- Using the task definition, e.g. add `"mapreduce.job.classloader": "true"` to the `jobProperties` of the `tuningConfig` of your indexing task (see the [batch ingestion documentation](../ingestion/batch-ingestion.html)).
- Using system properties, e.g. on the middleManager set `druid.indexer.runner.javaOpts=... -Dhadoop.mapreduce.job.classloader=true`.
When `mapreduce.job.classloader = true`, it is also possible to specifically define which classes should be loaded from the hadoop system classpath and which should be loaded from job-supplied JARs.
This is controlled by defining class inclusion/exclusion patterns in the `mapreduce.job.classloader.system.classes` property in the `jobProperties` of `tuningConfig`.
For example, some community members have reported version incompatibility errors with the Validator class:
The following `jobProperties` excludes `javax.validation.` classes from being loaded from the system classpath, while including those from `java.,javax.,org.apache.commons.logging.,org.apache.log4j.,org.apache.hadoop.`.
[mapred-default.xml](https://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/mapred-default.xml) documentation contains more information about this property.
Next, use `hadoopDependencyCoordinates` in [Hadoop Index Task](../ingestion/batch-ingestion.html) to specify the Hadoop dependencies you want Druid to load.
1. Set `druid.indexer.task.defaultHadoopCoordinates=[]`. By setting this to an empty list, Druid will not load any other Hadoop dependencies except the ones specified in the classpath.
2. Append your Hadoop jars to Druid's classpath. Druid will load them into the system.
java.lang.VerifyError: class com.fasterxml.jackson.datatype.guava.deser.HostAndPortDeserializer overrides final method deserialize.(Lcom/fasterxml/jackson/core/JsonParser;Lcom/fasterxml/jackson/databind/DeserializationContext;)Ljava/lang/Object;
Another workaround solution is to build a custom fat jar of Druid using [sbt](http://www.scala-sbt.org/), which manually excludes all the conflicting Jackson dependencies, and then put this fat jar in the classpath of the command that starts overlord indexing service. To do this, please follow the following steps.
(10) Include the fat jar in the classpath when you start the indexing service. Make sure you've removed 'lib/*' from your classpath because now the fat jar includes all you need.
If sbt is not your choice, you can also use `maven-shade-plugin` to make a fat jar: relocation all jackson packages will resolve it too. In this way, druid will not be affected by jackson library embedded in hadoop. Please follow the steps below:
(1) Add all extensions you needed to `services/pom.xml` like
```xml
<dependency>
<groupId>io.druid.extensions</groupId>
<artifactId>druid-avro-extensions</artifactId>
<version>${project.parent.version}</version>
</dependency>
<dependency>
<groupId>io.druid.extensions.contrib</groupId>
<artifactId>druid-parquet-extensions</artifactId>
<version>${project.parent.version}</version>
</dependency>
<dependency>
<groupId>io.druid.extensions</groupId>
<artifactId>druid-hdfs-storage</artifactId>
<version>${project.parent.version}</version>
</dependency>
<dependency>
<groupId>io.druid.extensions</groupId>
<artifactId>mysql-metadata-storage</artifactId>
<version>${project.parent.version}</version>
</dependency>
```
(2) Shade jackson packages and assemble a fat jar.
Copy out `services/target/xxxxx-selfcontained.jar` after `mvn install` in project root for further usage.
(3) run hadoop indexer (post an indexing task is not possible now) as below. `lib` is not needed anymore. As hadoop indexer is a standalone tool, you don't have to replace the jars of your running services:
