Static Code Blocks

2015-02-04

In a presentation that I gave recently, I noticed that some of the participants never heard of static code blocks before. Static code blocks let you execute something at class loading time (assign a value for example). They are like “Class-Constructors”. In this example i created a @Default-Configuration-annotation. Once the Application was loaded, the static code block was looking for the annotation and set the default configuration (a property-file)

The annotation:

@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ TYPE, METHOD, FIELD, PARAMETER })
public @interface DefaultConfiguration {
}

The annotated method:

public class Configurator {

  public void setConfigTest() {
    AppConfig.loadProperties("ConfigTest.properties");
  }

  public void setConfigDev() {
    AppConfig.loadProperties("ConfigDev.properties");
  }

  @DefaultConfiguration
  public void setConfigProduction() {
    AppConfig.loadProperties("ConfigProduction.properties");
  }
}

The configuriation entry point of the application:

public class AppConfig {

  private static Properties properties;

  static {
    AppConfig.initDefaultConfig();
  }

  private static void initDefaultConfig() {
    Configurator configurator = new Configurator();
    Method[] methods = configurator.getClass().getMethods();

    for (Method method : methods) {
      DefaultConfiguration annos = method.getAnnotation(DefaultConfiguration.class);

      if (annos != null) {
        try {
	  method.invoke(configurator);
	  break;

	} catch (Exception e) {
	  ExceptionHandler.handleException(e, AppConfig.class);
        }
      }
    }	
  }

  protected static void loadProperties(String fileName) {
  (...)
  }
}

The only pitfall you face: If you have more than one static code block, the execution order is the same as the order in the code.