Apache Log4j is a Java-based logging utility. It was originally written by Ceki Gülcü and is now a project of the Apache Software Foundation. Log4j is one of several Java logging frameworks.
Gülcü has since started the SLF4J and Logback projects, with the intention of offering a successor to Log4j.
The Apache Log4j team has created a successor to Log4j 1 with version number 2. Log4j 2 was developed with a focus on the problems of Log4j 1.2, 1.3, java.util.logging and Logback, and addresses issues which appeared in those frameworks. In addition, Log4j 2 offers a plugin architecture which makes it more extensible than its predecessor. Log4j 2 is not backwards compatible with 1.x versions, although an "adapter" is available.
On August 5, 2015 the Apache Logging Services Project Management Committee announced that Log4j 1 had reached end of life and that users of Log4j 1 are recommended to upgrade to Apache Log4j 2.
Apache Log4j 2 is the successor of Log4j 1 which was released as GA version in July 2014. The framework was rewritten from scratch and has been inspired by existing logging solutions, including Log4j 1 and java.util.logging. The main differences to Log4j 1 are:Improved reliability. Messages are not lost while reconfiguring the framework like in Log4j 1 or Logback
Extensibility: Log4j 2 supports a plugin system to let users define and configure custom components
Simplified configuration syntax
Support for xml, json, yaml and properties configurations
Property lookup support for values defined in the configuration file, system properties, environment variables, the ThreadContext Map, and data present in the event
Support for multiple APIs: Log4j 2 can be used with applications using the Log4j 2, Log4j 1.2, SLF4J, Commons Logging and java.util.logging (JUL) APIs.
Custom Log Levels
Java 8-style lambda support for "lazy logging"
Support for user-defined Message objects
"Garbage-free or low garbage" in common configurations
One of the most recognized features of Log4j 2 is the performance of the "Asynchronous Loggers". Log4j 2 makes use of the LMAX Disruptor. The library reduces the need for kernel locking and increases the logging performance by a factor 12. For example, in the same environment Log4j 2 can write more than 18,000,000 messages per second, whereas other frameworks like Logback and Log4j 1 just write < 2,000,000 messages per second.
The following table defines the built-in log levels and messages in Log4j, in decreasing order of severity. The left column lists the log level designation in Log4j and the right column provides a brief description of each log level.
Log4j 2 allows users to define their own log levels. A source code generator tool is provided to create Loggers that support custom log levels identically to the built-in log levels. Custom log levels can either complement or replace the built-in log levels.
Log4j can be configured through a configuration file or through Java code. Configuration files can be written in XML, JSON, YAML, or properties file format. Within a configuration you can define three main components: Loggers, Appenders and Layouts. Configuring logging via a file has the advantage that logging can be turned on or off without modifying the application that uses Log4j. The application can be allowed to run with logging off until there's a problem, for example, and then logging can be turned back on simply by modifying the configuration file.
Loggers are named log message destinations. They are the names that are known to the Java application. Each logger is independently configurable as to what level of logging (FATAL, ERROR, etc.) it currently logs. In early versions of Log4j, these were called category and priority, but now they're called logger and level, respectively. A Logger can send log messages to multiple Appenders.
The actual outputs are done by Appenders. There are numerous Appenders available, with descriptive names, such as FileAppender, RollingFileAppender, ConsoleAppender, SocketAppender, SyslogAppender, and SMTPAppender. Logj 2 added Appenders that write to Apache Flume, the Java Persistence API, Apache Kafka, NoSQL databases, Memory-mapped files, Random Access files and ZeroMQ endpoints. Multiple Appenders can be attached to any Logger, so it's possible to log the same information to multiple outputs; for example to a file locally and to a socket listener on another computer.
Appenders use Layouts to format log entries. A popular way to format one-line-at-a-time log files is PatternLayout, which uses a pattern string, much like the C / C++ function printf. There are also HTMLLayout and XMLLayout formatters for use when HTML or XML formats are more convenient, respectively. Log4j 2 added Layouts for CSV, Graylog Extended Log Format (GELF), JSON, YAML and RFC-5424.
In Log4j 2, Filters can be defined on configuration elements to give more fine-grained control over which log entries should be processed by which Loggers and Appenders. In addition to filtering by log level and regular expression matching on the message string, Log4j 2 added burst filters, time filters, filtering by other log event attributes like Markers or Thread Context Map and JSR 223 script filters.
To debug a misbehaving configuration:In Log4j 2 configurations set the
status attribute to TRACE to send internal status logging output to standard out. To enable status logging before the configuration is found, use the Java VM property
In Log4j 1, use the Java VM property
To find out where a log4j2.xml configuration file was loaded from inspect
There is also an implicit "unconfigured" or "default" configuration of Log4j, that of a Log4j-instrumented Java application which lacks any Log4j configuration. This prints to stdout a warning that the program is unconfigured, and the URL to the Log4j web site where details on the warning and configuration may be found. As well as printing this warning, an unconfigured Log4j application will only print ERROR or FATAL log entries to standard out.
TTCC is a message format used by log4j. TTCC is an acronym for Time Thread Category Component. It uses the following pattern:
%r [%t] %-5p %c %x - %m%n
Example outputlog4c - A port for C. Log4C is a C-based logging library, released on SourceForge under the LGPL license. For various Unix operating systems the autoconf and automake files are provided. On Windows a Makefile is provided for use with MSVC. Developers may also choose to use their own make system to compile the source, depending on their build engineering requirements. An instance of the log4c library may be configured via three methods: using environment variables, programmatically, or via XML configuration file. Last version is 1.2.4, released in 2013, and the project is no longer actively developed.
Apache Log4net - A port to the Microsoft .NET Framework. The initial work was done by Neoworks and was donated to the Apache Software Foundation in February 2004. The framework is similar to the original log4j while taking advantage of new features in the .NET runtime. Provides Nested Diagnostic Context (NDC) and Mapped Diagnostic Context (MDC). Last version is 1.2.15, released in 2015.
log4perl - A Perl port of the widely popular log4j logging package. Last version is 1.44, released in May 2014.
Apache log4php - "A versatile logging framework for PHP. Originally a port of Apache log4j to PHP, it has grown to include various PHP specific features."
PL-SQL-Logging-Utility is an adaptation of log4j in PL/SQL.
Log4db2 is a logging utility for DB2 for LUW that uses SQL instructions with SQL PL code.
Apache Log4cxx - A logging framework for C++ patterned after Apache log4j, which uses Apache Portable Runtime for most platform-specific code and should be usable on any platform supported by APR. It is currently undergoing Incubation, The latest version is 0.10.0, released in 2013.
467 [main] INFO org.apache.log4j.examples.Sort - Exiting main method.