The Apache Tomcat Servlet/JSP Container

Apache Tomcat 7

Version 7.0.75, Jan 18 2017
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Logging in Tomcat

Table of Contents
Introduction

Logging in Apache Tomcat is implemented with the help of Apache Commons Logging library. That library is a thin wrapper above different logging frameworks. It provides Tomcat with the ability to log hierarchically across various log levels without the need to rely on a particular logging implementation.

Since Tomcat 6.0, Tomcat uses a private package-renamed implementation of Apache Commons Logging, to allow web applications to use their own independent copies of the original Apache Commons Logging library. In the default distribution this private copy of the library is simplified and hardcoded to use the java.util.logging framework.

To configure Tomcat to use alternative logging frameworks for its internal logging, one has to replace the logging library with the one that is built with the full implementation. Such library is provided as an extras component. Instructions on how to configure Tomcat to use Log4j framework for its internal logging may be found below.

A web application running on Apache Tomcat can:

  • Use system logging API, java.util.logging.
  • Use the logging API provided by the Java Servlets specification, javax.servlet.ServletContext.log(...)
  • Use any logging framework of its choice.

The logging frameworks used by different web applications run independently of each other. See class loading for more details. The exception to this rule is java.util.logging, if it used directly or indirectly by your logging library. That is because it is loaded by the system and is shared across web applications.

Java logging API — java.util.logging

Apache Tomcat has its own implementation of several key elements of java.util.logging API. This implementation is called "JULI". The key component there is a custom LogManager implementation, that is aware of different web applications running on Tomcat (and their different class loaders). It supports private per-application logging configurations. It is also notified by Tomcat when a web application is unloaded from memory, so that the references to its classes can be cleared, preventing memory leaks.

This java.util.logging implementation is enabled by providing certain system properties when starting Java. The Apache Tomcat startup scripts do this for you, but if you are using different tools to run Tomcat (such as jsvc, or running Tomcat from within an IDE), you should take care of them by yourself.

More details about java.util.logging may be found in the documentation for your JDK and on its Javadoc pages for the java.util.logging package.

More details about Tomcat JULI may be found below.

Servlets logging API

The calls to javax.servlet.ServletContext.log(...) to write log messages are handled by internal Tomcat logging. Such messages are logged to the category named

org.apache.catalina.core.ContainerBase.[${engine}].[${host}].[${context}]

This logging is performed according to the Tomcat logging configuration. You cannot overwrite it in a web application.

The Servlets logging API predates the java.util.logging API that is now provided by Java. As such, it does not offer you much options. E.g., you cannot control the log levels. It can be noted, though, that in Apache Tomcat implementation the calls to ServletContext.log(String) or GenericServlet.log(String) are logged at the INFO level. The calls to ServletContext.log(String, Throwable) or GenericServlet.log(String, Throwable) are logged at the SEVERE level.