61. Logging

Spring Boot has no mandatory logging dependence, except for the commons-logging API, of which there are many implementations to choose from. To use Logback you need to include it, and some bindings for commons-logging on the classpath. The simplest way to do that is through the starter poms which all depend on spring-boot-starter-logging. For a web application you only need spring-boot-starter-web since it depends transitively on the logging starter. For example, using Maven:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

Spring Boot has a LoggingSystem abstraction that attempts to configure logging based on the content of the classpath. If Logback is available it is the first choice.

If the only change you need to make to logging is to set the levels of various loggers then you can do that in application.properties using the "logging.level" prefix, e.g.

logging.level.org.springframework.web: DEBUG
logging.level.org.hibernate: ERROR

You can also set the location of a file to log to (in addition to the console) using "logging.file".

To configure the more fine grained settings of a logging system you need to use the native configuration format supported by the LoggingSystem in question. By default Spring Boot picks up the native configuration from its default location for the system (e.g. classpath:/logback.xml for Logback), but you can set the location of the config file using the "logging.config" property.

61.1 Configure Logback for logging

If you put a logback.xml in the root of your classpath it will be picked up from there. Spring Boot provides a default base configuration that you can include if you just want to set levels, e.g.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/base.xml"/>
    <logger name="org.springframework.web" level="DEBUG"/>
</configuration>

If you look at the default logback.xml in the spring-boot jar you will see that it uses some useful System properties which the LoggingSystem takes care of creating for you. These are:

  • ${PID} the current process ID.
  • ${LOG_FILE} if logging.file was set in Boot’s external configuration.
  • ${LOG_PATH} if logging.path was set (representing a directory for log files to live in).

Spring Boot also provides some nice ANSI colour terminal output on a console (but not in a log file) using a custom Logback converter. See the default base.xml configuration for details.

If Groovy is on the classpath you should be able to configure Logback with logback.groovy as well (it will be given preference if present).

61.2 Configure Log4j for logging

Spring Boot supports Log4j for logging configuration, but it has to be on the classpath. If you are using the starter poms for assembling dependencies that means you have to exclude logback and then include log4j instead. If you aren’t using the starter poms then you need to provide commons-logging (at least) in addition to Log4j.

The simplest path to using Log4j is probably through the starter poms, even though it requires some jiggling with excludes, e.g. in Maven:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <exclusions>
            <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-logging</artifactId>
            </exclusion>
    </exclusions>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j</artifactId>
</dependency>
[Note]Note

The use of the log4j starter gathers together the dependencies for common logging requirements (e.g. including having Tomcat use java.util.logging but configure the output using Log4j). See the Actuator Log4j Sample for more detail and to see it in action.