Apache Tomcat 8 (8.0.53) - Logging in Tomcat
Author: Allistair Crossley

## 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, by default, 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, one has to replace the JULI implementation that is hard-coded to use `java.util.logging` with a JULI implementation that retains the full Commons Logging discovery mechanism. Such an implementation 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 any logging framework of its choice.
- Use system logging API, `java.util.logging`.
- Use the logging API provided by the Java Servlets specification, `javax.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 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 `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.

#### 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 `javax.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 `FileHandler`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 forever. 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 one is `org.apache.juli.FileHandler`.

`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.

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

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

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

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

    1catalina.org.apache.juli.FileHandler.level = FINE
    1catalina.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    1catalina.org.apache.juli.FileHandler.prefix = catalina.

    2localhost.org.apache.juli.FileHandler.level = FINE
    2localhost.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    2localhost.org.apache.juli.FileHandler.prefix = localhost.

    3manager.org.apache.juli.FileHandler.level = FINE
    3manager.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    3manager.org.apache.juli.FileHandler.prefix = manager.
    3manager.org.apache.juli.FileHandler.bufferSize = 16384

    java.util.logging.ConsoleHandler.level = FINE
    java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

    ############################################################
    # 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.FileHandler

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

    # 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.FileHandler, java.util.logging.ConsoleHandler

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

    org.apache.juli.FileHandler.level = FINE
    org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    org.apache.juli.FileHandler.prefix = ${classloader.webappName}.

    java.util.logging.ConsoleHandler.level = FINE
    java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

#### Documentation references

See the following resources for additional information:

- Apache Tomcat Javadoc for the `org.apache.juli` package.
- Oracle Java 6 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 `FileHandler` 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 `FileHandler`s for the applications that you do not use. E.g., the one for `host-manager`.
- The handlers by default use the system default encoding to write the log files. It can be configured with `encoding` property. See Javadoc for details.
- Consider configuring an Access log.

### Using Log4j

This section explains how to configure Tomcat to use log4j rather than java.util.logging for all Tomcat's internal logging.

Note: The steps described in this section are needed when you want to reconfigure Tomcat to use Apache log4j for its own logging. These steps are not needed if you just want to use log4j in your own web application. — In that case, just put `log4j.jar` and `log4j.properties` into `WEB-INF/lib` and `WEB-INF/classes` of your web application.

The following steps describe configuring log4j to output Tomcat's internal logging.

1. Create a file called `log4j.properties` with the following content and save it into `$CATALINA_BASE/lib`

    log4j.rootLogger = INFO, CATALINA

    # Define all the appenders
    log4j.appender.CATALINA = org.apache.log4j.DailyRollingFileAppender
    log4j.appender.CATALINA.File = ${catalina.base}/logs/catalina
    log4j.appender.CATALINA.Append = true
    log4j.appender.CATALINA.Encoding = UTF-8
    # Roll-over the log once per day
    log4j.appender.CATALINA.DatePattern = '.'yyyy-MM-dd'.log'
    log4j.appender.CATALINA.layout = org.apache.log4j.PatternLayout
    log4j.appender.CATALINA.layout.ConversionPattern = %d [%t] %-5p %c- %m%n

    log4j.appender.LOCALHOST = org.apache.log4j.DailyRollingFileAppender
    log4j.appender.LOCALHOST.File = ${catalina.base}/logs/localhost
    log4j.appender.LOCALHOST.Append = true
    log4j.appender.LOCALHOST.Encoding = UTF-8
    log4j.appender.LOCALHOST.DatePattern = '.'yyyy-MM-dd'.log'
    log4j.appender.LOCALHOST.layout = org.apache.log4j.PatternLayout
    log4j.appender.LOCALHOST.layout.ConversionPattern = %d [%t] %-5p %c- %m%n

    log4j.appender.MANAGER = org.apache.log4j.DailyRollingFileAppender
    log4j.appender.MANAGER.File = ${catalina.base}/logs/manager
    log4j.appender.MANAGER.Append = true
    log4j.appender.MANAGER.Encoding = UTF-8
    log4j.appender.MANAGER.DatePattern = '.'yyyy-MM-dd'.log'
    log4j.appender.MANAGER.layout = org.apache.log4j.PatternLayout
    log4j.appender.MANAGER.layout.ConversionPattern = %d [%t] %-5p %c- %m%n

    log4j.appender.HOST-MANAGER = org.apache.log4j.DailyRollingFileAppender
    log4j.appender.HOST-MANAGER.File = ${catalina.base}/logs/host-manager
    log4j.appender.HOST-MANAGER.Append = true
    log4j.appender.HOST-MANAGER.Encoding = UTF-8
    log4j.appender.HOST-MANAGER.DatePattern = '.'yyyy-MM-dd'.log'
    log4j.appender.HOST-MANAGER.layout = org.apache.log4j.PatternLayout
    log4j.appender.HOST-MANAGER.layout.ConversionPattern = %d [%t] %-5p %c- %m%n

    log4j.appender.CONSOLE = org.apache.log4j.ConsoleAppender
    log4j.appender.CONSOLE.Encoding = UTF-8
    log4j.appender.CONSOLE.layout = org.apache.log4j.PatternLayout
    log4j.appender.CONSOLE.layout.ConversionPattern = %d [%t] %-5p %c- %m%n

    # Configure which loggers log to which appenders
    log4j.logger.org.apache.catalina.core.ContainerBase.[Catalina].[localhost] = INFO, LOCALHOST
    log4j.logger.org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager] =\
      INFO, MANAGER
    log4j.logger.org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/host-manager] =\
      INFO, HOST-MANAGER

2. Download Log4J (Tomcat requires v1.2.x).

3. Download or build `tomcat-juli.jar` and `tomcat-juli-adapters.jar` that are available as an "extras" component for Tomcat. See Additional Components documentation for details.

   This `tomcat-juli.jar` differs from the default one. It contains the full Apache Commons Logging implementation and thus is able to discover the presence of log4j and configure itself.

4. If you want to configure Tomcat to use log4j globally:

   - Put `log4j.jar` and `tomcat-juli-adapters.jar` from "extras" into `$CATALINA_HOME/lib`.
   - Replace `$CATALINA_HOME/bin/tomcat-juli.jar` with `tomcat-juli.jar` from "extras".

5. If you are running Tomcat with separate `$CATALINA_HOME` and `$CATALINA_BASE` and want to configure to use log4j in a single `$CATALINA_BASE` only:

   - Create `$CATALINA_BASE/bin` and `$CATALINA_BASE/lib` directories if they do not exist.
   - Put `log4j.jar` and `tomcat-juli-adapters.jar` from "extras" into `$CATALINA_BASE/lib`
   - Put `tomcat-juli.jar` from "extras" as `$CATALINA_BASE/bin/tomcat-juli.jar`
   - If you are running with a security manager, you would need to edit the `$CATALINA_BASE/conf/catalina.policy` file to adjust it to using a different copy of tomcat-juli.jar.

   Note: This works because libraries, if they exist in `$CATALINA_BASE`, are loaded in preference to the same library in `$CATALINA_HOME`.

   Note: tomcat-juli.jar is loaded from `$CATALINA_BASE`/bin not `$CATALINA_BASE`/lib as it is loaded as part of the bootstrap process and all the bootstrap classes are loaded from bin.

6. Delete `$CATALINA_BASE/conf/logging.properties` to prevent java.util.logging generating zero length log files.

7. Start Tomcat

This log4j configuration mirrors the default java.util.logging setup that ships with Tomcat: both the manager and host-manager apps get an individual log file, and everything else goes to the "catalina.log" log file. Each file is rolled-over once per day.

You can (and should) be more picky about which packages to include in the logging. Tomcat defines loggers by Engine and Host names. For example, for a more detailed Catalina localhost log, add this to the end of the log4j.properties above. Note that there are known issues with using this naming convention (with square brackets) in log4j XML based configuration files, so we recommend you use a properties file as described until a future version of log4j allows this convention.

    log4j.logger.org.apache.catalina.core.ContainerBase.[Catalina].[localhost]=DEBUG
    log4j.logger.org.apache.catalina.core=DEBUG
    log4j.logger.org.apache.catalina.session=DEBUG

Be warned: a level of DEBUG will produce megabytes of logging and slow startup of Tomcat. This level should be used sparingly when debugging of internal Tomcat operations is required.

Your web applications should certainly use their own log4j configuration. This is valid with the above configuration. You would place a similar log4j.properties file in your web application's WEB-INF/classes directory, and log4jx.y.z.jar into WEB-INF/lib. Then specify your package level logging. This is a basic setup of log4j which does *not* require Commons-Logging, and you should consult the log4j documentation for more options. This page is intended only as a bootstrapping guide.

Additional notes

- This exposes log4j libraries to the web applications through the Common classloader. See class loading documentation for details.

  Because of that, the web applications and libraries using Apache Commons Logging library are likely to automatically choose log4j as the underlying logging implementation.

- The `java.util.logging` API is still available for those web applications that use it directly. The `${catalina.base}/conf/logging.properties` file is still referenced by Tomcat startup scripts. For more information, see the subsections of the Introduction to this page.

  Removal of `${catalina.base}/conf/logging.properties` file, mentioned as one of the steps above, causes `java.util.logging` to fallback to the default configuration for the JRE, which is to use a ConsoleHandler and therefore not create any standard log files. You should confirm that all your log files are being created by log4j before disabling the standard mechanism.

- The Access Log Valve and ExtendedAccessLogValve use their own self-contained logging implementation, so they cannot be configured to use log4j. Refer to Valves for specific configuration details.

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