Java Setting Static field with spring beans

1. In AppConstants change setAppName() to receive a String (why Properties?)

   public final class AppConstants {
       // ...
   
       public static void setAppName(String appName) {
           APP_NAME = appName;
       }
   }
   

2. Why use reflection? you can simply invoke the static method.
   Your entire AppConfig class can be as simple as this:

   @Configuration
   public class AppConfig {
       @Value("${app.constants.name}")
       public void setAppName(String appName) {
           AppConstants.setAppName(appName);
       }
   }
   

P.S. Settings static fields from Spring is not considered a best practice.

Edit: Answering "I would like to know what would be best practice"

A: First, why is injecting to static fields isn't recommended:

1. It makes testing a bit harder:

   • When using configurable static fields, it's pretty easy to forget to set one and screw up tests.
   • It makes running tests (with different configs) in parallel much harder.

2. Theoretically, you can have 2 Spring contexts loaded, each with a different config, then when the second context loads it overrides values set by the first context (so you can't be using static fields in that situation and you'll need to replace all usages).

   In practice this could happen, for instance, when you have two webapps (with different configs) sharing a jar containing AppConstants.

3. (This is no so common, but - somewhere down the line you might want to change your config scope (e.g. from Singleton to Prototype or Session scope). This will be harder with static fields.)

What is the best practice?

The best practice is simple (but might be hard to move to) - just use injection all the way:

Where you have AppConstants.APP_NAME replace it with:

@Inject AppConstants appConstants;
// ...
appConstants.getAppName()

But only change it if you feel what I said makes sense and worth the effort in your case.

If you do consider changing it, I recommend the following:

• Consider renaming AppConstants to AppProperties. By convention such beans, containing injected properties, are usually named XxxProperties.

• Consider using @ConfigurationProperties like so:
  (Removes some boilerplate).

  @EnableConfigurationProperties
  public class AppConfig {
  }
  
  @ConfigurationProperties("app.constants")
  public class AppProperties {
      private String name;        // = ${app.constants.name}
      private String version;     // = ${app.constants.version}
      private int port;           // = ${app.constants.port}
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      public String getVersion() {
          return version;
      }
  
      public void setVersion(String version) {
          this.version = version;
      }
  
      public int getPort() {
          return port;
      }
  
      public void setPort(int port) {
          this.port = port;
      }
  }