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Using @Value
@Value
is typically used to inject externalized properties:
-
Java
-
Kotlin
@Component
public class MovieRecommender {
private final String catalog;
public MovieRecommender(@Value("${catalog.name}") String catalog) {
this.catalog = catalog;
}
}
@Component
class MovieRecommender(@Value("\${catalog.name}") private val catalog: String)
With the following configuration:
-
Java
-
Kotlin
@Configuration
@PropertySource("classpath:application.properties")
public class AppConfig { }
@Configuration
@PropertySource("classpath:application.properties")
class AppConfig
And the following application.properties
file:
catalog.name=MovieCatalog
In that case, the catalog
parameter and field will be equal to the MovieCatalog
value.
A default lenient embedded value resolver is provided by Spring. It will try to resolve the
property value and if it cannot be resolved, the property name (for example ${catalog.name}
)
will be injected as the value. If you want to maintain strict control over nonexistent
values, you should declare a PropertySourcesPlaceholderConfigurer
bean, as the following
example shows:
-
Java
-
Kotlin
@Configuration
public class AppConfig {
@Bean
public static PropertySourcesPlaceholderConfigurer propertyPlaceholderConfigurer() {
return new PropertySourcesPlaceholderConfigurer();
}
}
@Configuration
class AppConfig {
@Bean
fun propertyPlaceholderConfigurer() = PropertySourcesPlaceholderConfigurer()
}
When configuring a PropertySourcesPlaceholderConfigurer using JavaConfig, the
@Bean method must be static .
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Using the above configuration ensures Spring initialization failure if any ${}
placeholder could not be resolved. It is also possible to use methods like
setPlaceholderPrefix
, setPlaceholderSuffix
, or setValueSeparator
to customize
placeholders.
Spring Boot configures by default a PropertySourcesPlaceholderConfigurer bean that
will get properties from application.properties and application.yml files.
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Built-in converter support provided by Spring allows simple type conversion (to Integer
or int
for example) to be automatically handled. Multiple comma-separated values can be
automatically converted to String
array without extra effort.
It is possible to provide a default value as following:
-
Java
-
Kotlin
@Component
public class MovieRecommender {
private final String catalog;
public MovieRecommender(@Value("${catalog.name:defaultCatalog}") String catalog) {
this.catalog = catalog;
}
}
@Component
class MovieRecommender(@Value("\${catalog.name:defaultCatalog}") private val catalog: String)
A Spring BeanPostProcessor
uses a ConversionService
behind the scenes to handle the
process for converting the String
value in @Value
to the target type. If you want to
provide conversion support for your own custom type, you can provide your own
ConversionService
bean instance as the following example shows:
-
Java
-
Kotlin
@Configuration
public class AppConfig {
@Bean
public ConversionService conversionService() {
DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService();
conversionService.addConverter(new MyCustomConverter());
return conversionService;
}
}
@Configuration
class AppConfig {
@Bean
fun conversionService(): ConversionService {
return DefaultFormattingConversionService().apply {
addConverter(MyCustomConverter())
}
}
}
When @Value
contains a SpEL
expression the value will be dynamically
computed at runtime as the following example shows:
-
Java
-
Kotlin
@Component
public class MovieRecommender {
private final String catalog;
public MovieRecommender(@Value("#{systemProperties['user.catalog'] + 'Catalog' }") String catalog) {
this.catalog = catalog;
}
}
@Component
class MovieRecommender(
@Value("#{systemProperties['user.catalog'] + 'Catalog' }") private val catalog: String)
SpEL also enables the use of more complex data structures:
-
Java
-
Kotlin
@Component
public class MovieRecommender {
private final Map<String, Integer> countOfMoviesPerCatalog;
public MovieRecommender(
@Value("#{{'Thriller': 100, 'Comedy': 300}}") Map<String, Integer> countOfMoviesPerCatalog) {
this.countOfMoviesPerCatalog = countOfMoviesPerCatalog;
}
}
@Component
class MovieRecommender(
@Value("#{{'Thriller': 100, 'Comedy': 300}}") private val countOfMoviesPerCatalog: Map<String, Int>)