log4j - Interview Questions

log4j is a reliable, fast and flexible logging framework (APIs) written in Java, which is distributed under the Apache Software License.

log4j has been ported to the C, C++, C#, Perl, Python, Ruby, and Eiffel languages.

log4j is highly configurable through external configuration files at runtime. It views the logging process in terms of levels of priorities and offers mechanisms to direct logging information to a great variety of destinations, such as a database, file, console, UNIX Syslog, etc.

log4j has three main components −

• loggers: Responsible for capturing logging information.

• appenders: Responsible for publishing logging information to various preferred destinations.

• layouts: Responsible for formatting logging information in different styles.

Following are features of log4j −

• It is thread-safe.

• It is optimized for speed.

• It is based on a named logger hierarchy.

• It supports multiple output appenders per logger.

• It supports internationalization.

• It is not restricted to a predefined set of facilities.

• Logging behavior can be set at runtime using a configuration file.

• It is designed to handle Java Exceptions from the start.

• It uses multiple levels, namely ALL, TRACE, DEBUG, INFO, WARN, ERROR and FATAL.

• The format of the log output can be easily changed by extending the Layout class.

• The target of the log output as well as the writing strategy can be altered by implementations of the Appender interface.

• It is fail-stop. However, although it certainly strives to ensure delivery, log4j does not guarantee that each log statement will be delivered to its destination.

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.

Logger Object − The top-level layer of log4j architecture is the Logger which provides the Logger object. The Logger object is responsible for capturing logging information and they are stored in a namespace hierarchy.

Layout Object − The layout layer of log4j architecture provides objects which are used to format logging information in different styles. It provides support to appender objects before publishing logging information.

Layout objects play an important role in publishing logging information in a way that is human-readable and reusable.

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.

Level Object - The Level object defines the granularity and priority of any logging information. There are seven levels of logging defined within the API: OFF, DEBUG, INFO, ERROR, WARN, FATAL, and ALL.

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.

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.

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.

The log4j.properties file is a log4j configuration file which keeps properties in key-value pairs. By default, the LogManager looks for a file named log4j.properties in the CLASSPATH.

layout − Appender uses the Layout objects and the conversion pattern associated with them to format the logging information.

target − The target may be a console, a file, or another item depending on the appender.

level − The level is required to control the filtration of the log messages.

threshold − Appender can have a threshold level associated with it independent of the logger level. The Appender ignores any logging messages that have a level lower than the threshold level.

filter − The Filter objects can analyze logging information beyond level matching and decide whether logging requests should be handled by a particular Appender or ignored.

Following syntax defines the root logger with appender file:

# Define the root logger with appender file
log = /usr/home/log4j
log4j.rootLogger = DEBUG, FILE

Following syntax defines a file appender −

# Define the file appender
log4j.appender.FILE=org.apache.log4j.FileAppender
log4j.appender.FILE.File=${log}/log.out

Following syntax defines the layout of file appender −

# Define the layout for file appender
log4j.appender.FILE.layout=org.apache.log4j.PatternLayout
log4j.appender.FILE.layout.conversionPattern=%m%n

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());

public void debug(Object message) of Logger class prints messages with the level Level.DEBUG.

public void error(Object message) of Logger class prints messages with the level Level.ERROR.

public void fatal(Object message) of Logger class prints messages with the level Level.FATAL.

public void info(Object message) of Logger class prints messages with the level Level.INFO.

public void warn(Object message) of Logger class prints messages with the level Level.WARN.

public void trace(Object message) of Logger class prints messages with the level Level.TRACE.

ALL − All levels including custom levels.

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

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

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

INFO − Designates informational messages that highlight the progress of the application at coarse-grained level.

OFF − The highest possible rank and is intended to turn off logging.

TRACE − Designates finer-grained informational events than the DEBUG.

WARN − Designates potentially harmful situations.

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.

Following syntax defines the root logger with WARN mode turning DEBUG mode off.

# Define the root logger with appender file
log = /usr/home/log4j
log4j.rootLogger = WARN, FILE

If you want to generate your logging information in a particular format based on a pattern, then you can use org.apache.log4j.PatternLayout to format your logging information.

The PatternLayout class extends the abstract org.apache.log4j.Layout class and overrides the format() method to structure the logging information according to a supplied pattern.

c − 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".

C − Used to output the fully qualified class name of the caller issuing the logging request. For example, for the class name. "org.apache.xyz.SomeClass", the pattern %C{1} will output "SomeClass".

d − Used to output the date of the logging event. For example, %d{HH:mm:ss,SSS} or %d{dd MMM yyyy HH:mm:ss,SSS}.

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

l − Used to output location information of the caller which generated the logging event.

L − Used to output the line number from where the logging request was issued.

m − Used to output the application supplied message associated with the logging event.

M − Used to output the method name where the logging request was issued.

n − Outputs the platform dependent line separator character or characters.

p − Used to output the priority of the logging event.

r − Used to output the number of milliseconds elapsed from the construction of the layout until the creation of the logging event.

t − Used to output the name of the thread that generated the logging event.

x − Used to output the NDC (nested diagnostic context) associated with the thread that generated the logging event.

X − The X conversion character is followed by the key for the MDC. For example, X{clientIP} will print the information stored in the MDC against the key clientIP.

% − The literal percent sign. %% will print a % sign.

By default, the relevant information is displayed as output as is. However, with the aid of format modifiers, it is possible to change the minimum field width, the maximum field width, and justification.

%20c − Left pad with spaces if the category name is less than 20 characters long.

%-20c − Right pad with spaces if the category name is less than 20 characters long.

%.30c − Truncate from the beginning if the category name is longer than 30 characters.

%20.30c − Left pad with spaces if the category name is shorter than 20 characters. However, if the category name is longer than 30 characters, then truncate from the beginning.

%-20.30c − Right pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning.

If you want to generate your logging information in an HTML-formatted file, then you can use org.apache.log4j.HTMLLayout to format your logging information.

The HTMLLayout class extends the abstract org.apache.log4j.Layout class and overrides the format() method from its base class to provide HTML-style formatting.

It provides the following information to be displayed −

• The time elapsed from the start of the application before a particular logging event was generated.

• The name of the thread that invoked the logging request.

• The level associated with this logging request.

• The name of the logger and logging message.

• The optional location information for the program file and the line number from which this logging was invoked.

HTMLLayout.setContentType(String) − Sets the content type of the HTML content. Default is text/html.

HTMLLayout.setLocationInfo(String) − Sets the location information for the logging event. Default is false.

HTMLLayout.setTitle(String) − Sets the title for the HTML file. Default is log4j Log Messages.

immediateFlush − This flag is by default set to true, which means the output stream to the file being flushed with each append operation.

encoding − It is possible to use any character-encoding. By default, it is the platform-specific encoding scheme.

threshold − The threshold level for this appender.

Filename − The name of the log file.

fileAppend − This is by default set to true, which means the logging information being appended to the end of the same file.

bufferedIO − This flag indicates whether we need buffered writing enabled. By default, it is set to false.

bufferSize − If buffered I/O is enabled, it indicates the buffer size. By default, it is set to 8kb.

Following code configures immediate flush to true −

# Set the immediate flush to true (default)
log4j.appender.FILE.ImmediateFlush=true

Following code sets the threshold to debug mode −

# Set the threshold to debug mode
log4j.appender.FILE.Threshold=debug

Following code sets the append to false, overwrite −

# Set the append to false, overwrite
log4j.appender.FILE.Append=false

To write your logging information into multiple files, you would have to use org.apache.log4j.RollingFileAppender class which extends the FileAppender class and inherits all its properties.

This is the critical size of the file above which the file will be rolled.

Default value is 10 MB.

This property denotes the number of backup files to be created.

Default value is 1.

Following code configures a RollingFileAppender −

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FILE
# Define the file appender
log4j.appender.FILE=org.apache.log4j.RollingFileAppender

Following code configures maximum file size before rollover −

# Set the maximum file size before rollover
log4j.appender.FILE.MaxFileSize=5KB

Following code configures maximum files to be used −

# Set the the backup index
log4j.appender.FILE.MaxBackupIndex=2

A new log file will be created.

Once the last log file reaches the maximum size, the first log file will be erased and thereafter, all the logging information will be rolled back to the first log file.

To write your logging information into files on a daily basis, you would have to use org.apache.log4j.DailyRollingFileAppender class which extends the FileAppender class and inherits all its properties.

This indicates when to roll over the file and the naming convention to be followed. By default, roll over is performed at midnight each day.

'.' yyyy-MM − Roll over at the end of each month and at the beginning of the next month.

'.' yyyy-MM-dd − Roll over at midnight each day. This is the default value.

'.' yyyy-MM-dd-a − Roll over at midday and midnight of each day.

'.' yyyy-MM-dd-HH − Roll over at the top of every hour.

'.' yyyy-MM-dd-HH-mm − Roll over every minute.

'.' yyyy-ww − Roll over on the first day of each week depending upon the locale.

Following code configures a DailyRollingFileAppender −

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FILE 
# Define the file appender
log4j.appender.FILE = org.apache.log4j.DailyRollingFileAppender

Following code configures a DatePattern −

# Set the DatePattern
log4j.appender.FILE.DatePattern = '.' yyyy-MM-dd-a

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

driver − Sets the driver class to the specified string. If no driver class is specified, it defaults to sun.jdbc.odbc.JdbcOdbcDriver.

password − Sets the database password.

sql − Specifies the SQL statement to be executed every time a logging event occurs. This could be INSERT, UPDATE, or DELETE.

URL − Sets the JDBC URL.

user − Sets the database user name.