Mastering Spring @Value: From Basics to Advanced Techniques

1. Introduction

Spring's @Value annotation injects external configuration (e.g., from application.yml ) into bean fields, constructor parameters, or method parameters. It supports simple types, SpEL expressions, and default values, making it a convenient choice for straightforward configuration needs.

<code>@Value("${pack.title}")
private String title ;
</code>

Corresponding YAML:

<code>pack:
  title: xxxooo
</code>

The above assigns title the value xxxooo . Type conversion is automatic, e.g., to LocalDate :

<code>@Value("${pack.date}")
private LocalDate date ;
</code>

<code>pack:
  date: 2024-07-27
</code>

2. Advanced Usage

2.1 Default Value

If a property is missing, Spring Boot throws an exception. You can provide a default by appending a colon:

<code>@Value("${pack.version:}")
</code>

or with an actual fallback value:

<code>@Value("${pack.version:1.0.0}")
</code>

2.2 Nested Property

You can reference another property inside the default expression:

<code>@Value("${pack.title:${title}}")
private String title ;
</code>

YAML example:

<code>title: xxxooo
pack:
  date: 2024-07-27
</code>

2.3 Using SpEL Expressions

Inject system properties:

<code>@Value("#{systemProperties['java.home']}")
private String javaHome ;
</code>

Result example:

<code>D:\devsoft\jdk-21_windows-x64_bin\jdk-21.0.1</code>

Inject other beans:

<code>@RestController
public class UserController {
  @Value("#{us}")
  private UserService us ;

  @Override
  public String toString() {
    return "UserController [us=" + us + "]";
  }
}
</code>

Result:

<code>com.pack.UserService@54e1c68b</code>

2.4 Custom Type Conversion

Register a custom converter:

<code>@Bean
public ConversionService conversionService() {
    DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService();
    conversionService.addConverter(new StringToDateConverter());
    return conversionService;
}
public class StringToDateConverter implements Converter<String, Date> {
  public Date convert(String source) {
    try {
      return new SimpleDateFormat("yyyy-MM-dd").parse(source);
    } catch (ParseException e) {
      throw new RuntimeException(e);
    }
  }
}
</code>

Then:

<code>@Value("${pack.date}")
private Date productDate ;
</code>

Result:

<code>Sat Jul 27 00:00:00 CST 2024</code>

Note: In Spring Boot, using BeanFactoryPostProcessor is often recommended for such registrations.

2.5 Custom Default Behavior

Override placeholder handling to ignore unresolved placeholders:

<code>@Bean
public PropertySourcesPlaceholderConfigurer propertyPlaceholderConfigurer() {
  PropertySourcesPlaceholderConfigurer pspc = new PropertySourcesPlaceholderConfigurer();
  pspc.setIgnoreUnresolvablePlaceholders(true);
  return pspc;
}
</code>

When the property is absent, the expression itself is output:

<code>@Value("${pack.version}")
private String version ;
</code>

<code>${pack.version}</code>

You can also change the placeholder delimiters:

<code>pspc.setPlaceholderPrefix("&{");
pspc.setPlaceholderSuffix("}");
</code>

2.6 Custom @Value Annotation

Create a meta‑annotation to reuse a specific property key:

<code>@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Value("${pack.url}")
public @interface PackValue {}
</code>

Usage:

<code>@PackValue
private String url ;
</code>

2.7 Combining ${} and #{}

Compose a value from multiple properties using SpEL:

<code>@Value("#{'${pack.baseURL}' + '/api/v' + '${app.version}'}")
private String url ;
</code>

YAML:

<code>app:
  version: 1.0
pack:
  baseURL: https://www.pack.com
</code>

Result:

<code>http://www.pack.com/api/v1.0</code>