关于log4cpp

1. 基本概念

categories 日志的层级体系
appenders 日志打印到什么地方，可以是文件，也可以是终端
layouts 日志输出格式，定义类似于printf中的输出方式。有三种，后面都有例子，这三种分别是：basic，simple，pattern
priority 日志的级别，所有的级别有：EMERG、FATAL、ALERT、CRIT、ERROR、WARN、NOTICE、INFO、DEBUG、NOTSET，其中NOTSET < DEBUG < INFO < NOTICE < WARN < ERROR < CRIT < ALERT < FATAL = EMERG。这个优先级的含义是：配置文件中设置了级别是debug，则任意的log都能打出来；如果配置了级别是fatal，则只有高于他优先级的日志才可以打印出来
additivity 限制appender会不会被继承(后面还有补充说明)

2. 程序举例

#include <stdio.h>

#include <log4cpp/Category.hh>

#include <log4cpp/FileAppender.hh>

#include <log4cpp/SimpleLayout.hh>

#define LOGFILE "./test.log"

int main() {

/*Setting up Appender, layout and Category*/

log4cpp::Appender *appender = new log4cpp::FileAppender("FileAppender",LOGFILE);//第一个参数是appender的名字，第二个是log文件的名字

log4cpp::Layout *layout = new log4cpp::SimpleLayout();

//log4cpp::Layout *layout = new log4cpp::BasicLayout();

log4cpp::Category& category = log4cpp::Category::getInstance("abc");

appender->setLayout(layout);

category.setAppender(appender);

category.setPriority(log4cpp::Priority::INFO);

/*The actual logging*/

category.info("This is for tracing the flow");

category.notice("This is to notify certain events");

category.warn("This is to generate certain warnings");

}
以上程序将得到下面的log输出：

INFO : This is for tracing the flow

NOTICE : This is to notify certain events
WARN : This is to generate certain warnings
这个程序里面可以把 log4cpp::Layout *layout = new log4cpp::BasicLayout(); 的注释取消，注释掉上面的语句。将得到下面的log输出：

1308806376 INFO abc : This is for tracing the flow
1308806376 NOTICE abc : This is to notify certain events

1308806376 WARN abc : This is to generate certain warnings
两者的区别就在于使用了不同的layout，一个是simple一个是basic的。
要做到复杂格式的输出就要使用第三种，即：PatternLayout，下面借助配置文件来说明这种方式的使用：

3. 配置文件说明

配置文件log4cpp.conf：

# 定义了3个category sub1, sub2, sub3

# 其中sub2和sub3设置了additivity属性为false;sub1的additivity属性默认为true

rootCategory=DEBUG, rootAppender

category.sub1=,A1

category.sub2=INFO, A2

additivity.sub2=false

category.sub3=ERROR, A3

additivity.sub3=false

# 定义rootAppender类型和layout属性，这里使用了BasicLayout

appender.rootAppender=org.apache.log4cpp.ConsoleAppender

appender.rootAppender.layout=org.apache.log4cpp.
BasicLayout

#定义A1的属性，

这里使用了

SimpleLayout

appender.A1=org.apache.log4cpp.FileAppender

appender.A1.fileName=./log/A1.log

appender.A1.layout=org.apache.log4cpp.SimpleLayout

#定义A2的属性，这里使用了

PatternLayout

appender.A2=org.apache.log4cpp.ConsoleAppender

appender.A2.layout=org.apache.log4cpp.PatternLayout

appender.A2.layout.ConversionPattern=The message '%m' at time %d%n

#定义A3的属性

appender.A3=org.apache.log4cpp.RollingFileAppender

appender.A3.fileName=./log/A3.log

appender.A3.maxFileSize=50

appender.A3.maxBackupIndex=3

appender.A3.backupPattern=%Y-%m-%d

appender.A3.layout=org.apache.log4cpp.PatternLayout

appender.A3.layout.ConversionPattern=%d{%Y-%m-%d %H:%M:%S} [%p]: [%c] %m%n

4. 带配置文件的程序举例

源代码：

#include "log4cpp/Category.hh"

#include "log4cpp/PropertyConfigurator.hh"

int main(int argc, char* argv[]) {

// 1 读取解析配置文件

// 读取出错, 完全可以忽略，可以定义一个缺省策略或者使用系统缺省策略

// BasicLayout输出所有优先级日志到ConsoleAppender

try

{

log4cpp::PropertyConfigurator::configure("./log4cpp.conf");

}

catch (log4cpp::ConfigureFailure& f)

{

std::cout << "Configure Problem " << f.what() << std::endl;

return -1;

}

//2 实例化category对象

// 这些对象即使配置文件没有定义也可以使用，不过其属性继承其父category

// 通常使用引用可能不太方便，可以使用指针，以后做指针使用

log4cpp::Category& root = log4cpp::Category::getRoot();

log4cpp::Category& sub1 = log4cpp::Category::getInstance(std::string("sub1"));

log4cpp::Category& sub2 = log4cpp::Category::getInstance(std::string("sub2"));

log4cpp::Category& sub3 = log4cpp::Category::getInstance(std::string("sub3"));

log4cpp::Category& sub4 = log4cpp::Category::getInstance(std::string("sub4"));

// 正常使用这些category对象进行日志处理。

root.fatal("root's log");

// sub1 has appender A1 and rootappender. since the additivity property is set true by default

sub1.info("sub1's log");

// sub2 has appender A2 appender. since the additivity property is set to false

sub2.alert("sub2's log");

// sub3 only has A3 appender. since the additivity property is set to false

sub3.debug("sub3's log");

sub3.alert("sub3's log");

// sub4 can not be found in the config file, so the root category's appender and layout are used

sub4.warn("sub4's log");

return 0;

}

终端打印出来的log：

1308828470 FATAL : root's log
1308828470 INFO sub1 : sub1's log
The message 'sub2's log' at time 2011-06-23 19:27:50,624
1308829427 WARN sub4 : sub4's log
log/A1.log的内容：
INFO : sub1's log
log/A3.log的内容：
2011-06-23 19:27:50 [ALERT]: [sub3] sub3's log

5. 配置参考

Appender Additivity
The output of a log statement of logger C will go to all the appenders in C and its ancestors. This is the meaning of the term "appender additivity".
However, if an ancestor of logger C, say P, has the additivity flag set to false, then C's output will be directed to all the appenders in C and its ancestors upto and including P but not the appenders in
any of the ancestors of P.

Loggers have their additivity flag set to true by default.
参考来自：http://logging.apache.org/log4j/1.2/manual.html

Sets the format of log lines handled by this PatternLayout.
By default, set to "%m%n".

Format characters are as follows:

%%%% - a single percent sign
%c - the category
%d - the date

Date format: The date format character may be followed by a date format specifier enclosed between braces. For example, %d{%H:%M:%S,%l} or %d{%d %m %Y %H:%M:%S,%l}. If no date format specifier is given then the following format is used: "Wed Jan 02 02:03:55
1980". The date format specifier admits the same syntax as the ANSI C function strftime, with 1 addition. The addition is the specifier %l for milliseconds, padded with zeros to make 3 digits.
%m - the message
%n - the platform specific line separator
%p - the priority
%r - milliseconds since this layout was created.
%R - seconds since Jan 1, 1970
%u - clock ticks since process start
%x - the NDC

参考来自：http://log4cpp.sourceforge.net/api/classlog4cpp_1_1PatternLayout.html