摘要: 線上的系統出現了bug,可能是請求的數據出現了問題,這個時候,日誌就爲我們提供瞭解決問題的辦法。但是線上的產品系統,一般的優先級都在INFO之上,如果修日日誌級別,獲取豐富的信息,可能需要重啓服務,對線上的影響比較大。如何能做到 動態的修改日誌的級別,而且不用重啓服務,對線上環境的影響減少到最小呢?Log4jConfigListener就上場了
之前就聽說有這麼個功能,一直沒有用上,這次線上產品出現了bug了,就趁這個機會使用下。
Log4jConfigListener在spring-web中,需要添加maven的依賴,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>
這樣幾配置好了,可以部署到服務器上去了。平時根據項目的需求配置日誌的輸出級別,如果想動態修改日誌級別,只需要修改log4j.xml就可以了。
那麼,Log4jConfigListener做了什麼,可以知道文件變化了並加以應用,難道是起了個線程來做的?
讓我們看看源碼吧,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());
}
}
這裏Log4jConfigListener使用了Log4jWebConfigure,讓我們繼續
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());
}
}
}
這裏有幾行代碼需要是重點,
String intervalString = servletContext.getInitParameter(REFRESH_INTERVAL_PARAM);
Log4jConfigurer.initLogging(location, refreshInterval);
那Log4jConfigure.initLogging有幹了啥呢?
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);
}
}
獲取配置文件,根據log4配置文件的格式(xml,properties)方式進行加載xml,那麼一定是在DOMConfigurator.configureAndWatch 或者PropertyConfigurator.configureAndWatch裏面有個線程在做幕後工作,由於LZ
採用的是XML格式的配置文件,那就看下DOMConfigurator.configureAndWatch,看看它到底怎麼實現的吧。
public static void configureAndWatch(String configFilename, long delay) {
XMLWatchdog xdog = new XMLWatchdog(configFilename);
xdog.setDelay(delay);
xdog.start();
}
XMLWatchdog,這是個WatchDog,哈哈,有啥動靜,自然躲不過watchDog的眼睛,還有start方法,看起來應該是Thread類,讓我們看看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有個抽象方法,doOnChange,就是對文件變化後的響應,抽象方法的定義,爲子類的擴展提供了可能。
我們看到,Log4jConfirgureListener也就是通過線程的方式掃描log4j.xml,當發現log4j的配置文件發生變化後就作出響應,從而做到了不重啓應用修改日誌的輸出級別。
通過閱讀源碼,我們更清楚的知道web.xml中的配置參數
log4jRefreshInterval的時間單位是MS
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