Spring's variant of the
Commons Logging API: with special support for Log4J 2, SLF4J and java.util.logging. 
This is a custom bridge along the lines of jcl-over-slf4j. You may exclude spring-jcl and switch to jcl-over-slf4j instead if you prefer the hard-bound SLF4J bridge. However, Spring's own bridge provides a better out-of-the-box experience when using Log4J 2 or java.util.logging, with no extra bridge jars necessary, and also easier setup of SLF4J with Logback (no JCL exclude, no JCL bridge). 
Log is equivalent to the original. However, LogFactory is a very different implementation which is minimized and optimized for Spring's purposes, detecting Log4J 2.x and SLF4J 1.7 in the framework classpath and falling back to java.util.logging. If you run into any issues with this implementation, consider excluding spring-jcl and switching to the standard commons-logging artifact or to jcl-over-slf4j. 
Note that this Commons Logging bridge is only meant to be used for framework logging purposes, both in the core framework and in extensions. For applications, prefer direct use of Log4J/SLF4J or java.util.logging. /**
 * Spring's variant of the
 * <a href="https://commons.apache.org/logging">Commons Logging API</a>:
 * with special support for Log4J 2, SLF4J and {@code java.util.logging}.
 *
 * <p>This is a custom bridge along the lines of {@code jcl-over-slf4j}.
 * You may exclude {@code spring-jcl} and switch to {@code jcl-over-slf4j}
 * instead if you prefer the hard-bound SLF4J bridge. However, Spring's own
 * bridge provides a better out-of-the-box experience when using Log4J 2
 * or {@code java.util.logging}, with no extra bridge jars necessary, and
 * also easier setup of SLF4J with Logback (no JCL exclude, no JCL bridge).
 *
 * <p>{@link org.apache.commons.logging.Log} is equivalent to the original.
 * However, {@link org.apache.commons.logging.LogFactory} is a very different
 * implementation which is minimized and optimized for Spring's purposes,
 * detecting Log4J 2.x and SLF4J 1.7 in the framework classpath and falling
 * back to {@code java.util.logging}. If you run into any issues with this
 * implementation, consider excluding {@code spring-jcl} and switching to the
 * standard {@code commons-logging} artifact or to {@code jcl-over-slf4j}.
 *
 * <p>Note that this Commons Logging bridge is only meant to be used for
 * framework logging purposes, both in the core framework and in extensions.
 * For applications, prefer direct use of Log4J/SLF4J or {@code java.util.logging}.
 */
package org.apache.commons.logging;