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SLF4J allows the end-user to plug in the desired logging framework at deployment time. 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. It follows that SLF4J version 1. Given the extent of the improvement, users are highly encouraged to migrate to SLF4J 1. 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". To run this example, you first need to download the slf4j distribution , and then to unpack it.
Once that is done, add the file slf4j-api Compiling and running HelloWorld will result in the following output being printed on the console. The warning will disappear as soon as you add a binding to your class path. Assuming you add slf4j-simple Compiling and running HelloWorld will now result in the following output on the console. The sample code below illustrates the typical usage pattern for SLF4J.
See the question "What is the fastest way of logging? As mentioned previously, SLF4J supports various logging frameworks. To switch logging frameworks, just replace slf4j bindings on your class path. For example, to switch from java. 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-log4j In your code, in addition to slf4j-api Do not place more than one binding on your class path. Here is a graphical illustration of the general idea. 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. 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.
Authors of widely-distributed components and libraries may code against the SLF4J interface in order to avoid imposing an 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. In order to avoid imposing a logging framework on the end-user, Wombat's distribution includes slf4j-api. Even in the absence of any SLF4J binding on the class path, Wombat's distribution will still work out-of-the-box, and without requiring the end-user to download a binding from SLF4J's 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 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.
Given Maven's transitive dependency rules, for "regular" projects not libraries or frameworks declaring logging dependencies can be accomplished with a single dependency declaration. In addition to logback-classic Note that explicitly declaring a dependency on logback-core In addition to slf4j-log4j Note that explicitly declaring a dependency on log4j In addition to slf4j-jdk Note that explicitly declaring a dependency on slf4j-api An SLF4J binding designates an artifact such as slf4j-jdk From the client's perspective all versions of slf4j-api are compatible.
Client code compiled with slf4j-api-N. You only need to ensure that the version of your binding matches that of the slf4j-api. You do not have to worry about the version of slf4j-api. Mixing different versions of slf4j-api. For example, if you are using slf4j-api However, from the client's perspective all versions of slf4j-api are compatible.
You can always use any version of slf4j-api. At initialization time, if SLF4J suspects that there may be an slf4j-api vs. It is common to find projects depending on a combination of JCL, java. It then becomes desirable to consolidate logging through a single channel. For more details, please refer to the page on Bridging legacy APIs. MDC data can also be highly helpful in filtering messages or triggering certain actions. Note that at this time, only log4j and logback offer MDC functionality.
If the underlying framework does not offer MDC, for example java. 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. See the page on Bridging legacy APIs for more details.
Hello World As customary in programming tradition, here is an example illustrating the simplest way to output "Hello world" using SLF4J. Failed to load class "org. This warning is printed because no slf4j binding could be found on your class path. You also need to place log4j. Only messages of level INFO and higher are printed. This binding may be useful in the context of small applications.
Logger class is a direct implementation of SLF4J's org. Thus, using SLF4J in conjunction with logback involves strictly zero memory and computational overhead. Libraries Authors of widely-distributed components and libraries may code against the SLF4J interface in order to avoid imposing an logging framework on their end-user. Declaring project dependencies for logging Given Maven's transitive dependency rules, for "regular" projects not libraries or frameworks declaring logging dependencies can be accomplished with a single dependency declaration.
Mapped Diagnostic Context MDC support "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. Executive summary Advantage Description Select your logging framework at deployment time The desired logging framework can be plugged in at deployment time by inserting the appropriate jar file binding on your class path.
Fail-fast operation 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.
Bindings for popular logging frameworks SLF4J supports popular logging frameworks, namely log4j, java. The logback project supports SLF4J natively. Support for parameterized log messages All SLF4J bindings support parameterized log messages with significantly improved performance results.
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. SLF4J supports popular logging frameworks, namely log4j, java. 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.