Error scanning entry META-INF/versions/9/org/apach

This is the directory layout I have in my maven project:

.
├── pom.xml
└── src
    └── main
        ├── java
        │   └── biz
        │       └── tugay
        │           └── web
        │               └── servlet
        │                   └── MainServlet.java
        ├── resources
        │   └── log4j.properties
        └── webapp
            └── WEB-INF
                └── web.xml

pom.xml relavent parts as follows:

</dependencies>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>3.1.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-core</artifactId>
        <version>${log4j.version}</version>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-jcl</artifactId>
        <version>${log4j.version}</version>
        <scope>runtime</scope>
    </dependency>
</dependencies>

<build>
    <finalName>bare-webapp</finalName>
    <plugins>
        <plugin>
            <groupId>org.eclipse.jetty</groupId>
            <artifactId>jetty-maven-plugin</artifactId>
            <version>9.2.1.v20140609</version>
        </plugin>
    </plugins>
</build>

<properties>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <log4j.version>2.8.2</log4j.version>
</properties>

And here is my web.xml

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                             http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
</web-app>

and finally this is what log4j.properties look like:

# suppress inspection "UnusedProperty" for whole file

# logger level configurations
log4j.rootLogger=ALL, console

# configuration for console outputs
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.conversionPattern=[%t] %-5p %c - %m%n

So currently everything runs fine (I start jetty with: mvn jetty:start) and when I visit "/" I see in console:

20:38:09.433 [qtp1503674351-26] ERROR biz.tugay.web.servlet.MainServlet - Serving request!

This is all fine..

<log4j.version>2.9.1</log4j.version>

after jetty:start I get:

[WARNING] Failed startup of context o.e.j.m.p.JettyWebAppContext@fb0d6fe{/,file:/Users/koraytugay/bare-webapp/src/main/webapp/,STARTING}{file:/Users/koraytugay/bare-webapp/src/main/webapp/}
MultiException[java.lang.RuntimeException: Error scanning entry META-INF/versions/9/org/apache/logging/log4j/util/ProcessIdUtil.class from jar file:/Users/koraytugay/.m2/repository/org/apache/logging/log4j/log4j-api/2.9.1/log4j-api-2.9.1.jar, java.lang.RuntimeException: Error scanning entry META-INF/versions/9/org/apache/logging/log4j/util/StackLocator.class from jar file:/Users/koraytugay/.m2/repository/org/apache/logging/log4j/log4j-api/2.9.1/log4j-api-2.9.1.jar]
    at org.eclipse.jetty.annotations.AnnotationConfiguration.scanForAnnotations(AnnotationConfiguration.java:530)
    at org.eclipse.jetty.annotations.AnnotationConfiguration.configure(AnnotationConfiguration.java:441)
    at org.eclipse.jetty.webapp.WebAppContext.configure(WebAppContext.java:471)
    at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1329)
    at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:744)
    at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:497)
    at org.eclipse.jetty.maven.plugin.JettyWebAppContext.doStart(JettyWebAppContext.java:281)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:132)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:114)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
    at org.eclipse.jetty.server.handler.ContextHandlerCollection.doStart(ContextHandlerCollection.java:154)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:132)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:114)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:132)
    at org.eclipse.jetty.server.Server.start(Server.java:357)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:114)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
    at org.eclipse.jetty.server.Server.doStart(Server.java:324)
    at org.eclipse.jetty.maven.plugin.JettyServer.doStart(JettyServer.java:68)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.eclipse.jetty.maven.plugin.AbstractJettyMojo.startJetty(AbstractJettyMojo.java:564)
    at org.eclipse.jetty.maven.plugin.AbstractJettyMojo.execute(AbstractJettyMojo.java:360)
    at org.eclipse.jetty.maven.plugin.JettyRunMojo.execute(JettyRunMojo.java:168)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:207)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
    at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
    at org.apache.maven.cli.MavenCli.execute(MavenCli.java:863)
    at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
    at org.apache.maven.cli.MavenCli.main(MavenCli.java:199)
    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 org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
    at org.codehaus.classworlds.Launcher.main(Launcher.java:47)
java.lang.RuntimeException: Error scanning entry META-INF/versions/9/org/apache/logging/log4j/util/ProcessIdUtil.class from jar file:/Users/koraytugay/.m2/repository/org/apache/logging/log4j/log4j-api/2.9.1/log4j-api-2.9.1.jar
    at org.eclipse.jetty.annotations.AnnotationParser.parseJar(AnnotationParser.java:913)
    at org.eclipse.jetty.annotations.AnnotationParser.parse(AnnotationParser.java:831)
    at org.eclipse.jetty.annotations.AnnotationConfiguration$ParserTask.call(AnnotationConfiguration.java:159)
    at org.eclipse.jetty.annotations.AnnotationConfiguration$1.run(AnnotationConfiguration.java:542)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:607)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:536)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.eclipse.jetty.annotations.AnnotationParser.scanClass(AnnotationParser.java:973)
    at org.eclipse.jetty.annotations.AnnotationParser.parseJarEntry(AnnotationParser.java:956)
    at org.eclipse.jetty.annotations.AnnotationParser.parseJar(AnnotationParser.java:909)
    ... 6 more
java.lang.RuntimeException: Error scanning entry META-INF/versions/9/org/apache/logging/log4j/util/StackLocator.class from jar file:/Users/koraytugay/.m2/repository/org/apache/logging/log4j/log4j-api/2.9.1/log4j-api-2.9.1.jar
    at org.eclipse.jetty.annotations.AnnotationParser.parseJar(AnnotationParser.java:913)
    at org.eclipse.jetty.annotations.AnnotationParser.parse(AnnotationParser.java:831)
    at org.eclipse.jetty.annotations.AnnotationConfiguration$ParserTask.call(AnnotationConfiguration.java:159)
    at org.eclipse.jetty.annotations.AnnotationConfiguration$1.run(AnnotationConfiguration.java:542)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:607)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:536)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.objectweb.asm.ClassReader.<init>(Unknown Source)
    at org.eclipse.jetty.annotations.AnnotationParser.scanClass(AnnotationParser.java:973)
    at org.eclipse.jetty.annotations.AnnotationParser.parseJarEntry(AnnotationParser.java:956)
    at org.eclipse.jetty.annotations.AnnotationParser.parseJar(AnnotationParser.java:909)
    ... 6 more
[WARNING] !RequestLog
[INFO] Started ServerConnector@42c81ca{HTTP/1.1}{0.0.0.0:8080}
[INFO] Started @2609ms
[INFO] Started Jetty Server

How can I make jetty work with log4j 2.9.1? Is there a problem in my configuration or is this a bug in jetty or log4j?

log4j 2.9 and later are multi-release jars for Java 9.

Your Jetty version don't support that. Either upgrade Jetty to a Java 9 compatible version, or use log4j 2.8.x.

See Jetty issue #1797: JEP 238 - Multi-Release JAR files break bytecode scanning.