Log4J: Spring Boot - Using Log4J with Spring Boot

A common debugging technique in any application, language or framework is to put in “debugging print statements”, e.g.

   System.out.println("Inside method foo, myVariable = " + myVariable);

Of course, these can get noisy if there are too many of them. And the burden of adding and deleting these can get cumbersome. Therefore, many frameworks provide a way to manage these debugging print statements with a logging framework that allows the developer to turn off and turn off various levels of debugging output.

For Java applications, a commonly used logging framework is called Log4J.

Using Log4J with the Lombok Log4J annotation

In our code base, the simplest way to use Log4J is to add the @Log4J annotation to the top of the class, like this:

@RestController
@Log4J
public class HelloController {
...

This annotation requires the following import:

import lombok.extern.log4j.Log4J;

Then, anytime you want logging information to appear, you can use statements such as these:

 log.trace("A TRACE Message");
 log.debug("A DEBUG Message");
 log.info("An INFO Message... profile.getUsername()="+profile.getUsername());

If the import is not able to be resolved, click the triangle for advice on what needs to be in your pom.xml for Maven to find Project Lombok imports.

package lombok.extern.log4j does not exist

    <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
    <dependency>
      <groupId>org.projectlombok</groupId>
      <artifactId>lombok</artifactId>
      <version>1.18.26</version>
      <scope>provided</scope>
    </dependency>

More on log4j

See also: http://www.springboottutorial.com/logging-with-spring-boot-logback-slf4j-and-log4j

Log4J is short for Apache Logging For Java

Setting the debugging level

To set the level of debugging, you can set the value in one of these ways:

1. In the file src/main/resources/application.properties like this:

   logging.level.org.pac4j.springframework.web=DEBUG
   logging.level.org.pac4j.core.engine=DEBUG
   logging.level.path.to.your.class=INFO
   

2. If you have a way of setting up the environment variable SPRING_APPLICATION_JSON, you can set the variables there also.

Setting up log4j the old fashioned way

You can also use log4j without Lombok. Here’s how:

In a Spring Boot application, inside any class where you want to do logging, you need these two import statements:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

And then you need to put this line of code inside the class. Substitute the actual name of the class followed by .class inside the parameter to LoggerFactory.getLogger. It is a good practice to put this first in the class, so that if the class name changes, you remember to change the name of the class on the next line.

public class ThisIsTheClassName {
    private Logger logger = LoggerFactory.getLogger(ThisIsTheClassName.class);
...

Then, anytime you want logging information to appear, you can use statements such as these:

 logger.trace("A TRACE Message");
 logger.debug("A DEBUG Message");
 logger.info("An INFO Message... profile.getUsername()="+profile.getUsername());