Introduction to @Value Annotation

The @Value annotation in Spring Boot is used for injecting values into fields in a Spring-managed bean. It supports retrieving values from application properties files, environment variables, or even expressions. This is particularly useful for configuring application settings and externalizing configuration.

Basic Usage of @Value

Injecting a Property Value

To inject a value from the application.properties file, you can use the @Value annotation. Here’s an example:

application.properties:

properties

app.name=My Spring Boot Application

MyService.java:

java

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class MyService {    @Value("${app.name}")
    private String appName;    public void printAppName() {
        System.out.println("Application Name: " + appName);
    }
}

In this example, the value of app.name from the application.properties file is injected into the appName field.

Default Values

You can also provide a default value in case the property is not defined:

MyService.java:

java

Copy code

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class MyService {    @Value("${app.description:Default Application Description}")
    private String appDescription;    public void printAppDescription() {
        System.out.println("Application Description: " + appDescription);
    }
}

Here, if app.description is not defined in the properties file, the default value “Default Application Description” will be used.

Advanced Usage of @Value

Injecting System Properties and Environment Variables

You can inject system properties and environment variables using the @Value annotation:

MyService.java:

java

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class MyService {    @Value("${user.home}")
    private String userHome;    @Value("${JAVA_HOME}")
    private String javaHome;    public void printSystemProperties() {
        System.out.println("User Home: " + userHome);
        System.out.println("Java Home: " + javaHome);
    }
}

Using SpEL (Spring Expression Language)

The @Value annotation supports SpEL expressions for more complex scenarios:

MyService.java:

java

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class MyService {    @Value("#{systemProperties['user.name']}")
    private String userName;    @Value("#{T(java.lang.Math).random() * 100.0}")
    private double randomValue;    public void printSpelValues() {
        System.out.println("User Name: " + userName);
        System.out.println("Random Value: " + randomValue);
    }
}

Injecting List or Map Values

You can also inject lists or maps from properties:

application.properties:

properties

app.servers=server1,server2,server3
app.settings.key1=value1
app.settings.key2=value2

MyService.java:

java

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Map;@Service
public class MyService {    @Value("#{'${app.servers}'.split(',')}")
    private List<String> servers;    @Value("#{${app.settings}}")
    private Map<String, String> settings;    public void printListsAndMaps() {
        System.out.println("Servers: " + servers);
        System.out.println("Settings: " + settings);
    }
}

Conclusion

The @Value annotation is a powerful tool in Spring Boot for injecting configuration values into your beans. Whether you need to inject simple properties, system properties, environment variables, or more complex SpEL expressions, @Value makes it easy to externalize configuration and keep your application flexible and maintainable. The Spring Boot @Value annotation streamlines the process, ensuring that your configuration management is efficient and straightforward.