are functionally equivalent; YAML has a clearer hierarchy, while Properties are parsed faster.
Customize the banner using banner.txt; the startup log can help diagnose startup issues

📝 Exercises

Basic Exercise (Difficulty: ⭐): Create a Spring Boot project, customize banner.txt, change the service port to 9090, and after starting the project, verify that the /api/hello endpoint is accessible.
Advanced Exercise (Difficulty ⭐⭐): Replace SpringApplicationBuilder with SpringApplication.run(), disable the banner, set the log level to DEBUG, and customize a /api/app-info endpoint to return the application name and Java version.
Challenge (Difficulty: ⭐⭐⭐): Implement StartupListener and ApplicationListener<ApplicationStartedEvent>, record the application startup time, and print it to the log. Reflect on the design intent behind Spring Boot's event mechanism.

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404 Not Found

Your First Spring Boot Application

1. What You'll Learn

2. A True Story About a New Project

(1) Pain Point: It takes half a day to set up a project

(2) The Spring Boot Solution

(3) Revenue

3. Breaking Down the @SpringBootApplication Annotation

(1) A combination of three annotations

(2) @ComponentScan scanning rules

4. SpringApplication.run() Startup Process

(1) Startup Sequence

(2) Determining the Web Application Type

(1) ▶ Example: Customizing SpringApplication Startup

5. Configuration File Format

(1) Properties vs. YAML

(1) ▶ Example: How to write application.properties

(2) ▶ Example: How to write application.yml

(2) Quick Reference for Common Configuration Options

6. Customizing the Banner and Startup Logs

(1) Custom Banner

(1) ▶ Example: OrderFlow Custom Banner

(2) Interpreting the Startup Log

7. Run Your First "Hello OrderFlow" Interface

(1) ▶ Example: Hello OrderFlow Controller

(2) ▶ Example: Using @Value for configuration injection

8. Comprehensive Example: Complete OrderFlow Startup Configuration

❓ FAQ

📖 Summary

📝 Exercises

404 Not Found

Your First Spring Boot Application

1. What You'll Learn

2. A True Story About a New Project

(1) Pain Point: It takes half a day to set up a project

(2) The Spring Boot Solution

(3) Revenue

3. Breaking Down the @SpringBootApplication Annotation

(1) A combination of three annotations

(2) @ComponentScan scanning rules

4. SpringApplication.run() Startup Process

(1) Startup Sequence

(2) Determining the Web Application Type

(1) ▶ Example: Customizing SpringApplication Startup

5. Configuration File Format

(1) Properties vs. YAML

(1) ▶ Example: How to write application.properties

(2) ▶ Example: How to write application.yml

(2) Quick Reference for Common Configuration Options

6. Customizing the Banner and Startup Logs

(1) Custom Banner

(1) ▶ Example: OrderFlow Custom Banner

(2) Interpreting the Startup Log

7. Run Your First "Hello OrderFlow" Interface

(1) ▶ Example: Hello OrderFlow Controller

(2) ▶ Example: Using @Value for configuration injection

8. Comprehensive Example: Complete OrderFlow Startup Configuration

❓ FAQ

📖 Summary

📝 Exercises

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