ExecutorService must be final?

[enar wang]

By testing the sample questions at Wiley, I noticed that the IDE complains that ExecutorService must be final or effectively final, so that all the examples in our book must make a temp final copy of previous 'service' to let it compile and run?

import java.util.ArrayList;
import java.util.concurrent.*;
import java.util.stream.IntStream;

public class PrintCounter {

    static int count = 0;

    public static void main(String[] args) throws InterruptedException, ExecutionException {

        ExecutorService service = null;

        try {
            service = Executors.newSingleThreadExecutor();
            var r = new ArrayList<Future<?>>();
            ExecutorService finalService = service;
            IntStream
                    .iterate(0, i -> i + 1)
                    .limit(5)
                    .forEach(i -> r.add(finalService.submit(()-> {count++;})));
            for (Future<?> future : r) {
                System.out.println(future.get() + " ");
            }
        } finally {
            if (service != null) service.shutdown();
        }
    }
}

[Edmund Yong]

enar wang wrote:By testing the sample questions at Wiley, I noticed that the IDE complains that ExecutorService must be final or effectively final, so that all the examples in our book must make a temp final copy of previous 'service' to let it compile and run?

import java.util.ArrayList;
import java.util.concurrent.*;
import java.util.stream.IntStream;

public class PrintCounter {

    static int count = 0;

    public static void main(String[] args) throws InterruptedException, ExecutionException {

        ExecutorService service = null;

        try {
            service = Executors.newSingleThreadExecutor();
            var r = new ArrayList<Future<?>>();
            ExecutorService finalService = service;
            IntStream
                    .iterate(0, i -> i + 1)
                    .limit(5)
                    .forEach(i -> r.add(finalService.submit(()-> {count++;})));
            for (Future<?> future : r) {
                System.out.println(future.get() + " ");
            }
        } finally {
            if (service != null) service.shutdown();
        }
    }
}

If you are using "service" inside a lambda in the forEach statement, then it must be effectively final. You initialized "service" to null, and then you assign it to Executors.newSingleThreadExecutor(), so that means it is no longer effectively final.

[Ron McLeod]

enar wang wrote:By testing the sample questions at Wiley, I noticed that the IDE complains that ExecutorService must be final or effectively final, so that all the examples in our book must make a temp final copy of previous 'service' to let it compile and run?

If this is a test question (rather than an exercise), then maybe it is not supposed to compile??

[Stephan van Hulst]

Agree with Edmund. This has nothing to do with the class ExecutorService. You're not allowed to write lambda expressions or anonymous classes that close over variables that are not effectively final. The reason for this is that the compiler doesn't know when the function will get executed, so it needs a fixed value to ensure that the closure agrees with what you expect.

You can easily solve your problem by assigning your service variable just once. There is no point in calling newSingleThreadExecutor() inside the try-block, because either it succeeds and you can start the try-finally statement immediately afterwards, or it fails and you don't have anything to shut down:

var service = Executors.newSingleThreadExecutor();

try {
  // use service;
}

finally {
  service.shutdown();
}

[Scott Selikoff]

I really wish ExecutorService supported AutoCloseable.... then you could use it in a try-with-resources statement.  Since it does not, it is strongly recommended in practice you wrap it in try/finally block if you're not going to keep the service open after the method finishes.

On the exam, they rarely do this but it's a good practice in real life.

[enar wang]

Thank you for all replies!