log4j的详细使用
一：配置
A:)在Web Application 中使用log4j
Step 1:配置log4j配置文件
=========================
# Set root logger level to DEBUG and its only appender to A1
#log4j中有五级logger
#FATAL0
#ERROR3
#WARN4
#INFO6
#DEBUG7
log4j.rootLogger=DEBUG, A1
# A1 is set to be a ConsoleAppender.
#log4j中Appender有几层
#ConsoleAppender输出到控制台
log4j.appender.A1=org.apache.log4j.ConsoleAppender
# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
#输出格式 具体查看log4j javadoc org.apache.log4j.PatternLayout
#d 时间 ....
log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

================================另外一种配置
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=%5p [%t] (%F:%L) - %m%n
#R 输出到文件
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=%p %t %c - %m%n

Step 2:写启动的Servlet

public class InitServlet extends HttpServlet {

public void init() {
ServletContext sct = getServletContext();
String prefix = sct.getRealPath("/");
//log4j 配置文件存放目录
System.out.println("[....Log4j]: The Root Path: " + prefix);
System.out.println("[....Log4j]: InitServlet init start...");
String file = getInitParameter("log4j");
//log4j 配置文件
if (file != null) {
PropertyConfigurator.configure(prefix + file);
//根据配置初始化log4j
}

System.out.println("[....Log4j]: InitServlet Init Sucess...");

}

public void doGet(HttpServletRequest req, HttpServletResponse res) {

}

public void doPost(HttpServletRequest req, HttpServletResponse res) {

}
}

B:在一般的IDE中使用log4j,因为使用log4j需要做一些初始化，在Web Application
中这一部分初始化是有AppServer 启动是加载的。在IDE中，则我们在使用log4j
之前需要自己配置一下。
如下文件：
public final class Lo4jTest {
private static String CLASS_NAME = "com.bingo.test.Lo4jTest";
private static Logger log = Logger.getLogger(CLASS_NAME);
//在Web Application 中下面的pattern是在log4j的配置文件中配置的。
//现在我们手动配置
private static String pattern="%-4r [%-t] [%p] [%c] [%x] - [%m]%n";
private static String pattern2="%-d{yyyy-MM-dd HH:mm:ss} [%c]-[%p] %m%n";
//设置输出层
//
private static ConsoleAppender consAppender =
new ConsoleAppender(
new PatternLayout(pattern2));
public static void main(String[] args) {
//配置log4j
BasicConfigurator.configure(consAppender);
// void configure(Appender appender);
// void configure();
//配置时log4j提供了两种方式
//后一种比较简单，输出的信息不够详细
//我们可以使用前一种输出我们希望的格式
log.debug("Log4j Debug.");
log.error("Log4j Error.");
log.info("Log4j Info.");
log.fatal("Log4j Fatal.");
}
}
//使用pattern2上面的输出结果如下
2002-08-30 13:49:09 [com.bingo.test.Lo4jTest]-[DEBUG] Log4j Debug.
2002-08-30 13:49:09 [com.bingo.test.Lo4jTest]-[ERROR] Log4j Error.
2002-08-30 13:49:09 [com.bingo.test.Lo4jTest]-[INFO] Log4j Info.
2002-08-30 13:49:09 [com.bingo.test.Lo4jTest]-[FATAL] Log4j Fatal.
//假如我们使用pattern输出如下:
0 [main] [DEBUG] [com.bingo.test.Lo4jTest] [] - [Log4j Debug.]
0 [main] [ERROR] [com.bingo.test.Lo4jTest] [] - [Log4j Error.]
0 [main] [INFO] [com.bingo.test.Lo4jTest] [] - [Log4j Info.]
0 [main] [FATAL] [com.bingo.test.Lo4jTest] [] - [Log4j Fatal.