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= Overview
Ralph Goers <rgoers@apache.org>

== Welcome to Log4j!

Almost every large application includes its own logging or tracing API.
In conformance with this rule, the E.U. http://www.semper.org[SEMPER]
project decided to write its own tracing API. This was in early 1996.
After countless enhancements, several incarnations and much work that
API has evolved to become log4j, a popular logging package for Java. The
package is distributed under the link:../LICENSE[Apache Software
License], a fully-fledged open source license certified by the
http://www.opensource.org[open source] initiative. The latest log4j
version, including full-source code, class files and documentation can
be found at
https://logging.apache.org/log4j/2.x/index.html[*https://logging.apache.org/log4j/2.x/index.html*].

Inserting log statements into code is a low-tech method for debugging
it. It may also be the only way because debuggers are not always
available or applicable. This is usually the case for multithreaded
applications and distributed applications at large.

Experience indicates that logging was an important component of the
development cycle. It offers several advantages. It provides precise
_context_ about a run of the application. Once inserted into the code,
the generation of logging output requires no human intervention.
Moreover, log output can be saved in persistent medium to be studied at
a later time. In addition to its use in the development cycle, a
sufficiently rich logging package can also be viewed as an auditing
tool.

As Brian W. Kernighan and Rob Pike put it in their truly excellent book
_"The Practice of Programming":_

____
As personal choice, we tend not to use debuggers beyond getting a stack
trace or the value of a variable or two. One reason is that it is easy
to get lost in details of complicated data structures and control flow;
we find stepping through a program less productive than thinking harder
and adding output statements and self-checking code at critical places.
Clicking over statements takes longer than scanning the output of
judiciously-placed displays. It takes less time to decide where to put
print statements than to single-step to the critical section of code,
even assuming we know where that is. More important, debugging
statements stay with the program; debugging sessions are transient.
____

Logging does have its drawbacks. It can slow down an application. If too
verbose, it can cause scrolling blindness. To alleviate these concerns,
log4j is designed to be reliable, fast and extensible. Since logging is
rarely the main focus of an application, the log4j API strives to be
simple to understand and to use.

== Log4j 2

Log4j 1.x has been widely adopted and used in many applications.
However, through the years development on it has slowed down. It has
become more difficult to maintain due to its need to be compliant with
very old versions of Java and became
https://blogs.apache.org/foundation/entry/apache_logging_services_project_announces[End
of Life] in August 2015. Its alternative, SLF4J/Logback made many needed
improvements to the framework. So why bother with Log4j 2? Here are a
few of the reasons.

1.  Log4j 2 is designed to be usable as an audit logging framework. Both
Log4j 1.x and Logback will lose events while reconfiguring. Log4j 2 will
not. In Logback, exceptions in Appenders are never visible to the
application. In Log4j 2 Appenders can be configured to allow the
exception to percolate to the application.
2.  Log4j 2 contains next-generation link:async.html[Asynchronous
Loggers] based on the https://lmax-exchange.github.io/disruptor/[LMAX
Disruptor library]. In multi-threaded scenarios Asynchronous Loggers
have 10 times higher throughput and orders of magnitude lower latency
than Log4j 1.x and Logback.
3.  Log4j 2 is link:garbagefree.html[garbage free] for stand-alone
applications, and low garbage for web applications during steady state
logging. This reduces pressure on the garbage collector and can give
better response time performance.
4.  Log4j 2 uses a link:plugins.html[Plugin system] that makes it
extremely easy to link:extending.html[extend the framework] by adding
new link:appenders.html[Appenders], link:filters.html[Filters],
link:layouts.html[Layouts], link:lookups.html[Lookups], and Pattern
Converters without requiring any changes to Log4j.
5.  Due to the Plugin system configuration is simpler. Entries in the
configuration do not require a class name to be specified.
6.  Support for link:customloglevels.html[custom log levels]. Custom log
levels can be defined in code or in configuration.
7.  Support for link:api.html#LambdaSupport[lambda expressions]. Client
code running on Java 8 can use lambda expressions to lazily construct a
log message only if the requested log level is enabled. Explicit level
checks are not needed, resulting in cleaner code.
8.  Support for link:messages.html[Message objects]. Messages allow
support for interesting and complex constructs to be passed through the
logging system and be efficiently manipulated. Users are free to create
their own `Message` types and write custom link:layouts.html[Layouts],
link:filters.html[Filters] and link:lookups.html[Lookups] to manipulate
them.
9.  Log4j 1.x supports Filters on Appenders. Logback added TurboFilters
to allow filtering of events before they are processed by a Logger.
Log4j 2 supports Filters that can be configured to process events before
they are handled by a Logger, as they are processed by a Logger or on an
Appender.
10. Many Logback Appenders do not accept a Layout and will only send
data in a fixed format. Most Log4j 2 Appenders accept a Layout, allowing
the data to be transported in any format desired.
11. Layouts in Log4j 1.x and Logback return a String. This resulted in
the problems discussed at
http://logback.qos.ch/manual/encoders.html[Logback Encoders]. Log4j 2
takes the simpler approach that link:layouts.html[Layouts] always return
a byte array. This has the advantage that it means they can be used in
virtually any Appender, not just the ones that write to an OutputStream.
12. The link:appenders.html#SyslogAppender[Syslog Appender] supports
both TCP and UDP as well as support for the BSD syslog and the
http://tools.ietf.org/html/rfc5424[RFC 5424] formats.
13. Log4j 2 takes advantage of Java 5 concurrency support and performs
locking at the lowest level possible. Log4j 1.x has known deadlock
issues. Many of these are fixed in Logback but many Logback classes
still require synchronization at a fairly high level.
14. It is an Apache Software Foundation project following the community
and support model used by all ASF projects. If you want to contribute or
gain the right to commit changes just follow the path outlined at
http://jakarta.apache.org/site/contributing.html[Contributing].