how to use strings which annotation with @Value in order to join the other strings?

Question

I want to use Strings which have Annotation @Value but If I try them join to another a string they turns a null object.If I use just them they works well.What I need to do use them to join another a string or those strings cannot join other strings?

 @Value("${Example.com}")
private String HOST_NAME;

private String BASE_URL="http://"+HOST_NAME+":8080";

System.out.println(HOST_NAME);
// Output is 123.123.123.1123;(For example)
System.out.println(BASE_URL);
// Output like that http://null:8080;

Answer 1

Here :

private String BASE_URL="http://"+HOST_NAME+":8080";

HOST_NAME is null because in Java, field initializers are evaluated before the class constructor but at the Spring startup, Spring values fields annotated with @Value such as :

@Value("${Example.com}")
private String HOST_NAME;

only after the constructor be invoked.

So to solve your requirement, set BASE_URL in the constructor such as :

private String BASE_URL;
public FooClass(){
   BASE_URL="http://"+HOST_NAME+":8080";
}

Not your question but you could use a constructor injection with @Value. It makes the class less Spring dependent and more unit-testable.

private String BASE_URL;
private String HOST_NAME;

public FooClass(@Value("${Example.com}") String hostName){
   this.HOST_NAME = hostName;
   this.BASE_URL="http://"+HOST_NAME+":8080";
}