log4net is a tool to help the programmer output log statements to a variety of output targets. In case of problems with an application, it is helpful to enable logging so that the problem can be located. With log4net it is possible to enable logging at runtime without modifying the application binary. The log4net package is designed so that log statements can remain in shipped code without incurring a high performance cost. It follows that the speed of logging (or rather not logging) is crucial.
At the same time, log output can be so voluminous that it quickly becomes overwhelming. One of the distinctive features of log4net is the notion of hierarchical loggers. Using these loggers it is possible to selectively control which log statements are output at arbitrary granularity.
log4net is designed with two distinct goals in mind: speed and flexibility
log4net runs on all ECMA CLI 1.0 compatible runtimes. log4net has specific builds for the following frameworks:
The "Client Profile" builds are stripped down versions of the "normal" builds that don't contain any ASP.NET releated code - which for example means the %aspnet-* patterns and the AspNetTraceAppender are not available.
*Not supported by the binary release but can be built from the source release.
log4net ships with the following appenders (not on all frameworks):
| Type | Description |
|---|---|
| log4net.Appender.AdoNetAppender | Writes logging events to a database using either prepared statements or stored procedures. |
| log4net.Appender.AnsiColorTerminalAppender | Writes color highlighted logging events to a an ANSI terminal window. |
| log4net.Appender.AspNetTraceAppender | Writes logging events to the ASP trace context. These can then be rendered at the end of the ASP page or on the ASP trace page. |
| log4net.Appender.ColoredConsoleAppender | Writes color highlighted logging events to the application's Windows Console. |
| log4net.Appender.ConsoleAppender | Writes logging events to the application's Console. The events may go to either the standard our stream or the standard error stream. |
| log4net.Appender.DebugAppender | Writes logging events to the .NET system. |
| log4net.Appender.EventLogAppender | Writes logging events to the Windows Event Log. |
| log4net.Appender.FileAppender | Writes logging events to a file in the file system. |
| log4net.Appender.LocalSyslogAppender | Writes logging events to the local syslog service (UNIX only). |
| log4net.Appender.MemoryAppender | Stores logging events in an in memory buffer. |
| log4net.Appender.NetSendAppender | Writes logging events to the Windows Messenger service. These messages are displayed in a dialog on a users terminal. |
| log4net.Appender.OutputDebugStringAppender | Writes logging events to the debugger. If the application has no debugger, the system debugger displays the string. If the application has no debugger and the system debugger is not active, the message is ignored. |
| log4net.Appender.RemoteSyslogAppender | Writes logging events to a remote syslog service using UDP networking. |
| log4net.Appender.RemotingAppender | Writes logging events to a remoting sink using .NET remoting. |
| log4net.Appender.RollingFileAppender | Writes logging events to a file in the file system. The RollingFileAppender can be configured to log to multiple files based upon date or file size constraints. |
| log4net.Appender.SmtpAppender | Sends logging events to an email address. |
| log4net.Appender.SmtpPickupDirAppender | Sends logging events to an email address but writes the emails to a configurable directory rather than sending them directly via SMTP. |
| log4net.Appender.TelnetAppender | Clients connect via Telnet to receive logging events. |
| log4net.Appender.TraceAppender | Writes logging events to the .NET trace system. |
| log4net.Appender.UdpAppender | Sends logging events as connectionless UDP datagrams to a remote host or a multicast group using a UdpClient. |
A special log4net.Appender.ForwardingAppender can be used to wrap another appender, for example to attach additional filters.
Hierarchical logging is an ideal fit with component based development. Each component has its own of logger. When individually tested, the properties of these loggers may be set as the developer requires. When combined with other components, the loggers inherit the properties determined by the integrator of the components. One can selectively elevate logging priorities on one component without affecting the other components. This is useful when you need a detailed trace from just a single component without crowding the trace file with messages from other components. All this can be done through configuration files; no code changes are required.
log4net is configured using an XML configuration file. The configuration information can be embedded within other XML configuration files (such as the application's .config file) or in a separate file. The configuration is easily readable and updateable while retaining the flexibility to express all configurations.
Alternatively log4net can be configured programmatically.
log4net can monitor its configuration file for changes and dynamically apply changes made by the configurator. The logging levels, appenders, layouts, and just about everything else can be adjusted at runtime. In many cases it is possible to diagnose application issues without terminating the process in question. This can a very valuable tool in investigating issues with deployed applications.
log4net can be used to collect logging context data in a way that is transparent to the developer at the point of logging. The GlobalContext and the ThreadContext allow the application to store contextual data that is attached to logging messages. For instance, in a web service, once the caller is authenticated the username of the caller could be stored in a ThreadContext property. This property would then be automatically logged as part of each subsequent logging message made from the same thread.
log4net is based on the highly successful Apache log4j™ logging library, in development since 1996. This popular and proven architecture has so far been ported to 12 languages.