Conflicting log4j implementations

Fixing log4j conflicts can be time consuming. 

I was getting the below exception in my maven build:

Failed to instantiate SLF4J LoggerFactory
Reported exception:
java.lang.NoClassDefFoundError: org/apache/log4j/Level
    at org.slf4j.LoggerFactory.bind(LoggerFactory.java:150)
    at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:124)
    at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:412)
    at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:357)
    at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:383)
    at org.apache.drill.jdbc.Driver.(Driver.java:34)
    at edu.emory.cci.bindaas.datasource.provider.drill.DrillProvider.getDatabaseDriver(DrillProvider.java:50)
    at edu.emory.cci.bindaas.datasource.provider.drill.DrillProvider.getDatabaseDriver(DrillProvider.java:13)
    at edu.emory.cci.bindaas.datasource.provider.genericsql.AbstractSQLProvider.init(AbstractSQLProvider.java:31)
    at edu.emory.cci.bindaas.datasource.provider.drill.DrillProvider.init(DrillProvider.java:21)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeCustomInitMethod(AbstractAutowireCapableBeanFactory.java:1614)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1555)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1485)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:524)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:461)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:626)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932)
    at org.eclipse.gemini.blueprint.context.support.AbstractDelegatedExecutionApplicationContext.access$1600(AbstractDelegatedExecutionApplicationContext.java:60)
    at org.eclipse.gemini.blueprint.context.support.AbstractDelegatedExecutionApplicationContext$4.run(AbstractDelegatedExecutionApplicationContext.java:325)
    at org.eclipse.gemini.blueprint.util.internal.PrivilegedUtils.executeWithCustomTCCL(PrivilegedUtils.java:85)
    at org.eclipse.gemini.blueprint.context.support.AbstractDelegatedExecutionApplicationContext.completeRefresh(AbstractDelegatedExecutionApplicationContext.java:290)
    at org.eclipse.gemini.blueprint.extender.internal.dependencies.startup.DependencyWaiterApplicationContextExecutor$CompleteRefreshTask.run(DependencyWaiterApplicationContextExecutor.java:137)
    at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.ClassNotFoundException: org.apache.log4j.Level
    at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:501)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:421)
    at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:412)
    at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 31 more
14 Jun 2016 15:19:46,059 ERROR activator.ContextLoaderListener - Application context refresh failed (OsgiBundleXmlApplicationContext(bundle=drill-datasource-provider, config=osgibundle:/META-INF/spring/*.xml))
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'drillProvider' defined in URL [bundleentry://36.fwk1254526270/META-INF/spring/drill-beans.xml]: Invocation of init method failed; nested exception is java.lang.NoClassDefFoundError: org/apache/log4j/Level


Choosing  log4j-over-slf4j.jar instead of slf4j-log4j12.jar fixed the error. Means, I should have both log4j-over-slf4j-1.7.21.jar and slf4j-api-1.7.21.jar. Not slf4j-log4j12.jar and slf4j-api-1.7.21.jar.

Including both implementations (log4j-over-slf4j.jar AND slf4j-log4j12.jar) will generate the below exception:
Caused by: java.lang.IllegalStateException: Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the class path, preempting StackOverflowError. See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more details.
    at org.slf4j.impl.Log4jLoggerFactory.(Log4jLoggerFactory.java:54)