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Logging refers to the recording of activity. Logging is a common issue for development teams. Several frameworks ease and standardize the process of logging for the Java platform. This article covers general purpose logging frameworks.
Logging is broken into three major pieces: the Logger, Formatter and the Handler (Appender). The Logger is responsible for capturing the message to be logged along with certain metadata and passing it to the logging framework. After receiving the message, the framework calls the Formatter with the message. The Formatter formats it for output. The framework then hands the formatted message to the appropriate Appender for disposition. This might include a console display, writing to disk, appending to a database, or email.
Simpler logging frameworks, like Java Logging Framework by the Object Guy, combine the logger and the appender. This simplifies default operation, but it is less configurable, especially if the project is moved across environments.
A Logger is an object that allows the application to log without regard to where the output is sent/stored. The application logs a message by passing an object or an object and an exception with an optional severity level to the logger object under a given a name/identifier.
A logger has a name. The name is usually structured hierarchically, with periods (.) separating the levels. A common scheme is to use the name of the class or package that is doing the logging. Both log4j and the Java logging API support defining Handlers higher up the hierarchy.
For example, the logger might be named "
com.sun.some.UsefulClass". The handler can be defined for any of the following:
The message is logged at a certain level. Common levels are from Apache Commons Logging:
|FATAL||Severe errors that cause premature termination. Expect these to be immediately visible on a status console.|
|ERROR||Other runtime errors or unexpected conditions. Expect these to be immediately visible on a status console.|
|WARNING||Use of deprecated APIs, poor use of API, 'almost' errors, other runtime situations that are undesirable or unexpected, but not necessarily "wrong". Expect these to be immediately visible on a status console.|
|INFO||Interesting runtime events (startup/shutdown). Expect these to be immediately visible on a console, so be conservative and keep to a minimum.|
|DEBUG||detailed information on the flow through the system. Expect these to be written to logs only.|
|TRACE||more detailed information. Expect these to be written to logs only.|
The logging framework maintains the current logging level for each logger. The logging level can be set more or less restrictive. For example, if the logging level is set to "WARNING", then all messages of that level or higher are logged, ERROR and FATAL.
Formatters or renderers
A Formatter is an object that formats a given object. Mostly this consists of taking the binary object and converting it to a string representation.
Appenders or handlers
Appenders listen for messages at or above a specified minimum severity level. The Appender takes the message it is passed and posts it appropriately. Message dispositions include:
- display on the console
- write to a file or syslog
- append to a database table
- distribute via Java Messaging Services
- send via email
- write to a socket
- discard to the "bit-bucket" (/dev/null)
|Framework||Supported log levels||Standard appenders||Popularity||Cost / licence|
|Log4J||FATAL ERROR WARN INFO DEBUG TRACE||AsyncAppender, JDBCAppender, JMSAppender, LF5Appender, NTEventLogAppender, NullAppender, SMTPAppender, SocketAppender, SocketHubAppender, SyslogAppender, TelnetAppender, WriterAppender||Widely used in many projects and platforms||Apache License, Version 2.0|
|Java Logging API||SEVERE WARNING INFO CONFIG FINE FINER FINEST||Sun's default Java Virtual Machine (JVM) has the following: ConsoleHandler, FileHandler, SocketHandler, MemoryHandler||Comes with the JRE|
|Apache Commons Logging||FATAL ERROR WARN INFO DEBUG TRACE||Depends on the underlying framework||Widely used, in conjunction with log4j||Apache License, Version 2.0|
|SLF4J||ERROR WARN INFO DEBUG TRACE||Depends on the underlying framework, which is pluggable||MIT License|
|tinylog||ERROR WARNING INFO DEBUG TRACE||ConsoleWriter, FileWriter, LogcatWriter, JdbcWriter, RollingFileWriter, SharedFileWriter and null (discards all log entries) ||Apache License, Version 2.0|
|Logback||ERROR WARN INFO DEBUG TRACE||Too many to list: see Appender JavaDoc||Used in many projects like Akka, Apache Camel, Apache Cocoon, Artifactory, Gradle, Lift Framework, Play Framework, Scalatra, SonarQube, etc...||LGPL, Version 2.1|
Apache Commons Logging isn't really a logging framework, but a wrapper for one. As such, it requires a logging framework underneath it. It is particularly useful when developing reusable libraries which need to write to whichever underlying logging system is being used by the application. It also provides flexibility in heterogeneous environments where the logging framework is likely to change, although in most cases, once a logging framework has been chosen, there is little need to change it over the life of the project.
The Java Logging API is also not a logging framework, but a standard API for accessing a logging framework. Compatible frameworks can be loaded into JVM and accessed via the API. There is also a logging implementation supplied with the Sun JVM which is the default logging framework accessed by the API. Many developers confuse this implementation with the Java Logging API.
- Javolution LogContext based on context programming (actual logging framework selectable at run-time).
- Runtime intelligence
- Java 6.0 Logging API
- Commons Logging
- Open Source Logging Tools in Java
- The Apache 2.0 license.
- Logback - A successor to the popular log4j project
- tinylog - Minimalist logging utility with a static logger
- Loggifier A tool that inserts logging code into .class, .jar and .ear files
- JLV - Java logging viewer which is currently available as a plugin for Eclipse IDE
- Log4j 2