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| |
| <document> |
| <properties> |
| <title>Log4J 2.0 API</title> |
| <author email="rgoers@apache.org">Ralph Goers</author> |
| </properties> |
| |
| <body> |
| <section name="Log4j 2.0 API"> |
| <a name="Overview"/> |
| <subsection name="Overview"> |
| <p> |
| The Log4Jj 2.0 API provides the interface that applications should code to and provides the |
| adapter components required for implementers to create a logging implementation. Although Log4j 2 |
| is broken up between an API and an implementation, the primary purpose of doing so was not to |
| allow multiple implementations, although that is certainly possible, but to clearly define |
| what classes and methods are safe to use in "normal" application code. |
| </p> |
| <h4>Hello World!</h4> |
| <p> |
| No introduction would be complete without the customary Hello, World example. Here is ours. First, |
| a Logger with the name "HelloWorld" is obtained from the LogManager. Next, the logger is used to |
| write the "Hello, World!" message, however the message will be written only if the Logger is |
| configured to allow informational messages. |
| </p> |
| <source> import org.apache.logging.log4j.LogManager; |
| import org.apache.logging.log4j.Logger; |
| |
| public class HelloWorld { |
| private static Logger logger = LogManager.getLogger("HelloWorld"); |
| public static void main(String[] args) { |
| logger.info("Hello, World!"); |
| } |
| }</source> |
| <p> |
| The output from the call to logger.info() will vary significantly depending on the configuration |
| used. See the <a href="./configuration.html">Configuration</a> section for more details. |
| </p> |
| <h4>Parameter Substitution</h4> |
| <p> |
| Frequently the purpose of logging is to provide information about what is happening in the system, |
| which requires including information about the objects being manipulated. In Log4j 1.x this could |
| be accomplished by doing: |
| </p> |
| <source> if (logger.isDebugEnabled()) { |
| logger.debug("Logging in user " + user.getName() + " with id " + user.getId()); |
| }</source> |
| <p> |
| Doing this repeatedly has the effect of making the code feel like it is more about logging than the |
| actual task at hand. In addition, it results in the logging level being checked twice; once on the |
| call to isDebugEnabled and once on the debug method. A better alternative would be: |
| </p> |
| <source> logger.debug("Logging in user {} with id {}", user.getName(), user.getId());</source> |
| <p> |
| With the code above the logging level will only be checked once and the String construction will |
| only occur when debug logging is enabled. |
| </p> |
| </subsection> |
| </section> |
| </body> |
| </document> |
