Log4jConfigListener动态改变记录级别及实现
线上的系统出现了bug，可能是请求的数据出现了问题，这个时候，日志就为我们提供了解决问题的办法。但是线上的产品系统，一般的优先级都在INFO之上，如果修日日志级别，获取丰富的信息，可能需要重启服务，对线上的影响比较大。如何能做到 动态的修改日志的级别，而且不用重启服务，对线上环境的影响减少到最小呢？Log4jConfigListener就上场了
之前就听说有这么个功能，一直没有用上，这次线上产品出现了bug了，就趁这个机会使用下。
Log4jConfigListener在spring-web中，需要添加maven的依赖,在pom中添加
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
<version>${spring.version}</version>
</dependency>
在web.xml中配置
<context-param> <param-name>log4jConfigLocation</param-name> <param-value>classpath:log4j.xml</param-value> </context-param> <context-param> <param-name>log4jRefreshInterval</param-name> <param-value>60000</param-value> </context-param> <listener> <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class> </listener>
这样几配置好了，可以部署到服务器上去了。平时根据项目的需求配置日志的输出级别，如果想动态修改日志级别，只需要修改log4j.xml就可以了。
那么，Log4jConfigListener做了什么，可以知道文件变化了并加以应用，难道是起了个线程来做的？
让我们看看源码吧，首先看下Log4jConfigListener
public class Log4jConfigListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent event) {
Log4jWebConfigurer.initLogging(event.getServletContext());
}
public void contextDestroyed(ServletContextEvent event) {
Log4jWebConfigurer.shutdownLogging(event.getServletContext());
}
}
这里Log4jConfigListener使用了Log4jWebConfigure,让我们继续
public static void initLogging(ServletContext servletContext) {
// Expose the web app root system property.
if (exposeWebAppRoot(servletContext)) {
WebUtils.setWebAppRootSystemProperty(servletContext);
}
// Only perform custom log4j initialization in case of a config file.
String location = servletContext.getInitParameter(CONFIG_LOCATION_PARAM);
if (location != null) {
// Perform actual log4j initialization; else rely on log4j's default initialization.
try {
// Resolve system property placeholders before potentially
// resolving a real path.
location = SystemPropertyUtils.resolvePlaceholders(location);
// Leave a URL (e.g. "classpath:" or "file:") as-is.
if (!ResourceUtils.isUrl(location)) {
// Consider a plain file path as relative to the web
// application root directory.
location = WebUtils.getRealPath(servletContext, location);
}
// Write log message to server log.
servletContext.log("Initializing log4j from [" + location + "]");
// Check whether refresh interval was specified.
String intervalString = servletContext.getInitParameter(REFRESH_INTERVAL_PARAM);
if (intervalString != null) {
// Initialize with refresh interval, i.e. with log4j's watchdog thread,
// checking the file in the background.
try {
long refreshInterval = Long.parseLong(intervalString);
Log4jConfigurer.initLogging(location, refreshInterval);
}
catch (NumberFormatException ex) {
throw new IllegalArgumentException("Invalid 'log4jRefreshInterval' parameter: " + ex.getMessage());
}
}
else {
// Initialize without refresh check, i.e. without log4j's watchdog thread.
Log4jConfigurer.initLogging(location);
}
}
catch (FileNotFoundException ex) {
throw new IllegalArgumentException("Invalid 'log4jConfigLocation' parameter: " + ex.getMessage());
}
}
}
这里有几行代码需要是重点，
String intervalString = servletContext.getInitParameter(REFRESH_INTERVAL_PARAM); Log4jConfigurer.initLogging(location, refreshInterval);
那Log4jConfigure.initLogging有干了啥呢？
public static void initLogging(String location, long refreshInterval) throws FileNotFoundException {
String resolvedLocation = SystemPropertyUtils.resolvePlaceholders(location);
File file = ResourceUtils.getFile(resolvedLocation);
if (!file.exists()) {
throw new FileNotFoundException("Log4j config file [" + resolvedLocation + "] not found");
}
if (resolvedLocation.toLowerCase().endsWith(XML_FILE_EXTENSION)) {
DOMConfigurator.configureAndWatch(file.getAbsolutePath(), refreshInterval);
}
else {
PropertyConfigurator.configureAndWatch(file.getAbsolutePath(), refreshInterval);
}
}
获取配置文件，根据log4配置文件的格式(xml，properties)方式进行加载xml，那么一定是在DOMConfigurator.configureAndWatch 或者PropertyConfigurator.configureAndWatch里面有个线程在做幕后工作，由于LZ
采用的是XML格式的配置文件,那就看下DOMConfigurator.configureAndWatch，看看它到底怎么实现的吧。
static
public
void configureAndWatch(String configFilename, long delay) {
XMLWatchdog xdog = new XMLWatchdog(configFilename);
xdog.setDelay(delay);
xdog.start();
}
XMLWatchdog，这是个WatchDog，哈哈，有啥动静，自然躲不过watchDog的眼睛，还有start方法，看起来应该是Thread类，让我们看看WatchDog的真面目吧。
class XMLWatchdog extends FileWatchdog {
XMLWatchdog(String filename) {
super(filename);
}
/**
Call {@link DOMConfigurator#configure(String)} with the
<code>filename</code> to reconfigure log4j. */
public
void doOnChange() {
new DOMConfigurator().doConfigure(filename,
LogManager.getLoggerRepository());
}
}
FileWatchDog
public abstract class FileWatchdog extends Thread{
.......
abstract
protected
void doOnChange();
public void run() {
while(!interrupted) {
try {
Thread.sleep(delay);
} catch(InterruptedException e) {
// no interruption expected
}
checkAndConfigure();
}
}
protected void checkAndConfigure() {
........
if(fileExists) {
long l = file.lastModified(); // this can also throw a SecurityException
if(l > lastModif) { // however, if we reached this point this
lastModif = l; // is very unlikely.
doOnChange();
warnedAlready = false;
}
} else {
if(!warnedAlready) {
LogLog.debug("["+filename+"] does not exist.");
warnedAlready = true;
}
}
}
}
FileWatchDog有个抽象方法，doOnChange，就是对文件变化后的响应，抽象方法的定义，为子类的扩展提供了可能。
我们看到，Log4jConfirgureListener也就是通过线程的方式扫描log4j.xml，当发现log4j的配置文件发生变化后就作出响应，从而做到了不重启应用修改日志的输出级别。
通过阅读源码，我们更清楚的知道web.xml中的配置参数
log4jRefreshInterval的时间单位是MS
如果你有空，不妨阅读下源码，这样更有收获。
由于本人水平有限，如果不对的地方或需要补充的地方，请您指出。如果您是大牛，可以忽略本文。
谢谢