There are many good benefits in using
slf4j library as your Java
application logging API layer. Here I will show few examples on how
to use and configure it with different loggers.
You can think of
slf4j as an Java interface, and then you would
need an implementation (ONLY ONE) at runtime to provide the actual
logging details, such as writing to STDOUT or to a file etc. Each
logging implementation (or called binding) would obviously have their
own way of configuring the log output, but your application will remain
agnostic and always use the same org.slf4j.Logger API. Let’s see how
this works in practice.
Using slf4j with Simple logger
Create a Maven based project and this in your
pom.xml. <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.5</version>
</dependency>
Now you may use
Logger in your Java code like this.package deng;
import org.slf4j.*;
public class Hello {
static Logger LOGGER = LoggerFactory.getLogger(Hello.class);
public static void main(String[] args) {
for (int i = 0; i < 10; i++)
if (i % 2 == 0)
LOGGER.info("Hello {}", i);
else
LOGGER.debug("I am on index {}", i);
}
}
The above will get your program compiled, but when you run it, you will see these output.
bash> java deng.Hello SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder". SLF4J: Defaulting to no-operation (NOP) logger implementation SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
What it’s saying is that at runtime, you are missing the logging "implementation" (or the
logger binding), so
slf4j simply use a "NOP" implmentation, which does nothing. In order
to see the output properly, you may try use an simple implementation that does not require
any configuration at all! Just go back to your pom.xml and add the following: <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
<version>1.7.5</version>
</dependency>
Now you see logging output on STDOUT with INFO level. This simple logger will default
show any INFO level message or higher. In order to see DEBUG messages, you would
need to pass in this System Property
-Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG
at your Java startup.
Using slf4j with Log4j logger
Now we can experiment and swap different logger implementations, but your application code
can remain the same. All we need is to replace
slf4j-simple with another popular logger
implementation, such as the Log4j. <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.5</version>
</dependency>
Again, we must configure logging per implementation that we picked.
In this case, we need an
src/main/resources/log4j.properties file. log4j.rootLogger=DEBUG, STDOUT
log4j.logger.deng=INFO
log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
log4j.appender.STDOUT.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n
Re-run your program, and you should see similar output.
Using slf4j with JDK logger
The JDK actually comes with a logger package, and you can replace
pom.xml
with this logger implementation. <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-jdk14</artifactId>
<version>1.7.5</version>
</dependency>
Now the configuration for JDK logging is a bit difficult to work with. Not only need a
config file, such as
src/main/resources/logging.properties, but you would also need
to add a System properties -Djava.util.logging.config.file=logging.properties
in order to have it pick it up. Here is an example to get you started:.level=INFOhandlers=java.util.logging.ConsoleHandler
java.util.logging.ConsoleHandler.level=FINEST
deng.level=FINEST
Using slf4j with Logback logger
The logback logger implementation is a super dupa quality implementation. If you intend
to write serious code that go into production, you may want to evaluate this option. Again
modify your
pom.xml to replace with this: <dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.0.13</version>
</dependency>
Here is a sample of configuration
src/main/resources/logback.xml to get things started.<configuration>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
</encoder>
</appender>
<logger name="deng" level="DEBUG"/>
<root level="INFO">
<appender-ref ref="STDOUT" />
</root>
</configuration>
Writing your own library with slf4j logger
If you are providing an Java library for large end users consumption, it’s good idea to
set your project to depend on
slf4j-api only, and then let your user choose any
logger implementation at their development or runtime environment. As end users, they
may quickly select one of option above and take advatage of their own favorite logging
implementation features.