Spring constructor injection of SLF4J logger - how to get injection target class?

Question

I\'m trying to use Spring to inject a SLF4J logger into a class like so:

@Component
public class Example {

  private final Logger logger;

  @Autowired
  pu         


        

Answer 1

Here is an alternative to your solution. You could achieve your goal with BeanFactoryPostProcessor implementation.

Let's assume you want to have a class with logging. Here it is:

  package log;
  import org.apache.log4j.Logger;

  @Loggable
  public class MyBean {

     private Logger logger;
  }

As you could see this class does nothing and created just to be a logger container for simplicity. The only remarkable thing here is @Loggable annotation. Here its source code:

package log;

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

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Loggable {
}

This annotation is only a marker for further processing. And here is a most interesting part:

package log;

import org.apache.log4j.Logger;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;

import java.lang.reflect.Field;

public class LoggerBeanFactoryPostProcessor implements BeanFactoryPostProcessor{

    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        String[] names = beanFactory.getBeanDefinitionNames();
        for(String name : names){
            Object bean = beanFactory.getBean(name);
            if(bean.getClass().isAnnotationPresent(Loggable.class)){
                try {
                    Field field = bean.getClass().getDeclaredField("logger");
                    field.setAccessible(true);
                    field.set(bean, Logger.getLogger(bean.getClass()));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

It searches through all beans, and if a bean is marked as @Loggable, it initialize its private field with name logger. You could go even further and pass some parameters in @Loggable annotation. For example, it could be a name of field corresponding to logger.

I used Log4j in this example, but I guess it should work exactly the same way with slf4j.

Answer 2

Since Spring 4.3.0 you can use InjectionPoint or DependencyDescriptor as parameters for bean producing methods:

@Component
public class LoggingFactoryBean {
    @Bean
    public Logger logger(InjectionPoint injectionPoint) {
        Class<?> targetClass = injectionPoint.getMember().getDeclaringClass();
        return LoggerFactory.getLogger(targetClass);
    }
}

Answer 3

1. Why are you creating a new logger for each instance? The typical pattern is to have one logger per class (as a private static member).

2. If you really do want to do it that way: Maybe you can write a logger factory class, and inject that? Something like:

   @Singleton 
   public class LogFactory { 
       public Logger getLogger(Object o) {  
           return LoggerFactory.getLogger(o.getClass());  
       }  
   }
   

Answer 4

I am trying to get this feature into official SLF4J API. Please support/vote/contribute: https://issues.jboss.org/browse/JBLOGGING-62

(this feature is already implemented by JBoss Logging + Seam Solder, see http://docs.jboss.org/seam/3/latest/reference/en-US/html/solder-logging.html)

11.4. Native logger API

You can also inject a "plain old" Logger (from the JBoss Logging API):

import javax.inject.Inject;
import org.jboss.logging.Logger;

public class LogService {

    @Inject
    private Logger log;

    public void logMessage() {
        log.info("Hey sysadmins!");
    }

}

Log messages created from this Logger will have a category (logger name) equal to the fully-qualified class name of the bean implementation class. You can specify a category explicitly using an annotation.

@Inject @Category("billing")
private Logger log;

You can also specify a category using a reference to a type:

@Inject @TypedCategory(BillingService.class)
private Logger log;

Sorry for not providing a relevant answer.

Answer 5

I resolved it with a custom BeanFactory. If anyone comes up with a better solution, I would be happy to hear it. Anyway, here's the bean factory:

import java.util.Set;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.TypeConverter;
import org.springframework.beans.factory.config.DependencyDescriptor;
import org.springframework.beans.factory.support.DefaultListableBeanFactory;

public class CustomBeanFactory extends DefaultListableBeanFactory {

    public CustomBeanFactory() {
    }

    public CustomBeanFactory(DefaultListableBeanFactory delegate) {
        super(delegate);
    }

    @Override
    public Object resolveDependency(DependencyDescriptor descriptor,
            String beanName, Set<String> autowiredBeanNames,
            TypeConverter typeConverter) throws BeansException {
        //Assign Logger parameters if required      
        if (descriptor.isRequired()
                && Logger.class.isAssignableFrom(descriptor
                        .getMethodParameter().getParameterType())) {            
            return LoggerFactory.getLogger(descriptor.getMethodParameter()
                    .getDeclaringClass());
        } else {
            return super.resolveDependency(descriptor, beanName,
                    autowiredBeanNames, typeConverter);
        }
    }
}

Example usage with an XML config:

        CustomBeanFactory customBeanFactory = new CustomBeanFactory();      
        GenericApplicationContext ctx = new GenericApplicationContext(customBeanFactory);
        XmlBeanDefinitionReader xmlReader = new XmlBeanDefinitionReader(ctx);
        xmlReader.loadBeanDefinitions(new ClassPathResource("beans.xml"));
        ctx.refresh();

EDIT:

Below you can find Arend v. Reinersdorffs improved version (see the comments for an explanation).

import java.lang.reflect.Field;
import java.util.Set;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.TypeConverter;
import org.springframework.beans.factory.config.DependencyDescriptor;
import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.core.MethodParameter;

public class CustomBeanFactory extends DefaultListableBeanFactory {

    public CustomBeanFactory() {
    }

    public CustomBeanFactory(DefaultListableBeanFactory delegate) {
        super(delegate);
    }

    @Override
    public Object resolveDependency(DependencyDescriptor descriptor,
            String beanName, Set<String> autowiredBeanNames,
            TypeConverter typeConverter) throws BeansException {
        //Assign Logger parameters if required      
        if (Logger.class == descriptor.getDependencyType()) {            
            return LoggerFactory.getLogger(getDeclaringClass(descriptor));
        } else {
            return super.resolveDependency(descriptor, beanName,
                    autowiredBeanNames, typeConverter);
        }
    }

    private Class<?> getDeclaringClass(DependencyDescriptor descriptor) {
        MethodParameter methodParameter = descriptor.getMethodParameter();
        if (methodParameter != null) {
            return methodParameter.getDeclaringClass();
        }
        Field field = descriptor.getField();
        if (field != null) {
            return field.getDeclaringClass();
        }
        throw new AssertionError("Injection must be into a method parameter or field.");
    }
}

Answer 6

Try something like:

@Component
public class Example {

  @Autowired
  @Qualifier("exampleLogger")
  private final Logger logger;

}

And:

<bean id="exampleLogger" class="org.slf4j.LoggerFactory" factory-method="getLogger">
  <constructor-arg type="java.lang.Class" value="package.Example"/>        
</bean>