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| |
| <section> |
| <h2><a name="Welcome_to_Log4j_2.21"></a>Welcome to Log4j 2!</h2> |
| <section> |
| <h3><a name="Introduction"></a>Introduction</h3> |
| |
| <p>Almost every large application includes its own logging or tracing |
| API. In conformance with this rule, the E.U. <a class="externalLink" href="http://www.semper.org">SEMPER</a> 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 <a href="../LICENSE">Apache Software License</a>, a |
| fully-fledged open source license certified by the <a class="externalLink" href="http://www.opensource.org">open source</a> initiative. The |
| latest log4j version, including full-source code, class files and |
| documentation can be found at <a class="externalLink" href="http://logging.apache.org/log4j/2.x/index.html"><b>http://logging.apache.org/log4j/2.x/index.html</b></a>. |
| </p> |
| |
| |
| <p>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.</p> |
| |
| |
| <p>Experience indicates that logging was an important component of the |
| development cycle. It offers several advantages. It provides precise |
| <i>context</i> 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.</p> |
| |
| |
| <p>As Brian W. Kernighan and Rob Pike put it in their truly excellent |
| book <i>"The Practice of Programming":</i></p> |
| |
| |
| <div class="well"> |
| 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. |
| </div> |
| |
| |
| <p>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.</p> |
| </section> |
| <section> |
| <h3><a name="Log4j_2"></a>Log4j 2</h3> |
| |
| <p> |
| 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 |
| <a class="externalLink" href="https://blogs.apache.org/foundation/entry/apache_logging_services_project_announces">End |
| of Life</a> 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. |
| </p> |
| |
| <ol style="list-style-type: decimal"> |
| |
| <li>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.</li> |
| |
| <li>Log4j 2 contains next-generation <a href="async.html">Asynchronous Loggers</a> based |
| on the <a class="externalLink" href="https://lmax-exchange.github.io/disruptor/">LMAX Disruptor library</a>. |
| In multi-threaded scenarios Asynchronous Loggers have 10 times higher throughput and |
| orders of magnitude lower latency than Log4j 1.x and Logback.</li> |
| |
| <li>Log4j 2 is <a href="garbagefree.html">garbage free</a> 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.</li> |
| |
| <li>Log4j 2 uses a <a href="plugins.html">Plugin system</a> that makes it extremely easy to |
| <a href="extending.html">extend the framework</a> by adding new <a href="appenders.html">Appenders</a>, |
| <a href="filters.html">Filters</a>, <a href="layouts.html">Layouts</a>, <a href="lookups.html">Lookups</a>, |
| and Pattern Converters without requiring any changes to Log4j.</li> |
| |
| <li>Due to the Plugin system configuration is simpler. Entries in the configuration |
| do not require a class name to be specified.</li> |
| |
| <li>Support for <a href="customloglevels.html">custom log levels</a>. |
| Custom log levels can be defined in code or in configuration.</li> |
| |
| <li>Support for <a href="api.html#LambdaSupport">lambda expressions</a>. |
| 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.</li> |
| |
| <li>Support for <a href="messages.html">Message objects</a>. 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 |
| <a href="../log4j-api/apidocs/org/apache/logging/log4j/message/Message.html">Message</a> |
| types and write custom <a href="layouts.html">Layouts</a>, <a href="filters.html">Filters</a> and |
| <a href="lookups.html">Lookups</a> to manipulate them.</li> |
| |
| <li>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.</li> |
| |
| <li>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.</li> |
| |
| <li>Layouts in Log4j 1.x and Logback return a String. This resulted in the problems |
| discussed at <a class="externalLink" href="http://logback.qos.ch/manual/encoders.html">Logback Encoders</a>. |
| Log4j 2 takes the simpler approach that <a href="layouts.html">Layouts</a> 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.</li> |
| |
| |
| <li>The <a href="appenders.html#SyslogAppender">Syslog Appender</a> supports both TCP and UDP as well as |
| support for the BSD syslog |
| and the <a class="externalLink" href="http://tools.ietf.org/html/rfc5424">RFC 5424</a> formats.</li> |
| |
| <li>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.</li> |
| |
| <li>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 |
| <a class="externalLink" href="http://jakarta.apache.org/site/contributing.html">Contributing</a>.</li> |
| </ol> |
| </section> |
| </section> |
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