@Value Annotation in Spring boot

📌 What it is

@Value is a Spring Framework annotation used for dependency injection of values into fields, method parameters, or constructor arguments.
It allows you to read values from:

• application.properties or application.yml
• System environment variables
• Command-line arguments
• SpEL (Spring Expression Language) expressions

💡 Why we use it

1. Externalize Configuration
   Instead of hardcoding values (like URLs, usernames, etc.) in code, you can keep them in application.properties or environment variables.

   name=Gaurav

Then inject it:

   @Value("${name}")
   private String name;

1. Easier Maintenance
   When configuration changes (like database URL, API key, etc.), you can just update the .properties file — no code change required.

2. Environment Flexibility
   Helps when deploying the same app in multiple environments — dev, test, prod — each with different configurations.

🕐 When to use it

Use @Value when:

• You need simple static configuration values (e.g., strings, numbers, booleans).
• You want to inject a single property directly from application.properties.
• You need to evaluate small expressions (e.g., ${timeout:30} means use 30 if timeout is missing).

Example scenarios:

• Injecting API base URLs
• Setting retry counts, timeouts, feature toggles
• Reading environment-specific constants

⚙️ How it works (internally)

1. When Spring Boot starts, it loads all configuration files (application.properties, application.yml, etc.).
2. It creates a Spring Environment object that holds key–value pairs.
3. When Spring encounters @Value("${name}"), it:

• Looks up "name" in the Environment.
• Retrieves the value.
• Injects it into the constructor argument (or field).
  1. The bean is then created with that injected value.

🧩 Example in full context

application.properties

app.title=Backend

Java class

@Component
public class ConfigTestClass {

    private final String appTitle;

    public ConfigTestClass(@Value("${app.title}") String appTitle) {
        this.appTitle = appTitle;
    }

    @PostConstruct
    public void printValue() {
        System.out.println("App Title: " + appTitle);
    }
}

🟢 Output on startup:

App Title: Backend

🧠 Extra: Alternatives to @Value

For injecting multiple properties or grouping them:

@ConfigurationProperties(prefix = "app")
public class AppConfig {
    private String title;
    private String version;
}

Then in application.properties:

app.title= Backend
app.version=1.0

✅ Best practice for larger configurations.

📚 Summary