@Value

The @Value annotation in Spring is used to inject values into fields, constructor parameters, or method parameters. These values can come from various sources, including:

• Properties files: Typically application.properties or application.yml.
• Environment variables: Operating system environment variables.
• System properties: Java system properties.
• SpEL (Spring Expression Language): For more complex value resolution.

Common Use Cases and Examples

1. Injecting Values from Properties Files

Assume you have an application.properties file like this:

app.name=My Awesome Application
app.version=1.0.0
database.url=jdbc:postgresql://localhost:5432/mydb
database.username=admin

You can inject these values into your Spring beans like this:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;
 
@Component
public class AppConfig {
 
    @Value("${app.name}")
    private String applicationName;
 
    @Value("${app.version}")
    private String version;
 
    @Value("${database.url}")
    private String dbUrl;
 
    @Value("${database.username}")
    private String dbUser;
}

2. Injecting Environment Variables and System Properties

You can also access environment variables and system properties using @Value:

@Component
public class SystemInfo {
 
    @Value("${JAVA_HOME}") // Accessing an environment variable
    private String javaHome;
 
    @Value("${user.dir}")   // Accessing a system property
    private String currentWorkingDirectory;
}

3. Using Default Values

Sometimes, a configuration property might not be defined. You can provide a default value using the following syntax:

@Component
public class FeatureToggle {
 
    @Value("${feature.new-dashboard:false}") // Default to false
    private boolean newDashboardEnabled;
 
}

4. Injecting into Constructor Parameters

You can also use @Value to inject values into constructor parameters:

@Service
public class NotificationService {
 
    private final String senderEmail;
 
    public NotificationService(@Value("${notification.sender.email}") String senderEmail) {
        this.senderEmail = senderEmail;
    }
 
    public void sendNotification(String recipient, String message) {
        System.out.println("Sending email from: " + senderEmail + " to: " + recipient + " with message: " + message);
    }
}

5. Injecting into Method Parameters

Less commonly used, but you can also inject values into method parameters:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;
 
@Component
public class ReportGenerator {
 
    public void generateMonthlyReport(@Value("${report.month}") String month,
                                     @Value("${report.year}") int year) {
        System.out.println("Generating report for " + month + ", " + year);
    }
}