Log4j是Apache的著名專案，隨著Java應用的越來越廣泛，對日誌效能等方面的要求也越來越高。Log4j的升級版本Log4j2在前些年釋出。

在log4j2釋出前，應經有了logback和SLF4J，功能也是非常強大，那麼為什麼作者還要釋出log4j2呢？在《log4j-users-guide》對這個問題做了解釋。

Its alternative, SLF4J/Logback made many needed improvements to the framework. So why bother with Log4j 2? Here are a few of the reasons.

下面是官方使用者手冊2.5版中，給出的說明。原文和自己的翻譯都放在這：

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被設計成一個具有審計功能的日誌框架。在配置更新時，Log4j 1.x and Logback對日誌事件沒有反應，Log4j 2則會正常工作。另外，Logback不允許Appenders中的異常是不可見的，而Log4j2可以配置成傳送異常給應用程式。)

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. (主要說明Log4j2有強大的基於LMAX Disruptor library的非同步日誌功能。)

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. (基於外掛的軟體架構，使得Log4j2更易於擴充套件，而不需會改變Log4j2自身的功能。也就是遵循了設計模式的一個重要原則，對修改封閉，對擴充套件開放。)

4. Due to the Plugin system configuration is simpler. Entries in the configuration do not require a class name to be specified. (基於外掛的配置檔案更加簡單，配置檔案中的實體不必指定一個類名。--這一點暫時沒有搞清楚。)

5. Support for custom log levels. Custom log levels can be defined in code or in configuration. (支援使用者自定義的日誌等級，在程式碼或者配置檔案中都可以定義。)

6. Support for 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. (支援lambda表示式。--這是Java 8的新特性，不瞭解的同學可以查一下。)

7. 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. (支援訊息物件。...。使用者可以自由的建立他們自己的訊息型別，寫使用者自定義的輸出，過濾器和Lookups來操作這些訊息型別。)

8. 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. (比Log4j1和Logback更加全面的過濾器，Log4j2可以在Logger處理前，處理中和到Appender後都新增過濾器。)

9. 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. (很多Logback Appenders不接收Layout引數，只用固定的格式傳送資料。大多數的Log4j2的Appenders接收Layout引數，允許以任何預期的格式傳送。)

10.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. (Log4j1和Logback中的Layouts都返回String型別值。這導致了Logback Encoders中討論的一些問題。Log4j2用一個簡單的方法，Layouts返回位元組陣列。這使得Log4j2的Layouts可以用於任何Appender，而不只是使用OutputStream寫日誌的Appenders.)

11.The Syslog Appender supports both TCP and UDP as well as support for the BSD syslog and the RFC 5424 formats. (Syslog Appender即支援TCP和UDP，同時還支援BSD syslog和RFC 5424格式。)

12.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. (Log4j2使用了Java 5對併發性的支援，而且可以在最低的等級執行枷鎖操作。Log4j1中有很多死鎖的問題。其中一些在Logback中得到了解決，但是很多Logback的類在相對高的層級，仍然需要同步保證。)

13.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. (Log4j2是Apache的專案，所以同所有的Apache軟體基金會的支援方案相同。 按照Contributing路徑下的步驟，就可以獲得修改軟體的許可。)

終於翻譯完了，有些地方可能與原文字意有出入，歡迎大家指正：）。

後續的文章中，我會根據專案中Log4j2的使用，寫一些例子和Log4j2新特性的實驗。