Log4j file sample

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Software Engineering. Web Technology. Cyber Security. C Programming. This is because the appender associated with logger com. Bar is first used, which writes the first instance to the Console.

Next, the parent of com. Bar, which in this case is the root logger, is referenced. The event is then passed to its appender, which is also writes to the Console, resulting in the second instance.

This is known as additivity. While additivity can be quite a convenient feature as in the first previous example where no appender reference needed to be configured , in many cases this behavior is considered undesirable and so it is possible to disable it by setting the additivity attribute on the logger to false:.

Once an event reaches a logger with its additivity set to false the event will not be passed to any of its parent loggers, regardless of their additivity setting. When configured from a File, Log4j has the ability to automatically detect changes to the configuration file and reconfigure itself. The example below shows how to configure the attribute so that the configuration file will be checked for changes only after at least 30 seconds have elapsed.

The minimum interval is 5 seconds. Log4j provides the ability to 'advertise' appender configuration details for all file-based appenders as well as socket-based appenders.

For example, for file-based appenders, the file location and the pattern layout in the file are included in the advertisement. Chainsaw and other external systems can discover these advertisements and use that information to intelligently process the log file. The mechanism by which an advertisement is exposed, as well as the advertisement format, is specific to each Advertiser implementation. An external system which would like to work with a specific Advertiser implementation must understand how to locate the advertised configuration as well as the format of the advertisement.

For example, a 'database' Advertiser may store configuration details in a database table. An external system can read that database table in order to discover the file location and the file format.

Chainsaw automatically discovers log4j's multicastdns-generated advertisements and displays those discovered advertisements in Chainsaw's Zeroconf tab if the jmdns library is in Chainsaw's classpath.

To begin parsing and tailing a log file provided in an advertisement, just double-click the advertised entry in Chainsaw's Zeroconf tab. Currently, Chainsaw only supports FileAppender advertisements. FileAppender-based configurations require an additional 'advertiseURI' attribute to be specified on the appender.

The 'advertiseURI' attribute provides Chainsaw with information on how the file can be accessed. As of version 2. If you want to split the configuration in multiple files, use XInclude or Composite Configuration.

As the previous examples have shown as well as those to follow, Log4j allows you to easily redefine logging behavior without needing to modify your application. It is possible to disable logging for certain parts of the application, log only when specific criteria are met such as the action being performed for a specific user, route output to Flume or a log reporting system, etc. Being able to do this requires understanding the syntax of the configuration files.

The level of internal Log4j events that should be logged to the console. Valid values for this attribute are "off", "trace", "debug", "info", "warn", "error", "fatal", and "all".

Log4j will log details about initialization, rollover and other internal actions to the status logger. Alternatively, setting system property log4j2. Log4j can be configured using two XML flavors; concise and strict. Concise Syntax The concise format makes configuration very easy as the element names match the components they represent however it cannot be validated with an XML schema.

For example, the ConsoleAppender is configured by declaring an XML element named Console under its parent appenders element. However, element and attribute names are are not case sensitive. In addition, attributes can either be specified as an XML attribute or as an XML element that has no attributes and has a text value. The file below represents the structure of an XML configuration, but note that the elements in italics below represent the concise element names that would appear in their place.

See the many examples on this page for sample appender, filter and logger declarations. This is accomplished by replacing the friendly element names above with their object type as shown below. For example, instead of the ConsoleAppender being configured using an element named Console it is instead configured as an appender element with a type attribute containing "Console".

Where a key contains more than a simple value it itself will be a subordinate plugin. Note that in the RoutingAppender the Route element has been declared as an array. This is valid because each array element will be a Route component. This won't work for elements such as appenders and filters, where each element has a different name in the concise format.

Appenders and filters can be defined as array elements if each appender or filter declares an attribute named "type" that contains the type of the appender. The following example illustrates this as well as how to declare multiple loggers as an array. Additional runtime dependencies are required for using JSON configuration files. Log4j also supports using YAML for configuration files. For example:. Additional runtime dependencies are required for using YAML configuration files.

Note that the property syntax is NOT the same as the syntax used in Log4j 1. Like the XML and JSON configurations, properties configurations define the configuration in terms of plugins and attributes to the plugins. Prior to version 2. Each of those components will then be expected to be defined in sets of properties that begin with component.

The identifier does not have to match the name of the component being defined but must uniquely identify all the attributes and subcomponents that are part of the component. If the list of identifiers is not present the identifer must not contain a '. Each individual component MUST have a "type" attribute specified that identifies the component's Plugin type.

If the list is present it will be used. Unlike the base components, when creating subcomponents you cannot specify an element containing a list of identifiers. Instead, you must define the wrapper element with its type as is shown in the policies definition in the rolling file appender below.

You then define each of the subcomponents below that wrapper element, as the TimeBasedTriggeringPolicy and SizeBasedTriggeringPolicy are defined below. Properties configuration files support the advertiser, monitorInterval, name, packages, shutdownHook, shutdownTimeout, status, verbose, and dest attrbutes. See Configuration Syntax for the definitions of these attributes. An understanding of how loggers work in Log4j is critical before trying to configure them.

Please reference the Log4j architecture if more information is required. Trying to configure Log4j without understanding those concepts will lead to frustration. A LoggerConfig is configured using the logger element. The logger element must have a name attribute specified, will usually have a level attribute specified and may also have an additivity attribute specified. The additivity attribute may be assigned a value of true or false.

If the attribute is omitted the default value of true will be used. Capturing location information the class name, file name, method name, and line number of the caller can be slow.

Log4j tries to optimize this by reducing the size of the stack that must be traversed to find the caller of the logging method. It does this by determining if any component that might be accessed requires location information.

This can cause performance issues if a logger is configured at a level like trace or debug with the expectation that most logs will be filtered on an Appender reference or Appender as Log4j will calculate the location information even though the log event is going to be discarded.

To disable this behavior the includeLocation attribute can be set to false on the LoggerConfig. This will cause Log4j to defer calculating the location information until absolutely necessary. A LoggerConfig including the root LoggerConfig can be configured with properties that will be added to the properties copied from the ThreadContextMap.

These properties can be referenced from Appenders, Filters, Layouts, etc just as if they were part of the ThreadContext Map.

The properties can contain variables that will be resolved either when the configuration is parsed or dynamically when each event is logged. See Property Substitution for more information on using variables.

The LoggerConfig may also be configured with one or more AppenderRef elements. Each appender referenced will become associated with the specified LoggerConfig. If multiple appenders are configured on the LoggerConfig each of them be called when processing logging events.

Every configuration must have a root logger. The main differences between the root logger and other loggers are. An appender is configured either using the specific appender plugin's name or with an appender element and the type attribute containing the appender plugin's name. In addition each appender must have a name attribute specified with a value that is unique within the set of appenders. The name will be used by loggers to reference the appender as described in the previous section.

Most appenders also support a layout to be configured which again may be specified either using the specific Layout plugin's name as the element or with "layout" as the element name along with a type attribute that contains the layout plugin's name. The various appenders will contain other attributes or elements that are required for them to function properly. Although only a single filter element can be configured, that element may be the filters element which represents the CompositeFilter.

The filters element allows any number of filter elements to be configured within it. The following example shows how multiple filters can be configured on the ConsoleAppender. Log4j 2 supports the ability to specify tokens in the configuration as references to properties defined elsewhere.

Some of these properties will be resolved when the configuration file is interpreted while others may be passed to components where they will be evaluated at runtime.

For example, the following example shows the filename for the rolling file appender being declared as a property. While this is useful, there are many more places properties can originate from. See the Lookups manual page for more details. The contexts that are built in to Log4j are:. A default property map can be declared in the configuration file by placing a Properties element directly after the Configuration element and before any Loggers, Filters, Appenders, etc.

If the value cannot be located in the specified lookup the value in the default property map will be used. The default map is pre-populated with a value for "hostName" that is the current system's host name or IP address and the "contextName" with is the value of the current logging context.

See many places a Properties element is used in this section for examples. In some cases the key might contain a leading '-'. This would use the MainMapLookup for a key named --file. If the key is not found then app.

In the previous example the "Routes" element is capable of resolving the variable at runtime. Not all elements support resolving variables at runtime. Components that do will specifically call that out in their documentation. If no value is found for the key in the Lookup associated with the prefix then the value associated with the key in the properties declaration in the configuration file will be used. If no value is found the variable declaration will be returned as the value.

Default values may be declared in the configuration by doing:. As a footnote, it is worth pointing out that the variables in the RollingFile appender declaration will also not be evaluated when the configuration is processed. This is simply because the resolution of the whole RollingFile element is deferred until a match occurs.

See RoutingAppender for more information. Log4j provides support for JSR scripting languages to be used in some of its components. Any language that provides support for the JSR scripting engine may be used. A list of the languages and bindings for them can be found at the Scripting Engine web site. However, some of the languages listed there, such as JavaScript, Groovy and Beanshell, directly support the JSR scripting framework and only require that the jars for that language be installed.

The script element contains a name for the script, the language of the script, and the script text. The scriptFile element contains the name of the script, its location, its language, its charset, and whether the file should be watched for changes.

The name of the script is used to store the script, along with its ScriptEngine, so it can quickly be located each time the script needs to be run. While the name is not required, providing it will help in debugging problems when the script is running. The language must be provided on the script element and must specify one of the language names that appear in the Configuration status log as described in the next section.

If the language is not specified on the scriptFile element the language will be determined by the file extension of the script path. If file monitoring is requested it will only be enabled if a non-zero monitorInterval is specified on the configuration element. That interval will be used to check for changes in the file.

Log4j is a library that is used by many Java applications. The challenge here is finding Log4j because of the way Java packaging works. In the Java ecosystem, dependencies are distributed as Java archive JAR files, which are packages that can be used as a Java library. In some situations, one dependency pulls in hundreds of other dependencies making it even more difficult to find. This creates many layers that all need to be investigated.

There are two open source tools led by Anchore that have the ability to scan a large number of packaged dependency formats, identify their existence, and report if they contain vulnerabilities. Both of these tools are able to inspect multiple nested layers of JAR archives to uncover and identify versions of Log4j. Syft is also able to discern which version of Log4j a Java application contains.

The Log4j JAR can be directly included in our project, or it can be hidden away in one of the dependencies we include. For example, using Syft to scan this sample Java project shows that it includes Log4j version 2. Regardless of the version of Log4j that is included, there is value in generating and storing an SBOM to keep a record of everything that is included in any software component or application you deliver.

Grype is a scanner that has the ability to tell us which specific vulnerabilities our software contains. When you include a dependency in your application you can also identify the vulnerabilities that the dependency contains, and so on through multiple levels of nesting.