logger(一)slf4j简介及其实现原理

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;

1. 提供日志接口 (Logger.class)
2. 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);

}

LoggerFactory.class

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);
            fixSubstituteLoggers();
            replayEvents();
            // release all resources in SUBST_FACTORY
            SUBST_FACTORY.clear();
        } 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);
        }
    }

    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");
    }
}

这里看下获取getLogger函数的源码，我们发现LoggerFactory.getLogger()首先获取一个ILoggerFactory接口，然后使用该接口获取具体的Logger。获取ILoggerFactory的时候用到了一个StaticLoggerBinder类，仔细研究我们会发现StaticLoggerBinder这个类并不是slf4j-api这个包中的类，而是实现日志框架里的类，所有的实现框架都需要定义该类，提供具体的Logger实现。

接下来具体分析日志实现框架 logback和log4j2。