1 logback概述
SLF4J与其它日志组件调用关系图如下所示。
SLF4J，即Java中的简单日志门面（Simple Logging Facade for Java），不是具体的日志解决方案，它只服务于各种各样的日志系统。
SLF4J最常用的日志实现框架是：log4j、logback。一般有slf4j+log4j、slf4j+log4j2、slf4j+logback三种日志组合。下面将介绍logback的简单使用和使用中遇到的常用问题的排查。
2 引入依赖
<!--logback-->
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
</dependency>3 新增配置
在resources文件夹下面创建logback.xml配置文件。举例如下。
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
<encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{1000}-%M:%L - %msg%n</Pattern>
</layout>
</encoder>
</appender>
<appender name="logstash" class="ch.qos.logback.core.rolling.RollingFileAppender">
<file>/app/logs/demoForStudy/logstash.log</file>
<rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
<!-- rollover daily -->
<FileNamePattern>/app/logs/demoForStudy/logstash.%d{yyyy-MM-dd}-%i.log</FileNamePattern>
<!-- each file should be at most 10MB, keep 2 days worth of history, but at most 100MB -->
<maxFileSize>10MB</maxFileSize>
<maxHistory>2</maxHistory>
<totalSizeCap>100MB</totalSizeCap>
<!-- archive removal will be executed on appender start up -->
<cleanHistoryOnStart>true</cleanHistoryOnStart>
</rollingPolicy>
<layout class="ch.qos.logback.classic.PatternLayout">
<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{1000}-%M:%L - %msg%n</pattern>
</layout>
</appender>
<root level="INFO">
<appender-ref ref="stdout" /> <!-- 输出到控制台 -->
<appender-ref ref="logstash" /> <!-- 输出到logstash -->
</root>
</configuration>4 问题排查
可能出现的问题如下。
4.1 LoggerFactory is not a Logback LoggerContext but Logback is on the classpath
4.1.1 报错
启动报错如下：
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/softwareInstall/maven/repository/org/slf4j/slf4j-log4j12/1.7.30/slf4j-log4j12-1.7.30.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/softwareInstall/maven/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.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.slf4j.impl.Log4jLoggerFactory]
Exception in thread "main" java.lang.IllegalArgumentException: LoggerFactory is not a Logback LoggerContext but Logback is on the classpath. Either remove Logback or the competing implementation (class org.slf4j.impl.Log4jLoggerFactory loaded from file:/softwareInstall/maven/repository/org/slf4j/slf4j-log4j12/1.7.30/slf4j-log4j12-1.7.30.jar). If you are using WebLogic you will need to add 'org.slf4j' to prefer-application-packages in WEB-INF/weblogic.xml: org.slf4j.impl.Log4jLoggerFactory
at org.springframework.util.Assert.instanceCheckFailed(Assert.java:702)
at org.springframework.util.Assert.isInstanceOf(Assert.java:621)
at org.springframework.boot.logging.logback.LogbackLoggingSystem.getLoggerContext(LogbackLoggingSystem.java:290)
at org.springframework.boot.logging.logback.LogbackLoggingSystem.beforeInitialize(LogbackLoggingSystem.java:117)
at org.springframework.boot.context.logging.LoggingApplicationListener.onApplicationStartingEvent(LoggingApplicationListener.java:232)
at org.springframework.boot.context.logging.LoggingApplicationListener.onApplicationEvent(LoggingApplicationListener.java:213)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
at org.springframework.boot.context.event.EventPublishingRunListener.starting(EventPublishingRunListener.java:76)
at org.springframework.boot.SpringApplicationRunListeners.lambda$starting$0(SpringApplicationRunListeners.java:53)
at java.util.ArrayList.forEach(ArrayList.java:1257)
at org.springframework.boot.SpringApplicationRunListeners.doWithListeners(SpringApplicationRunListeners.java:117)
at org.springframework.boot.SpringApplicationRunListeners.starting(SpringApplicationRunListeners.java:53)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:317)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1309)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1298)
at com.hn.liao.study.DemoForStudyApplication.main(DemoForStudyApplication.java:37)
4.1.2 问题原因
从报错可知，因为在项目中存在SLF4J的两种日志桥接绑定或日志实现组件：slf4j-log4j12和logback-classic，即日志实现组件的包冲突导致项目启动报错。
4.1.3 排除slf4j-log4j12依赖
- 首先右键pom.xml文件，通过选项“Maven->show Depedencies”显示项目中包的依赖关系；
- 然后在依赖关系文件中直接搜索“slf4j-log4j12”找到该包。
- 最后选中该包右键，通过选项“Exclude”直接排除此依赖。
操作截图如下所示。
5 参考文献
（2）logback详解
（3）IDEA 报错 LoggerFactory is not a Logback LoggerContext but Logback is on the classpath
