Java日志实战及解析
日志是程序员必须掌握的基础技能之一，如果您写的软件没有日志，可以说你没有成为一个真正意义上的程序员。
为什么要记日志？
• 监控代码
• 变量变化情况，系统运行过程。
• 产线环境，不太好调试。
• 分布式环境下，调试更困难，日志就是非常好的帮手。
• 统计分析
• 日后审计
• 实际中有4%的代码是日志！
Java日志框架主要有log4j，logback，及其他不常用的官方日志及apachelogging等。
Log4j和LogBack的原作者为同一作者CekiGülcü。主流使用的一般是log4j的居多点，所以本文主要也讲解log4j为主。
配置文件log4j.properites文件，一般放倒classpath目录下即可，无需自启。
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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 |
log4j.rootLogger=debug, stdout,R
log4j.rootLogger=[Level], Appender1, Appender2
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.Appender1=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.Appender1.layout=org.apache.log4j.PatternLayout
其他类似
基本是三大主件的配置：
Logger
日志类型和日志级别（TRACE < DEBUG < INFO < WARN <ERROR < FATAL）
Appenders
输出到哪里，可以有多个目的地（ Console，File，GUI 组件，remote socket servers，JMS，NT Event Loggers，remote Unix Syslog daemons。也可以异步）
Appenders的一般Console（ConsoleAppender）用来调试，File有每天更新（DailyRollingFileAppender）和最大多大文件（RollingFileAppender）滚动的设置，也有邮件的告警设置。基本理解上面的即可。
Layouts
%r [%t] %-5p %c - %m%n
176 [main] INFO org.foo.Bar - Located nearest gasstation.
Layouts说明
|
Conversion Character |
Effect |
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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". |
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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. |
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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. |
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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. |
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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. |
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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. |
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m |
Used to output the application supplied message associated with the logging event. |
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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. |
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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. |
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p |
Used to output the priority of the logging event. |
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r |
Used to output the number of milliseconds elapsed from the construction of the layout until the creation of the logging event. |
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t |
Used to output the name of the thread that generated the logging event. |
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x |
Used to output the NDC (nested diagnostic context) associated with the thread that generated the logging event. |
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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. |
更多说明：
|
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. |
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%-20c |
true |
20 |
none |
Right pad with spaces if the category name is less than 20 characters long. |
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%.30c |
NA |
none |
30 |
Truncate from the beginning if the category name is longer than 30 characters. |
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%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. |
Maven引用
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<dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.8</version> </dependency> |
个人认为必须了解和掌握的日志特性：
日志级别
继承
JMX管理
Appenders的Additivity
其他的需要了解
Filter
NDC
MDC
日志调优
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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。 |
在了解Log4j之后，也必须了解一下SLF4j，大家一般是使用这个进行整合内部不同日志的具体实现的。
Maven使用
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• <dependency> • <groupId>org.slf4j</groupId> • <artifactId>slf4j-api</artifactId> • <version>1.7.21</version> • </dependency> |
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• Logger logger = LoggerFactory.getLogger(HelloWorld.class); • logger.info("Hello World"); |
常用记录日志的地方：
• 方法入口
• 方法出口
• 异常
• 自己需要跟踪的信息
• 距离日志最近的地方记录日志
• 先记录日志，后抛异常，可以把异常往上抛
• 去除冗余日志
日志库的使用情况
日志一般会使用即可。掌握本课程基本够用，剩下来就看你自己了。
著名的solr使用日志的情况
一般大企业，会收集日志，使用ELK等技术查看日志，或sink到hadoop进行数据分析和挖掘，甚至使用storm进行实时统计。
更多内容可以参考视频：
CSDN学 院： http://edu.csdn.net/course/detail/2890
网易云课堂： http://study.163.com/course/introduction/1003149011.htm