Issue#6377 changed slf4j LoggingEvent to apache log4j LoggingEvent (#6378)

Co-authored-by: NMargaux Cabrera <margaux.cabrera@skulibrary.com>
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

CHANGES.md

@@ -9,8 +9,8 @@ Release Notes.
 
 #### Java Agent
 * Remove invalid mysql configuration in agent.config.
-* Add net.bytebuddy.agent.builder.AgentBuilder.RedefinitionStrategy.Listener to show detail message when redefine errors occur
-
+* Add net.bytebuddy.agent.builder.AgentBuilder.RedefinitionStrategy.Listener to show detail message when redefine errors occur.
+* Fix ClassCastException of log4j gRPC reporter.
 
 #### OAP-Backend
 * Allow user-defined `JAVA_OPTS` in the startup script.

apm-sniffer/apm-toolkit-activation/apm-toolkit-log4j-1.x-activation/pom.xml

@@ -36,7 +36,7 @@
             <groupId>log4j</groupId>
             <artifactId>log4j</artifactId>
             <version>${log4j.version}</version>
-            <scope>test</scope>
+            <scope>provided</scope>
         </dependency>
         <dependency>
             <groupId>org.apache.skywalking</groupId>

apm-sniffer/apm-toolkit-activation/apm-toolkit-log4j-1.x-activation/src/main/java/org/apache/skywalking/apm/toolkit/activation/log/log4j/v1/x/log/GRPCLogAppenderInterceptor.java

@@ -35,7 +35,7 @@ import org.apache.skywalking.apm.network.logging.v3.LogDataBody;
 import org.apache.skywalking.apm.network.logging.v3.LogTags;
 import org.apache.skywalking.apm.network.logging.v3.TextLog;
 import org.apache.skywalking.apm.network.logging.v3.TraceContext;
-import org.slf4j.event.LoggingEvent;
+import org.apache.log4j.spi.LoggingEvent;
 
 public class GRPCLogAppenderInterceptor implements InstanceMethodsAroundInterceptor {
 
@@ -103,6 +103,8 @@ public class GRPCLogAppenderInterceptor implements InstanceMethodsAroundIntercep
     }
 
     private String transformLogText(final LoggingEvent event) {
-        return event.getMessage() + "\n" + ThrowableTransformer.INSTANCE.convert2String(event.getThrowable(), 2048);
+        final String throwableString = Objects.isNull(event.getThrowableInformation()) ? "" : 
+            ThrowableTransformer.INSTANCE.convert2String(event.getThrowableInformation().getThrowable(), 2048);
+        return event.getMessage() + "\n" + throwableString;
     }
 }