# Groovy ExecutorService submit Callable

## the little annoying thing

Posted by ChenRiang on June 20, 2021

I have been using Spock instead of JUnit for a few months now. Today, I’m trying something around thread safety and I found something about Groovy that annoyed me. In my unit test, I need to submit a Callable task to ExecutorService.

# The Problem

In Java, normally we will normally do this:

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ExecutorService executor = Executors.newSingleThreadExecutor();
Callable<Integer> c = () -> 1 + 2;
Future<Integer> future = executor.submit(c);
System.out.println("Result: " + future.get());


Output:

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Result: 3


In Groovy(Spock),

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ExecutorService executor = Executors.newSingleThreadExecutor()
Callable<Integer> c = { -> return 1 + 2 }
Future<Integer> future = executor.submit(c)
System.out.println("Result: " + future.get())


Output:

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Result: null


Why??!?

Root Cause

In Groovy, the closure is always default as Runnable instead of Callable. (See more in ticket GROOVY-3295) Thus, when we execute submit method Groovy will auto convert it to submit(Runnable) instead of submit(Callable).

This happen even I assigned it to a variable c with type Callable<Integer>.

# The Solution

Use as Callable.

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ExecutorService executor = Executors.newSingleThreadExecutor()
Callable<Integer> c = { -> return 1 + 2 }
Future<Integer> future = executor.submit(c as Callable<Integer>)
System.out.println("Result: " + future.get())


Output:

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Result: 3