## Logging in Tomcat

### Table of Contents

- Introduction
  1. Java logging API — java.util.logging
  2. Servlets logging API
  3. Console
  4. Access logging
- Using java.util.logging (default)
  1. Documentation references
  2. Considerations for production usage
- Using Log4j

### Introduction

The internal logging for Apache Tomcat uses JULI, a packaged renamed fork of Apache Commons Logging that is hard-coded to use the `java.util.logging` framework. This ensures that Tomcat's internal logging and any web application logging will remain independent, even if a web application uses Apache Commons Logging.

To configure Tomcat to use an alternative logging framework for its internal logging, follow the instructions provided by the alternative logging framework for redirecting logging for applications that use `java.util.logging`. Links to some of these instructions are provided towards the end of this page. Keep in mind that the alternative logging framework will need to be capable of working in an environment where different loggers with the same name may exist in different class loaders.

A web application running on Apache Tomcat can:

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

The logging frameworks used by different web applications are independent. See class loading for more details. The exception to this rule is `java.util.logging`. If it is used directly or indirectly by your logging library then elements of it will be shared across web applications because it is loaded by the system class loader.

#### 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 `jakarta.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.

#### Console

When running Tomcat on unixes, the console output is usually redirected to the file named `catalina.out`. The name is configurable using an environment variable. (See the startup scripts). Whatever is written to `System.err/out` will be caught into that file. That may include:

- Uncaught exceptions printed by `java.lang.ThreadGroup.uncaughtException(..)`
- Thread dumps, if you requested them via a system signal

When running as a service on Windows, the console output is also caught and redirected, but the file names are different.

The default logging configuration in Apache Tomcat writes the same messages to the console and to a log file. This is great when using Tomcat for development, but usually is not needed in production.

Old applications that still use `System.out` or `System.err` can be tricked by setting `swallowOutput` attribute on a Context. If the attribute is set to `true`, the calls to `System.out/err` during request processing will be intercepted, and their output will be fed to the logging subsystem using the `jakarta.servlet.ServletContext.log(...)` calls.
Note, that the `swallowOutput` feature is actually a trick, and it has its limitations. It works only with direct calls to `System.out/err`, and only during request processing cycle. It may not work in other threads that might be created by the application. It cannot be used to intercept logging frameworks that themselves write to the system streams, as those start early and may obtain a direct reference to the streams before the redirection takes place.

#### Access logging

Access logging is a related but different feature, which is implemented as a `Valve`. It uses self-contained logic to write its log files. The essential requirement for access logging is to handle a large continuous stream of data with low overhead, so it only uses Apache Commons Logging for its own debug messages. This implementation approach avoids additional overhead and potentially complex configuration. Please refer to the Valves documentation for more details on its configuration, including the various report formats.

### Using java.util.logging (default)

The default implementation of java.util.logging provided in the JDK is too limited to be useful. The key limitation is the inability to have per-web application logging, as the configuration is per-VM. As a result, Tomcat will, in the default configuration, replace the default LogManager implementation with a container friendly implementation called JULI, which addresses these shortcomings.

JULI supports the same configuration mechanisms as the standard JDK `java.util.logging`, using either a programmatic approach, or properties files. The main difference is that per-classloader properties files can be set (which enables easy redeployment friendly webapp configuration), and the properties files support extended constructs which allows more freedom for defining handlers and assigning them to loggers.

JULI is enabled by default, and supports per classloader configuration, in addition to the regular global java.util.logging configuration. This means that logging can be configured at the following layers:

- Globally. That is usually done in the `${catalina.base}/conf/logging.properties` file. The file is specified by the `java.util.logging.config.file` System property which is set by the startup scripts. If it is not readable or is not configured, the default is to use the `${java.home}/lib/logging.properties` file in the JRE.
- In the web application. The file will be `WEB-INF/classes/logging.properties`

The default `logging.properties` in the JRE specifies a `ConsoleHandler` that routes logging to System.err. The default `conf/logging.properties` in Apache Tomcat also adds several `AsyncFileHandler`s that write to files.

A handler's log level threshold is `INFO` by default and can be set using `SEVERE`, `WARNING`, `INFO`, `CONFIG`, `FINE`, `FINER`, `FINEST` or `ALL`. You can also target specific packages to collect logging from and specify a level.

To enable debug logging for part of Tomcat's internals, you should configure both the appropriate logger(s) and the appropriate handler(s) to use the `FINEST` or `ALL` level. e.g.:

    org.apache.catalina.session.level=ALL
    java.util.logging.ConsoleHandler.level=ALL

When enabling debug logging it is recommended that it is enabled for the narrowest possible scope as debug logging can generate large amounts of information.

The configuration used by JULI is the same as the one supported by plain `java.util.logging`, but uses a few extensions to allow better flexibility in configuring loggers and handlers. The main differences are:

- A prefix may be added to handler names, so that multiple handlers of a single class may be instantiated. A prefix is a String which starts with a digit, and ends with '.'. For example, `22foobar.` is a valid prefix.
- System property replacement is performed for property values which contain `${systemPropertyName}`.
- If using a class loader that implements the `org.apache.juli.WebappProperties` interface (Tomcat's web application class loader does) then property replacement is also performed for `${classloader.webappName}`, `${classloader.hostName}` and `${classloader.serviceName}` which are replaced with the web application name, the host name and the service name respectively.
- By default, loggers will not delegate to their parent if they have associated handlers. This may be changed per logger using the `loggerName.useParentHandlers` property, which accepts a boolean value.
- The root logger can define its set of handlers using the `.handlers` property.
- By default the log files will be kept on the file system for `90` days. This may be changed per handler using the `handlerName.maxDays` property. If the specified value for the property is `≤0` then the log files will be kept on the file system forever, otherwise they will be kept the specified maximum days.

There are several additional implementation classes, that can be used together with the ones provided by Java. The notable ones are `org.apache.juli.FileHandler` and `org.apache.juli.AsyncFileHandler`.

`org.apache.juli.FileHandler` supports buffering of the logs. The buffering is not enabled by default. To configure it, use the `bufferSize` property of a handler. The value of `0` uses system default buffering (typically an 8K buffer will be used). A value of `<0` forces a writer flush upon each log write. A value `>0` uses a BufferedOutputStream with the defined value but note that the system default buffering will also be applied.

`org.apache.juli.AsyncFileHandler` is a subclass of `FileHandler` that queues the log messages and writes them asynchronously to the log files. Its additional behaviour can be configured by setting some system properties.

Example logging.properties file to be placed in $CATALINA_BASE/conf:

    handlers = 1catalina.org.apache.juli.AsyncFileHandler, \
               2localhost.org.apache.juli.AsyncFileHandler, \
               3manager.org.apache.juli.AsyncFileHandler, \
               java.util.logging.ConsoleHandler

    .handlers = 1catalina.org.apache.juli.AsyncFileHandler, java.util.logging.ConsoleHandler

    ############################################################
    # Handler specific properties.
    # Describes specific configuration info for Handlers.
    ############################################################

    1catalina.org.apache.juli.AsyncFileHandler.level = ALL
    1catalina.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
    1catalina.org.apache.juli.AsyncFileHandler.prefix = catalina.
    1catalina.org.apache.juli.AsyncFileHandler.maxDays = 90
    1catalina.org.apache.juli.AsyncFileHandler.encoding = UTF-8

    2localhost.org.apache.juli.AsyncFileHandler.level = ALL
    2localhost.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
    2localhost.org.apache.juli.AsyncFileHandler.prefix = localhost.
    2localhost.org.apache.juli.AsyncFileHandler.maxDays = 90
    2localhost.org.apache.juli.AsyncFileHandler.encoding = UTF-8

    3manager.org.apache.juli.AsyncFileHandler.level = ALL
    3manager.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
    3manager.org.apache.juli.AsyncFileHandler.prefix = manager.
    3manager.org.apache.juli.AsyncFileHandler.bufferSize = 16384
    3manager.org.apache.juli.AsyncFileHandler.maxDays = 90
    3manager.org.apache.juli.AsyncFileHandler.encoding = UTF-8

    java.util.logging.ConsoleHandler.level = ALL
    java.util.logging.ConsoleHandler.formatter = org.apache.juli.OneLineFormatter
    java.util.logging.ConsoleHandler.encoding = UTF-8

    ############################################################
    # Facility specific properties.
    # Provides extra control for each logger.
    ############################################################

    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].level = INFO
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].handlers = \
       2localhost.org.apache.juli.AsyncFileHandler

    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].level = INFO
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].handlers = \
       3manager.org.apache.juli.AsyncFileHandler

    # For example, set the org.apache.catalina.util.LifecycleBase logger to log
    # each component that extends LifecycleBase changing state:
    #org.apache.catalina.util.LifecycleBase.level = FINE

Example logging.properties for the servlet-examples web application to be placed in WEB-INF/classes inside the web application:

    handlers = org.apache.juli.AsyncFileHandler, java.util.logging.ConsoleHandler

    ############################################################
    # Handler specific properties.
    # Describes specific configuration info for Handlers.
    ############################################################

    org.apache.juli.AsyncFileHandler.level = ALL
    org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
    org.apache.juli.AsyncFileHandler.prefix = ${classloader.webappName}.
    org.apache.juli.AsyncFileHandler.encoding = UTF-8

    java.util.logging.ConsoleHandler.level = ALL
    java.util.logging.ConsoleHandler.formatter = org.apache.juli.OneLineFormatter
    java.util.logging.ConsoleHandler.encoding = UTF-8

#### Documentation references

See the following resources for additional information:

- Apache Tomcat Javadoc for the `org.apache.juli` package.
- Oracle Java 17 Javadoc for the `java.util.logging` package.

#### Considerations for production usage

You may want to take note of the following:

- Consider removing `ConsoleHandler` from configuration. By default (thanks to the `.handlers` setting) logging goes both to a `AsyncFileHandler` and to a `ConsoleHandler`. The output of the latter one is usually captured into a file, such as `catalina.out`. Thus you end up with two copies of the same messages.
- Consider removing `AsyncFileHandler`s for the applications that you do not use. E.g., the one for `host-manager`.
- Consider configuring an Access log.

### Using Log4j

The Log4j project provides instructions for using Log4J for Tomcat's internal logging.