SLF4J error: class loader have different class objects for the type

Question

trying to figure out why I'm getting the following error in tomcat logs:

Caused by: java.lang.LinkageError: loader constraint violation: when resolving method "org.slf4j.impl.StaticLoggerBinder.getLoggerFactory()Lorg/slf4j/ILoggerFactory;" the class loader (instance of org/apache/catalina/loader/WebappClassLoader) of the current class, org/slf4j/LoggerFactory, and the class loader (instance of sun/misc/Launcher$AppClassLoader) for the method's defining class, org/slf4j/impl/StaticLoggerBinder, have different Class objects for the type org/slf4j/ILoggerFactory used in the signature

In my gradle files I'm using slf4j version 1.7.7, and in tomcat I have jul-to-slf4j-1.7.7.jar and slf4j-api-1.7.7.jar, so I'm pretty confused.

Here is the stack trace - thanks for any help!

Total time: 2.982 secs
Using CATALINA_BASE:   /usr/local/Cellar/tomcat/8.0.18/libexec
Using CATALINA_HOME:   /usr/local/Cellar/tomcat/8.0.18/libexec
Using CATALINA_TMPDIR: /usr/local/Cellar/tomcat/8.0.18/libexec/temp
Using JRE_HOME:        /Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home
Using CLASSPATH:       /usr/local/Cellar/tomcat/8.0.18/libexec/bin/jul-to-slf4j-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/slf4j-api-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-classic-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-core-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-config/:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/bootstrap.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/tomcat-juli.jar
19:03:55.979 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Server version:        Apache Tomcat/8.0.18
19:03:55.981 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Server built:          Jan 23 2015 11:56:07 UTC
19:03:55.981 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Server number:         8.0.18.0
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - OS Name:               Mac OS X
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - OS Version:            10.10
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Architecture:          x86_64
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Java Home:             /Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home/jre
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - JVM Version:           1.8.0_25-b17
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - JVM Vendor:            Oracle Corporation
19:03:55.982 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - CATALINA_BASE:         /usr/local/Cellar/tomcat/8.0.18/libexec
19:03:55.983 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - CATALINA_HOME:         /usr/local/Cellar/tomcat/8.0.18/libexec
19:03:55.983 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.util.logging.config.file=/usr/local/Cellar/tomcat/8.0.18/libexec/conf/logging.properties
19:03:55.983 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
19:03:55.983 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.endorsed.dirs=/usr/local/Cellar/tomcat/8.0.18/libexec/endorsed
19:03:55.983 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Dcatalina.base=/usr/local/Cellar/tomcat/8.0.18/libexec
19:03:55.984 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Dcatalina.home=/usr/local/Cellar/tomcat/8.0.18/libexec
19:03:55.984 [main] INFO  org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.io.tmpdir=/usr/local/Cellar/tomcat/8.0.18/libexec/temp
19:03:55.984 [main] INFO  org.apache.catalina.core.AprLifecycleListener - The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: /Users/jadekler/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.
19:03:56.076 [main] INFO  org.apache.coyote.http11.Http11NioProtocol - Initializing ProtocolHandler ["http-nio-8080"]
19:03:56.092 [main] INFO  org.apache.tomcat.util.net.NioSelectorPool - Using a shared selector for servlet write/read
19:03:56.094 [main] INFO  org.apache.coyote.ajp.AjpNioProtocol - Initializing ProtocolHandler ["ajp-nio-8009"]
19:03:56.096 [main] INFO  org.apache.tomcat.util.net.NioSelectorPool - Using a shared selector for servlet write/read
19:03:56.096 [main] INFO  org.apache.catalina.startup.Catalina - Initialization processed in 510 ms
19:03:56.115 [main] INFO  org.apache.catalina.core.StandardService - Starting service Catalina
19:03:56.115 [main] INFO  org.apache.catalina.core.StandardEngine - Starting Servlet Engine: Apache Tomcat/8.0.18
19:03:56.132 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application archive /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/websiteskeleton.war
19:03:56.812 [localhost-startStop-1] ERROR org.apache.catalina.core.StandardContext - Error during ServletContainerInitializer processing
javax.servlet.ServletException: Failed to instantiate WebApplicationInitializer class
    at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:160) ~[spring-web-4.1.0.RELEASE.jar:4.1.0.RELEASE]
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5143) ~[catalina.jar:8.0.18]
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) [catalina.jar:8.0.18]
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725) [catalina.jar:8.0.18]
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701) [catalina.jar:8.0.18]
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:714) [catalina.jar:8.0.18]
    at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:917) [catalina.jar:8.0.18]
    at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1701) [catalina.jar:8.0.18]
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_25]
    at java.util.concurrent.FutureTask.run(FutureTask.java:266) [na:1.8.0_25]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_25]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_25]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_25]
Caused by: java.lang.LinkageError: loader constraint violation: when resolving method "org.slf4j.impl.StaticLoggerBinder.getLoggerFactory()Lorg/slf4j/ILoggerFactory;" the class loader (instance of org/apache/catalina/loader/WebappClassLoader) of the current class, org/slf4j/LoggerFactory, and the class loader (instance of sun/misc/Launcher$AppClassLoader) for the method's defining class, org/slf4j/impl/StaticLoggerBinder, have different Class objects for the type org/slf4j/ILoggerFactory used in the signature
    at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:306) ~[slf4j-api-1.7.7.jar:1.7.7]
    at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:276) ~[slf4j-api-1.7.7.jar:1.7.7]
    at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:288) ~[slf4j-api-1.7.7.jar:1.7.7]
    at com.websiteskeleton.core.HelloInitializer.<init>(HelloInitializer.java:13) ~[HelloInitializer.class:na]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_25]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_25]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_25]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:408) ~[na:1.8.0_25]
    at java.lang.Class.newInstance(Class.java:438) ~[na:1.8.0_25]
    at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:157) ~[spring-web-4.1.0.RELEASE.jar:4.1.0.RELEASE]
    ... 12 common frames omitted
19:03:56.821 [localhost-startStop-1] ERROR org.apache.catalina.core.StandardContext - Context [/websiteskeleton] startup failed due to previous errors
19:03:56.825 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application archive /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/websiteskeleton.war has finished in 693 ms
19:03:56.826 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/docs
19:03:56.893 [localhost-startStop-1] INFO  org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
19:03:56.904 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/docs has finished in 78 ms
19:03:56.904 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/examples
19:03:57.133 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/examples has finished in 228 ms
19:03:57.133 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/host-manager
19:03:57.192 [localhost-startStop-1] INFO  org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
19:03:57.195 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/host-manager has finished in 62 ms
19:03:57.195 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/manager
19:03:57.252 [localhost-startStop-1] INFO  org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
19:03:57.254 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/manager has finished in 59 ms
19:03:57.254 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/ROOT
19:03:57.307 [localhost-startStop-1] INFO  org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
19:03:57.308 [localhost-startStop-1] INFO  org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/ROOT has finished in 54 ms
19:03:57.310 [main] INFO  org.apache.coyote.http11.Http11NioProtocol - Starting ProtocolHandler ["http-nio-8080"]
19:03:57.314 [main] INFO  org.apache.coyote.ajp.AjpNioProtocol - Starting ProtocolHandler ["ajp-nio-8009"]
19:03:57.315 [main] INFO  org.apache.catalina.startup.Catalina - Server startup in 1218 ms

Answer 1

The Tomcat log shows you have another slf4j-1.7.7.jar on the classpath:

Using CLASSPATH: /usr/local/Cellar/tomcat/8.0.18/libexec/bin/jul-to-slf4j-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/slf4j-api-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-classic-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-core-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-config/:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/bootstrap.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/tomcat-juli.jar

When Tomcat is been initialized, it initialize jul-to-slf4j and slf4j-api. These jars are been loading by the Classloader which loaded the Tomcat jars (sun/misc/Launcher$AppClassLoader).

When your application tries to access the slf4j-api classes, it uses the WebApplication Classloader (org/apache/catalina/loader/WebappClassLoader), which by default search for classes definition on the app classpath before the system classpath. Note this can be changed: https://tomcat.apache.org/tomcat-8.0-doc/class-loader-howto.html

In Java, the class signature is composed of the class full qualified name and the classloader that loaded the class. If a class is loaded two times from two different classloaders (same hierarchy), there will be two different classes for the JVM. Trying to point a object reference of one class to a variable of the other will trhow a ClassCastException. Check this blog post (item 5) for more info on this: https://techblug.wordpress.com/2012/03/17/classloader-tips/

So, as far as I know, you can fix this by removing the slf4j-*.jar from the Tomcat classpath, or if you have more then one webapp using the same version of slf4j, you can remove the jars from the webapp, so it will use those loaded by Tomcat classloader.