slf4j簡介和技術選型

市面上的日誌框架：

JUL、JCL、Jboss-logging、logback、log4j、log4j2、slf4j....

左邊選一個門面（抽象層）、右邊來選一個實現；

日誌門面： SLF4J；

日誌實現：Logback；

SpringBoot：底層是Spring框架，Spring框架預設是用JCL；‘

SpringBoot選用 SLF4j和logback；

一、slf4j簡介

slf4j(Simplelogging facade for Java)是對所有日誌框架制定的一種規範、標準、介面，並不是一個框架的具體的實現，因為介面並不能獨立使用，需要和具體的日誌框架實現配合使用

slf4j是門面模式的典型應用，外部與一個子系統的通訊必須通過一個統一的外觀物件進行，使得子系統更易於使用。

日誌實現(log4j、logback、log4j2)

• log4j是apache實現的一個開源日誌元件
• logback同樣是由log4j的作者設計完成的，擁有更好的特性，用來取代log4j的一個日誌框架，是slf4j的原生實現
• log4j2是log4j 1.x和logback的改進版，據說採用了一些新技術（無鎖非同步、等等），使得日誌的吞吐量、效能比log4j 1.x提高10倍，並解決了一些死鎖的bug，而且配置更加簡單靈活。

為什麼要使用slf4j

• SLF4J提供了統一的記錄日誌的介面，對不同日誌系統的具體實現進行了抽象化，只要按照其提供的方法記錄即可，最終日誌的格式、記錄級別、輸出方式等通過繫結具體的日誌系統來實現。在專案中使用了SLF4J記錄日誌，並且綁定了log4j，則日誌會以log4j的風格輸出；後期需要改為以logback的風格輸出日誌，只需要將jar包log4j替換成logback即可，不用修改專案中的程式碼。
• SLF4J支援{}作為佔位符，等價於C語言中的%s，而不必再進行字串的拼接，效率有顯著的提升

maven jar包

springBoot版本：

<!--boot版本:-->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.13.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

<!--使用的log版本是（boot這一個就可以了）：以下內容會自動匯入-->
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-logging</artifactId>
        <version>2.2.13.RELEASE</version>
      </dependency>

<!--logback-classic和logback-core版本是：-->

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
      <scope>compile</scope>
    </dependency>

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-core</artifactId>
      <version>1.2.3</version>
      <scope>compile</scope>
    </dependency>

    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>jul-to-slf4j</artifactId>
      <version>1.7.30</version>
      <scope>compile</scope>
    </dependency>

二、實現原理

slf4j只是一個日誌標準，並不是日誌系統的具體實現。slf4j只提做兩件事情：Logger類用來打日誌，LoggerFactory類用來獲取Logger;

1. 提供日誌介面 (Logger.class)
2. 提供獲取具體日誌物件的方法 (LoggerFactory.class)

Logger.class

/**
 * Copyright (c) 2004-2011 QOS.ch
 * All rights reserved.
 *
 * Permission is hereby granted, free  of charge, to any person obtaining
 * a  copy  of this  software  and  associated  documentation files  (the
 * "Software"), to  deal in  the Software without  restriction, including
 * without limitation  the rights to  use, copy, modify,  merge, publish,
 * distribute,  sublicense, and/or sell  copies of  the Software,  and to
 * permit persons to whom the Software  is furnished to do so, subject to
 * the following conditions:
 *
 * The  above  copyright  notice  and  this permission  notice  shall  be
 * included in all copies or substantial portions of the Software.
 *
 * THE  SOFTWARE IS  PROVIDED  "AS  IS", WITHOUT  WARRANTY  OF ANY  KIND,
 * EXPRESS OR  IMPLIED, INCLUDING  BUT NOT LIMITED  TO THE  WARRANTIES OF
 * MERCHANTABILITY,    FITNESS    FOR    A   PARTICULAR    PURPOSE    AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE,  ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */

package org.slf4j;

/**
 * The org.slf4j.Logger interface is the main user entry point of SLF4J API.
 * It is expected that logging takes place through concrete implementations
 * of this interface.
 * <p/>
 * <h3>Typical usage pattern:</h3>
 * <pre>
 * import org.slf4j.Logger;
 * import org.slf4j.LoggerFactory;
 *
 * public class Wombat {
 *
 *   <span style="color:green">final static Logger logger = LoggerFactory.getLogger(Wombat.class);</span>
 *   Integer t;
 *   Integer oldT;
 *
 *   public void setTemperature(Integer temperature) {
 *     oldT = t;
 *     t = temperature;
 *     <span style="color:green">logger.debug("Temperature set to {}. Old temperature was {}.", t, oldT);</span>
 *     if(temperature.intValue() > 50) {
 *       <span style="color:green">logger.info("Temperature has risen above 50 degrees.");</span>
 *     }
 *   }
 * }
 * </pre>
 *
 * Be sure to read the FAQ entry relating to <a href="../../../faq.html#logging_performance">parameterized
 * logging</a>. Note that logging statements can be parameterized in
 * <a href="../../../faq.html#paramException">presence of an exception/throwable</a>.
 *
 * <p>Once you are comfortable using loggers, i.e. instances of this interface, consider using
 * <a href="MDC.html">MDC</a> as well as <a href="Marker.html">Markers</a>.</p>
 *
 * @author Ceki G&uuml;lc&uuml;
 */
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);

}

LoggerFactory.classs

/**
 * Copyright (c) 2004-2011 QOS.ch
 * All rights reserved.
 *
 * Permission is hereby granted, free  of charge, to any person obtaining
 * a  copy  of this  software  and  associated  documentation files  (the
 * "Software"), to  deal in  the Software without  restriction, including
 * without limitation  the rights to  use, copy, modify,  merge, publish,
 * distribute,  sublicense, and/or sell  copies of  the Software,  and to
 * permit persons to whom the Software  is furnished to do so, subject to
 * the following conditions:
 *
 * The  above  copyright  notice  and  this permission  notice  shall  be
 * included in all copies or substantial portions of the Software.
 *
 * THE  SOFTWARE IS  PROVIDED  "AS  IS", WITHOUT  WARRANTY  OF ANY  KIND,
 * EXPRESS OR  IMPLIED, INCLUDING  BUT NOT LIMITED  TO THE  WARRANTIES OF
 * MERCHANTABILITY,    FITNESS    FOR    A   PARTICULAR    PURPOSE    AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE,  ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */
package org.slf4j;

import java.io.IOException;
import java.net.URL;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Enumeration;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.LinkedBlockingQueue;

import org.slf4j.event.SubstituteLoggingEvent;
import org.slf4j.helpers.NOPLoggerFactory;
import org.slf4j.helpers.SubstituteLogger;
import org.slf4j.helpers.SubstituteLoggerFactory;
import org.slf4j.helpers.Util;
import org.slf4j.impl.StaticLoggerBinder;

/**
 * The <code>LoggerFactory</code> is a utility class producing Loggers for
 * various logging APIs, most notably for log4j, logback and JDK 1.4 logging.
 * Other implementations such as {@link org.slf4j.impl.NOPLogger NOPLogger} and
 * {@link org.slf4j.impl.SimpleLogger SimpleLogger} are also supported.
 * <p/>
 * <p/>
 * <code>LoggerFactory</code> is essentially a wrapper around an
 * {@link ILoggerFactory} instance bound with <code>LoggerFactory</code> at
 * compile time.
 * <p/>
 * <p/>
 * Please note that all methods in <code>LoggerFactory</code> are static.
 * 
 * 
 * @author Alexander Dorokhine
 * @author Robert Elliot
 * @author Ceki G&uuml;lc&uuml;
 * 
 */
public final class LoggerFactory {

    static final String CODES_PREFIX = "http://www.slf4j.org/codes.html";

    static final String NO_STATICLOGGERBINDER_URL = CODES_PREFIX + "#StaticLoggerBinder";
    static final String MULTIPLE_BINDINGS_URL = CODES_PREFIX + "#multiple_bindings";
    static final String NULL_LF_URL = CODES_PREFIX + "#null_LF";
    static final String VERSION_MISMATCH = CODES_PREFIX + "#version_mismatch";
    static final String SUBSTITUTE_LOGGER_URL = CODES_PREFIX + "#substituteLogger";
    static final String LOGGER_NAME_MISMATCH_URL = CODES_PREFIX + "#loggerNameMismatch";
    static final String REPLAY_URL = CODES_PREFIX + "#replay";

    static final String UNSUCCESSFUL_INIT_URL = CODES_PREFIX + "#unsuccessfulInit";
    static final String UNSUCCESSFUL_INIT_MSG = "org.slf4j.LoggerFactory in failed state. Original exception was thrown EARLIER. See also " + UNSUCCESSFUL_INIT_URL;

    static final int UNINITIALIZED = 0;
    static final int ONGOING_INITIALIZATION = 1;
    static final int FAILED_INITIALIZATION = 2;
    static final int SUCCESSFUL_INITIALIZATION = 3;
    static final int NOP_FALLBACK_INITIALIZATION = 4;

    static volatile int INITIALIZATION_STATE = UNINITIALIZED;
    static final SubstituteLoggerFactory SUBST_FACTORY = new SubstituteLoggerFactory();
    static final NOPLoggerFactory NOP_FALLBACK_FACTORY = new NOPLoggerFactory();

    // Support for detecting mismatched logger names.
    static final String DETECT_LOGGER_NAME_MISMATCH_PROPERTY = "slf4j.detectLoggerNameMismatch";
    static final String JAVA_VENDOR_PROPERTY = "java.vendor.url";

    static boolean DETECT_LOGGER_NAME_MISMATCH = Util.safeGetBooleanSystemProperty(DETECT_LOGGER_NAME_MISMATCH_PROPERTY);

    /**
     * It is LoggerFactory's responsibility to track version changes and manage
     * the compatibility list.
     * <p/>
     * <p/>
     * It is assumed that all versions in the 1.6 are mutually compatible.
     */
    static private final String[] API_COMPATIBILITY_LIST = new String[] { "1.6", "1.7" };

    // private constructor prevents instantiation
    private LoggerFactory() {
    }

    /**
     * Force LoggerFactory to consider itself uninitialized.
     * <p/>
     * <p/>
     * This method is intended to be called by classes (in the same package) for
     * testing purposes. This method is internal. It can be modified, renamed or
     * removed at any time without notice.
     * <p/>
     * <p/>
     * You are strongly discouraged from calling this method in production code.
     */
    static void reset() {
        INITIALIZATION_STATE = UNINITIALIZED;
    }

    private final static void performInitialization() {
        bind();
        if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
            versionSanityCheck();
        }
    }

    private static boolean messageContainsOrgSlf4jImplStaticLoggerBinder(String msg) {
        if (msg == null)
            return false;
        if (msg.contains("org/slf4j/impl/StaticLoggerBinder"))
            return true;
        if (msg.contains("org.slf4j.impl.StaticLoggerBinder"))
            return true;
        return false;
    }

    private final static void bind() {
        try {
            Set<URL> staticLoggerBinderPathSet = null;
            // skip check under android, see also
            // http://jira.qos.ch/browse/SLF4J-328
            if (!isAndroid()) {
                staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
                reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
            }
            // the next line does the binding
            StaticLoggerBinder.getSingleton();
            INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
            reportActualBinding(staticLoggerBinderPathSet);
        } catch (NoClassDefFoundError ncde) {
            String msg = ncde.getMessage();
            if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
                INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
                Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
                Util.report("Defaulting to no-operation (NOP) logger implementation");
                Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
            } else {
                failedBinding(ncde);
                throw ncde;
            }
        } catch (java.lang.NoSuchMethodError nsme) {
            String msg = nsme.getMessage();
            if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
                INITIALIZATION_STATE = FAILED_INITIALIZATION;
                Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
                Util.report("Your binding is version 1.5.5 or earlier.");
                Util.report("Upgrade your binding to version 1.6.x.");
            }
            throw nsme;
        } catch (Exception e) {
            failedBinding(e);
            throw new IllegalStateException("Unexpected initialization failure", e);
        } finally {
            postBindCleanUp();
        }
    }

    private static void postBindCleanUp() {
        fixSubstituteLoggers();
        replayEvents();
        // release all resources in SUBST_FACTORY
        SUBST_FACTORY.clear();
    }

    private static void fixSubstituteLoggers() {
        synchronized (SUBST_FACTORY) {
            SUBST_FACTORY.postInitialization();
            for (SubstituteLogger substLogger : SUBST_FACTORY.getLoggers()) {
                Logger logger = getLogger(substLogger.getName());
                substLogger.setDelegate(logger);
            }
        }

    }

    static void failedBinding(Throwable t) {
        INITIALIZATION_STATE = FAILED_INITIALIZATION;
        Util.report("Failed to instantiate SLF4J LoggerFactory", t);
    }

    private static void replayEvents() {
        final LinkedBlockingQueue<SubstituteLoggingEvent> queue = SUBST_FACTORY.getEventQueue();
        final int queueSize = queue.size();
        int count = 0;
        final int maxDrain = 128;
        List<SubstituteLoggingEvent> eventList = new ArrayList<SubstituteLoggingEvent>(maxDrain);
        while (true) {
            int numDrained = queue.drainTo(eventList, maxDrain);
            if (numDrained == 0)
                break;
            for (SubstituteLoggingEvent event : eventList) {
                replaySingleEvent(event);
                if (count++ == 0)
                    emitReplayOrSubstituionWarning(event, queueSize);
            }
            eventList.clear();
        }
    }

    private static void emitReplayOrSubstituionWarning(SubstituteLoggingEvent event, int queueSize) {
        if (event.getLogger().isDelegateEventAware()) {
            emitReplayWarning(queueSize);
        } else if (event.getLogger().isDelegateNOP()) {
            // nothing to do
        } else {
            emitSubstitutionWarning();
        }
    }

    private static void replaySingleEvent(SubstituteLoggingEvent event) {
        if (event == null)
            return;

        SubstituteLogger substLogger = event.getLogger();
        String loggerName = substLogger.getName();
        if (substLogger.isDelegateNull()) {
            throw new IllegalStateException("Delegate logger cannot be null at this state.");
        }

        if (substLogger.isDelegateNOP()) {
            // nothing to do
        } else if (substLogger.isDelegateEventAware()) {
            substLogger.log(event);
        } else {
            Util.report(loggerName);
        }
    }

    private static void emitSubstitutionWarning() {
        Util.report("The following set of substitute loggers may have been accessed");
        Util.report("during the initialization phase. Logging calls during this");
        Util.report("phase were not honored. However, subsequent logging calls to these");
        Util.report("loggers will work as normally expected.");
        Util.report("See also " + SUBSTITUTE_LOGGER_URL);
    }

    private static void emitReplayWarning(int eventCount) {
        Util.report("A number (" + eventCount + ") of logging calls during the initialization phase have been intercepted and are");
        Util.report("now being replayed. These are subject to the filtering rules of the underlying logging system.");
        Util.report("See also " + REPLAY_URL);
    }

    private final static void versionSanityCheck() {
        try {
            String requested = StaticLoggerBinder.REQUESTED_API_VERSION;

            boolean match = false;
            for (String aAPI_COMPATIBILITY_LIST : API_COMPATIBILITY_LIST) {
                if (requested.startsWith(aAPI_COMPATIBILITY_LIST)) {
                    match = true;
                }
            }
            if (!match) {
                Util.report("The requested version " + requested + " by your slf4j binding is not compatible with "
                                + Arrays.asList(API_COMPATIBILITY_LIST).toString());
                Util.report("See " + VERSION_MISMATCH + " for further details.");
            }
        } catch (java.lang.NoSuchFieldError nsfe) {
            // given our large user base and SLF4J's commitment to backward
            // compatibility, we cannot cry here. Only for implementations
            // which willingly declare a REQUESTED_API_VERSION field do we
            // emit compatibility warnings.
        } catch (Throwable e) {
            // we should never reach here
            Util.report("Unexpected problem occured during version sanity check", e);
        }
    }

    // We need to use the name of the StaticLoggerBinder class, but we can't
    // reference
    // the class itself.
    private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";

    static Set<URL> findPossibleStaticLoggerBinderPathSet() {
        // use Set instead of list in order to deal with bug #138
        // LinkedHashSet appropriate here because it preserves insertion order
        // during iteration
        Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
        try {
            ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
            Enumeration<URL> paths;
            if (loggerFactoryClassLoader == null) {
                paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
            } else {
                paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
            }
            while (paths.hasMoreElements()) {
                URL path = paths.nextElement();
                staticLoggerBinderPathSet.add(path);
            }
        } catch (IOException ioe) {
            Util.report("Error getting resources from path", ioe);
        }
        return staticLoggerBinderPathSet;
    }

    private static boolean isAmbiguousStaticLoggerBinderPathSet(Set<URL> binderPathSet) {
        return binderPathSet.size() > 1;
    }

    /**
     * Prints a warning message on the console if multiple bindings were found
     * on the class path. No reporting is done otherwise.
     * 
     */
    private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) {
        if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Class path contains multiple SLF4J bindings.");
            for (URL path : binderPathSet) {
                Util.report("Found binding in [" + path + "]");
            }
            Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
        }
    }

    private static boolean isAndroid() {
        String vendor = Util.safeGetSystemProperty(JAVA_VENDOR_PROPERTY);
        if (vendor == null)
            return false;
        return vendor.toLowerCase().contains("android");
    }

    private static void reportActualBinding(Set<URL> binderPathSet) {
        // binderPathSet can be null under Android
        if (binderPathSet != null && isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Actual binding is of type [" + StaticLoggerBinder.getSingleton().getLoggerFactoryClassStr() + "]");
        }
    }

    /**
     * Return a logger named according to the name parameter using the
     * statically bound {@link ILoggerFactory} instance.
     * 
     * @param name
     *            The name of the logger.
     * @return logger
     */
    public static Logger getLogger(String name) {
        ILoggerFactory iLoggerFactory = getILoggerFactory();
        return iLoggerFactory.getLogger(name);
    }

    /**
     * Return a logger named corresponding to the class passed as parameter,
     * using the statically bound {@link ILoggerFactory} instance.
     * 
     * <p>
     * In case the the <code>clazz</code> parameter differs from the name of the
     * caller as computed internally by SLF4J, a logger name mismatch warning
     * will be printed but only if the
     * <code>slf4j.detectLoggerNameMismatch</code> system property is set to
     * true. By default, this property is not set and no warnings will be
     * printed even in case of a logger name mismatch.
     * 
     * @param clazz
     *            the returned logger will be named after clazz
     * @return logger
     * 
     * 
     * @see <a
     *      href="http://www.slf4j.org/codes.html#loggerNameMismatch">Detected
     *      logger name mismatch</a>
     */
    public static Logger getLogger(Class<?> clazz) {
        Logger logger = getLogger(clazz.getName());
        if (DETECT_LOGGER_NAME_MISMATCH) {
            Class<?> autoComputedCallingClass = Util.getCallingClass();
            if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
                Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                                autoComputedCallingClass.getName()));
                Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
            }
        }
        return logger;
    }

    private static boolean nonMatchingClasses(Class<?> clazz, Class<?> autoComputedCallingClass) {
        return !autoComputedCallingClass.isAssignableFrom(clazz);
    }

    /**
     * Return the {@link ILoggerFactory} instance in use.
     * <p/>
     * <p/>
     * ILoggerFactory instance is bound with this class at compile time.
     * 
     * @return the ILoggerFactory instance in use
     */
    public static ILoggerFactory getILoggerFactory() {
        if (INITIALIZATION_STATE == UNINITIALIZED) {
            synchronized (LoggerFactory.class) {
                if (INITIALIZATION_STATE == UNINITIALIZED) {
                    INITIALIZATION_STATE = ONGOING_INITIALIZATION;
                    performInitialization();
                }
            }
        }
        switch (INITIALIZATION_STATE) {
        case SUCCESSFUL_INITIALIZATION:
            return StaticLoggerBinder.getSingleton().getLoggerFactory();
        case NOP_FALLBACK_INITIALIZATION:
            return NOP_FALLBACK_FACTORY;
        case FAILED_INITIALIZATION:
            throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
        case ONGOING_INITIALIZATION:
            // support re-entrant behavior.
            // See also http://jira.qos.ch/browse/SLF4J-97
            return SUBST_FACTORY;
        }
        throw new IllegalStateException("Unreachable code");
    }
}

這裡看下獲取getLogger函式的原始碼，我們發現LoggerFactory.getLogger()首先獲取一個ILoggerFactory介面，然後使用該介面獲取具體的Logger。

獲取ILoggerFactory的時候用到了一個LoggerContext類，仔細研究我們會發現LoggerContext這個類並不是slf4j-api這個包中的類，而是實現日誌框架裡的類，所有的實現框架都需要定義該類，提供具體的Logger實現。

三、SLF4j使用

1、如何在系統中使用SLF4j https://www.slf4j.org

以後開發的時候，日誌記錄方法的呼叫，不應該來直接呼叫日誌的實現類，而是呼叫日誌抽象層裡面的方法；

給系統裡面匯入slf4j的jar和 logback的實現jar

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class HelloWorld {
  public static void main(String[] args) {
    Logger logger = LoggerFactory.getLogger(HelloWorld.class);
    logger.info("Hello World");
  }
}

每一個日誌的實現框架都有自己的配置檔案。使用slf4j以後，配置檔案還是做成日誌實現框架自己本身的配置檔案；

a（slf4j+logback）: Spring（commons-logging）、Hibernate（jboss-logging）、MyBatis、xxxx

統一日誌記錄，即使是別的框架和我一起統一使用slf4j進行輸出？

如何讓系統中所有的日誌都統一到slf4j；

1、將系統中其他日誌框架先排除出去；

2、用中間包來替換原有的日誌框架；

3、我們匯入slf4j其他的實現

二、SpringBoot日誌關係

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.13.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

SpringBoot使用它來做日誌功能：

<!--不需要匯入--> 
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-logging</artifactId>
    <version>2.2.13.RELEASE</version>
</dependency>

底層依賴關係

貌似新版本的 boot 不需要匯入jcl-over-self4j

總結：

1）、SpringBoot底層也是使用slf4j+logback的方式進行日誌記錄

2）、SpringBoot也把其他的日誌都替換成了slf4j；

3）、中間替換包？

@SuppressWarnings("rawtypes")
public abstract class LogFactory {

    static String UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J = "http://www.slf4j.org/codes.html#unsupported_operation_in_jcl_over_slf4j";

    static LogFactory logFactory = new SLF4JLogFactory();

4）、如果我們普通的 maven 專案要引入其他框架？一定要把spring框架的預設日誌依賴移除掉？

Spring框架用的是commons-logging；

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>commons-logging</groupId>
                    <artifactId>commons-logging</artifactId>
                </exclusion>
            </exclusions>
        </dependency>

SpringBoot能自動適配所有的日誌，而且底層使用slf4j+logback的方式記錄日誌，引入其他框架的時候，只需要把這個框架依賴的日誌框架排除掉即可

三、logback簡介

logback是log4j創始人寫的，效能比log4j要好，目前主要分為3個模組

1. logback-core:核心程式碼模組
2. logback-classic:log4j的一個改良版本，同時實現了slf4j的介面，這樣你如果之後要切換其他日誌元件也是一件很容易的事
3. logback-access:訪問模組與Servlet容器整合提供通過Http來訪問日誌的功能

三、日誌使用

1、logback-spring.xml配置

<?xml version="1.0" encoding="UTF-8"?>
<!--
scan：當此屬性設定為true時,配置檔案如果發生改變,將會被重新載入,預設值為true。
scanPeriod：設定監測配置檔案是否有修改的時間間隔,如果沒有給出時間單位,預設單位是毫秒當scan為true時,此屬性生效。預設的時間間隔為1分鐘。
debug：當此屬性設定為true時,將打印出logback內部日誌資訊,實時檢視logback執行狀態。預設值為false。
-->
<configuration scan="false" scanPeriod="60 seconds" debug="false">
    <!-- 定義日誌的根目錄為專案的根目錄,前面不要加"/",加了會預設會認為是根目錄,提示 classnotfond -->
    <property name="LOG_HOME" value="app/log"/>
    <!-- 定義日誌檔名稱 -->
    <property name="appName" value="logbackBootText"/>

    <!-- ConsoleAppender 用於控制檯輸出 -->
    <appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
        <!--
        日誌輸出格式：
            %d表示日期時間,
            %thread表示執行緒名,
            %-5level：級別從左顯示5個字元寬度
            %logger{50} 表示logger名字最長50個字元,否則按照句點分割。
            %msg：日誌訊息,
            %n是換行符
        -->
        <layout class="ch.qos.logback.classic.PatternLayout">
            <!--springProfile可以指定某段配置只在某個環境下生效-->
            <!--如果使用logback.xml作為日誌配置檔案,還要使用profile功能,會有以下錯誤no applicable action for [springProfile]-->
            <springProfile name="dev">
                <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} ----> [%thread] ---> %-5level %logger{50} - %msg%n</pattern>
            </springProfile>
            <springProfile name="!dev">
                <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} ==== [%thread] ==== %-5level %logger{50} - %msg%n</pattern>
            </springProfile>
        </layout>
    </appender>

    <!-- 滾動記錄檔案,先將日誌記錄到指定檔案,當符合某個條件時,將日誌記錄到其他檔案 -->
    <appender name="appLogAppender" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <!--定義日誌輸出的路徑和名稱-->
        <!-- 指定日誌檔案的名稱 -->
        <!--這裡的scheduler.manager.server.home 沒有在上面的配置中設定,所以會使用java啟動時配置的值-->
        <!--比如通過 java -Dscheduler.manager.server.home=/path/to XXXX 配置該屬性-->
        <!--<file>${scheduler.manager.server.home}/${LOG_HOME}/${appName}.log</file>-->


        <!--當發生滾動時,決定 RollingFileAppender 的行為,涉及檔案移動和重新命名
            TimeBasedRollingPolicy： 最常用的滾動策略,它根據時間來制定滾動策略,既負責滾動也負責出發滾動。“%d”可以包含一個java.text.SimpleDateFormat指定的時間格式,-->
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <!--滾動時產生的檔案的存放位置及檔名稱 %d{yyyy-MM-dd}：按天進行日誌滾動
            %i：當檔案大小超過maxFileSize時,按照i進行檔案滾動-->
            <fileNamePattern>${LOG_HOME}/${appName}-%d{yyyy-MM-dd}-%i.log</fileNamePattern>

            <!--可選節點,控制保留的歸檔檔案的最大數量,超出數量就刪除舊檔案。假設設定每天滾動,
        且maxHistory是365,則只儲存最近365天的檔案,刪除之前的舊檔案。注意,刪除舊檔案是,
        那些為了歸檔而建立的目錄也會被刪除。-->
            <MaxHistory>365</MaxHistory>
            <!-- 該屬性在 1.1.6版本後 才開始支援,日誌量最大20GB-->
            <totalSizeCap>20GB</totalSizeCap>

            <!--當日志文件超過maxFileSize指定的大小是,根據上面提到的%i進行日誌檔案滾動 注意此處配置 SizeBasedTriggeringPolicy 是無法實現按檔案大小進行滾動的,必須配置 timeBasedFileNamingAndTriggeringPolicy-->
            <timeBasedFileNamingAndTriggeringPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
                <maxFileSize>100MB</maxFileSize>
            </timeBasedFileNamingAndTriggeringPolicy>
        </rollingPolicy>
        <!-- 日誌輸出格式： -->
        <layout class="ch.qos.logback.classic.PatternLayout">
            <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [ %thread ] ->> [ %-5level ] [ %logger{50} : %line ] ->> %msg%n
            </pattern>
        </layout>
    </appender>

    <!-- 滾動記錄檔案,先將日誌記錄到指定檔案,當符合某個條件時,將日誌記錄到其他檔案 -->
    <appender name="appLogAppenderBoot" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">

            <fileNamePattern>${LOG_HOME}/boot-${appName}-%d{yyyy-MM-dd}-%i.log</fileNamePattern>

            <MaxHistory>365</MaxHistory>

            <timeBasedFileNamingAndTriggeringPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
                <maxFileSize>100MB</maxFileSize>
            </timeBasedFileNamingAndTriggeringPolicy>
        </rollingPolicy>
        <layout class="ch.qos.logback.classic.PatternLayout">
            <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [ %thread ] -> [ %-5level ] [ %logger{50} : %line ] -> %msg%n</pattern>
        </layout>
    </appender>

    <!--root是預設的logger,root與logger是父子關係,沒有特別定義則預設為root,任何一個類只會和一個logger對應,
    要麼是定義的logger,要麼是root,判斷的關鍵在於找到這個logger,然後判斷這個logger的appender和level。 -->
    <root level="DEBUG">
        <!--定義了三個appender,日誌會通過往這這些appender裡面寫-->
        <appender-ref ref="stdout"/>
        <appender-ref ref="appLogAppender"/>
        <appender-ref ref="appLogAppenderBoot"/>
    </root>

    <!--logger主要用於存放日誌物件,也可以定義日誌型別、級別-->
    <!--name：表示匹配的logger型別字首,也就是包的前半部分-->
    <!--level：要記錄的日誌級別,包括 TRACE < DEBUG < INFO < WARN < ERROR 越往後越精簡-->
    <!--additivity：作用在於children-logger是否使用 rootLogger配置的appender進行輸出,-->
    <!--false：表示只用當前logger的appender-ref,true：表示當前logger的appender-ref和rootLogger的appender-ref都有效-->
    <!--    Spring framework logger 記錄 spring 的日誌 -->
    <logger name="org.springframework.boot" level="debug" additivity="false">
        <appender-ref ref="appLogAppenderBoot"/>
    </logger>

    <!--通過 LoggerFactory.getLogger("mytest") 可以獲取到這個logger-->
    <!--由於這個logger自動繼承了root的appender，root中已經有stdout的appender了，自己這邊又引入了stdout的appender-->
    <!--如果沒有設定 additivity="false" ,就會導致一條日誌在控制檯輸出兩次的情況,原因是:沒有寫additivity會用 rootlogger 輸出日誌,而它下面有三個appender,單appLogAppenderBoot經過自定義,不會輸出,stdout則會列印兩遍-->
    <!--additivity表示要不要使用rootLogger配置的appender進行輸出-->
    <logger name="com.tuniu" level="debug" additivity="false">
        <!--即輸出到appLogAppender,又輸出到stdout-->
        <appender-ref ref="appLogAppender"/>
        <appender-ref ref="stdout"/>
    </logger>


    <!--對於類路徑以 com.tuniu 開頭的Logger,輸出級別設定為 warn,-->
    <!--這個logger沒有指定appender，它會繼承root節點中定義的那些appender-->
    <!--注意,如果名稱相同,兩個 logger 會有屬性互補機制;而且以最後一個載入的屬性為準,可以理解為 boot的 bootstrap.properties與 application.yml-->
    <!--
        <logger name="com.tuniu" level="warn"/>
        如果將上面的這段配置放開,就如同寫了:
        <logger name="com.tuniu" level="warn" additivity="false">
            <appender-ref ref="appLogAppender"/>
        </logger>
    -->

    <!--由於設定了 additivity="false" ，所以輸出時不會使用rootLogger的appender-->
    <!--但是這個logger本身又沒有配置 appender，所以使用這個logger輸出日誌的話就不會輸出到任何地方-->
<!--    <logger name="mytest2" level="info" additivity="false"/>-->

</configuration>

後續有原始碼閱讀部分