Using `@Value`

@Value 通常用于注入外部化属性：

@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)

使用以下配置：

With the following configuration:

Java
Kotlin

@Configuration
@PropertySource("classpath:application.properties")
public class AppConfig { }

@Configuration
@PropertySource("classpath:application.properties")
class AppConfig

和以下 application.properties 文件：

And the following application.properties file:

catalog.name=MovieCatalog

在这种情况下，catalog 参数和字段将等于 MovieCatalog 值。

In that case, the catalog parameter and field will be equal to the MovieCatalog value.

Spring 提供了默认的宽松嵌入式值解析器。它将尝试解析属性值，如果无法解析，将注入属性名称（例如 ${catalog.name}）作为值。如果您想要对不存在的值进行严格控制，则应声明一个 PropertySourcesPlaceholderConfigurer bean，如下例所示：

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()
}

使用 JavaConfig 配置 PropertySourcesPlaceholderConfigurer 时，@Bean 方法必须是 static。

When configuring a PropertySourcesPlaceholderConfigurer using JavaConfig, the @Bean method must be static.

使用上述配置可确保在无法解析任何 ${} 占位符时，Spring 初始化失败。还可以使用诸如 setPlaceholderPrefix、setPlaceholderSuffix、setValueSeparator 或 setEscapeCharacter 的方法来自定义占位符。

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, setValueSeparator, or setEscapeCharacter to customize placeholders.

Spring Boot 默认配置一个 PropertySourcesPlaceholderConfigurer bean，该 bean 将从 application.properties 和 application.yml 文件中获取属性。

Spring Boot configures by default a PropertySourcesPlaceholderConfigurer bean that will get properties from application.properties and application.yml files.

Spring 提供的内置转换器支持允许自动处理简单类型转换（例如，转换为 Integer 或 int）。可以自动将多个逗号分隔值转换为 String 数组，而无需其他操作。

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.

可以提供默认值，如下所示：

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)

Spring BeanPostProcessor 在后台使用 ConversionService 来处理将 @Value 中的 String 值转换为目标类型的过程。如果您想为自己的自定义类型提供转换支持，则可以使用自己的 ConversionService bean 实例，如下例所示：

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())
		}
	}
}

当 @Value 包含一个 `SpEL`表达式时，如下面的示例所示，该值会在运行时被动态计算：

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 还支持使用更复杂的数据结构：

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>)

Using @Value

Using `@Value`