java开发过程中为了找问题方便或是统计信息方便，总免不了要打各种日志。之前的各个项目都是配置好的，maven配置的时候，会看到各种类型的Log包：slf4j、log4j、commons-log、logback等等，看着就烦，今天仔细了解了一下，有点收获，记录一下
起因
使用JAVA的kafka api写main方法进行开发测试，发现总报错
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log4j:WARN No appenders could be found for logger (kafka.utils.VerifiableProperties).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
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解决过程
第一直觉是缺少log4j的配置。好办，classpath下加入log4j.properties(log4j默认应该是直接找classpath下的这个文件，当然，可以直接指定加载哪个文件)，OK，不报错了
刚好最近的一个项目使用的是logback，而且网上也查了，logback性能要优于log4j(没有做过测试，先盲目从众吧)，就想直接引入logback。
POM中加入logback的依赖
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<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-access</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.1.2</version>
</dependency>
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启动，报错
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log4j:WARN No appenders could be found for logger (kafka.utils.VerifiableProperties).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/C:/Users/zhaohuayu/.m2/repository/org/slf4j/slf4j-log4j12/1.6.1/slf4j-log4j12-1.6.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/C:/Users/zhaohuayu/.m2/repository/ch/qos/logback/logback-classic/1.1.2/logback-classic-1.1.2.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]
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哦，Class path contains multiple SLF4J bindings。
原来slf4j提供的是一个抽象的接口，实现可以是log4j、logback等，但是，运行时候只能有一个接口实现类。之前引入的kafka包，已经包含了对log4j的依赖，我又引入logback的包，冲突了
好办，去掉对Log4j的依赖
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<dependency>
<groupId>org.apache.kafka</groupId>
<artifactId>kafka_2.10</artifactId>
<version>0.8.2.0</version>
<exclusions>
<exclusion>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
</exclusion>
<exclusion>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-access</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.1.2</version>
</dependency>
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这下总OK了吧，运行：
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Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/log4j/Logger
at kafka.utils.Logging$class.logger(Logging.scala:24)
at kafka.utils.VerifiableProperties.logger$lzycompute(VerifiableProperties.scala:26)
at kafka.utils.VerifiableProperties.logger(VerifiableProperties.scala:26)
at kafka.utils.Logging$class.info(Logging.scala:67)
at kafka.utils.VerifiableProperties.info(VerifiableProperties.scala:26)
at kafka.utils.VerifiableProperties.verify(VerifiableProperties.scala:217)
at kafka.producer.ProducerConfig.<init>(ProducerConfig.scala:57)
at KafkaProducer.getClient(KafkaProducer.java:26)
at KafkaProducer.sendMsg(KafkaProducer.java:35)
at KafkaProducer.main(KafkaProducer.java:41)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
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what?
kafka.utils.Logging里包含了对log4j类的引用……
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package kafka.utils;
import kafka.utils.Log4jController.;
import org.apache.log4j.Logger;
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怎么办？难道使用kafka的项目必须使用log4j？
总有解决方案，
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<dependency>
<groupId>org.slf4j</groupId>
<artifactId>log4j-over-slf4j</artifactId>
<version>1.7.7</version>
</dependency>
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再次运行，OK，日志正常打印了
总结
1、slf4j提供各种抽象接口，日志应该基于slf4j的API进行日志打印，这样无论迁移到那个项目，只需要配一个实现类log4j or logback,都能正常打印日志
2、slf4j的实现类不能有多个，不然冲突
3、如果项目中有直接引用log4j的，可以加入log4j-over-slf4j，把旧的日志log4j适配到slf4j,这时候，再使用logback就可以了。
分享一个博客中解决各个项目模块中，各种日志杂乱的方案
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<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>${org.slf4j-version}</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jcl-over-slf4j</artifactId>
<version>${org.slf4j-version}</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>log4j-over-slf4j</artifactId>
<version>${org.slf4j-version}</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jul-to-slf4j</artifactId>
<version>${org.slf4j-version}</version>
</dependency>
<dependency>
<groupId>org.jboss.logging</groupId>
<artifactId>jboss-logging</artifactId>
<version>3.1.4.GA</version>
</dependency>
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当然，前提是需要把各个项目中依赖的log实现exclusion掉