spring @value

转自：http://www.mkyong.com/spring3/spring-value-default-value/

In this tutorial, we will show you how to set a default value for @Value

1. @Value Examples

To set a default value in Spring expression, use Elvis operator :

	#{expression?:default value}

Few examples :

	@Value("#{systemProperties['mongodb.port'] ?: 27017}")
	private String mongodbPort;

	@Value("#{config['mongodb.url'] ?: '127.0.0.1'}")
	private String mongodbUrl;	
	
	@Value("#{aBean.age ?: 21}")
	private int age;

P.S @Value has been available since Spring 3.0

2. @Value and Property Examples

To set a default value for property placeholder :

	${property:default value}

Few examples :

	//@PropertySource("classpath:/config.properties}")
	//@Configuration
	
	@Value("${mongodb.url:127.0.0.1}")
	private String mongodbUrl;
	
	@Value("#{'${mongodb.url:172.0.0.1}'}")
	private String mongodbUrl;
	
	@Value("#{config['mongodb.url']?:'127.0.0.1'}")
	private String mongodbUrl;

config.properties

mongodb.url=1.2.3.4
mongodb.db=hello

For “config” bean.

	<util:properties id="config" location="classpath:config.properties"/>

Follow up
Must register a static PropertySourcesPlaceholderConfigurer bean in either XML or annotation, so that Spring @Valueknow how to interpret ${}

  //@PropertySource("classpath:/config.properties}")
  //@Configuration

  @Bean
  public static PropertySourcesPlaceholderConfigurer propertyConfigIn() {
	return new PropertySourcesPlaceholderConfigurer();
  }