logger(一)slf4j简介及其实现原理

我会带着你远行 2023-08-17 15:42 62阅读 0赞

### 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

![1050601-20190730181608559-1795431550.png][]

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*

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

![1050601-20190730183312215-522107675.png][]

### 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

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the TRACE level. 
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the TRACE level. 
 *
 * @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.
 * 
 * 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.
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the DEBUG level. 
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the DEBUG level. 
 *
 * @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.
 * 
 * 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.
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the INFO level. 
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the INFO level. 
 *
 * @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.
 * 
 * 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.
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the WARN level. 
 *
 * @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.
 * 
 * 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.
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the WARN level. 
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the ERROR level. 
 *
 * @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.
 * 
 * This form avoids superfluous object creation when the logger
 * is disabled for the ERROR level. 
 *
 * @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.
 * 
 * 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.
 *
 * @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

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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.
 * 
 * 
 * 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.
 * 
 * 
 * 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.
 * 
 * 
 * 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.
 * 
 * 
 * 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.
 * 
 * 
 * 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。

转载于:https://www.cnblogs.com/pjfmeng/p/11272068.html

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[1050601-20190730183312215-522107675.png]: /images/20230810/13cc5bd87c734280a518c710fdcc5ea3.png
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