Spring搭建：log4j的配置

在spring搭建项目时，一般都是需要使用log4j。此时，需要在web.xml中配置好log4j，或者log4j.xml中进行路径配置，此时就需要进行log4j的配置了解。

web.xml中的配置

在spring中，使用log4j时，主要2点。一是web.xml中配置log4j；二是项目的webapp下配置log4j.xml。首先是web.xml中配置。如下：

   <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>

web.xml最前面写入以上的配置就可以了。
参数：log4jConfigLocation主要是配置log4j.xml的文件位置。可以指定方式如下：
1.类路径，例如:classpath:log4j.xml；
2.绝对文件路径，例如:d:/log4j.xml；
3.如果不指定，默认采取class路径中的”log4j.properties” 或 “log4j.xml”
参数：log4jRefreshInterval主要检测log4j配置文件刷新时间，动态修改log4j配置。
指定参数：配置文件刷新时间（单位是毫秒）
不指定参数：不再检查log4j配置文件有没有发生，同时log4j的监督线程将不会启动。

注意点：
log4j的listener配置要优先于spring的ContextLoaderListener配置。

log4j.xml中的路径问题

在log4j.xml中配置文件时，可以如下配置：

log4j.appender.myfile.File=${webapp.root}/WEB-INF/demo.log

将路径指定为动态的{webapp.root}，它会在项目运行时，进行替换操作，它允许日志文件路径相对于web应用程序的根目录。
当需要使用多个webapp.root时，或者需要换个名字时，可以在web.xml中如下配置：

   <context-param><param-name>webAppRootKey</param-name><param-value>demo.root</param-value></context-param>

如此，log4j中就可以采取如下方式指定了：

log4j.appender.myfile.File=${demo.root}/WEB-INF/demo.log

Log4jConfigListener说明

采取Log4jConfigListener配置时，它的源码如下：

/*** <p>This listener should be registered before ContextLoaderListener in {@code web.xml}* when using custom log4j initialization.*/
public class Log4jConfigListener implements ServletContextListener {public Log4jConfigListener() {}public void contextInitialized(ServletContextEvent event) {Log4jWebConfigurer.initLogging(event.getServletContext());}public void contextDestroyed(ServletContextEvent event) {Log4jWebConfigurer.shutdownLogging(event.getServletContext());}
}

主要使用了Log4jWebConfigurer进行初始化等操作。类说明了，应该在ContextLoaderListener 之前进行初始化操作。其余介绍可参考源码。

Log4jWebConfigurer说明

Log4jWebConfigurer简化源码如下所示：

public abstract class Log4jWebConfigurer {/** Parameter specifying the location of the log4j config file */public static final String CONFIG_LOCATION_PARAM = "log4jConfigLocation";/** Parameter specifying the refresh interval for checking the log4j config file */public static final String REFRESH_INTERVAL_PARAM = "log4jRefreshInterval";/** Parameter specifying whether to expose the web app root system property */public static final String EXPOSE_WEB_APP_ROOT_PARAM = "log4jExposeWebAppRoot";/*** Initialize log4j, including setting the web app root system property.* @param servletContext the current ServletContext* @see WebUtils#setWebAppRootSystemProperty*/public static void initLogging(ServletContext servletContext) {.....}/*** Shut down log4j, properly releasing all file locks* and resetting the web app root system property.* @param servletContext the current ServletContext* @see WebUtils#removeWebAppRootSystemProperty*/public static void shutdownLogging(ServletContext servletContext) {.......}/*** Return whether to expose the web app root system property,* checking the corresponding ServletContext init parameter.* @see #EXPOSE_WEB_APP_ROOT_PARAM*/private static boolean exposeWebAppRoot(ServletContext servletContext) {String exposeWebAppRootParam = servletContext.getInitParameter(EXPOSE_WEB_APP_ROOT_PARAM);return (exposeWebAppRootParam == null || Boolean.valueOf(exposeWebAppRootParam));}}

前面的三个参数就是我们在web.xml中进行配置的参数，后面的2个方法是log4j的初始化方法。

log4jConfigLocation说明：

    /** Parameter specifying the location of the log4j config file */public static final String CONFIG_LOCATION_PARAM = "log4jConfigLocation";
/*  * Location of the log4j config file; either a "classpath:" location (e.g.* "classpath:myLog4j.properties"), an absolute file URL (e.g. "file:C:/log4j.properties),* or a plain path relative to the web application root directory (e.g.* "/WEB-INF/log4j.properties"). If not specified, default log4j initialization* will apply ("log4j.properties" or "log4j.xml" in the class path; see the* log4j documentation for details).*/

log4jRefreshInterval说明：

/** Parameter specifying the refresh interval for checking the log4j config file */public static final String REFRESH_INTERVAL_PARAM = "log4jRefreshInterval";/** Interval between config file refresh checks, in milliseconds. If not specified,* no refresh checks will happen, which avoids starting log4j's watchdog thread.*/

log4jExposeWebAppRoot说明：

    /** Parameter specifying whether to expose the web app root system property */public static final String EXPOSE_WEB_APP_ROOT_PARAM = "log4jExposeWebAppRoot";/*** Whether the web app root system property should be exposed, allowing for log* file paths relative to the web application root directory. Default is "true";* specify "false" to suppress expose of the web app root system property. See* below for details on how to use this system property in log file locations.*/

这个参数一般不需要配置，默认是”true”，是否暴漏log4j配置。在 Log4jWebConfigurer的initLogging(ServletContext servletContext)中会去检查，如下所示：

public static void initLogging(ServletContext servletContext) {// Expose the web app root system property.if (exposeWebAppRoot(servletContext)) {WebUtils.setWebAppRootSystemProperty(servletContext);}.......
}

setWebAppRootSystemProperty方法就说明了webapp的设置和webAppRootKey的设置问题。

    /*** Web app root key parameter at the servlet context level* (i.e. a context-param in {@code web.xml}): "webAppRootKey".*/public static final String WEB_APP_ROOT_KEY_PARAM = "webAppRootKey";/** Default web app root key: "webapp.root" */public static final String DEFAULT_WEB_APP_ROOT_KEY = "webapp.root";/*** Set a system property to the web application root directory.* The key of the system property can be defined with the "webAppRootKey"* context-param in {@code web.xml}. Default is "webapp.root".* <p>Can be used for tools that support substition with {@code System.getProperty}* values, like log4j's "${key}" syntax within log file locations.* @param servletContext the servlet context of the web application* @throws IllegalStateException if the system property is already set,* or if the WAR file is not expanded* @see #WEB_APP_ROOT_KEY_PARAM* @see #DEFAULT_WEB_APP_ROOT_KEY* @see WebAppRootListener* @see Log4jWebConfigurer*/public static void setWebAppRootSystemProperty(ServletContext servletContext) throws IllegalStateException {Assert.notNull(servletContext, "ServletContext must not be null");String root = servletContext.getRealPath("/");if (root == null) {throw new IllegalStateException("Cannot set web app root system property when WAR file is not expanded");}String param = servletContext.getInitParameter(WEB_APP_ROOT_KEY_PARAM);String key = (param != null ? param : DEFAULT_WEB_APP_ROOT_KEY);String oldValue = System.getProperty(key);if (oldValue != null && !StringUtils.pathEquals(oldValue, root)) {throw new IllegalStateException("Web app root system property already set to different value: '" +key + "' = [" + oldValue + "] instead of [" + root + "] - " +"Choose unique values for the 'webAppRootKey' context-param in your web.xml files!");}System.setProperty(key, root);servletContext.log("Set web app root system property: '" + key + "' = [" + root + "]");}

log4j的init方法如下：

/*** Initialize log4j, including setting the web app root system property.* @param servletContext the current ServletContext* @see WebUtils#setWebAppRootSystemProperty*/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 property placeholders before potentially resolving a real path.location = ServletContextPropertyUtils.resolvePlaceholders(location, servletContext);// 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());}}}

主要：1.检查是否暴漏；2.读取log4j文件；3.替换log4j文件的占位符；4.读取刷新时间，初始化log4j。最终调用Log4jConfigurer方法。

shutdownLogging主要就是关闭log4j了。

    /*** Shut down log4j, properly releasing all file locks* and resetting the web app root system property.* @param servletContext the current ServletContext* @see WebUtils#removeWebAppRootSystemProperty*/public static void shutdownLogging(ServletContext servletContext) {servletContext.log("Shutting down log4j");try {Log4jConfigurer.shutdownLogging();}finally {// Remove the web app root system property.if (exposeWebAppRoot(servletContext)) {WebUtils.removeWebAppRootSystemProperty(servletContext);}}}

以上基本介绍了log4j，具体的实现待考虑下。

参考文档：官方文档—https://docs.spring.io/spring/docs/2.5.x/javadoc-api/org/springframework/web/util/Log4jWebConfigurer.html