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The Simple Logging Facade for Java (SLF4J) serves as a simple
facade or abstraction for various logging frameworks, such as
java.util.logging, logback and reload4j. SLF4J allows the end-user
to plug in the desired logging framework at deployment
time. Note that SLF4J-enabling your library/application implies
the addition of only a single mandatory dependency, namely
slf4j-api-2.0.6.jar.
Where are the Maven coordinates?
At this time if you are only interested in obtaining the
coordinates for using SLF4J API with a logging backend, you can jump to the relevant section.
Salient historical changes
since 1.6.0 If no binding is found on the
class path, then SLF4J will default to a no-operation
implementation.
since 1.7.0 Printing methods in the
Logger
interface now offer variants accepting varargs
instead of Object[]. This change implies that SLF4J
requires JDK 1.5 or later. Under the hood the Java compiler
transforms the varargs part in methods into
Object[]. Thus, the Logger interface generated by the
compiler is indistinguishable in 1.7.x from its 1.6.x
counterpart. It follows that SLF4J version 1.7.x is totally 100%
no-ifs-or-buts compatible with SLF4J version 1.6.x.
since 1.7.5 Significant improvement
in logger retrieval times. Given the extent of the improvement,
users are highly encouraged to migrate to SLF4J 1.7.5 or later.
since 1.7.9 By setting the
slf4j.detectLoggerNameMismatch system property to
true, SLF4J can automatically spot incorrectly named loggers.
since 2.0.0 SLF4J API version 2.0.0
requires Java 8 and introduces a backward-compatible fluent
logging API. By backward-compatible, we mean that existing logging
frameworks do not have to be changed in order for the user to
benefit from the fluent logging API.
since 2.0.0 SLF4J API version 2.0.0
relies on the ServiceLoader
mechanism to find its logging backend. See the relevant FAQ entry for more
details.
Hello World
As customary in programming tradition, here is an example
illustrating the simplest way to output "Hello world" using SLF4J.
It begins by getting a logger with the name "HelloWorld". This
logger is in turn used to log the message "Hello World".
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class HelloWorld {
public static void main(String[] args) {
Logger logger = LoggerFactory.getLogger(HelloWorld.class);
logger.info("Hello World");
}
To run this example, you first need to obtain slf4j artifacts. Once that is
done, add the file
slf4j-api-2.0.6.jar to your class
path.
Compiling and running HelloWorld will result in the
following output being printed on the console.
SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See https://www.slf4j.org/codes.html#noProviders for further details.
If you are using SLF4J 1.7 or earlier, the message would be:
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See https://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
This warning is printed because no slf4j provider (or binding)
could be found on your class path.
The warning will disappear as soon as you add a provider to your class path. Assuming you add
slf4j-simple-2.0.6.jar so that your class
path contains:
Compiling and running HelloWorld will now result in
the following output on the console.
0 [main] INFO HelloWorld - Hello World
Typical usage
pattern
The sample code below illustrates the typical usage pattern
for SLF4J. Note the use of {}-placeholders on line 15. See the
question "What is the fastest way of logging?" in the FAQ for more details.
1: import org.slf4j.Logger;
2: import org.slf4j.LoggerFactory;
4: public class Wombat {
6: final Logger logger = LoggerFactory.getLogger(Wombat.class);
7: Integer t;
8: Integer oldT;
10: public void setTemperature(Integer temperature) {
12: oldT = t;
13: t = temperature;
15: logger.debug("Temperature set to {}. Old value was {}.", t, oldT);
17: if(temperature.intValue() > 50) {
18: logger.info("Temperature has risen above 50 degrees.");
19: }
20: }
21: }
since 2.0.0 SLF4J API version
2.0.0 requires Java 8 and introduces a backward-compatible
fluent logging API. By backward-compatible, we mean that
existing logging frameworks do not have to be changed in order
for the user to benefit from the fluent logging API.
The idea is to build a logging event piece by piece with a LoggingEventBuilder
and to log once the event is fully built. The
atTrace(), atDebug(),
atInfo(), atWarn() and
atError() methods, all new in the
org.slf4j.Logger interface, return an instance of
LoggingEventBuilder.
For disabled log levels, the returned
LoggingEventBuilder instance does nothing, thus
preserving the nanosecond level performance of the traditional
logging interface.
When using the fluent API, you must terminate the invocation
chain by calling one of the log() method
variants. Forgetting to call any of the log()
method variants will result in no logging regardless of the
logging level. Fortunately, many IDEs will alert you with a "No
return value" compiler warning.
Here are few usage examples:
The statement
logger.atInfo().log("Hello world.");
is equivalent to:
logger.info("Hello world.");
The following log statements are equivalent in their output
(for the default implementation):
int newT = 15;
int oldT = 16;
// using traditional API
logger.debug("Temperature set to {}. Old value was {}.", newT, oldT);
// using fluent API, log message with arguments
logger.atDebug().log("Temperature set to {}. Old value was {}.", newT, oldT);
// using fluent API, add arguments one by one and then log message
logger.atDebug().setMessage("Temperature set to {}. Old value was {}.").addArgument(newT).addArgument(oldT).log();
// using fluent API, add one argument with a Supplier and then log message with one more argument.
// Assume the method t16() returns 16.
logger.atDebug().setMessage("Temperature set to {}. Old value was {}.").addArgument(() -> t16()).addArgument(oldT).log();
The fluent logging API allows the specification of many
different types of data to a org.slf4j.Logger
without a combinatorial explosion in the number of methods in
the Logger interface.
It is now possible to pass multiple Markers, pass arguments
with a Supplier
or pass multiple key-value pairs. Key-value pairs are
particularly useful in conjunction with log data analysers which
can interpret them automatically.
The following log statements are equivalent:
int newT = 15;
int oldT = 16;
// using classical API
logger.debug("oldT={} newT={} Temperature changed.", oldT, newT);
// using fluent API
logger.atDebug().setMessge("Temperature changed.").addKeyValue("oldT", oldT).addKeyValue("newT", newT).log();
The key-value pair variant of the API stores the key-value
pairs as separates objects. The default implementation as
shipping in the org.slf4j.Logger class
prefixes key-value pairs to the message. Logging
backends are free and even encouraged to offer a more
customizable behaviour.
As mentioned previously, SLF4J supports various logging
frameworks. The SLF4J distribution ships with several jar files
referred to as "SLF4J provider", with each provider
corresponding to a supported framework. SLF4J 1.7 and earlier
used the term "binding" for providers.
Binding/provider for log4j version 1.2, a widely used logging framework. Given that log4j 1.x has been declared EOL in 2015
and again in 2022, as of SLF4J 1.7.35, the slf4j-log4j
module automatically redirects to the
slf4j-reload4j module at build time.
Assuming you wish to continue to use the log4j 1.x framework,
we strongly encourage you to use slf4j-reload4j
instead. See below.
Binding/provider for reload4j
framework. Reload4j is a drop-in replacement for log4j
version 1.2.7. You also need to place reload4j.jar
on your class path.
slf4j-jdk14-2.0.6.jar
Binding/provider for java.util.logging, also referred to as JDK 1.4
logging
slf4j-nop-2.0.6.jar
Binding/provider for NOP,
silently discarding all logging.
slf4j-simple-2.0.6.jar
Binding/provider for Simple implementation, which outputs all events to
System.err. Only messages of level INFO and higher are
printed. This binding may be useful in the context of small
applications.
slf4j-jcl-2.0.6.jar
Binding/provider for Jakarta Commons Logging. This binding will delegate all SLF4J logging to
JCL.
logback-classic-1.4.5.jar for use with Jakarta EE, requires logback-core-1.4.5.jar
logback-classic-1.3.5.jar for use with Javax EE, requires logback-core-1.3.5.jar
Native implementation There are also
SLF4J bindings/providers external to the SLF4J project, e.g. logback which implements
SLF4J natively. Logbacks
ch.qos.logback.classic.Logger class is a
direct implementation of SLF4Js org.slf4j.Logger interface. Thus, using SLF4J
in conjunction with logback involves strictly zero memory and
computational overhead.
To switch logging frameworks, just replace slf4j bindings on
your class path. For example, to switch from java.util.logging
to log4j, just replace slf4j-jdk14-2.0.6.jar with
slf4j-log4j12-2.0.6.jar.
SLF4J does not rely on any special class loader machinery. In
fact, each SLF4J binding is hardwired at compile time
to use one and only one specific logging framework. For
example, the slf4j-log4j12-2.0.6.jar binding is
bound at compile time to use log4j. In your code, in addition
to slf4j-api-2.0.6.jar, you simply drop
one and only one binding of your choice onto the
appropriate class path location. Do not place more than one
binding on your class path.
since 2.0.0 As of version 2.0.0,
SLF4J bindings are called providers. Nevertheless, the general
idea remains the same. SLF4J API version 2.0.0 relies on the ServiceLoader
mechanism to find its logging backend. See the relevant FAQ entry for more
details.
The SLF4J interfaces and their various adapters are extremely
simple. Most developers familiar with the Java language should
be able to read and fully understand the code in less than one
hour. No knowledge of class loaders is necessary as SLF4J does
not make use nor does it directly access any class loaders. As a
consequence, SLF4J suffers from none of the class loader
problems or memory leaks observed with Jakarta Commons Logging
(JCL).
Given the simplicity of the SLF4J interfaces and its
deployment model, developers of new logging frameworks should
find it very easy to write SLF4J bindings.
Libraries
Authors of widely-distributed components and libraries may
code against the SLF4J interface in order to avoid imposing a
logging framework on their end-user. Thus, the end-user may
choose the desired logging framework at deployment time by
inserting the corresponding slf4j binding on the classpath,
which may be changed later by replacing an existing binding with
another on the class path and restarting the application. This
approach has proven to be simple and very robust.
As of SLF4J version 1.6.0, if no binding is found on
the class path, then slf4j-api will default to a no-operation
implementation discarding all log requests. Thus, instead of
throwing a NoClassDefFoundError because the
org.slf4j.impl.StaticLoggerBinder class is missing,
SLF4J version 1.6.0 and later will emit a single warning message
about the absence of a binding and proceed to discard all log
requests without further protest. For example, let Wombat be
some biology-related framework depending on SLF4J for
logging. In order to avoid imposing a logging framework on the
end-user, Wombats distribution includes slf4j-api.jar
but no binding. Even in the absence of any SLF4J binding on the
class path, Wombats distribution will still work
out-of-the-box, and without requiring the end-user to download a
binding from SLF4Js web-site. Only when the end-user decides to
enable logging will she need to install the SLF4J binding
corresponding to the logging framework chosen by her.
Basic rule Embedded components
such as libraries or frameworks should not declare a dependency
on any SLF4J binding/provider but only depend on
slf4j-api. When a library declares a transitive dependency
on a specific binding, that binding is imposed on the end-user
negating the purpose of SLF4J. Note that declaring a
non-transitive dependency on a binding, for example for testing,
does not affect the end-user.
SLF4J usage in embedded components is also discussed in the
FAQ in relation with logging configuration, dependency reduction and
testing.
Declaring project
dependencies for logging
Given Mavens transitive dependency rules, for "regular"
projects (not libraries or frameworks) declaring logging
dependencies can be accomplished with a single dependency
declaration.
SLF4J API SLF4J API ships
within the "org.slf4j:slf4j-api" artifact. You can explicitly
declare a dependency to it in your pom.xml file as
shown below. Note that most logging implementations will
automatically pull-in slf4j-api as a dependency. However, it is
often a good idea to declare an explicit dependency to slf4j-api
in order to fix the correct version of slf4j-api your project by
virtue of of Mavens "nearest definition" dependency mediation
rule.
logback-classic 1.3.x (Javax
EE) If you wish to use logback-classic for Javax EE as the underlying logging
framework, all you need to do is to declare
"ch.qos.logback:logback-classic" as a dependency in your
pom.xml file as shown below. In addition to
logback-classic-1.3.5.jar, this will
pull in slf4j-api-2.0.6.jar as well
as logback-core-1.3.5.jar into your
project. Note that explicitly declaring a dependency on
logback-core-1.3.5 or
slf4j-api-2.0.6.jar is not wrong and
may be necessary to impose the correct version of said artifacts
by virtue of Mavens "nearest definition" dependency mediation
rule.
dependency>
groupId>ch.qos.logback /groupId>
artifactId>logback-classic /artifactId>
version>1.3.5 /version>
/dependency>
If you wish to use logback-classic for Jakarta EE as the underlying logging framework,
all you need to do is to declare
"ch.qos.logback:logback-classic" as a dependency in your
pom.xml file as shown below. In addition to
logback-classic-1.4.5.jar, this
will pull in slf4j-api-2.0.6.jar as
well as logback-core-1.4.5.jar
into your project. Note that explicitly declaring a
dependency on logback-core-1.4.5
or slf4j-api-2.0.6.jar is not wrong
and may be necessary to impose the correct version of said
artifacts by virtue of Mavens "nearest definition" dependency
mediation rule.
dependency>
groupId>ch.qos.logback /groupId>
artifactId>logback-classic /artifactId>
version>1.4.5 /version>
/dependency>
reload4j If you wish to use
reload4j as the underlying logging framework, all you need to do
is to declare "org.slf4j:slf4j-reload4j" as a dependency in your
pom.xml file as shown below. In addition to
slf4j-reload4j-2.0.6.jar, this will
pull in slf4j-api-2.0.6.jar as well
as reload4j-1.2.24.jar into your project.
Note that explicitly declaring a dependency on
reload4j-1.2.24.jar or
slf4j-api-2.0.6.jar is not wrong and
may be necessary to impose the correct version of said artifacts
by virtue of Mavens "nearest definition" dependency mediation
rule.
dependency>
groupId>org.slf4j /groupId>
artifactId>slf4j-reload4j /artifactId>
version>2.0.6 /version>
/dependency>
wish to use java.util.logging as the underlying logging
framework, all you need to do is to declare
"org.slf4j:slf4j-jdk14" as a dependency in your pom.xml
file as shown below. In addition to
slf4j-jdk14-2.0.6.jar, this will
pull in slf4j-api-2.0.6.jar into
your project. Note that explicitly declaring a dependency on
slf4j-api-2.0.6.jar is not wrong and
may be necessary to impose the correct version of said artifact
by virtue of Mavens "nearest definition" dependency mediation
rule.
dependency>
groupId>org.slf4j /groupId>
artifactId>slf4j-jdk14 /artifactId>
version>2.0.6 /version>
/dependency>
wish to use org.slf4j.simple as the underlying logging
implementation, all you need to do is to declare
"org.slf4j:slf4j-simple" as a dependency in your pom.xml
file as shown below. In addition to
slf4j-simple-2.0.6.jar, this will
pull in slf4j-api-2.0.6.jar into
your project. Note that explicitly declaring a dependency on
slf4j-api-2.0.6.jar is not wrong and
may be necessary to impose the correct version of said artifact
by virtue of Mavens "nearest definition" dependency mediation
rule.
dependency>
groupId>org.slf4j /groupId>
artifactId>slf4j-simple /artifactId>
version>2.0.6 /version>
/dependency>
An SLF4J binding designates an artifact such as
slf4j-jdk14.jar or slf4j-log4j12.jar used to
bind slf4j to an underlying logging framework, say,
java.util.logging and respectively log4j.
From the clients perspective all versions
of slf4j-api are compatible. Client code compiled with
slf4j-api-N.jar will run perfectly fine with slf4j-api-M.jar for
any N and M. You only need to ensure that the version of your
binding matches that of the slf4j-api.jar. You do not have to
worry about the version of slf4j-api.jar used by a given
dependency in your project.
Mixing different versions of slf4j-api.jar and SLF4J
binding can cause problems. For example, if you are using
slf4j-api-2.0.6.jar, then you should also use
slf4j-simple-2.0.6.jar, using
slf4j-simple-1.5.5.jar will not work.
However, from the clients perspective all versions of
slf4j-api are compatible. Client code compiled with
slf4j-api-N.jar will run perfectly fine with
slf4j-api-M.jar for any N and M. You only need to
ensure that the version of your binding matches that of the
slf4j-api.jar. You do not have to worry about the version of
slf4j-api.jar used by a given dependency in your project. You
can always use any version of slf4j-api.jar, and as
long as the version of slf4j-api.jar and its binding
match, you should be fine.
At initialization time, if SLF4J suspects that there may be
an slf4j-api vs. binding version mismatch problem, it will emit
a warning about the suspected mismatch.
Consolidate logging via
SLF4J
Often times, a given project will depend on various
components which rely on logging APIs other than SLF4J. It is
common to find projects depending on a combination of JCL,
java.util.logging, log4j and SLF4J. It then becomes desirable to
consolidate logging through a single channel. SLF4J caters for
this common use-case by providing bridging modules for JCL,
java.util.logging and log4j. For more details, please refer to
the page on Bridging legacy APIs.
Support for JDK Platform Logging (JEP 264)
SLF4J
Since 2.0.0-alpha5 The
slf4j-jdk-platform-logging module adds support for JDK Platform Logging.
dependency>
groupId>org.slf4j /groupId>
artifactId>slf4j-jdk-platform-logging /artifactId>
version>2.0.6 /version>
/dependency>
"Mapped Diagnostic Context" is essentially a map maintained
by the logging framework where the application code provides
key-value pairs which can then be inserted by the logging
framework in log messages. MDC data can also be highly helpful
in filtering messages or triggering certain actions.
SLF4J supports MDC, or mapped diagnostic context. If the
underlying logging framework offers MDC functionality, then
SLF4J will delegate to the underlying frameworks MDC. Note that
at this time, only log4j and logback offer MDC functionality. If
the underlying framework does not offer MDC, for example
java.util.logging, then SLF4J will still store MDC data but the
information therein will need to be retrieved by custom user
code.
Thus, as a SLF4J user, you can take advantage of MDC
information in the presence of log4j or logback, but without
forcing these logging frameworks upon your users as
dependencies.
For more information on MDC please see the chapter on MDC
in the logback manual.
The desired logging framework can be plugged in at
deployment time by inserting the appropriate jar file
(binding) on your class path.
Due to the way that classes are loaded by the JVM, the
framework binding will be verified automatically very early
on. If SLF4J cannot find a binding on the class path it
will emit a single warning message and default to
no-operation implementation.
SLF4J supports popular logging frameworks, namely log4j,
java.util.logging, Simple logging and NOP. The logback project supports
SLF4J natively.
The implementation of JCL over SLF4J, i.e
jcl-over-slf4j.jar, will allow your project to
migrate to SLF4J piecemeal, without breaking compatibility
with existing software using JCL. Similarly,
log4j-over-slf4j.jar and jul-to-slf4j modules will allow
you to redirect log4j and respectively java.util.logging
calls to SLF4J. See the page on Bridging legacy APIs for more
details.
The slf4j-migrator utility
can help you migrate your source to use SLF4J.
All SLF4J bindings support parameterized log messages
with significantly improved performance
results.
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