场景

一个简单的spring-boot程序，需要用kafka做消息队列，于是在maven中引入kafka依赖，一切看似没问题，在启动时，打印出Warning信息：

SLF4J: Class path contains multiple SLF4J bindings. SLF4J: Found binding in [jar:file:/xxx/learning-slf4j-multiple-bindings/WEB-INF/lib/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: Found binding in [jar:file:/xxx/learning-slf4j-multiple-bindings/WEB-INF/lib/slf4j-log4j12-1.7.25.jar!/org/slf4j/impl/StaticLoggerBinder.class] SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.

原因分析

通过警告消息，可以简单的看出是slf4j绑定发生问题，有多个StaticLoggerBinder.class存在，即slf4j-log4j12和logback-classic冲突。

• 疑惑点1是我并没有手动引入slf4j-log4j12依赖，依赖jar包是被自动引入的，通过maven自带工具分析依赖路径，可以看出是kafka依赖于slf4j-log4j12，自动导入的依赖包。
  
  
• 日志绑定的机制分析
  从日志对象开始探究slf4j的绑定方式。

import org.slf4j.Logger; import org.slf4j.LoggerFactory; ··· private final Logger logs = LoggerFactory.getLogger(***.class);

LoggerFactory.getLogger()方法：(下述均只保留关键逻辑代码 )

public static Logger getLogger(Class<?> clazz) {         Logger logger = getLogger(clazz.getName());         ···         return logger; } public static Logger getLogger(String name) {         ILoggerFactory iLoggerFactory = getILoggerFactory();//看这里         return iLoggerFactory.getLogger(name);//根据名字返回一个Logger实例对象 }

ILoggerFactory是一个接口，归属package org.slf4j;仅存在一个方法为：
public Logger getLogger(String name);
接下来就是看看getILoggerFactory()的真面目：

public static ILoggerFactory getILoggerFactory() {     if (INITIALIZATION_STATE == UNINITIALIZED) {         synchronized (LoggerFactory.class) {             if (INITIALIZATION_STATE == UNINITIALIZED) {                 INITIALIZATION_STATE = ONGOING_INITIALIZATION;                 performInitialization();//看这里             }         }     }     switch (INITIALIZATION_STATE) {     case SUCCESSFUL_INITIALIZATION:         return StaticLoggerBinder.getSingleton().getLoggerFactory();     case NOP_FALLBACK_INITIALIZATION:         

                    

                        
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