When I try to run "exec" in SBT, I get " Error running exec: java.lang.ArrayIndexOutOfBoundsException: 0". How to fix?

Question

If I create an SBT project, even a simple "hello world", compile (successfully) and then exec, the folowing error is thrown. WHat may the reason be and how to fix this?

java.lang.ArrayIndexOutOfBoundsException: 0
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:459)
    at sbt.SimpleProcessBuilder.run(ProcessImpl.scala:381)
    at sbt.AbstractProcessBuilder.run(ProcessImpl.scala:130)
    at sbt.AbstractProcessBuilder.$bang(ProcessImpl.scala:158)
    at sbt.ExecProject$$anonfun$execOut$1.apply(ScalaProject.scala:436)
    at sbt.ExecProject$$anonfun$execOut$1.apply(ScalaProject.scala:435)
    at sbt.TaskManager$Task.invoke(TaskManager.scala:62)
    at sbt.impl.RunTask.doRun$1(RunTask.scala:77)
    at sbt.impl.RunTask.runTask(RunTask.scala:85)
    at sbt.impl.RunTask.run(RunTask.scala:32)
    at sbt.impl.RunTask$.apply(RunTask.scala:17)
    at sbt.impl.RunTask$.apply(RunTask.scala:16)
    at sbt.Project$class.run(Project.scala:98)
    at sbt.Project$class.call(Project.scala:93)
    at sbt.BasicScalaProject.call(DefaultProject.scala:21)
    at sbt.xMain$$anonfun$7.apply(Main.scala:512)
    at sbt.xMain$$anonfun$7.apply(Main.scala:512)
    at sbt.xMain.withAction(Main.scala:541)
    at sbt.xMain.sbt$xMain$$handleAction(Main.scala:512)
    at sbt.xMain.handleCommand(Main.scala:502)
    at sbt.xMain.processAction(Main.scala:441)
    at sbt.xMain.process$1(Main.scala:257)
    at sbt.xMain$Continue$1.apply(Main.scala:132)
    at sbt.xMain.run$1(Main.scala:136)
    at sbt.xMain.processArguments(Main.scala:266)
    at sbt.xMain.startProject(Main.scala:107)
    at sbt.xMain.run(Main.scala:84)
    at sbt.xMain.run0$1(Main.scala:35)
    at sbt.xMain.run(Main.scala:42)
    at xsbt.boot.Launch$.run(Launch.scala:53)
    at xsbt.boot.Launch$$anonfun$explicit$1.apply(Launch.scala:42)
    at xsbt.boot.Launch$$anonfun$explicit$1.apply(Launch.scala:42)
    at xsbt.boot.Launch$.launch(Launch.scala:57)
    at xsbt.boot.Launch$.explicit(Launch.scala:42)
    at xsbt.boot.Launch$.initialized(Launch.scala:38)
    at xsbt.boot.Launch$.parsed(Launch.scala:31)
    at xsbt.boot.Launch$.configured(Launch.scala:21)
    at xsbt.boot.Launch$.apply(Launch.scala:16)
    at xsbt.boot.Launch$.apply(Launch.scala:13)
    at xsbt.boot.Boot$.runImpl(Boot.scala:24)
    at xsbt.boot.Boot$.run(Boot.scala:19)
    at xsbt.boot.Boot$.main(Boot.scala:15)
    at xsbt.boot.Boot.main(Boot.scala)
[info] == exec ==
[error] Error running exec: java.lang.ArrayIndexOutOfBoundsException: 0

Answer 1

The purpose of the build action exec is to execute a command on the underlying shell. As such it needs to be followed with a command. EG:

exec killall firefox

Under the covers, SBT calls java.lang.ProcessBuilder, which throws this Exception if the caller tries to start it but has not provided any parameters.

IndexOutOfBoundsException - If the command is an empty list (has size 0)

I reckon SBT should not be propagating this exception and that this is a bug. You should get an error message instead.

Perhaps you were looking for the build action run, which will invoke your main class.