Spring Boot – Logging
Logging in Spring Boot uses Apache Commons Logging for all internal logging but leaves the underlying log implementation open. The Commons Logging (JCL) provides a Log interface that is intended to be both light-weight and an independent abstraction of other logging toolkits.
We shouldn’t worry about importing spring-jcl at all if we’re using a Spring Boot Starter Project. That’s because every starter, like our spring-boot-starter-web, depends on spring-boot-starter-logging, which already pulls in spring-jcl dependencies for us.
When using starters, Logback is used for logging in spring boot by default.
The default log output from Spring Boot resembles the following example:
2019-09-15 17:59:56.002 INFO 13108 --- [ main] c.talksinfo.SpringPropertiesApplication : Starting SpringApplication on PID 13108 2019-09-15 17:59:56.004 INFO 13108 --- [ main] c.talksinfo.SpringPropertiesApplication : The following profiles are active: test 2019-09-15 17:59:56.597 INFO 13108 --- [ main] c.talksinfo.SpringPropertiesApplication : Started SpringApplication in 0.951 seconds
The following items are output:
- Date and Time: Millisecond precision and easily sortable.
- Log Level:
- Process ID.
---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.
It is important to ensure that log message are appropriate in content and severity. The following guidelines are suggested:
- error – Severe errors that cause premature termination. Expect these to be immediately visible on a status console. Also runtime errors or unexpected conditions. Expect these to be immediately visible on a status console.
- warn – Use of deprecated APIs, poor use of API, ‘almost’ errors, other runtime situations that are undesirable or unexpected, but not necessarily “wrong”. Expect these to be immediately visible on a status console.
- info – Interesting runtime events (startup/shutdown). Expect these to be immediately visible on a console, so be conservative and keep to a minimum.
- debug – detailed information on the flow through the system. Expect these to be written to logs only.
- trace – more detailed information. Expect these to be written to logs only.
Logback does not have a
FATALlevel. It is mapped to
Default Message Priority/Level
The default log configuration echoes messages to the console as they are written. By default,
INFO-level messages are logged. You can also enable a “debug” mode by starting your application with a
You can also specify
When the debug mode is enabled, a selection of core loggers (embedded container, Hibernate, and Spring Boot) are configured to output more information. Enabling the debug mode does not configure your application to log all messages with
Alternatively, you can enable a “trace” mode by starting your application with a
--trace flag (or
trace=true in your
application.properties). Doing so enables trace logging for a selection of core loggers (embedded container, Hibernate schema generation, and the whole Spring portfolio).
Example of declaring Log Level in application.properties
Example of declaring Log Level in application.yaml
logging: level: root: INFO
Setting log level to
trace will show
trace log something like below in console,
Above image shows truncated lines of logs as it is just an example. Original record has still more number of lines.
Custom Logging Properties
Other properties available for customization of logging are below ones,
Grouping Packages for Setting Log Levels
Packages can be grouped together so that logging can all be configured at the same time.
For example, you can change the logging levels for all Tomcat related loggers at one place. To help with this, Spring Boot allows you to define logging groups in your Spring
Environment. For example, here’s how you could define a “tomcat” group by adding it to your
logging.group.tomcat=org.apache.catalina, org.apache.coyote, org.apache.tomcat
Once defined, you can change the level for all the loggers in the group with a single line:
Spring Boot includes the following pre-defined logging groups that can be used out-of-the-box:
Custom Logging using Java Util logging
Spring Boot also supports JDK logging, through the logging.properties configuration file. There are cases when it’s not a good idea to use it, though. From the documentation:
There are known classloading issues with Java Util Logging that cause problems when running from an ‘executable jar’. We recommend that you avoid it when running from an ‘executable jar’ if at all possible.
In this article, we had a quick look at how spring-jcl is included in starters, log format, different log levels, setting log levels, grouping package for log levels and available custom logging properties.
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