log4j logging hierarchy order

Question

This table might be helpful for you:

Going down the first column, you will see how the log works in each level. i.e for WARN, (FATAL, ERROR and WARN) will be visible. For OFF, nothing will be visible.

Use the force, read the source (excerpt from the Priority and Level class compiled, TRACE level was introduced in version 1.2.12):

public final static int OFF_INT = Integer.MAX_VALUE;
public final static int FATAL_INT = 50000;
public final static int ERROR_INT = 40000;
public final static int WARN_INT  = 30000;
public final static int INFO_INT  = 20000;
public final static int DEBUG_INT = 10000;
public static final int TRACE_INT = 5000; 
public final static int ALL_INT = Integer.MIN_VALUE; 

or the log4j API for the Level class, which makes it quite clear.

When the library decides whether to print a certain statement or not, it computes the effective level of the responsible Logger object (based on configuration) and compares it with the LogEvent's level (depends on which method was used in the code – trace/debug/.../fatal). If LogEvent's level is greater or equal to the Logger's level, the LogEvent is sent to appender(s) – "printed". At the core, it all boils down to an integer comparison and this is where these constants come to action.

OFF
FATAL
ERROR
WARN
INFO
DEBUG
TRACE
ALL

Hierarchy of log4j logging levels are as follows in Highest to Lowest order :

• TRACE
• DEBUG
• INFO
• WARN
• ERROR
• FATAL
• OFF

TRACE log level provides highest logging which would be helpful to troubleshoot issues. DEBUG log level is also very useful to trouble shoot the issues.

You can also refer this link for more information about log levels : https://logging.apache.org/log4j/2.0/manual/architecture.html

[Taken from http://javarevisited.blogspot.com/2011/05/top-10-tips-on-logging-in-java.html]

DEBUG is the lowest restricted java logging level and we should write everything we need to debug an application, this java logging mode should only be used on Development and Testing environment and must not be used in production environment.

INFO is more restricted than DEBUG java logging level and we should log messages which are informative purpose like Server has been started, Incoming messages, outgoing messages etc in INFO level logging in java.

WARN is more restricted than INFO java logging level and used to log warning sort of messages e.g. Connection lost between client and server. Database connection lost, Socket reaching to its limit. These messages and java logging level are almost important because you can setup alert on these logging messages in java and let your support team monitor health of your java application and react on this warning messages. In Summary WARN level is used to log warning message for logging in Java.

ERROR is the more restricted java logging level than WARN and used to log Errors and Exception, you can also setup alert on this java logging level and alert monitoring team to react on this messages. ERROR is serious for logging in Java and you should always print it.

FATAL java logging level designates very severe error events that will presumably lead the application to abort. After this mostly your application crashes and stopped.

OFF java logging level has the highest possible rank and is intended to turn off logging in Java.

Hierarchy order

1. ALL
2. TRACE
3. DEBUG
4. INFO
5. WARN
6. ERROR
7. FATAL
8. OFF

To make it clear, there is no defined term like 'hierarchy' in log4j. Yes there are different levels. For troubleshooting you have to select which level you want the logs.