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Lombok @Slf4j Examples

In this tutorial we will investigate how Lombok works with @Slf4j for logging with several examples and also we will see how to use custom Logger using @CustomLog and Lombok’s configuration support for logging.

1. Maven dependencies for Lombok’s @Slf4j example

There are several logging frameworks implements Slf4j, we have used Logback in this example. Logback is intended as a successor to the popular log4j project which implements Slf4j natively. We have used logback-classic maven dependency, it will pull the logback-core and slf4j-api dependencies.

Make sure you already installed Lombok setup for your IDE. To Setup in Eclipse or in Spring Tool Suite refer to our Lombok Maven example setup with Eclipse.

	<dependencies>
		<!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
			<version>1.18.10</version>
			<scope>provided</scope>
		</dependency>

		<!-- https://mvnrepository.com/artifact/ch.qos.logback/logback-classic -->
		<dependency>
			<groupId>ch.qos.logback</groupId>
			<artifactId>logback-classic</artifactId>
			<version>1.2.3</version>
		</dependency>

	</dependencies>

2. A simple Lombok @Slf4j example

2.1. Lombok’s @Slf4j generates a logger instance with default name log using the SLF4J API. Note that we must also include a library that implements the Slf4j API.

@Slf4j
public class LombokSlf4jDemo1 {

	public Double getSurcharge(Double transaction) {

		log.info("Surcharge Caliculation begins");
		log.debug("Surcharge amount " + transaction);

		if (transaction.isNaN(transaction)) {
			log.error(transaction + " is not a valid amount");
			throw new RuntimeException("Invalid Transaction");
		}

		return Math.PI;
	}
}

2.2. Lombok generated / DeLomoked code for LombokSlf4jDemo1.java

public class LombokSlf4jDemo1 {
	private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(LombokSlf4jDemo1.class);

	public Double getSurcharge(Double transaction) {
		log.info("Surcharge Caliculation begins");
		log.debug("Surcharge amount " + transaction);
		if (transaction.isNaN(transaction)) {
			log.error(transaction + " is not a valid amount");
			throw new RuntimeException("Invalid Transaction");
		}
		return Math.PI;
	}
}

2.3. Testing LombokSlf4jDemo1.java :

public class LombokSlf4JDemo1Test {

	public static void main(String[] args) {
		
		new LombokSlf4jDemo1()
		.getSurcharge(0/0.0);
		
	}
}

2.4. Output Results :

20:15:33.631 [main] INFO  c.j.lombok.log.LombokSlf4jDemo1 - Surcharge Caliculation begins
20:15:33.674 [main] DEBUG c.j.lombok.log.LombokSlf4jDemo1 - Surcharge amount NaN
20:15:33.677 [main] ERROR c.j.lombok.log.LombokSlf4jDemo1 - NaN is not a valid amount
Exception in thread "main" java.lang.RuntimeException: Invalid Transaction
	at com.javabydeveloper.lombok.log.LombokSlf4jDemo1.getSurcharge(LombokSlf4jDemo1.java:15)
	at com.javabydeveloper.demo.log.LombokSlf4JDemo1Test.main(LombokSlf4JDemo1Test.java:10)

2. Changing Logger field name using Lombok configuration

By default Lombok generates static Logger instance with name log. If you wants to change Logger filed name, following configuration in lombok.config file allows you to to do so. Now we have to get Logger by using name logger.

lombok.config :

# default value is "log" 
lombok.log.fieldName = logger

# default value is "true"
lombok.log.fieldIsStatic = false

DeLomoked code for LombokSlf4jDemo1.java after config change :

public class LombokSlf4jDemo1 {
	private final org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(LombokSlf4jDemo1.class);

	public Double getSurcharge(Double transaction) {
		log.info("Surcharge Caliculation begins");
		log.debug("Surcharge amount " + transaction);
		if (transaction.isNaN(transaction)) {
			log.error(transaction + " is not a valid amount");
			throw new RuntimeException("Invalid Transaction");
		}
		return Math.PI;
	}
}

3. Custom Logger using Lombok @CustomLog

To use custom logger we need to use @CustomLog annotation and following configuration in lombok.config file. lombok.log.custom.declaration = LoggerType LoggerFactoryType.factoryMethod(TYPE|NAME|TOPIC|NULL)(TYPE|NAME|TOPIC|NULL)

Example : I am using java.util.logging.Logger with following config for the logging to run a quick test. lombok.log.custom.declaration = java.util.logging.Logger java.util.logging.Logger.getLogger(NAME)(TOPIC)

lombok.config :

# default value is "log" 
lombok.log.fieldName = logger

# default value is "true"
lombok.log.fieldIsStatic = true

lombok.log.custom.declaration = java.util.logging.Logger java.util.logging.Logger.getLogger(NAME)(TOPIC)

Lomboked LombokSlf4jDemo2.java :

@CustomLog
public class LombokSlf4jDemo2 {

	public Double getSurcharge(Double transaction) {
		
		logger.info("Surcharge Caliculation begins");
		logger.info("Surcharge amount " + "transaction"); 

		if (transaction.isNaN(transaction)) {
			logger.warning(transaction + " is not a valid amount"); 
			throw new RuntimeException("Invalid Transaction");
		}

		return Math.PI;
	}
}

DeLomboked LombokSlf4jDemo2.java :

public class LombokSlf4jDemo2 {
	private static final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(LombokSlf4jDemo2.class.getName());

	public Double getSurcharge(Double transaction) {
		logger.info("Surcharge Caliculation begins");
		logger.info("Surcharge amount " + "transaction");
		if (transaction.isNaN(transaction)) {
			logger.warning(transaction + " is not a valid amount");
			throw new RuntimeException("Invalid Transaction");
		}
		return Math.PI;
	}
}

4. Change Lombok’s log topic

By default generated Logger takes the type of class, to change that we need to set topic attribute value with desired name.

@Slf4j(topic = "PaymentSurcharge")
public class LombokSlf4jDemo1 {

	public Double getSurcharge(Double transaction) {
		
		logger.info("Surcharge Calculation begins");
		logger.debug("Surcharge amount " + transaction); 

		if (transaction.isNaN(transaction)) {
			logger.debug(transaction + " is not a valid amount"); 
			throw new RuntimeException("Invalid Transaction");
		}

		return Math.PI;
	}
}

DeLomboked Code :

public class LombokSlf4jDemo3 {
	private static final org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger("PaymentSurcharge");

	public Double getSurcharge(Double transaction) {
		logger.info("Surcharge Calculation begins");
		logger.debug("Surcharge amount " + transaction);
		return Math.PI;
	}
}

Output Results :

12:05:58.412 [main] INFO  PaymentSurcharge - Surcharge Calculation begins
12:05:58.439 [main] DEBUG PaymentSurcharge - Surcharge amount NaN
12:05:58.440 [main] DEBUG PaymentSurcharge - NaN is not a valid amount
Exception in thread "main" java.lang.RuntimeException: Invalid Transaction
	at com.javabydeveloper.lombok.log.LombokSlf4jDemo1.getSurcharge(LombokSlf4jDemo1.java:16)
	at com.javabydeveloper.demo.log.LombokSlf4JDemo1Test.main(LombokSlf4JDemo1Test.java:10)

5. Conclusion

In this guide we have investigated how to work with Lombok @Slf4j with several example for logging in application and also we have covered how to work with Lombok’s @CustomLog for the custom logging. You can refer Delombok Maven example to see how looks like lombok generated code for your Lomboked classes.

You might be interested in our other following Lombok Tutorials :

  1. Lombok @Data
  2. Lombok @Builder
  3. Lombok @Builder.Default
  4. Lombok @Singular
  5. Lombok @NonNull
  6. Lombok Spring Boot example
Satish Varma
Satish Varmahttps://javabydeveloper.com
Satish is post graduated in master of computer applications and experienced software engineer with focus on Spring, JPA, REST, TDD and web development. Also founder of javabydeveloper.com. Follow him on LinkedIn or Twitter or Facebook

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