Druid bundles all core extensions out of the box. See the list of extensions for your options. You can load bundled extensions by adding their names to your common.runtime.properties druid.extensions.loadList property. For example, to load the postgresql-metadata-storage and druid-hdfs-storage extensions, use the configuration:
druid.extensions.loadList=["postgresql-metadata-storage", "druid-hdfs-storage"]
These extensions are located in the extensions directory of the distribution.
You can also load community and third-party extensions not already bundled with Druid. To do this, first download the extension and then install it into your extensions directory. You can download extensions from their distributors directly, or if they are available from Maven, the included pull-deps can download them for you. To use pull-deps, specify the full Maven coordinate of the extension in the form groupId:artifactId:version. For example, for the (hypothetical) extension com.example:druid-example-extension:1.0.0, run:
java \ -cp "lib/*" \ -Ddruid.extensions.directory="extensions" \ -Ddruid.extensions.hadoopDependenciesDir="hadoop-dependencies" \ org.apache.druid.cli.Main tools pull-deps \ --no-default-hadoop \ -c "com.example:druid-example-extension:1.0.0"
You only have to install the extension once. Then, add "druid-example-extension" to druid.extensions.loadList in common.runtime.properties to instruct Druid to load the extension.
If you add your extension jar to the classpath at runtime, Druid will also load it into the system. This mechanism is relatively easy to reason about, but it also means that you have to ensure that all dependency jars on the classpath are compatible. That is, Druid makes no provisions while using this method to maintain class loader isolation so you must make sure that the jars on your classpath are mutually compatible.