Spring Expression Language (SpEL)

The Spring Expression Language is a very powerful feature that can be a lifesaver when used correctly and a nightmare when not. It is Spring-specific syntax that adds a level dynamic behavior at runtime. You would typically interact with it in two primary ways - the SpelExpressionParser or through an annotation.

Read: Spring Expression Language Read: Spring Expression Language Guide

SpelExpressionParser

Here is an example using SpelExpressionParser from the Spring documentation:

var parser: ExpressionParser = SpelExpressionParser()
var exp: Expression = parser.parseExpression("'Hello World'.concat('!')")
var message: String = (String) exp.getValue() 
println(message) // prints "Hello World!"

Annotation with #{ }

Here is an example doing the same thing with an annotation. SpEL is trigger with syntax #{ }.

@Component
class MyConfig {

  @Value("#{'Hello World'.concat('!')}")
  lateinit var hello: String
  
  @PostConstruct fun init() = println(message) // prints "Hello World!"
  
}