In spring, there are two approaches to get value in runtime:

• Property placeholder
• Spring Expression Language(EL)

Property placeholder

1. create a property file, like test.properties

   jdbc.username=root
   ...
   

2. define PropertySourcesPlaceholderConfigurer

   <context:property-placeholder location="test.properties"/>

   or

   @PropertySource("classpath:test.properties")

   With annotation, developer have to define

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

3. get value by ${variable-name}

   <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
   
           <property name="username" value="${jdbc.username}" />
           ....
     </bean>
   

   @Autowired
   private Environment env;
   ...
   dataSource.setUrl(env.getProperty("jdbc.username"));
   

### Spring EL

Spring EL has to be defined in #{spring-expression-language}

For example, #{user.name}, #{'hello'}, #{usr.setName()}, #{user.getName()?.toUpperCase()}