Interview Questions

Interview Questions

Logging

• What types of logs exist?
• What are the components of the log4j logging system?
• What is a Logger in log4j?
• What is an Appender in log4j?
• What is a Layout in log4j?
• List the logging levels in log4j. What is their priority order?
• What configuration methods exist for log4j?

What types of logs exist?

• System;
• Security;
• Application (Business).

A user logs into the application, and their password is verified. This action pertains to security. Next, they launch a module. This event is at the application level. The module then requests additional data from another module and makes further calls—these are system actions.

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What are the components of the log4j logging system?

The logging system consists of three main parts:

• A logging controller - logger;
• A mechanism that writes to the log - appender;
• A format definition for added logs - layout.

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What is a Logger in log4j?

A Logger is an object of the class org.apache.log4j.Logger, which serves as the control interface for logging messages and can set the level of detail. The logger checks whether a message needs to be processed, and if logging is necessary, it passes the message to the appender; if not, processing of that message is terminated.

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What is an Appender in log4j?

An Appender is a named event logging object that implements the org.apache.log4j.Appender interface and writes events to the log. The appender can call various helper tools—like a layout, filter, or error handler (if defined and needed). During this process, the necessity of recording the message is definitively established, and the message is given its final content and form.

In log4j, a log can represent:

• Console;
• File;
• Socket;
• An object of a class implementing java.io.Writer or java.io.OutputStream;
• JDBC storage;
• JMS topic;
• NT Event Log;
• SMTP;
• Syslog;
• Telnet.

The most commonly used log4j appenders include:

• org.apache.log4j.ConsoleAppender - for console output;
• org.apache.log4j.FileAppender - for file logging;
• org.apache.log4j.DailyRollingFileAppender - for file logging with file rotation at specified intervals;
• org.apache.log4j.RollingFileAppender - for file logging with file rotation upon reaching a certain size;
• org.apache.log4j.varia.ExternallyRolledFileAppender - an extension of RollingFileAppender that rotates files based on commands received from a specified port;
• org.apache.log4j.net.SMTPAppender - for message sending via SMTP;
• org.apache.log4j.AsyncAppender - allows asynchronous logging using a separate thread, where messages are only recorded when a certain level of the intermediate buffer is filled.
• org.apache.log4j.nt.NTEventLogAppender - for logging to NT Event Log;
• org.apache.log4j.net.SyslogAppender - for logging to Syslog;
• org.apache.log4j.jdbc.JDBCAppender - for JDBC storage logging;
• org.apache.log4j.lf5.LF5Appender - sends messages to a special GUI interface called LogFactor5;
• org.apache.log4j.net.SocketAppender - for broadcasting messages to a specified address and port;
• org.apache.log4j.net.SocketHubAppender - for sending messages to multiple remote servers connected via a specified port;
• org.apache.log4j.net.TelnetAppender - for message sending via the Telnet protocol;
• org.apache.log4j.net.JMSAppender - for logging messages in JMS.

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What is a Layout in log4j?

A Layout is a subclass of org.apache.log4j.Layout that provides the ability to format messages before adding them to the log.

There are the following types of layouts in log4j:

• org.apache.log4j.SimpleLayout - produces a string containing only the output level and the message;
• org.apache.log4j.HTMLLayout - formats the message as an HTML table element;
• org.apache.log4j.xml.XMLLayout - formats the message in XML format;
• org.apache.log4j.TTCCLayout - appends time, thread, logger name, and nested diagnostic context information to the output message;
• org.apache.log4j.PatternLayout / org.apache.log4j.EnhancedPatternLayout - allows custom formatting of messages using user-defined templates.

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List the logging levels in log4j. What is their priority order?

• OFF - no logging;
• FATAL - severe error;
• ERROR - error;
• WARN - warning;
• INFO - informational message;
• DEBUG - detailed debugging message;
• TRACE - tracing all messages.

The priority order among the logging levels is as follows:

ALL < TRACE < DEBUG < INFO < WARN < ERROR < FATAL < OFF

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What configuration methods exist for log4j?

To begin working with log4j, you need to provide it with a configuration. This can be done in several ways:

• Create the configuration programmatically, i.e., obtain a logger, set the logging level, attach an appender, and specify the formatting method.
• Provide a file or URL as an argument when starting the Java machine using -Dlog4j.configuration=path/to/configuration/file, then read it in the program using PropertyConfigurator.configure(...) / DOMConfigurator.configure(...) for .properties or XML formats, respectively.
• Load the configuration from an XML file or .properties file: log4j searches for the configuration file in the classpath. It first looks for log4j.xml, and if it does not find it, it looks for log4j.properties.

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Sources

• Quizful
• Skipy

Interview Questions