背景
项目使用@slf4j注解，注入日志组件进行日志打印。POM中引入了spring-boot-starter-logging、spring-boot-starter-log4j2、log4j2、slf4j、logback等多种日志组件。
某次修改业务代码后，系统无法启动，并报错log4j-slf4j-impl cannot be present with log4j-to-slf4j。
SLF4J: Class path contains multiple SLF4J bindings.SLF4J: Found binding in [jar:file:XXXXXXXXX.jar!/BOOT-INF/lib/log4j-slf4j-impl-2.17.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]SLF4J: Found binding in [jar:file:XXXXXXXXX.jar!/BOOT-INF/lib/logback-classic-1.2.9.jar!/org/slf4j/impl/StaticLoggerBinder.class]SLF4J: See /codes.html#multiple_bindings for an explanation.SLF4J: Actual binding is of type [org.apache.logging.slf4j.Log4jLoggerFactory]Exception in thread "main" java.lang.reflect.InvocationTargetExceptionat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:498)at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:48)at org.springframework.boot.loader.Launcher.launch(Launcher.java:87)at org.springframework.boot.loader.Launcher.launch(Launcher.java:51)at org.springframework.boot.loader.PropertiesLauncher.main(PropertiesLauncher.java:597)Caused by: java.lang.ExceptionInInitializerErrorat com.easemob.daas.gateway.server.ServerStarter.main(ServerStarter.java:25)... 8 moreCaused by: org.apache.logging.log4j.LoggingException: log4j-slf4j-impl cannot be present with log4j-to-slf4jat org.apache.logging.slf4j.Log4jLoggerFactory.validateContext(Log4jLoggerFactory.java:60)at org.apache.logging.slf4j.Log4jLoggerFactory.newLogger(Log4jLoggerFactory.java:44)at org.apache.logging.slf4j.Log4jLoggerFactory.newLogger(Log4jLoggerFactory.java:33)at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getLogger(AbstractLoggerAdapter.java:53)at org.apache.logging.slf4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:33)at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:363)at mons.logging.LogAdapter$Slf4jAdapter.createLocationAwareLog(LogAdapter.java:130)at mons.logging.LogAdapter.createLog(LogAdapter.java:91)at mons.logging.LogFactory.getLog(LogFactory.java:67)at mons.logging.LogFactory.getLog(LogFactory.java:59)at org.springframework.boot.SpringApplication.<clinit>(SpringApplication.java:196)... 9 more
定位过程
查看slf4j使用方法
通过百度，了解到slf4j为门面模式，其定义了日志组件API规范，并交由具体的类实现。
确定报错原因
先查看前面的warning日志：Class path contains multiple SLF4J bindingsslf4j的入口是 private static final Logger log = LoggerFactory.getLogger(xx.class);先定位到LoggerFactory。
从slf4j的LoggerFactory的源码可以看到，在获取Logger时会先绑定具体的日志实现（slf4j本身只定义了API规范，具体由其他日志组件实现，因此需要动态的加载实现类）
实现slf4j的日志组件必须要实现org/slf4j/impl/StaticLoggerBinder.class。（package还得保持一致）
从slf4j的源码来看，即使有多个实现，也不会产生冲突，它自己会随机选择一种进行绑定（类加载器先加载那个，则绑定那个日志组件）。
private final static void bind() {try {Set<URL> staticLoggerBinderPathSet = null;if (!isAndroid()) {# 通过类文件搜索，具体实现staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);}# 类加载器先加载那个，则绑定那个日志组件StaticLoggerBinder.getSingleton();INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;reportActualBinding(staticLoggerBinderPathSet);} }private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";static Set<URL> findPossibleStaticLoggerBinderPathSet() {Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();try {ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();Enumeration<URL> paths;if (loggerFactoryClassLoader == null) {## 通过classloader去查询这个类文件org/slf4j/impl/StaticLoggerBinder.classpaths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);} else {paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);}while (paths.hasMoreElements()) {URL path = paths.nextElement();staticLoggerBinderPathSet.add(path);}} return staticLoggerBinderPathSet;}
确认log4j-slf4j-impl、log4j-to-slf4j包的作用
log4j-slf4j-impl:log4j本身是一套自闭环的日志组件，为了接入slf4j的体系，需要实现slf4j的规范，log4j-slf4j-impl就是log4j实现slf4j规范的jar。slf4j只是一层壳子，log4j-slf4j-impl还是引用log4j-core来进行日志打印。
log4j-to-slf4j:有些开源组件是讲log4j作为日志组件的，为了兼容这些开源组件，需要将log4j的打印桥接到slf4j上。和log4j-slf4j-impl的作用是相反的。
从两个jar的作用来看，天生就是冲突的。因此不能同时引用。
解决方案
通过查阅spring文档，spring为日志也准备了对应的解决方案。
如果需要使用logback作为日志组件，则引入spring-boot-starter-logging即可，该组件包含了所有依赖，同时兼容slf4j，也就是说可以使用logback的api打印日志，也可以使用slf4j的api打印日志。
如果需要使用log4j作为日志组件，则引入spring-boot-starter-log4j2，该组件包含了所有依赖，也兼容slf4j。
