log4j-slf4j-impl cannot be present with log4j-to-slf4j

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 http://www.slf4j.org/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.InvocationTargetException
        at 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.ExceptionInInitializerError
        at com.easemob.daas.gateway.server.ServerStarter.main(ServerStarter.java:25)
        ... 8 more
Caused by: org.apache.logging.log4j.LoggingException: log4j-slf4j-impl cannot be present with log4j-to-slf4j
        at 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 org.apache.commons.logging.LogAdapter$Slf4jAdapter.createLocationAwareLog(LogAdapter.java:130)
        at org.apache.commons.logging.LogAdapter.createLog(LogAdapter.java:91)
        at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:67)
        at org.apache.commons.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

1. 先查看前面的warning日志：Class path contains multiple SLF4J bindings
   slf4j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.class
                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);
            }
        } 
        return staticLoggerBinderPathSet;
    }

2. 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:有些开源组件是将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。