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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.
Log4j Stdout Appender
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.
Log4j is a very popular logging library for Java development.It provides convenient and flexible logging mechanism as well as fast performance. Putting log4j into your code is pretty easy as it requires minimum of work which mostly involves in configuring the logging system. RequiresLayout boolean requiresLayout Configurators call this method to determine if the appender requires a layout. If this method returns true, meaning that layout is required, then the configurator will configure an layout using the configuration information at its disposal.
PatternLayout is also a simple Layout object that provides the following-Bean Property which can be set using the configuration file:
Sr.No. | Property & Description |
---|---|
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Pattern Conversion Characters
The following table explains the characters used in the above pattern and all other characters that you can use in your custom pattern:
Conversion Character | Meaning |
---|---|
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. |
Format Modifiers
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.
Following table covers various modifiers scenarios:
Format modifier | left justify | minimum width | maximum width | comment |
---|---|---|---|---|
%20c | false | 20 | none | Left pad with spaces if the category name is less than 20 characters long. |
%-20c | true | 20 | none | Right pad with spaces if the category name is less than 20 characters long. |
%.30c | NA | none | 30 | Truncate from the beginning if the category name is longer than 30 characters. |
%20.30c | false | 20 | 30 | 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 | true | 20 | 30 | 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. |
PatternLayout Example
Following is a simple configuration file for PatternLayout:
Now consider the following Java Example which would generate logging information:
Compile and run the above program. It would create a log.out file in /usr/home/log4j directory which would have the following log information:
log4j_log_formatting.htm
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The log4j API provides the org.apache.log4j.jdbc.JDBCAppender object, which can put logging information in a specified database.
JDBCAppender Configuration
Property | Description |
---|---|
bufferSize | Sets the buffer size. Default size is 1. |
driver | Sets the driver class to the specified string. If no driver class is specified, it defaults to sun.jdbc.odbc.JdbcOdbcDriver. |
layout | Sets the layout to be used. Default layout is org.apache.log4j.PatternLayout. |
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. |
Log Table Configuration
Before you start using JDBC based logging, you should create a table to maintain all the log information. Following is the SQL Statement for creating the LOGS table −
Sample Configuration File
Following is a sample configuration file log4j.properties for JDBCAppender which will is be used to log messages to a LOGS table.
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For MySQL database, you would have to use the actual DBNAME, user ID and password, where you have created LOGS table. The SQL statement is to execute an INSERT statement with the table name LOGS and the values to be entered into the table.
JDBCAppender does not need a layout to be defined explicitly. Instead, the SQL statement passed to it uses a PatternLayout.
If you wish to have an XML configuration file equivalent to the above log4j.properties file, then here is the content −
Sample Program
The following Java class is a very simple example that initializes and then uses the Log4J logging library for Java applications.
Compile and Execute
Here are the steps to compile and run the above-mentioned program. Make sure you have set PATH and CLASSPATH appropriately before proceeding for compilation and execution.
All the libraries should be available in CLASSPATH and your log4j.properties file should be available in PATH. Follow the given steps −
- Create log4j.properties as shown above.
- Create log4jExample.java as shown above and compile it.
- Execute log4jExample binary to run the program.
Now check your LOGS table inside DBNAME database and you would find the following entries −
Note − Here x is used to output the Nested diagnostic Context (NDC) associated with the thread that generated the logging event. We use NDC to distinguish clients in server-side components handling multiple clients. Check Log4J Manual for more information on this.