在用springmvc开发项目的时候，在日志管理这一块，我们一般用的都是log4j进行日志管理，但是我们在导入spring相关的jar的时候，都会看到commons-logging.jar包，为什么我们使用log4j的同时还要引入commons-logging.jar包，它们到底是一种什么关系呢?
接下来我们看看commons-logging中的org.apache.commons.logging.Log的源码
/*** Eclipse Class Decompiler plugin, copyright (c) 2016 Chen Chao (cnfree2000@hotmail.com) ***/ package org.apache.commons.logging; public abstract interface Log { public abstract void debug(Object paramObject); public abstract void debug(Object paramObject, Throwable paramThrowable); public abstract void error(Object paramObject); public abstract void error(Object paramObject, Throwable paramThrowable); public abstract void fatal(Object paramObject); public abstract void fatal(Object paramObject, Throwable paramThrowable); public abstract void info(Object paramObject); public abstract void info(Object paramObject, Throwable paramThrowable); public abstract boolean isDebugEnabled(); public abstract boolean isErrorEnabled(); public abstract boolean isFatalEnabled(); public abstract boolean isInfoEnabled(); public abstract boolean isTraceEnabled(); public abstract boolean isWarnEnabled(); public abstract void trace(Object paramObject); public abstract void trace(Object paramObject, Throwable paramThrowable); public abstract void warn(Object paramObject); public abstract void warn(Object paramObject, Throwable paramThrowable); }
很显然，commons-logging中j的log类就是一个接口，只要实现了该接j它口，那么它就是一个logger组件，继续看commons-logging下的org.apache.commons.logging.impl.Log4JLogger源码
/*** Eclipse Class Decompiler plugin, copyright (c) 2016 Chen Chao (cnfree2000@hotmail.com) ***/ package org.apache.commons.logging.impl; import java.io.Serializable; import java.lang.reflect.Field; import org.apache.commons.logging.Log; import org.apache.log4j.Level; import org.apache.log4j.Logger; import org.apache.log4j.Priority; public class Log4JLogger implements Log, Serializable { private static final long serialVersionUID = 5160705895411730424L; private static final String FQCN = Log4JLogger.class.getName(); private volatile transient Logger logger = null; private final String name; private static final Priority traceLevel; public Log4JLogger() { this.name = null; } public Log4JLogger(String name) { this.name = name; this.logger = getLogger(); } public Log4JLogger(Logger logger) { if (logger == null) { throw new IllegalArgumentException( "Warning - null logger in constructor; possible log4j misconfiguration."); } this.name = logger.getName(); this.logger = logger; } public void trace(Object message) { getLogger().log(FQCN, traceLevel, message, null); } public void trace(Object message, Throwable t) { getLogger().log(FQCN, traceLevel, message, t); } public void debug(Object message) { getLogger().log(FQCN, Level.DEBUG, message, null); } public void debug(Object message, Throwable t) { getLogger().log(FQCN, Level.DEBUG, message, t); } public void info(Object message) { getLogger().log(FQCN, Level.INFO, message, null); } public void info(Object message, Throwable t) { getLogger().log(FQCN, Level.INFO, message, t); } public void warn(Object message) { getLogger().log(FQCN, Level.WARN, message, null); } public void warn(Object message, Throwable t) { getLogger().log(FQCN, Level.WARN, message, t); } public void error(Object message) { getLogger().log(FQCN, Level.ERROR, message, null); } public void error(Object message, Throwable t) { getLogger().log(FQCN, Level.ERROR, message, t); } public void fatal(Object message) { getLogger().log(FQCN, Level.FATAL, message, null); } public void fatal(Object message, Throwable t) { getLogger().log(FQCN, Level.FATAL, message, t); } public Logger getLogger() { Logger result = this.logger; if (result == null) { synchronized (this) { result = this.logger; if (result == null) { this.logger = (result = Logger.getLogger(this.name)); } } } return result; } public boolean isDebugEnabled() { return getLogger().isDebugEnabled(); } public boolean isErrorEnabled() { return getLogger().isEnabledFor(Level.ERROR); } public boolean isFatalEnabled() { return getLogger().isEnabledFor(Level.FATAL); } public boolean isInfoEnabled() { return getLogger().isInfoEnabled(); } public boolean isTraceEnabled() { return getLogger().isEnabledFor(traceLevel); } public boolean isWarnEnabled() { return getLogger().isEnabledFor(Level.WARN); } static { if (!(Priority.class.isAssignableFrom(Level.class))) { throw new InstantiationError("Log4J 1.2 not available"); } Priority _traceLevel; try { _traceLevel = (Priority) Level.class.getDeclaredField("TRACE").get( null); } catch (Exception ex) { _traceLevel = Level.DEBUG; } traceLevel = _traceLevel; } }
/*** Eclipse Class Decompiler plugin, copyright (c) 2016 Chen Chao (cnfree2000@hotmail.com) ***/ package org.apache.commons.logging.impl; import java.io.Serializable; import java.util.logging.Level; import java.util.logging.Logger; import org.apache.commons.logging.Log; public class Jdk14Logger implements Log, Serializable { private static final long serialVersionUID = 4784713551416303804L; protected static final Level dummyLevel = Level.FINE; protected transient Logger logger = null; protected String name = null; public Jdk14Logger(String name) { this.name = name; this.logger = getLogger(); } protected void log(Level level, String msg, Throwable ex) { Logger logger = getLogger(); if (!(logger.isLoggable(level))) return; Throwable dummyException = new Throwable(); StackTraceElement[] locations = dummyException.getStackTrace(); String cname = this.name; String method = "unknown"; if ((locations != null) && (locations.length > 2)) { StackTraceElement caller = locations[2]; method = caller.getMethodName(); } if (ex == null) logger.logp(level, cname, method, msg); else logger.logp(level, cname, method, msg, ex); } public void debug(Object message) { log(Level.FINE, String.valueOf(message), null); } public void debug(Object message, Throwable exception) { log(Level.FINE, String.valueOf(message), exception); } public void error(Object message) { log(Level.SEVERE, String.valueOf(message), null); } public void error(Object message, Throwable exception) { log(Level.SEVERE, String.valueOf(message), exception); } public void fatal(Object message) { log(Level.SEVERE, String.valueOf(message), null); } public void fatal(Object message, Throwable exception) { log(Level.SEVERE, String.valueOf(message), exception); } public Logger getLogger() { if (this.logger == null) { this.logger = Logger.getLogger(this.name); } return this.logger; } public void info(Object message) { log(Level.INFO, String.valueOf(message), null); } public void info(Object message, Throwable exception) { log(Level.INFO, String.valueOf(message), exception); } public boolean isDebugEnabled() { return getLogger().isLoggable(Level.FINE); } public boolean isErrorEnabled() { return getLogger().isLoggable(Level.SEVERE); } public boolean isFatalEnabled() { return getLogger().isLoggable(Level.SEVERE); } public boolean isInfoEnabled() { return getLogger().isLoggable(Level.INFO); } public boolean isTraceEnabled() { return getLogger().isLoggable(Level.FINEST); } public boolean isWarnEnabled() { return getLogger().isLoggable(Level.WARNING); } public void trace(Object message) { log(Level.FINEST, String.valueOf(message), null); } public void trace(Object message, Throwable exception) { log(Level.FINEST, String.valueOf(message), exception); } public void warn(Object message) { log(Level.WARNING, String.valueOf(message), null); } public void warn(Object message, Throwable exception) { log(Level.WARNING, String.valueOf(message), exception); } }
我们看到有两个类，一个是Jdk14Logger，另一个是Log4JLogger。到这里我们应该知道，真正记录日志的是Jdk14Logger类和Log4JLogger类。而 commons-logging 把这两个(实际上，在 org.apache.commons.logging.impl 包下，commons-logging 仅仅为我们封装了 log4j 和 sun logger)记录日志的工具重新封装了一遍(Log4JLogger.java 和 Jdk14Logger.java)，可以认为 org.apache.commons.logging.Log 是个傀儡，它只是提供了对外的统一接口。因此我们只要能拿到 org.apache.commons.logging.Log，而不用关注到底使用的是 log4j 还是 sun logger。正如我们经常在项目中这样写：
private static final Log logger = LogFactory.getLog(Run.class);
既然如此，我们的项目有两个jar包，那么我们用的是log4j 还是 Jdk14Logger呢？我们在配置spring的web.xml文件时，经常会有如下配置：
<listener> <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class> </listener>
终于找到了 org.springframework.util.Log4jConfigurer，这正是 log4j 提供给我们的初始化日志的类。至此，我们终于明白了我们系统的的确确使用的是 log4j 的日志工具。
可是问题又来了，org.apache.commons.logging.Log 和 org.apache.log4j.Logger 这两个类，通过包名我们可以发现它们都是 apache 的项目，既然如下，为何要动如此大的动作搞两个东西(指的是 commons-logging 和 log4j)出来呢？事实上，在 sun 开发 logger 前，apache 项目已经开发了功能强大的 log4j 日志工具，并向 sun 推荐将其纳入到 jdk 的一部分，可是 sun 拒绝了 apache 的提议，sun 后来自己开发了一套记录日志的工具。可是现在的开源项目都使用的是 log4j，log4j 已经成了事实上的标准，但由于又有一部分开发者在使用 sun logger，因此 apache 才推出 commons-logging，使得我们不必关注我们正在使用何种日志工具。