How to throw a checked exception from a java thread?

Question

Hey, I\'m writing a network application, in which I read packets of some custom binary format. And I\'m starting a background thread to wait for incoming data. The problem i

Answer 1

This answer is based on Esko Luontola one but it provides a working example.

Unlike the run() method of the Runnable interface the call() method of Callable allows to throw some exceptions. Here is an implementation example :

public class MyTask implements Callable<Integer> {

    private int numerator;
    private int denominator;

    public MyTask(int n, int d) {
        this.numerator = n;
        this.denominator = d;
    }

    @Override
    // The call method may throw an exception
    public Integer call() throws Exception {
        Thread.sleep(1000);
        if (denominator == 0) {
            throw new Exception("cannot devide by zero");
        } else {
            return numerator / denominator;
        }
    }

}

Executor provides a mechanism to run a Callable inside a thread and to handle any kind of exceptions :

public class Main {

    public static void main(String[] args) {

        // Build a task and an executor
        MyTask task = new MyTask(2, 0);
        ExecutorService threadExecutor = Executors.newSingleThreadExecutor();

        try {
            // Start task on another thread
            Future<Integer> futureResult = threadExecutor.submit(task);

            // While task is running you can do asynchronous operations
            System.out.println("Something that doesn't need the tasks result");

            // Now wait until the result is available
            int result = futureResult.get();
            System.out.println("The result is " + result);
        } catch (ExecutionException e) {
            // Handle the exception thrown by the child thread
            if (e.getMessage().contains("cannot devide by zero"))
                System.out.println("error in child thread caused by zero division");
        } catch (InterruptedException e) {
            // This exception is thrown if the child thread is interrupted.
            e.printStackTrace();
        }
    }
}

Answer 2

To be able to send the exception to the parent thread, you can put your background thread in a Callable (it allows throwing also checked exceptions) which you then pass to the submit method of some Executor. The submit method will return a Future which you can then use to get the exception (its get method will throw an ExecutionException which contains the original exception).

Answer 3

If your thread's code throw a RuntimeExpection, you doesn't need to add run() throw Exception.

But use this solution only when appropriate because this can be a bad pratice: http://java.sun.com/docs/books/tutorial/essential/exceptions/runtime.html

Any RuntimeException or unchecked Exception can help you. Maybe you'll need to create your own RuntimeException