druid/docs/operations/java.md

id	title
java	Java runtime

Apache Druid is written in Java and requires a Java runtime. This page provides details about obtaining and configuring a Java runtime for Druid.

Selecting a Java runtime

Druid fully supports Java 8 and 11. The project team recommends Java 11. The project team does not recommend running with Java 17, because certain Druid functionality is not currently compatible with Java 17.

The project team recommends using an OpenJDK-based Java distribution. There are many free and actively-supported distributions available, including Amazon Corretto, Azul Zulu, and Eclipse Temurin. The project team does not recommend any specific distribution over any other.

Druid relies on the environment variables JAVA_HOME or DRUID_JAVA_HOME to find Java on the machine. You can set DRUID_JAVA_HOME if there is more than one instance of Java. To verify Java requirements for your environment, run the bin/verify-java script.

Garbage collection

In general, the project team recommends using the G1 collector with default settings. This is the default collector in Java 11. To enable G1 on Java 8, use -XX:+UseG1GC. There is no harm in explicitly specifying this on Java 11 as well.

Garbage collector selection and tuning is a form of sport in the Java community. There may be situations where adjusting garbage collection configuration improves or worsens performance. The project team's guidance is that most people do not need to stray away from G1 with default settings.

Strong encapsulation

Java 9 and beyond (including Java 11) include the capability for strong encapsulation of internal JDK APIs. Druid uses certain internal JDK APIs for functionality- and performance-related reasons. In Java 11, this leads to log messages like the following:

WARNING: An illegal reflective access operation has occurred
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

These warning messages are harmless, and can be ignored. However, you can avoid them entirely if you wish by adding the following Java command line parameters. These parameters are not part of the default configurations that ship with Druid, because Java 8 does not recognize these parameters and fails to start up if they are provided.

To do this, add the following lines to your jvm.config files:

--add-exports=java.base/jdk.internal.perf=ALL-UNNAMED
--add-exports=java.base/jdk.internal.ref=ALL-UNNAMED
--add-exports=java.base/jdk.internal.misc=ALL-UNNAMED
--add-opens=java.base/java.lang=ALL-UNNAMED
--add-opens=java.base/java.io=ALL-UNNAMED
--add-opens=java.base/java.nio=ALL-UNNAMED
--add-opens=java.base/jdk.internal.ref=ALL-UNNAMED
--add-opens=java.base/sun.nio.ch=ALL-UNNAMED
--add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED

Additionally, tasks run by MiddleManagers execute in separate JVMs. The command line for these JVMs is given by druid.indexer.runner.javaOptsArray or druid.indexer.runner.javaOpts in middleManager/runtime.properties. Java command line parameters for tasks must be specified here. For example, use a line like the following:

druid.indexer.runner.javaOptsArray=["-server","-Xms1g","-Xmx1g","-XX:MaxDirectMemorySize=1g","-Duser.timezone=UTC","-Dfile.encoding=UTF-8","-XX:+ExitOnOutOfMemoryError","-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager","--add-exports=java.base/jdk.internal.perf=ALL-UNNAMED","--add-exports=java.base/jdk.internal.ref=ALL-UNNAMED","--add-exports=java.base/jdk.internal.misc=ALL-UNNAMED","--add-opens=java.base/java.lang=ALL-UNNAMED","--add-opens=java.base/java.io=ALL-UNNAMED","--add-opens=java.base/java.nio=ALL-UNNAMED","--add-opens=java.base/jdk.internal.ref=ALL-UNNAMED","--add-opens=java.base/sun.nio.ch=ALL-UNNAMED","--add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED"]

The Xms, Xmx, and MaxDirectMemorySize parameters in the line above are merely an example. You may use different values in your specific environment.