springboot 日志问题记录

问题：新建工程busr，采用pandora boot，引入了需要的包，简单写了点代码发布，
spring-boot启动报错：

Caused by: java.lang.IllegalArgumentException: LoggerFactory is not a Logback LoggerContext but Logback is on the classpath.
Either remove Logback or the competing implementation (class org.apache.logging.slf4j.Log4jLoggerFactory loaded from
jar:file:/opt/**.jar!/BOOT-INF/lib/log4j-slf4j-impl-2.6.2.jar!/). If you are using WebLogic you will need to add 'org.slf4j'
to prefer-application-packages in WEB-INF/weblogic.xml Object of class [org.apache.logging.slf4j.Log4jLoggerFactory]
must be an instance of class ch.qos.logback.classic.LoggerContext
at org.springframework.util.Assert.isInstanceOf(Assert.java:346)
at org.springframework.boot.logging.logback.LogbackLoggingSystem.getLoggerContext(LogbackLoggingSystem.java:231)
at org.springframework.boot.logging.logback.LogbackLoggingSystem.beforeInitialize(LogbackLoggingSystem.java:97)
at org.springframework.boot.logging.LoggingApplicationListener.onApplicationStartedEvent(LoggingApplicationListener.java:226)
at org.springframework.boot.logging.LoggingApplicationListener.onApplicationEvent(LoggingApplicationListener.java:205)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:166)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:138)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:121)
at org.springframework.boot.context.event.EventPublishingRunListener.started(EventPublishingRunListener.java:63)
at org.springframework.boot.SpringApplicationRunListeners.started(SpringApplicationRunListeners.java:48)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:304)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1186)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1175)

看到错误 "LoggerFactory is not a Logback LoggerContext but Logback is on the classpath.
Either remove Logback"
大概意思是 可以remove掉logback 解决问题。 当时直接Google了下 Stack Overflow上有篇文章也说
排掉logback的包即可。

于是 我这么做了
http://ata2-img.cn-hangzhou.img-pub.aliyun-inc.com/5231e3284419eb9b203064fc694ae66d.png

发布启动 一切貌似正常。 正当高兴之余，突然发现不打印日志了。很明显与我刚才的操作有关系，看来刚才解决问题的办法似乎不正确。静下来分析问题的根本原因：
看看spring boot 是怎么加载日志的：
源码中 onApplicationStartedEvent方法，在系统启动时会被调用，LoggingSystem获取当然的日志系统

private void onApplicationStartedEvent(ApplicationStartedEvent event) {
this.loggingSystem = LoggingSystem
.get(event.getSpringApplication().getClassLoader());
this.loggingSystem.beforeInitialize();
}

默认的会引入下面3个日志系统 如果没有任何配置的化 其默认的是LogbackLoggingSystem
static {
Map systems = new LinkedHashMap();
systems.put("ch.qos.logback.core.Appender",
"org.springframework.boot.logging.logback.LogbackLoggingSystem");
systems.put("org.apache.logging.log4j.core.impl.Log4jContextFactory",
"org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
systems.put("java.util.logging.LogManager",
"org.springframework.boot.logging.java.JavaLoggingSystem");
SYSTEMS = Collections.unmodifiableMap(systems);
}

逐步跟踪到第一次系统报错的地方
private LoggerContext getLoggerContext() {
ILoggerFactory factory = StaticLoggerBinder.getSingleton().getLoggerFactory();
Assert.isInstanceOf(LoggerContext.class, factory,
String.format(
"LoggerFactory is not a Logback LoggerContext but Logback is on "

• "the classpath. Either remove Logback or the competing "
• "implementation (%s loaded from %s). If you are using "
• "WebLogic you will need to add 'org.slf4j' to "
• "prefer-application-packages in WEB-INF/weblogic.xml",
  factory.getClass(), getLocation(factory)));

return (LoggerContext) factory;
}

说明返回的factory不是logback的实例， StaticLoggerBinder.getSingleton().getLoggerFactory() 没有找到logback，
所以这个时候开始怀疑StaticLoggerBinder 是否是因为包冲突，果然 如下图
http://ata2-img.cn-hangzhou.img-pub.aliyun-inc.com/a9fbe3063e81267c31ec925c4b5e8c35.png

默认用了slf4j-log4j12 里面的StaticLoggerBinder 类，而没有用logback的 所以才会报上面的启动错误。

找到问题原因了 下面就是排包
http://ata2-img.cn-hangzhou.img-pub.aliyun-inc.com/1c7a0bf723ae52db221267fd93e1d7d1.png

来至
com.alibaba.trip.tripspider:tripspider-httpclient:jar:1.0.0-SNAPSHOT:compile
排掉即可

org.slf4j
slf4j-log4j12

启动一切正常，久违的日志又有了。

思考：遇到问题 google一下找解决办法或许是最快的，但是有时候往往解决方案因人而异，可能并不完全正确，明白问题出现的根本原因找解决办法才是最靠谱的。