Monday, July 16, 2012

Inject SLF4J logger by annotation

Actually, the following class is not written by me. It is written my colleage. I know he is referencing from another post from Java Geeks. I found this is quite handy so I put it here as a reference.

The main idea is make use of the BeanPostProcessors interface. We first create an "LoggableInjector" class which implement the BeanPostProcessors interface.
This injector class gets the SLF4J logger and assign to the beans property after the bean creation by the Spring IOC continaer.

1. Create Loggable annotation
@Retention(RetentionPolicy.RUNTIME)  
@Target(ElementType.FIELD)  
@Documented  
public @interface Loggable {
 //for slf4j
}  

2. Create LoggableInjector class to add logger to the bean
@Component
public class LoggableInjector implements BeanPostProcessor {        
 public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {  
  return bean;  
 }  
      
 public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {  
  ReflectionUtils.doWithFields(bean.getClass(), new FieldCallback() {  
   public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {  
    // make the field accessible if defined private  
    ReflectionUtils.makeAccessible(field);  
    if (field.getAnnotation(Loggable.class) != null) {
     Logger log = LoggerFactory.getLogger(bean.getClass());      
     field.set(bean, log);  
    }  
   }  
  });  
  return bean;  
 }  
} 

3. Usage Example

@Service
public class ReportServiceImpl {

 @Loggable
 private Logger logger;
 
}

No comments: