Inject Logger in Spring

Introduction

When outputting Log in Java, use libraries such as Log4j and Logback, or use java.util.logging. Most libraries and APIs get and generate Logger instances as follows.

Logger logger = Logger.getLogger(Hoge.class);

Writing these in every class is a hassle. Let's find a simpler way to write using Spring.

Former story

It's almost an introduction to the implementation of this site. http://memorynotfound.com/spring-inject-logger-annotation-example/

Thank you.

Logger Injection by Spring

If you are using Spring, you can inject Logger into the annotated filed as below.

    @Log
    private static Logger logger;

The amount of description is small, but it is not necessary to specify the argument class, so you can eliminate Logger's mistake (which should sometimes be copied and pasted).

I will explain how to implement it.

Log annotation

Create a Log annotation as shown below.

Log.java

@Retention(RUNTIME)
@Target(FIELD)
@Documented
public @interface Log {
}

Since it is referenced at runtime, it is @Retention (RUNTIME), and since the target is a field, it is @Target (FIELD).

LogInjector

Using Spring, write the code to inject Logger in the field with the @Log annotation created earlier.

Create a BeanPostProcessor implementation of Spring as follows, and create a Logger in it.

@Component
public class LogInjector implements BeanPostProcessor {

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    @Override
    public Object postProcessBeforeInitialization(final Object bean, String name) throws BeansException {
        ReflectionUtils.doWithFields(bean.getClass(), new ReflectionUtils.FieldCallback() {
            public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
                // make the field accessible if defined private
                ReflectionUtils.makeAccessible(field);
                if (field.getAnnotation(Log.class) != null) {
                    Logger log = Logger.getLogger(bean.getClass());
                    field.set(bean, log);
                }
            }
        });
        return bean;
    }

}

how to use

Now you are ready. After that, you can use it by following the steps below.

• Keep LogInjector under Spring control.
• Add @Log to the static filed of the class you want to inject Logger.

Now you don't have to do getLogger every time.