今年一直关注log4j 2，但至今还没有出正式版。等不及了，今天正式向大家介绍一下log4j的升级框架，log4j 2。
log4j，相信大家都熟悉，至今对java影响最大的logging系统，至今仍有很多系统在使用log4j，但毕竟这个版本出的太早，Java都从1.2到7.0了，log4j怎么总是在1.2的版本呢？
不得承认，写log4j的那个人确实很牛，之后又写了slf4j和logback作为log4j 1.x的替代品。但是，最终，原作者可能发现，还是log4j的江山大，所以，最终决定重写log4j，因此诞生了log4j 2。哈哈，以上都是个人杜撰。
下面简单介绍一下，log4j 2的一些新特性。原文在这里。
为什么要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. Its alternative, SLF4J/Logback made many needed improvements to the framework. So why bother with Log4j 2? Here are a few of the reasons.
Log4j1.x已经被广泛应用到各个系统及框架中。然后，1.x毕竟太古老，代码很久没有更新。目前，Log4j 1.x的代码已经很难维护，因为它依赖于很多Jdk老版本的api。作为Log4j 1.x的替代品，SLF4J/Logback已经对日志系统做了很大的改进，那么，为什么我们还需要Log4j 2？
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
Log4j 2被设计成对安全审计有用的日志框架。在Logback框架中，当输出日志产生异常时，从来不会告诉被调用方，而log4j 2，这个将是可配置的。
2、 Log4j 2 contains next-generation lock-free Asynchronous Loggers based on the 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.
Log4j 2使用了新一代的基于LMAX Disruptor的无锁异步日志系统。在多线程的程序中，异步日志系统吞吐量比Log4j 1.x和logback高10倍，而时间延迟却更低。
3、Log4j 2 uses a Plugin system that makes it extremely easy to extend the framework by adding new Appenders, Filters, Layouts, Lookups, and Pattern Converters without requiring any changes to Log4j.
Log4j 2使用插件机制，更灵活。扩展appenders,Filters,Layouts,Lookups和Pattern Converters将变得更加简单，而不用去更改任何Log4j本身。
4、Due to the Plugin system configuration is simpler. Entries in the configuration do not require a class name to be specified.
因为插件机制，在配置文件中，不再需要指定定制的类名称。
（下面这些个人感觉是一些在logback上面的小改进，我就不翻译了）
5、Support for 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 Layouts, Filters and Lookups to manipulate them.
6、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.
7、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.
8、 Layouts in Log4j 1.x and Logback return a String. This resulted in the problems discussed at Logback Encoders. Log4j 2 takes the simpler approach that 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.
9、The Syslog Appender supports both TCP and UDP as well as support for the BSD syslog and the RFC 5424 formats.
10、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.
Log4j 2在底层尽可能使用了Java5提供的对并发及锁支持的工具类。Lo4j 1.x有死锁的bug。Logback中修复了log4j 1.x的很多bug，但是，logback中的有很多类采用同步机制（这种机制导致性能下降）。
11、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 Contributing
另外，作者针对性能做了专门介绍：
One of the often-cited arguments against logging is its computational cost. This is a legitimate concern as even moderately sized applications can generate thousands of log requests. Much effort was spent measuring and tweaking logging performance. Log4j claims to be fast and flexible: speed first, flexibility second.
决定日志系统好坏的一个重要参数就是它的性能指标。这很重要，因为即使一个中等规模的应用程序每秒产生的日志都数以千计。我们花费了大量精力在测量和调整log4j 2的性能。log4j声称，日志框架应该很快，并且要求灵活：速度第一，灵活性第二。
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