Constants and how they can be misleading

Java Constant Gotchas

Sample code for ways Java Constants may not work as you would expect

App.java

public class App {

  public static void main(String[] args) {
    System.out.println(MyAnnotatedClass.class.getAnnotation(MyAnnotation.class).value());
    System.out.println(Constants.OUTPUT_DIRECTORY);
  }

}

Constants.java

public class Constants {
  // if you're wondering where the drive letter is, you have more reading to do :)
  public static final String OUTPUT_DIRECTORY = "/opt/application-output"; 
}

MyAnnotatedClass.java

@MyAnnotation(Constants.OUTPUT_DIRECTORY)
public class MyAnnotatedClass {

}

MyAnnotation.java

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;


@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
  String value();
}

running-without-constants

$ tree
.
|____App.java
|____Constants.java
$ javac -cp . *.java
$ tree
.
|____App.class
|____App.java
|____Constants.class
|____Constants.java
$ java -cp . SimpleApp
/opt/application-output
$ rm Constants.class
$ java -cp . SimpleApp
/opt/application-output

SimpleApp.java

public class SimpleApp {

  public static void main(String[] args) {
    System.out.println(Constants.OUTPUT_DIRECTORY);
  }

}