Introduction

Almost every large application includes its own logging or tracing API. In conformance with this rule, the E.U. 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 Apache Software License, a fully-fledged open source license certified by the 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.

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":

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 extensively adopted and utilized across various applications. Its final release occurred in 2012, and in August 2015, the announcement was made that Log4j 1.x had reached its End of Life.

So, what makes upgrading to Log4j 2 a wise decision? Here are a few compelling reasons.

1. Audit Logging: 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. Asynchronous Loggers: Log4j 2 contains next-generation Asynchronous Loggers based on the LMAX Disruptor library. In multithreaded scenarios Asynchronous Loggers have 10 times higher throughput and orders of magnitude lower latency than Log4j 1.x and Logback.
3. Garbage Free: Log4j 2 is 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. Plugins: 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.
5. Simple Configuration: Due to the Plugin system configuration is simpler. Entries in the configuration do not require a class name to be specified.
6. Custom Log Levels: Support for custom log levels. Custom log levels can be defined in code or in configuration.
7. Lambdas: 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.
8. Messages: 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.
9. Filters: 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. Appender Layout: 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. Advanced Layouts: 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.
12. Syslog Support: The Syslog Appender supports both TCP and UDP as well as support for the BSD syslog and the RFC 5424 formats.
13. Java Concurrency: 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. Community: 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.