Java日志实战及解析

log4j.rootLogger=debug, stdout,R

log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

# Pattern to output the caller’s file name and line number.

log4j.appender.stdout.layout.ConversionPattern=%-4r[%t]
%5p %c
%x –%m
– %X{userName}%n

log4j.appender.R=org.apache.log4j.RollingFileAppender

log4j.appender.R.File=example.log

log4j.appender.R.MaxFileSize=100KB

# Keep one backup file

log4j.appender.R.MaxBackupIndex=1

log4j.appender.R.layout=org.apache.log4j.PatternLayout

log4j.appender.R.layout.ConversionPattern=%-4r[%t]
%5p %c
%x –%m
– %X{userName}%n

Conversion Character

Effect

c

Used to output the category of the logging event. The category conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets.

If a precision specifier is given, then only the corresponding number of right most components of the category name will be printed. By default the category name is printed in full.

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. This conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets.

If a precision specifier is given, then only the corresponding number of right most components of the class name will be printed. By default the class name is output in fully qualified form.

For example, for the class name “org.apache.xyz.SomeClass”, the pattern %C{1} will output “SomeClass”.

WARNING Generating the caller class information is slow. Thus, use should be avoided unless execution speed is not an issue.

d

Used to output the date of the logging event. The date conversion specifier may be followed by a date format specifier enclosed between braces. For example,%d{HH:mm:ss,SSS} or %d{dd MMM yyyy HH:mm:ss,SSS}.
If no date format specifier is given then ISO8601 format is assumed.

The date format specifier admits the same syntax as the time pattern string of the SimpleDateFormat.
Although part of the standard JDK, the performance ofSimpleDateFormat is quite poor.

For better results it is recommended to use the log4j date formatters. These can be specified using one of the strings “ABSOLUTE”, “DATE” and “ISO8601” for specifyingAbsoluteTimeDateFormat, DateTimeDateFormat and
respectively ISO8601DateFormat. For example, %d{ISO8601} or %d{ABSOLUTE}.

These dedicated date formatters perform significantly better than SimpleDateFormat.

F

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

WARNING Generating caller location information is extremely slow and should be avoided unless execution speed is not an issue.

l

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

The location information depends on the JVM implementation but usually consists of the fully qualified name of the calling method followed by the callers source the file name and line number between parentheses.

The location information can be very useful. However, its generation is extremely slow and should be avoided unless execution speed is not an issue.

L

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

WARNING Generating caller location information is extremely slow and should be avoided unless execution speed is not an issue.

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.

WARNING Generating caller location information is extremely slow and should be avoided unless execution speed is not an issue.

n

Outputs the platform dependent line separator character or characters.

This conversion character offers practically the same performance as using non-portable line separator strings such as ”
“, or ”
“. Thus, it is the preferred way of specifying a line separator.

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

Used to output the MDC (mapped diagnostic context) associated with the thread that generated the logging event. The X conversion character must be followed by the key for the map placed between braces, as in %X{clientNumber} where clientNumber is
the key. The value in the MDC corresponding to the key will be output.

See MDC class for more details.

%

The sequence %% outputs a single percent sign.

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 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.

       <dependency>

           <groupId>log4j</groupId>

           <artifactId>log4j</artifactId>

           <version>1.2.8</version>

       </dependency>

示例：

logger.debug(“Entry number: ” + i + “ is ” + String.valueOf(entry[i]));

if(logger.isDebugEnabled() {

    logger.debug(“Entry number: ” + i + “ is ” + String.valueOf(entry[i]));

}

判断只是记录日志的1%时间，所以一般情况下是值得的。

但是一般使用SLF4J就自动集成该功能。

•       <dependency>

•       <groupId>org.slf4j</groupId>

•       <artifactId>slf4j-api</artifactId>

•       <version>1.7.21</version>

•       </dependency>

•       Logger logger = LoggerFactory.getLogger(HelloWorld.class);

•       logger.info(“Hello World”);