java.lang.NoClassDefFoundError is sub class of LinkageError. This exception thrown if Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.

The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.

Constructor

• NoClassDefFoundError():  Constructs a NoClassDefFoundError with no detail message.
• NoClassDefFoundError(String s):  Constructs a NoClassDefFoundError with specific detail message.

Exception Stack Trace

Exception in thread "elasticsearch[2befByj][clusterService#updateTask][T#1]" java.lang.NoClassDefFoundError: org/apache/logging/log4j/core/config/Configurator
    at org.elasticsearch.common.logging.Loggers.setLevel(Loggers.java:149)
    at org.elasticsearch.common.logging.Loggers.setLevel(Loggers.java:144)
    at org.elasticsearch.index.SearchSlowLog.setLevel(SearchSlowLog.java:111)
    at org.elasticsearch.index.SearchSlowLog.(SearchSlowLog.java:106)
    at org.elasticsearch.index.IndexModule.(IndexModule.java:127)
    at org.elasticsearch.indices.IndicesService.createIndexService(IndicesService.java:440)
    at org.elasticsearch.indices.IndicesService.createIndex(IndicesService.java:413)
    at org.elasticsearch.cluster.metadata.MetaDataCreateIndexService$1.execute(MetaDataCreateIndexService.java:378)
    at org.elasticsearch.cluster.ClusterStateUpdateTask.execute(ClusterStateUpdateTask.java:45)
    at org.elasticsearch.cluster.service.ClusterService.executeTasks(ClusterService.java:634)
    at org.elasticsearch.cluster.service.ClusterService.calculateTaskOutputs(ClusterService.java:612)
    at org.elasticsearch.cluster.service.ClusterService.runTasks(ClusterService.java:571)
    at org.elasticsearch.cluster.service.ClusterService$ClusterServiceTaskBatcher.run(ClusterService.java:263)
    at org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:150)
    at org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:188)
    at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:575)
    at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:247)
    at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:210)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: org.apache.logging.log4j.core.config.Configurator
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 21 more

Issue

Lets focus on particularly on this exception. This exception was occurred because trying to use Spring Boot with Elasticsearch framework. ElasticsearchTemaplate framework internally use log4j dependency which was not included on pom.xml

Solutions

To resolve this issue include below dependency in you pom.xml to add log4j2 jar to your application.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>
   

More Issues Solution

For more other JAVA/JDBC issues solution follow link JAVA/JDBC Issues.