imaffe edited a comment on issue #2091:
URL: https://github.com/apache/rocketmq/issues/2091#issuecomment-647877363
@heshengjun811 you can try add this to your logback.xml
```
<appender name="mqAsyncAppender1"
class="ch.qos.logback.classic.AsyncAppender">
<queueSize>1024</queueSize>
<discardingThreshold>80</discardingThreshold>
<maxFlushTime>2000</maxFlushTime>
<neverBlock>true</neverBlock>
<appender-ref ref="mqAppender1"/>
</appender>
```
according to [logback
example](https://rocketmq.apache.org/docs/logappender-example/)
The whole logback.xml looks like
```
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<appender name="mqAppender1"
class="org.apache.rocketmq.logappender.logback.RocketmqLogbackAppender">
<tag>TagA</tag>
<topic>01_test</topic>
<producerGroup>group1</producerGroup>
<nameServerAddress>127.0.0.1:9876</nameServerAddress>
<layout>
<pattern>%date %p %t - %m%n</pattern>
</layout>
</appender>
<appender name="mqAsyncAppender1"
class="ch.qos.logback.classic.AsyncAppender">
<queueSize>1024</queueSize>
<discardingThreshold>80</discardingThreshold>
<maxFlushTime>2000</maxFlushTime>
<neverBlock>true</neverBlock>
<appender-ref ref="mqAppender1"/>
</appender>
<root>
<level value="debug"/>
<appender-ref ref="mqAppender1"/>
</root>
<logger name="testLogger" level="debug" additivity="true">
<appender-ref ref="mqAppender1"/>
</logger>
</configuration>
```
Once I added this, it will work properly. The reason for
NoClassDefFoundError was because we missed some configuration entries, the
static initialization process of some class (I don't know which exact class it
is) failed, and throws NoClassDefFoundError.
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