Introduction

Logging in spring boot is very flexible and easy to configure. Spring Boot’s default configurations provides a support for the use of Java Util Logging, Log4j2, and Logback. In each case, loggers are pre-configured to use console output with optional file output also available.

A new, freshly-generated Spring Boot application doesn’t come with any explicit logging configuration, and it will use Logback by default, the logging framework for Java, and the successor to the old log4j.
Logging is configured automatically when you add the spring-boot-starter-web dependency to your project.
Spring boot’s internal logging is written with Apache Commons Logging so it is one and only mandatory dependency. Till, boot 1.x – we had to import it manually. Since boot 2.x, it is downloaded transitively. To be more precise, spring-boot-starter-web depends on spring-boot-starter-logging, which pulls in spring-jcl for us.

File Output

By default, Spring Boot logs only to the console and does not write log files. To make Spring Boot write to a log file, you can set the logging.path property in your application.properties file:

logging.path = ./                      #  spring.log in current folder
logging.path = relativepath/logs       #  relativepath/logs/spring.log
logging.path = /fullpath/logs          #  /fullpath/logs/spring.log

With this configuration, Spring Boot will write to the console and also to a log file called spring.log, at the path you specify.
If you want to choose your own log filename instead of spring.log, then you can set the property logging.file, in your application.properties, e.g.:

logging.file = logfile.log                  # in current folder
logging.file = relativepath/logfile.log     # relative path with filename
logging.file = /fullpath/logfile.log        # full path with filename

Log files rotate when they reach 10 MB and, as with console output, ERROR-level, WARN-level, and INFO-level messages are logged by default. Size limits can be changed using the logging.file.max-size property. Previously rotated files are archived indefinitely unless the logging.file.max-history property has been set.

Logback does not have a FATAL level. It is mapped to ERROR.

We can configure separate property files for each environment like application-dev.properties and application-prod.properties. Follow the naming convention of application-{environment}.properties where {environment} is replaced with the environment name. Depending on your VM options or environment variables, one of these can be chosen.

Spring Cloud Config doesn’t support default Logback properties.

Log Format

The default log output from Spring Boot resembles the following example:

2020-03-08 11:57:51.102  INFO 55469 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/7.0.52
2020-03-08 11:57:51.203  INFO 55469 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2019-03-08 11:57:51.203  INFO 55469 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1358 ms
2020-03-08 11:57:51.608  INFO 55469 --- [ost-startStop-1] o.s.b.c.e.ServletRegistrationBean        : Mapping servlet: 'dispatcherServlet' to [/]

The following items are output:

• Date and Time: Millisecond precision and easily sort-able.
• Log Level: ERROR, WARN, INFO, DEBUG, or TRACE.
• Process ID.
• A --- separator to distinguish the start of actual log messages.
• Thread name: Enclosed in square brackets (may be truncated for console output).
• Logger name: This is usually the source class name (often abbreviated).
• The log message.

Customization

To help with the customization, some other properties are transferred from the Spring Environment to System properties, as described in the following table:

Spring Environment	System Property	Comments
logging.exception-conversion-word	LOG_EXCEPTION_CONVERSION_WORD	The conversion word used when logging exceptions.
logging.file	LOG_FILE	If defined, it is used in the default log configuration.
logging.file.max-size	LOG_FILE_MAX_SIZE	Maximum log file size (if LOG_FILE enabled). (Only supported with the default Logback setup.)
logging.file.max-history	LOG_FILE_MAX_HISTORY	Maximum number of archive log files to keep (if LOG_FILE enabled). (Only supported with the default Logback setup.)
logging.path	LOG_PATH	If defined, it is used in the default log configuration.
logging.pattern.console	CONSOLE_LOG_PATTERN	The log pattern to use on the console (stdout). (Only supported with the default Logback setup.)
logging.pattern.dateformat	LOG_DATEFORMAT_PATTERN	Appender pattern for log date format. (Only supported with the default Logback setup.)
logging.pattern.file	FILE_LOG_PATTERN	The log pattern to use in a file (if LOG_FILE is enabled). (Only supported with the default Logback setup.)
logging.pattern.level	LOG_LEVEL_PATTERN	The format to use when rendering the log level (default %5p). (Only supported with the default Logback setup.)

You can customize the default logging configuration by:

• Setting some specific Spring Boot properties (these can go in your application.properties, application.yml or as environment variables)
• Adding a logback.xml onto the classpath, which Spring Boot will detect and use to configure Logback
• Adding a logback-spring.xml onto the classpath, which Spring Boot will detect and use to configure Logback.

Tagged : java / log / logback / slf4j / spring boot / spring cloud config