IntelliJ Setup
This document contains some examples and instructions on how to get IntelliJ setup to run local debugging and test setups of Druid.
Project SDK
The “SDK”, configured for the Druid project, must be called 1.8, to avoid annoying changes of .idea/misc.xml file, that you shouldn‘t check into your commits. If you don’t have an SDK on your dev machine with this name, you should either rename one of the existing SDKs to 1.8, or create another one with this name, and it could point to the same files as some other existing SDK, even if it's actually JDK 9+. 1.8 is just an alias name. You can do this in Using File -> Project Structure... -> Platform Settings -> SDKs (see also IntelliJ help page about this): 
Code Style
The Code Style is available in XML format at druid_intellij_formatting.xml and can be imported into IntelliJ.
Metadata
The installation of a MySQL metadata store is outside the scope of this document, but instructions on setting up MySQL can be found at docs/content/development/extensions-core/mysql.md. This assumes you followed the example there and have a database named druid with proper permissions for a user named druid and a password of diurd.
Use of other databases such as Postgres or Derby are entirely reasonable, but doing so is left as an excercise to the reader.
ZooKeeper
This also assumes you have ZooKeeper running locally, which usually just involves downloading the latst distribution of ZooKeeper, doing some minor configuration in ZooKeeper's conf/ directory (most defaults are fine), then running ./bin/zkServer.sh start in the ZooKeeper directory.
Initial Build
Before running the apps, you should do a mvn clean install -DskipTests in the Druid source in order to make sure directories are populated correctly.
XML App Def
You can configure application definitions in XML for import into IntelliJ. Below are a few examples. These should be placed in an XML file in .idea/runConfigurations in the Druid source code.
Historical.xml
<component name="ProjectRunConfigurationManager"> <configuration default="false" name="Historical" type="Application" factoryName="Application"> <extension name="coverage" enabled="false" merge="false" sample_coverage="true" runner="idea" /> <option name="MAIN_CLASS_NAME" value="org.apache.druid.cli.Main" /> <option name="VM_PARAMETERS" value="-server -Duser.timezone=UTC -Dfile.encoding=UTF-8 -Xmx2G -XX:MaxJavaStackTraceDepth=9999 -XX:+UseG1GC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintAdaptiveSizePolicy -XX:+PrintReferenceGC -verbose:gc -XX:+PrintFlagsFinal -Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager -Dorg.jboss.logging.provider=slf4j -Dlog4j.configurationFile=$PROJECT_DIR$/common/src/main/resources/log4j2.debug.xml -Ddruid.host=localhost -Ddruid.service=historical -Ddruid.server.maxSize=10000000000 -Ddruid.processing.buffer.sizeBytes=100000000 -Ddruid.extensions.hadoopDependenciesDir=$PROJECT_DIR$/distribution/target/hadoop-dependencies/ -Ddruid.extensions.directory=$PROJECT_DIR$/distribution/target/extensions/ -Ddruid.extensions.loadList=[\"druid-s3-extensions\",\"druid-histogram\",\"mysql-metadata-storage\"] -Ddruid.historical.cache.useCache=false -Ddruid.historical.cache.populateCache=false -Ddruid.segmentCache.locations="[{\"path\":\"/tmp/druid/indexCache\",\"maxSize\":10000000000}]" -Ddruid.zk.service.host=localhost -Ddruid.processing.numThreads=1 -Ddruid.server.http.numThreads=50 -Ddruid.announcer.type=batch -Ddruid.emitter=logging" /> <option name="PROGRAM_PARAMETERS" value="server historical" /> <option name="WORKING_DIRECTORY" value="file://$PROJECT_DIR$" /> <option name="ALTERNATIVE_JRE_PATH_ENABLED" value="false" /> <option name="ALTERNATIVE_JRE_PATH" value="1.8" /> <option name="ENABLE_SWING_INSPECTOR" value="false" /> <option name="ENV_VARIABLES" /> <option name="PASS_PARENT_ENVS" value="true" /> <module name="druid-services" /> <envs /> <method /> </configuration> </component>
Coordinator.xml
<component name="ProjectRunConfigurationManager"> <configuration default="false" name="Coordinator" type="Application" factoryName="Application"> <extension name="coverage" enabled="false" merge="false" sample_coverage="true" runner="idea" /> <option name="MAIN_CLASS_NAME" value="org.apache.druid.cli.Main" /> <option name="VM_PARAMETERS" value="-server -Duser.timezone=UTC -Dfile.encoding=UTF-8 -Xmx256M -Xmx256M -XX:+UseG1GC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintAdaptiveSizePolicy -XX:+PrintReferenceGC -verbose:gc -XX:+PrintFlagsFinal -Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager -Dorg.jboss.logging.provider=slf4j -Ddruid.host=localhost -Ddruid.service=coordinator -Ddruid.extensions.directory=$PROJECT_DIR$/distribution/target/extensions/ -Ddruid.extensions.loadList=[\"druid-s3-extensions\",\"druid-histogram\",\"mysql-metadata-storage\"] -Ddruid.zk.service.host=localhost -Ddruid.metadata.storage.type=mysql -Ddruid.metadata.storage.connector.connectURI="jdbc:mysql://localhost:3306/druid" -Ddruid.metadata.storage.connector.user=druid -Ddruid.metadata.storage.connector.password=diurd -Ddruid.announcer.type=batch -Ddruid.emitter=logging -Ddruid.coordinator.period=PT10S -Ddruid.coordinator.startDelay=PT5S" /> <option name="PROGRAM_PARAMETERS" value="server coordinator" /> <option name="WORKING_DIRECTORY" value="file://$PROJECT_DIR$" /> <option name="ALTERNATIVE_JRE_PATH_ENABLED" value="false" /> <option name="ALTERNATIVE_JRE_PATH" value="1.8" /> <option name="ENABLE_SWING_INSPECTOR" value="false" /> <option name="ENV_VARIABLES" /> <option name="PASS_PARENT_ENVS" value="true" /> <module name="druid-services" /> <envs /> <method /> </configuration> </component>