## SLF4J warning or error messages and their meanings

### The method `o.a.commons.logging.impl.SLF4FLogFactory#release` was invoked.

Given the structure of the commons-logging API, in particular as implemented by SLF4J, the `o.a.commons.logging.impl.SLF4FLogFactory#release()` method should never be called. However, depending on the deployment of commons-logging.jar files in your servlet container, `release()` method may be unexpectedly invoked by a copy of `org.apache.commons.logging.LogFactory` class shipping with commons-logging.jar.

This is a relatively common occurrence with recent versions of Tomcat, especially if you place jcl-over-slf4j.jar in WEB-INF/lib directory of your web-application instead of $TOMCAT_HOME/common/lib, where $TOMCAT_HOME stands for the directory where Tomcat is installed. In order to fully benefit from the stability offered by jcl-over-slf4j.jar, we recommend that you place jcl-over-slf4j.jar in $TOMCAT_HOME/common/lib without placing a copy in your web-applications.

Please also see bug #22.

### Operation [suchAndSuch] is not supported in jcl-over-slf4j.

An `UnsuportedOperationException` is thrown whenever one of the protected methods introduced in JCL 1.1 are invoked. These methods are invoked by `LogFactory` implementations shipping with commons-logging.jar. However, the `LogFactory` implemented by jcl-over-slf4j.jar, namely SLF4FLogFactory, does not call any of these methods.

If you observe this problem, then it is highly probable that you have a copy of commons-logging.jar in your class path overriding the classes shipping with jcl-over-slf4j.jar. Note that this issue is very similar in nature to the warning issued when the "o.a.commons.logging.impl.SLF4FLogFactory.release()" method is invoked, discussed in the previous item.

### Failedto load class `org.slf4j.impl.StaticLoggerBinder`

This error is reported when the `org.slf4j.impl.StaticLoggerBinder` class could not be loaded into memory. This happens when no appropriate SLF4J binding could be found on the class path. Placing one (and only one) of slf4j-nop.jar, slf4j-simple.jar, slf4j-log4j12.jar, slf4j-jdk14.jar or logback-classic.jar on the class path should solve the problem.

since 1.6.0 As of SLF4J version 1.6, in the absence of a binding, SLF4J will default to a no-operation (NOP) logger implementation.

You can download SLF4J bindings from the project download page.

### Multiple bindings were found on the class path

SLF4J API is desinged to bind with one and only one underlying logging framework at a time. If more than one binding is present on the class path, SLF4J will emit a warning, listing the location of those bindings.

When multiple bindings are available on the class path, select one and only one binding you wish to use, and remove the other bindings. For example, if you have both slf4j-simple-${version}.jar and slf4j-nop-${version}.jar on the class path and you wish to use the nop (no-operation) binding, then remove slf4j-simple-${version}.jar from the class path.

The list of locations that SLF4J provides in this warning usually provides sufficient information to identify the dependency transitively pulling in an unwanted SLF4J binding into your project. In your project's pom.xml file, exclude this SLF4J binding when declaring the unscrupulous dependency. For example, cassandra-all version 0.8.1 declares both log4j and slf4j-log4j12 as compile-time dependencies. Thus, when you include cassandra-all as a dependency in your project, the cassandra-all declaration will cause both slf4j-log4j12.jar and log4j.jar to be pulled in as dependencies. In case you do not wish to use log4j as the the SLF4J backend, you can instruct Maven to exclude these two artifacts as shown next:

    <dependencies>
      <dependency>
        <groupId> org.apache.cassandra</groupId>
        <artifactId>cassandra-all</artifactId>
        <version>0.8.1</version>

        <exclusions>
          <exclusion>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
          </exclusion>
          <exclusion>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
          </exclusion>
        </exclusions>

      </dependency>
    </dependencies>

Note The warning emitted by SLF4J is just that, a warning. Even when multiple bindings are present, SLF4J will pick one logging framework/implementation and bind with it. The way SLF4J picks a binding is determined by the JVM and for all practical purposes should be considered random. As of version 1.6.6, SLF4J will name the framework/implementation class it is actually bound to.

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 compile-time dependency on a SLF4J binding, it imposes that binding on the end-user, thus negating SLF4J's purpose. When you come across an embedded component declaring a compile-time dependency on any SLF4J binding, please take the time to contact the authors of said component/library and kindly ask them to mend their ways.

### slf4j-api version does not match that of the binding

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.

Mixing mixing different versions of slf4j-api.jar and SLF4J binding can cause problems. For example, if you are using slf4j-api-${project.version}.jar, then you should also use slf4j-simple-${project.version}.jar, using slf4j-simple-1.5.5.jar will not work.

Note From the client's 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 a api vs. binding version mismatch problem, it will emit a warning about the suspected mismatch.

### Logging factory implementation cannot be null

This error is reported when the `LoggerFactory` class could not find an appropriate binding. Placing one (and only one) of slf4j-nop.jar, slf4j-simple.jar, slf4j-log4j12.jar, slf4j-jdk14.jar or logback-classic.jar on the class path should prove to be an effective remedy.

### Detected both log4j-over-slf4j.jar AND slf4j-log4j12.jar on the class path, preempting `StackOverflowError`.

The purpose of slf4j-log4j12 module is to delegate or redirect calls made to an SLF4J logger to log4j. The purpose of the log4j-over-slf4j module is to redirect calls made to a log4j logger to SLF4J. If both slf4j-log4j12.jar and log4j-over-slf4j.jar are present on the class path, a `StackOverflowError` will inevitably occur immediately after the first invocation of an SLF4J or a log4j logger.

Here is how the exception might look like:

    Exception in thread "main" java.lang.StackOverflowError
      at java.util.Hashtable.containsKey(Hashtable.java:306)
      at org.apache.log4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:36)
      at org.apache.log4j.LogManager.getLogger(LogManager.java:39)
      at org.slf4j.impl.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:73)
      at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:249)
      at org.apache.log4j.Category.<init>(Category.java:53)
      at org.apache.log4j.Logger..<init>(Logger.java:35)
      at org.apache.log4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:39)
      at org.apache.log4j.LogManager.getLogger(LogManager.java:39)
      at org.slf4j.impl.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:73)
      at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:249)
      at org.apache.log4j.Category..<init>(Category.java:53)
      at org.apache.log4j.Logger..<init>(Logger.java:35)
      at org.apache.log4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:39)
      at org.apache.log4j.LogManager.getLogger(LogManager.java:39)
      subsequent lines omitted...

Since 1.5.11 SLF4J software preempts the inevitable stack overflow error by throwing an exception with details about the actual cause of the problem. This is deemed to be better than leaving the user wondering about the reasons of the `StackOverflowError`.

For more background on this topic see Bridging legacy APIs.

### Detected both jcl-over-slf4j.jar AND slf4j-jcl.jar on the class path, preempting `StackOverflowError`.

The purpose of slf4j-jcl module is to delegate or redirect calls made to an SLF4J logger to jakarta commons logging (JCL). The purpose of the jcl-over-slf4j module is to redirect calls made to a JCL logger to SLF4J. If both slf4j-jcl.jar and jcl-over-slf4j.jar are present on the class path, then a `StackOverflowError` will inevitably occur immediately after the first invocation of an SLF4J or a JCL logger.

Here is how the exception might look like:

    Exception in thread "main" java.lang.StackOverflowError
      at java.lang.String.hashCode(String.java:1482)
      at java.util.HashMap.get(HashMap.java:300)
      at org.slf4j.impl.JCLLoggerFactory.getLogger(JCLLoggerFactory.java:67)
      at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:249)
      at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:155)
      at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:289)
      at org.slf4j.impl.JCLLoggerFactory.getLogger(JCLLoggerFactory.java:69)
      at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:249)
      at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:155)
      subsequent lines omitted...

Since 1.5.11 SLF4J software preempts the inevitable stack overflow error by throwing an exception with details about the actual cause of the problem. This is deemed to be better than leaving the user wondering about the reasons of the `StackOverflowError`.

For more background on this topic see Bridging legacy APIs.

### Failed to load class "org.slf4j.impl.StaticMDCBinder"

This error indicates that appropriate SLF4J binding could not be found on the class path. Placing one (and only one) of slf4j-nop.jar, slf4j-simple.jar, slf4j-log4j12.jar, slf4j-jdk14.jar or logback-classic.jar on the class path should solve the problem.

### MDCAdapter cannot be null

This error is reported when `org.slf4j.MDC` class has not been initialized correctly. Same cause and remedy as the previously listed item.

### SLF4J versions 1.4.0 and later requires log4j 1.2.12 or later

The trace level was added to log4j in version 1.2.12 released on August 29, 2005. The trace level was added to the SLF4J API in version 1.4.0 on May 16th, 2007. Thus, starting with SLF4J 1.4.0, the log4j binding for SLF4J requires log4j version 1.2.12 or above.

However, as reported in bug 68, in some environments it may be difficult to upgrade the log4j version. To accommodate such circumstances, SLF4J's `Log4jLoggerAdapter` will map the TRACE level as DEBUG.

### Substitute loggers were created during the default configuration phase of the underlying logging system

Highly configurable logging systems such as logback and log4j may create components which invoke loggers during their own initialization. See issue lbcore-47 for a typical occurrence. However, since the binding process with SLF4J has not yet completed (because the underlying logging system was not yet completely loaded into memory), it is not possible to honor such logger creation requests, resulting in a `NullPointerException`.

To avoid this chicken-and-egg problem, SLF4J substitutes a no-operation logger factory during this initialization phase. However, the substitute loggers returned during this phase are not operational. They are nop implementations.

If any substitute logger had to be created, SLF4J will emit a warning listing such nop loggers. This warning is intended to let you know that you should not expect any logging output from these loggers.

To obtain output from the listed loggers, isolate the components invoking these loggers and to exclude them from the default configuration. Once default configuration is finished, those excluded components can be configured in a second-step configuration. In principle, both logback and log4j allow multi-step configuration. For logback, second-step configuration is described in the relevant section..

If you are not interested in the output from any of the substitute loggers, then no action is required on your part.