slf4j+log4j+logback總結

java開發過程中為了找問題方便或是統計信息方便，總免不了要打各種日志。之前的各個項目都是配置好的，maven配置的時候，會看到各種類型的Log包：slf4j、log4j、commons-log、logback等等，看着就煩，今天仔細了解了一下，有點收獲，記錄一下

起因

使用JAVA的kafka api寫main方法進行開發測試，發現總報錯

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.

解決過程

第一直覺是缺少log4j的配置。好辦，classpath下加入log4j.properties(log4j默認應該是直接找classpath下的這個文件，當然，可以直接指定加載哪個文件)，OK，不報錯了

剛好最近的一個項目使用的是logback，而且網上也查了，logback性能要優於log4j(沒有做過測試，先盲目從眾吧)，就想直接引入logback。

POM中加入logback的依賴

<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>

啟動，報錯

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]

哦，Class path contains multiple SLF4J bindings。

原來slf4j提供的是一個抽象的接口，實現可以是log4j、logback等，但是，運行時候只能有一個接口實現類。之前引入的kafka包，已經包含了對log4j的依賴，我又引入logback的包，沖突了

好辦，去掉對Log4j的依賴

<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>

這下總OK了吧，運行：

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)

what?

kafka.utils.Logging里包含了對log4j類的引用……

package kafka.utils;

import kafka.utils.Log4jController.;
import org.apache.log4j.Logger;

怎么辦？難道使用kafka的項目必須使用log4j？

總有解決方案，

 <dependency>
          <groupId>org.slf4j</groupId>
          <artifactId>log4j-over-slf4j</artifactId>
          <version>1.7.7</version>
      </dependency>

再次運行，OK，日志正常打印了

總結

1、slf4j提供各種抽象接口，日志應該基於slf4j的API進行日志打印，這樣無論遷移到那個項目，只需要配一個實現類log4j or logback,都能正常打印日志

2、slf4j的實現類不能有多個，不然沖突

3、如果項目中有直接引用log4j的，可以加入log4j-over-slf4j，把舊的日志log4j適配到slf4j,這時候，再使用logback就可以了。

分享一個博客中解決各個項目模塊中，各種日志雜亂的方案

<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>  

當然，前提是需要把各個項目中依賴的log實現exclusion掉