A Java geek

I mainly write articles for two reasons: to understand something that is new to me or to document something I regularly have to explain to others. This article definitely falls in the second category: in order to celebrate the new 1.0.0 version of Logback, I’ve decided to write down once and for all how to properly use SLF4J with Maven since it seems there’s no shortage of questions about it.

Basics

The basis of SLF4J is to have two separate components, one API and one implementation. That means that your code should solely be dependent on the API thus the implementation can be changed at your convenience. Remember that decoupling code from implementations by introducing an interface should be your first concern: at the library level, it’s the role of an API.

This is easily done like this in your POM:

<project ...>
	<dependencies>
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-api</artifactId>
			<version>1.6.4</version>
		</dependency>
		<dependency>
			<groupId>ch.qos.logback</groupId>
			<artifactId>logback-classic</artifactId>
			<scope>runtime</scope>
			<version>1.0.0</version>
		</dependency>
	<dependencies>
</project>

The runtime scope prevents us from using Logback’s implementation classes when coding, which achieves decoupling.

Note: if you’re using a parent POM for your project (because you have modules), it goes without saying the above dependencies should be under the dependencyManagement section.

Bridging

Since an application with only logging features is a bit limited, it’s highly likely we’ll need other dependencies. Of course, some of those dependencies may use other logging frameworks, such as Commons Logging or Log4J.

For example, Spring uses Commons Logging: thus, logs produced by our application would use logback while those produced by Spring would use Commons Logging. This strategy needs two different configurations files (as well as knowledge of both) which is not very intuitive. Moreover, this would be unwieldly when reading twothe flow on two different sources. Note that this could be mitigated by using logging targets other than files (which I haven’t seen yet).

Fortunately, SLF4J provides bridging components that let us wire third-party API calls directly to SLF4J. For Spring, that means removing all dependencies to Commons Logging and replacing them with a single dependency to the Commons Logging to SLF4 bridge.

<project ...>
	<dependencies>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-core</artifactId>
			<version>3.0.0</version>
			<exclusions>
				<exclusion>
					<groupId>commons-logging</groupId>
					<artifactId>commons-logging</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-beans</artifactId>
			<version>3.0.0</version>
			<exclusions>
				<exclusion>
					<groupId>commons-logging</groupId>
					<artifactId>commons-logging</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>3.0.0</version>
			<exclusions>
				<exclusion>
					<groupId>commons-logging</groupId>
					<artifactId>commons-logging</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>jcl-over-slf4j</artifactId>
			<version>1.6.2</version>
		</dependency>
	<dependencies>
</project>

Like in the section above, you should preferably use the

Note: although this knowledge is by no means necessary, know that since API and implementation are both packages together in the commons-logging.jar, the jcl-over-slf4.jar bridge completely replaces the API classes provided by Commons Logging. That means you shouldn’t have both JAR in your classpath!

Troubleshooting

Remember the Highlander rule: “there can be only one”. In this case, there can be only one implementation available on the classpath since SLF4J uses the Java Services Provider. If its the case, SLF4J will display a “Multiple bindings were found on the class path” warning and uses the first it finds (first on the classpath). Since classpaths are ordered by Maven, there’s little chance it will be the thing we want. Thus, take care of not putting slf4j-simple in the test scope along with logback or be prepared to face undetermined side-effects.

To go further:

• Logback Project
• SLF4J warning or error messages and their meanings

In this post, I will ramble on logging in Java, how it was done in the old days and what can a library like SLF4J can bring.

Logging is a basic need when you create software. Logging use-cases are:

• debugging the software during development,
• help diagnose bugs during production
• trace access for security purposes
• create data for statistical use
• etc.

Whatever the use, logs should be detailed, configurable and reliable.

History

Historically, Java logs where done with System.out.println(), System.err.println() or e.printStackTrace() statements. Debug logs where put in System.out and error logs in System.err. In production, both were redirected: the former on the null output, the latter to the desired error log file. So they were useful enough but they suffered from a big drawback: they were not very configurable. It was an all or nothing switch, either log or don’t. You could not focus detailed logs on a particular layer or package.

Log4J came to the rescue. It was everything one could ever want from a logging framework. It invented many concepts still used in today’s logging frameworks (it was the first framework I used so please bear with me if a concept was not invented by it):

• the concept of Logger so that each logger can be configured independently
• the concept of Appender so that each appender can log wherever it wants (file, database, message, etc.)
• the concept of Level so that one can configure the logging (or not) of each message separately

After that, Sun felt the need for a logging feature inside the JDK but, instead of using log4j directly, it created an API inspired by it. However, it was not not so well finished as Log4J. If you want to use JDK 1.4 logging, chances are you’ll have to write your own Appenders – called Handlers – since the only handlers available are Console and File out-of-the-box.

With both frameworks available, you needed to configure both of them since whatever framework you used, there was surely at least one dependency of your project that used the other. Apache Commons Logging is an API bridge that connects itself to supported logging frameworks. Libraries should use commons-logging so that the real framework used is the choice of the project, and is not imposed by the dependencies. This is not always the case, so Commons Logging does not solve the double configuration problem. Morevoer, Commons Logging suffer from some class-loading problems, leading to NoClassDefFoundError errors.

Finally, the lead programer from Log4J quit the project for reasons I will not detail here. He created another logging framework, namely SLF4J, that should have been Log4J v2.

Some strange facts

The following facts are things that bother me with the previous frameworks. They are not drawbacks per se but are worth mentioning:

• Log4J has Maven dependencies on JMS, Mail and JMX that are not optional, meaning they will appear on your classpath if you do not bother to exclude them
• Likewise, Commons Logging has Maven dependencies on Avalon (another logging framework), Log4J, LogKit and Servlet API (!) that are not optional
• A Swing log viewer is included in Log4J jar, even if you it is used in an headless environment, such as a batch or an application server
• Log4J v1.3 main page redirects to v1.2 while Log4 v2.0 is experimental

Which framework to use ?

Log4J would be the framework of choice (and is for most) but it is no longer developed. Version 1.2 is the reference, version 1.3 is abandoned and version 2.0 is still in its early stage.

Commons Logging is a good choice for a library (as opposed to an application) but I suffered once classloaders issues, and once is enough to veto it (finally, i threw Commons Logging out and used Log4J directly).

JDK 1.4 Logging is a standard and does not raise concurrent versions problems. But it lacks so many features, it cannot be used without redeveloping some such as a database adapter and such. Too bad… but it does not answers the question: which framework to use?

Recently, architects of my company decided for SLF4J. Why such a choice?

SLF4J

SLF4J is not as widespread as Log4J because most architects (and developers alike) know Log4J well and either don’t know about SLF4J, or don’t care and stick to Log4J anyway. Moreover, for most projects Log4J fulfills all logging’s needs. Yet, interestingly enough, Hibernate uses SLF4J. It has some nice features that are not present in Log4J.

Simple syntax

Take the following Log4J example:

LOGGER.debug("Hello " + name);

Since String concatenation is frowned upon, many companies enforce the following syntax, so that the concatenation only does take place when in DEBUG level:

if (logger.isDebugEnabled()) {

  LOGGER.debug("Hello " + name);
}

It avoids String concatenation but it’s a bit heavy, isn’t it? In contrast, SLF4J offers the following simple syntax:

LOGGER.debug("Hello {}", name);

It’s like the first syntax but without the concatenation cost nor the heavy syntax burden.

SLF4J API and implementation

Moreover, SLF4 nicely decouples API from implementation so that you can use the API that works best with your development with the back-end that suits best your production team. For example, you could enforce the use of the SL4J API while letting the production still reuse the old log4j.properties they’ve known for ages. SLF4J’s logging implementation is known as LogKit.

SLF4J bridging

SLF4J has a bridging feature sor you can remove all log4j and commons-logging dependencies from your project’s dependencies and use only SLF4J.

SLF4J offers a JAR for each logging framework: they mimic its API but reroute the calls to the SLF4J API (which in turn uses the real framework). A word of warning: you could run into a cycle so beware to not have the bridging library along the implementation library in your classpath. For example, if you use the Log4J bridge, each Log4J API call will be rerouted to SLF4J. But if the SLF4J Log4J implementation is present, it will be routed back to Log4J then again, and again.

SLF4J API with Log4J implementation

Taking all these facts into account, my advice is to use SLF4J API and Log4J implementation. This way, you still configure logging the old Log4J way but you have access to SLF4J’s simpler API. In order to do so, you’ll have to:

Action	Location	Description
Add to classpath	slf4j-api.jar*	Main API without which you cannot use SLF4J
	slf4j-log4j.jar*	SLF4J Log4J implementation
	jul-to-slf4j.jar*	Enables rerouting JDK 1.4 logging calls to SLF4J
	jcl-over-slf4j.jar*	Reroutes commons-logging calls to SLF4J
Remove from classpath	commons-logging.jar*	Would conflict with commons-logging API in jcl-over-slf4j.jar
SLF4JBridgeHandler.install()**	Main application	Redirect JDK 1.4 logging calls to SLF4J
* Jar name will likely includes version
** 20% overhead advertised so do only if you need the single entry point and if there are a few calls

In you run inside an application server, you’ll probably have to change its libraries and / or its configuration to reach this situation.

To go further:

• SLF4J project
• Log4J project
• Commons Logging project
• Commons Logging class loader issues