log4j interview question set 1/log4j Interview Questions and Answers for Freshers & Experienced

What is the purpose of F character used in the conversionPattern of PatternLayout object?

F − Used to output the file name where the logging request was issued.

Explain why do you get multiple copies of the message in log file sometime?

There could be two reasons why this may happen

Repeated configuration of log4j
Attaching the same appenders to multiple loggers

Mention what is the best way to migrate from java.util logging to log4j?

The best way to migrate from java.util logged to log4j is to use global file search/replace method. It will replace with “org.apache.log4j.Logger”

Explain what does WARN and TRACE level indicates in log4j?

Log4j level WARN gives a warning about an unpredicted event to the user. The messages coming out of this level may not stop the progress of the system. The TRACE level provides more detailed information than the DEBUG level, and it will stay on the top of the hierarchy.

Explain what is package level logging in log4j?

Package level logging is the standard logging of log4j, with this you would determine the package and the associated level.

Mention what is the difference between Threshold and LevelRangeFilter in log4j?

Both Threshold and LevelRangeFilter does the same thing. However threshold should be faster. Filters enable you to implement your own logic, and you can also link them together if required. If you need a basic threshold functionality, then “threshold” function will be enough.

Explain how can you get multiple processes to log to the same file?

You may have each process log to a socket Appender. The receiving socket server can receive all the events and send them to a single log file.

Explain what are the system properties checked by log4j?

The system properties checked by log4j are

1. Log4j debug, if true, log4j will show internal debugging messages to the console
2. defaultInitOverride, if true, log4j will not execute default initialization
3. configuration, URL for default initialization configuration file
4. configurationClass, Class name for configurator to execute default initialization configuration file
5. ignoreTCL, if true, the thread class loader will be overlooked when loading classes

Explain whether a log output format can be customized?

Yes, you can extend the layout class to create your own customized log format. Appenders can be parameterized to use the layout of your choice.

Explain what is the command to write your logging information into a file?

To write your logging information into a file, you would need to use a command org.apache.log4j.FileAppender

In log4j how can you log into the database?

The log4j API provides the object org.apache.log4j.jdbc. JDBCAppender object can put logging information in a particular database.

What is the purpose of LogManager object?

LogManager − The LogManager object manages the logging framework. It is responsible for reading the initial configuration parameters from a system-wide configuration file or a configuration class.

What is the purpose of Filter object?

Filter Object − The Filter object is used to analyze logging information and make further decisions on whether that information should be logged or not. An Appender objects can have several Filter objects associated with them. If logging information is passed to a particular Appender object, all the Filter objects associated with that Appender need to approve the logging information before it can be published to the attached destination.

What are Pros and Cons of Logging?

Following are the Pros and Cons of Logging −
Logging is an important component of the software development. A well-written logging code offers quick debugging, easy maintenance, and structured storage of an application's runtime information.Logging does have its drawbacks also. It can slow down an application. If too verbose, it can cause scrolling blindness. To alleviate these concerns, log4j is designed to be reliable, fast and extensible.Since logging is rarely the main focus of an application, the log4j API strives to be simple to understand and to use.

Can log4j logging be stored to database?

Yes. The logging can be written to a database, file, console, JMS and Apache flume.

What is purpose of ERROR log level?

ERROR − Designates error events that might still allow the application to continue running.

Explain why do you get multiple copies of the message in log file sometime?

There could be two reasons why this may happen

1. Repeated configuration of log4j
2. Attaching the same appenders to multiple loggers

List the output destinations of at least 3 Log4j and explain their meanings.

Analysis: The ability to output to different destinations is a major feature of Log4j.
(1)org.apache. log4j.ConsoleAppender: Outputs log information to the console. If the Logger does not explicitly add ( )addAppender to the Appender, the ConsoleAppender is used by default.
(2) org.apache. log4j.FileAppender: Output log information to a file.
(3) org.apache. log4j.DailyRollingFileAppender: output log information to
A log file, and ( ) according to the specified mode, the log information can be output to a new log file according to a certain date.

Mention larger logging domains for your applications.

Diagnostic logging are used to understand the state of the system at a particular point of interest.

Security logging establishes accountability and useful for forensics.

Audit logging is the digital evidence and used for auditing purposes.

What is SLF4J?

The Simple Logging Facade for Java (SLF4J) is a simple facade or abstraction for various logging frameworks such as java.util.logging, LogBack, log4j allowing the end user to plug in the desired logging framework at deployment time. It is not itself a logging library, but a generic interface to one of many logging libraries.

Why should we prefer log4j framework while Java provides logging API?

Log4j has following advantages over standard logging API:

<> robust logging.
<> log4j has more features available than standard logging API.
<> configuring and using log4j is easier.
<> log4j also has a much more robust log formatting.
<> more handlers are available for log4j.

Mention what is the difference between Threshold and LevelRangeFilter in log4j?

Both Threshold and LevelRangeFilter does the same thing. However threshold should be faster. Filters enable you to implement your own logic, and you can also link them together if required. If you need a basic threshold functionality, then “threshold” function will be enough.

How log4j file is defined?

Log4j file is defined by the name log4j.properties, it keeps properties in key-value pairs. By default, the log manager looks for a file name log4j.properties in the CLASSPATH.

Explain immediateFlush property of FileAppender.

ImmediateFlush property enables the output stream to the file being flushed with each append operation.

By default the property is set to true.

To switch off immediateFlush use the below configuration. When set to false, it may improve performance.

log4j.appender.FILE.ImmediateFlush=false

How does log Levels work in Log4J?

A log request of level n in a logger with level m is enabled if m >= n.

For the standard levels, we have ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF.

How can you log into the database using Log4j?

The log4j API provides the object org.apache.log4j.jdbc. JDBCAppender object that can put logging information in a database.

Is log4j thread safe?

Yes, Log4j is a thread-safe; its components are built to be used in multithreaded systems.

Explain different logging levels in log4j.

The DEBUG level designates fine-grained informational events useful to debug an application.

The INFO level designates informational messages that highlight the progress of the application at coarse-grained level.

The WARN level designates potentially harmful situations.

The ERROR level designates error events that might still allow the application to continue running.

The TRACE Level designates finer-grained informational events than the DEBUG.

FATAL level designates very severe error events that will presumably lead the application to abort.

The ALL Level has the lowest possible rank and is intended to turn on all logging.

The OFF Level has the highest possible rank and is intended to turn off logging.

Mention what are the other support objects in Log4j?

There are other support objects in Log4j framework they are

<< Level Object
<< Filter Object
<< Object Renderer
<< Log Manager

Mention what are the different types of Appenders?

Some of the Appenders type include

<> ConsoleAppender logs to standard output
<> FileAppender prints logs to some file
<> Rolling file appender to a file with maximum size

Explain what are the format characters used in log4j?

The format characters used in log4j are:

L: it is used to output the line number from where the logging request was processed or issued
M: It is used to output the application supplied message related with the logging event
P: It is used to output the priority of the logging event
C: It is used to output the class name of the caller issuing the logging request

What is the purpose of c character used in the conversionPattern of PatternLayout object?

Used to output the category of the logging event. For example, for the category name "a.b.c" the pattern %c{2} will output "b.c".

Advantages of using log4j framework.

Thread safe.
Optimized for performance.
Supports more than one appender per logger.
Extensible.
Logging behavior is specified in a configuration file loaded during runtime.
Supports different logging levels such as WARN, INFO, and ERROR.
The logging format can be easily changed my altering the Layout class or by extending it.
The destination and the log writing strategy can be altered by implementing Appender interface.

Components of log4j framework.

There are mainly 3 components.

Logger captures logging information.

Appender publishes logging information to various destinations.

Layout formats the logging data in different configured styles to support readability.

How will you create a logger in any class?

Any other named Logger object instance is obtained through the second method by passing the name of the logger. The name of the logger can be any string you can pass, usually a class or a package name as we have used in the last chapter and it is mentioned below −

static Logger log = Logger.getLogger(log4jExample.class.getName());

What is the purpose of ObjectRenderer object?

ObjectRenderer: The ObjectRenderer object is specialized in providing a String representation of different objects passed to the logging framework. This object is used by Layout objects to prepare the final logging information.

What is the purpose of Appender object?

Appender Object: This is a lower-level layer of log4j architecture which provides Appender objects. The Appender object is responsible for publishing logging information to various preferred destinations such as a database, file, console, UNIX Syslog, etc.

Mention what are the two static methods for obtaining a logger object?

The two static methods for obtaining a logger object are:

1. Public static Logger getRootLogger()
2. Public static Logger getLogger(String name)

Mention what is the best way to migrate from java.util logging to log4j?

The best way to migrate from java.util logged to log4j is to use global file search/replace method. It will replace with “org.apache.log4j.Logger”

What is purpose of FATAL log level?

FATAL: Designates very severe error events that will presumably lead the application to abort.

What is purpose of ERROR log level?

ERROR: Designates error events that might still allow the application to continue running.

How do Levels Works?

A log request of level p in a logger with level q is enabled if p >= q. This rule is at the heart of log4j. It assumes that levels are ordered. For the standard levels, we have ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF.

What are Pros and Cons of Logging?

Following are the Pros and Cons of Logging −
Logging is an important component of the software development. A well-written logging code offers quick debugging, easy maintenance, and structured storage of an application's runtime information.
Logging does have its drawbacks also. It can slow down an application. If too verbose, it can cause scrolling blindness. To alleviate these concerns, log4j is designed to be reliable, fast and extensible.
Since logging is rarely the main focus of an application, the log4j API strives to be simple to understand and to use.

Why to use Log4j?

>> Being open-source its completely free to use.
>> You can easily save log information into either files or even databases.
>> Can be used for projects of any sizes small or large.

What is purpose of DEBUG log level?

DEBUG: Designates fine-grained informational events that are most useful to debug an application.

What are the different logging levels?

There are several logging levels that you can configure in you applicaiton
Those are FATAL,ERROR,WARN,TRACE,DEBUG,INFO OR ALL in apache logging. Default logging level is INFO.

Inside logger component what are the different log levels?

Different log levels inside logger components are

All
Debug
Info
Warn
Error
Fatal
Off

Mention what are the three principal components of Log4j?

The three principal components of Log4j are

Loggers
Appenders
Layout

Explain why to use Apache Log4j?

<> Being open-source its completely free to use.
<> You can easily save log information into either files or even databases.
<> Can be used for projects of any sizes small or large.

Explain what is Log4j?

Log4j is a fast, flexible and reliable logging framework written in Java developed in early 1996. It is distributed under the Apache software license and can be used for small to large scale projects. It has been ported to the languages like C, C++, C#, Python, etc.

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