spark-class shell script

spark-class shell script is the Spark application command-line launcher that is responsible for setting up JVM environment and executing a Spark application.

Note	Ultimately, any shell script in Spark, e.g. spark-submit, calls `spark-class` script.

You can find spark-class script in bin directory of the Spark distribution.

When started, spark-class first loads $SPARK_HOME/bin/load-spark-env.sh, collects the Spark assembly jars, and executes org.apache.spark.launcher.Main.

Depending on the Spark distribution (or rather lack thereof), i.e. whether RELEASE file exists or not, it sets SPARK_JARS_DIR environment variable to [SPARK_HOME]/jars or [SPARK_HOME]/assembly/target/scala-[SPARK_SCALA_VERSION]/jars, respectively (with the latter being a local build).

If SPARK_JARS_DIR does not exist, spark-class prints the following error message and exits with the code 1.

Failed to find Spark jars directory ([SPARK_JARS_DIR]).
You need to build Spark with the target "package" before running this program.

spark-class sets LAUNCH_CLASSPATH environment variable to include all the jars under SPARK_JARS_DIR.

If SPARK_PREPEND_CLASSES is enabled, [SPARK_HOME]/launcher/target/scala-[SPARK_SCALA_VERSION]/classes directory is added to LAUNCH_CLASSPATH as the first entry.

Note	Use `SPARK_PREPEND_CLASSES` to have the Spark launcher classes (from `[SPARK_HOME]/launcher/target/scala-[SPARK_SCALA_VERSION]/classes`) to appear before the other Spark assembly jars. It is useful for development so your changes don’t require rebuilding Spark again.

SPARK_TESTING and SPARK_SQL_TESTING environment variables enable test special mode.

Caution

FIXME What’s so special about the env vars?

spark-class uses org.apache.spark.launcher.Main command-line application to compute the Spark command to launch. The Main class programmatically computes the command that spark-class executes afterwards.

Tip	Use `JAVA_HOME` to point at the JVM to use.

`org.apache.spark.launcher.Main` Standalone Application

org.apache.spark.launcher.Main is a Scala standalone application used in spark-class to prepare the Spark command to execute.

Main expects that the first parameter is the class name that is the "operation mode":

org.apache.spark.deploy.SparkSubmit — Main uses SparkSubmitCommandBuilder to parse command-line arguments. This is the mode spark-submit uses.
anything — Main uses SparkClassCommandBuilder to parse command-line arguments.

$ ./bin/spark-class org.apache.spark.launcher.Main
Exception in thread "main" java.lang.IllegalArgumentException: Not enough arguments: missing class name.
	at org.apache.spark.launcher.CommandBuilderUtils.checkArgument(CommandBuilderUtils.java:241)
	at org.apache.spark.launcher.Main.main(Main.java:51)

Main uses buildCommand method on the builder to build a Spark command.

If SPARK_PRINT_LAUNCH_COMMAND environment variable is enabled, Main prints the final Spark command to standard error.

Spark Command: [cmd]
========================================

If on Windows it calls prepareWindowsCommand while on non-Windows OSes prepareBashCommand with tokens separated by \0.

Caution

FIXME What’s prepareWindowsCommand? prepareBashCommand?

Main uses the following environment variables:

SPARK_DAEMON_JAVA_OPTS and SPARK_MASTER_OPTS to be added to the command line of the command.
SPARK_DAEMON_MEMORY (default: 1g) for -Xms and -Xmx.

spark-class shell script