一、slf4j简介
slf4j(Simple logging facade for Java)是对所有日志框架制定的一种规范、标准、接口，并不是一个框架的具体的实现，因为接口并不能独立使用，需要和具体的日志框架实现配合使用
slf4j是门面模式的典型应用，外部与一个子系统的通信必须通过一个统一的外观对象进行，使得子系统更易于使用。用一张图来表示门面模式的结构为
日志实现(log4j、logback、log4j2)
- log4j是apache实现的一个开源日志组件
- logback同样是由log4j的作者设计完成的，拥有更好的特性，用来取代log4j的一个日志框架，是slf4j的原生实现
- log4j2是log4j 1.x和logback的改进版，据说采用了一些新技术（无锁异步、等等），使得日志的吞吐量、性能比log4j 1.x提高10倍，并解决了一些死锁的bug，而且配置更加简单灵活。
为什么要使用slf4j
- SLF4J提供了统一的记录日志的接口，对不同日志系统的具体实现进行了抽象化，只要按照其提供的方法记录即可，最终日志的格式、记录级别、输出方式等通过绑定具体的日志系统来实现。在项目中使用了SLF4J记录日志，并且绑定了log4j，则日志会以log4j的风格输出；后期需要改为以logback的风格输出日志，只需要将jar包log4j替换成logback即可，不用修改项目中的代码。
- SLF4J支持
{}作为占位符，等价于C语言中的%s，而不必再进行字符串的拼接，效率有显著的提升
maven jar包
<dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.7.21</version> </dependency>
如下图所示，slf4j不能单独使用，比如要使用logback实现，需要导入logback的jar包
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.21</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.1.7</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.1.7</version>
</dependency>
比如要使用log4j或者 jdk自带的logging，需要使用桥接包如slf4j-jdk14.jar包
二、实现原理
slf4j只是一个日志标准，并不是日志系统的具体实现。slf4j只提做两件事情：Logger类用来打日志，LoggerFactory类用来获取Logger;
- 提供日志接口 (Logger.class)
- 提供获取具体日志对象的方法 (LoggerFactory.class)
Logger.class
public interface Logger { /** * Case insensitive String constant used to retrieve the name of the root logger. * * @since 1.3 */ final public String ROOT_LOGGER_NAME = "ROOT"; /** * Return the name of this <code>Logger</code> instance. * @return name of this logger instance */ public String getName(); /** * Is the logger instance enabled for the TRACE level? * * @return True if this Logger is enabled for the TRACE level, * false otherwise. * @since 1.4 */ public boolean isTraceEnabled(); /** * Log a message at the TRACE level. * * @param msg the message string to be logged * @since 1.4 */ public void trace(String msg); /** * Log a message at the TRACE level according to the specified format * and argument. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the TRACE level. </p> * * @param format the format string * @param arg the argument * @since 1.4 */ public void trace(String format, Object arg); /** * Log a message at the TRACE level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the TRACE level. </p> * * @param format the format string * @param arg1 the first argument * @param arg2 the second argument * @since 1.4 */ public void trace(String format, Object arg1, Object arg2); /** * Log a message at the TRACE level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous string concatenation when the logger * is disabled for the TRACE level. However, this variant incurs the hidden * (and relatively small) cost of creating an <code>Object[]</code> before invoking the method, * even if this logger is disabled for TRACE. The variants taking {@link #trace(String, Object) one} and * {@link #trace(String, Object, Object) two} arguments exist solely in order to avoid this hidden cost.</p> * * @param format the format string * @param arguments a list of 3 or more arguments * @since 1.4 */ public void trace(String format, Object... arguments); /** * Log an exception (throwable) at the TRACE level with an * accompanying message. * * @param msg the message accompanying the exception * @param t the exception (throwable) to log * @since 1.4 */ public void trace(String msg, Throwable t); /** * Similar to {@link #isTraceEnabled()} method except that the * marker data is also taken into account. * * @param marker The marker data to take into consideration * @return True if this Logger is enabled for the TRACE level, * false otherwise. * * @since 1.4 */ public boolean isTraceEnabled(Marker marker); /** * Log a message with the specific Marker at the TRACE level. * * @param marker the marker data specific to this log statement * @param msg the message string to be logged * @since 1.4 */ public void trace(Marker marker, String msg); /** * This method is similar to {@link #trace(String, Object)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg the argument * @since 1.4 */ public void trace(Marker marker, String format, Object arg); /** * This method is similar to {@link #trace(String, Object, Object)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg1 the first argument * @param arg2 the second argument * @since 1.4 */ public void trace(Marker marker, String format, Object arg1, Object arg2); /** * This method is similar to {@link #trace(String, Object...)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param argArray an array of arguments * @since 1.4 */ public void trace(Marker marker, String format, Object... argArray); /** * This method is similar to {@link #trace(String, Throwable)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param msg the message accompanying the exception * @param t the exception (throwable) to log * @since 1.4 */ public void trace(Marker marker, String msg, Throwable t); /** * Is the logger instance enabled for the DEBUG level? * * @return True if this Logger is enabled for the DEBUG level, * false otherwise. */ public boolean isDebugEnabled(); /** * Log a message at the DEBUG level. * * @param msg the message string to be logged */ public void debug(String msg); /** * Log a message at the DEBUG level according to the specified format * and argument. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the DEBUG level. </p> * * @param format the format string * @param arg the argument */ public void debug(String format, Object arg); /** * Log a message at the DEBUG level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the DEBUG level. </p> * * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void debug(String format, Object arg1, Object arg2); /** * Log a message at the DEBUG level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous string concatenation when the logger * is disabled for the DEBUG level. However, this variant incurs the hidden * (and relatively small) cost of creating an <code>Object[]</code> before invoking the method, * even if this logger is disabled for DEBUG. The variants taking * {@link #debug(String, Object) one} and {@link #debug(String, Object, Object) two} * arguments exist solely in order to avoid this hidden cost.</p> * * @param format the format string * @param arguments a list of 3 or more arguments */ public void debug(String format, Object... arguments); /** * Log an exception (throwable) at the DEBUG level with an * accompanying message. * * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void debug(String msg, Throwable t); /** * Similar to {@link #isDebugEnabled()} method except that the * marker data is also taken into account. * * @param marker The marker data to take into consideration * @return True if this Logger is enabled for the DEBUG level, * false otherwise. */ public boolean isDebugEnabled(Marker marker); /** * Log a message with the specific Marker at the DEBUG level. * * @param marker the marker data specific to this log statement * @param msg the message string to be logged */ public void debug(Marker marker, String msg); /** * This method is similar to {@link #debug(String, Object)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg the argument */ public void debug(Marker marker, String format, Object arg); /** * This method is similar to {@link #debug(String, Object, Object)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void debug(Marker marker, String format, Object arg1, Object arg2); /** * This method is similar to {@link #debug(String, Object...)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arguments a list of 3 or more arguments */ public void debug(Marker marker, String format, Object... arguments); /** * This method is similar to {@link #debug(String, Throwable)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void debug(Marker marker, String msg, Throwable t); /** * Is the logger instance enabled for the INFO level? * * @return True if this Logger is enabled for the INFO level, * false otherwise. */ public boolean isInfoEnabled(); /** * Log a message at the INFO level. * * @param msg the message string to be logged */ public void info(String msg); /** * Log a message at the INFO level according to the specified format * and argument. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the INFO level. </p> * * @param format the format string * @param arg the argument */ public void info(String format, Object arg); /** * Log a message at the INFO level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the INFO level. </p> * * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void info(String format, Object arg1, Object arg2); /** * Log a message at the INFO level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous string concatenation when the logger * is disabled for the INFO level. However, this variant incurs the hidden * (and relatively small) cost of creating an <code>Object[]</code> before invoking the method, * even if this logger is disabled for INFO. The variants taking * {@link #info(String, Object) one} and {@link #info(String, Object, Object) two} * arguments exist solely in order to avoid this hidden cost.</p> * * @param format the format string * @param arguments a list of 3 or more arguments */ public void info(String format, Object... arguments); /** * Log an exception (throwable) at the INFO level with an * accompanying message. * * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void info(String msg, Throwable t); /** * Similar to {@link #isInfoEnabled()} method except that the marker * data is also taken into consideration. * * @param marker The marker data to take into consideration * @return true if this logger is warn enabled, false otherwise */ public boolean isInfoEnabled(Marker marker); /** * Log a message with the specific Marker at the INFO level. * * @param marker The marker specific to this log statement * @param msg the message string to be logged */ public void info(Marker marker, String msg); /** * This method is similar to {@link #info(String, Object)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg the argument */ public void info(Marker marker, String format, Object arg); /** * This method is similar to {@link #info(String, Object, Object)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void info(Marker marker, String format, Object arg1, Object arg2); /** * This method is similar to {@link #info(String, Object...)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arguments a list of 3 or more arguments */ public void info(Marker marker, String format, Object... arguments); /** * This method is similar to {@link #info(String, Throwable)} method * except that the marker data is also taken into consideration. * * @param marker the marker data for this log statement * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void info(Marker marker, String msg, Throwable t); /** * Is the logger instance enabled for the WARN level? * * @return True if this Logger is enabled for the WARN level, * false otherwise. */ public boolean isWarnEnabled(); /** * Log a message at the WARN level. * * @param msg the message string to be logged */ public void warn(String msg); /** * Log a message at the WARN level according to the specified format * and argument. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the WARN level. </p> * * @param format the format string * @param arg the argument */ public void warn(String format, Object arg); /** * Log a message at the WARN level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous string concatenation when the logger * is disabled for the WARN level. However, this variant incurs the hidden * (and relatively small) cost of creating an <code>Object[]</code> before invoking the method, * even if this logger is disabled for WARN. The variants taking * {@link #warn(String, Object) one} and {@link #warn(String, Object, Object) two} * arguments exist solely in order to avoid this hidden cost.</p> * * @param format the format string * @param arguments a list of 3 or more arguments */ public void warn(String format, Object... arguments); /** * Log a message at the WARN level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the WARN level. </p> * * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void warn(String format, Object arg1, Object arg2); /** * Log an exception (throwable) at the WARN level with an * accompanying message. * * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void warn(String msg, Throwable t); /** * Similar to {@link #isWarnEnabled()} method except that the marker * data is also taken into consideration. * * @param marker The marker data to take into consideration * @return True if this Logger is enabled for the WARN level, * false otherwise. */ public boolean isWarnEnabled(Marker marker); /** * Log a message with the specific Marker at the WARN level. * * @param marker The marker specific to this log statement * @param msg the message string to be logged */ public void warn(Marker marker, String msg); /** * This method is similar to {@link #warn(String, Object)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg the argument */ public void warn(Marker marker, String format, Object arg); /** * This method is similar to {@link #warn(String, Object, Object)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void warn(Marker marker, String format, Object arg1, Object arg2); /** * This method is similar to {@link #warn(String, Object...)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arguments a list of 3 or more arguments */ public void warn(Marker marker, String format, Object... arguments); /** * This method is similar to {@link #warn(String, Throwable)} method * except that the marker data is also taken into consideration. * * @param marker the marker data for this log statement * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void warn(Marker marker, String msg, Throwable t); /** * Is the logger instance enabled for the ERROR level? * * @return True if this Logger is enabled for the ERROR level, * false otherwise. */ public boolean isErrorEnabled(); /** * Log a message at the ERROR level. * * @param msg the message string to be logged */ public void error(String msg); /** * Log a message at the ERROR level according to the specified format * and argument. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the ERROR level. </p> * * @param format the format string * @param arg the argument */ public void error(String format, Object arg); /** * Log a message at the ERROR level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous object creation when the logger * is disabled for the ERROR level. </p> * * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void error(String format, Object arg1, Object arg2); /** * Log a message at the ERROR level according to the specified format * and arguments. * <p/> * <p>This form avoids superfluous string concatenation when the logger * is disabled for the ERROR level. However, this variant incurs the hidden * (and relatively small) cost of creating an <code>Object[]</code> before invoking the method, * even if this logger is disabled for ERROR. The variants taking * {@link #error(String, Object) one} and {@link #error(String, Object, Object) two} * arguments exist solely in order to avoid this hidden cost.</p> * * @param format the format string * @param arguments a list of 3 or more arguments */ public void error(String format, Object... arguments); /** * Log an exception (throwable) at the ERROR level with an * accompanying message. * * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void error(String msg, Throwable t); /** * Similar to {@link #isErrorEnabled()} method except that the * marker data is also taken into consideration. * * @param marker The marker data to take into consideration * @return True if this Logger is enabled for the ERROR level, * false otherwise. */ public boolean isErrorEnabled(Marker marker); /** * Log a message with the specific Marker at the ERROR level. * * @param marker The marker specific to this log statement * @param msg the message string to be logged */ public void error(Marker marker, String msg); /** * This method is similar to {@link #error(String, Object)} method except that the * marker data is also taken into consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg the argument */ public void error(Marker marker, String format, Object arg); /** * This method is similar to {@link #error(String, Object, Object)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arg1 the first argument * @param arg2 the second argument */ public void error(Marker marker, String format, Object arg1, Object arg2); /** * This method is similar to {@link #error(String, Object...)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param format the format string * @param arguments a list of 3 or more arguments */ public void error(Marker marker, String format, Object... arguments); /** * This method is similar to {@link #error(String, Throwable)} * method except that the marker data is also taken into * consideration. * * @param marker the marker data specific to this log statement * @param msg the message accompanying the exception * @param t the exception (throwable) to log */ public void error(Marker marker, String msg, Throwable t); }