Question

We are working on a project to handle virtual machine managers from a Java application.

At the moment, we faced a problem where some instructions must be considered as a task because they require more time. Theses taks can also end with errors or be a success, and we can’t know the result from the application unless we request the state of the task to the VM hypervisor.

To have a smooth application we wanted to have a CommandManager to handle the differents request to theses hypervisors in separated threads. The problem is that these Command might return specific errors such as that we used to catch in the view to display revelant information to the user but when we implement Runnableon our Commands interface, we can’t throw any exception out of that thread back to reach the view.

What could I do to keep notifying the user of the errors that happened and their nature?

Sorry for my english!

Lets see the code below for a brief example :

First the Command

class ChangeMemorySize extends Command {
    private String name;
    private int memorySizeMb;

    public ChangeMemorySize(Hypervisor hypervisor, String server,
            String name, int memory) {
        super(hypervisor, server);
        this.name = name;
        this.memorySizeMb = memory;
    }
    protected void execute() throws ConnectionException,OperationException{

    //Some code here

    }
    public void run() //CANT THROW ANYTHING HERE :throws VmConfigFault, 
        try{
            execute();
        }catch{Exception e){
            // I have to catch it all here!
        }

This isn’t real code this is just an example. This command would then be passed to a manager who would run it in another thread. But I loose all the specifics exception that I used to notify the user of what really happened in there!

Answer 1

Use an ExecutorService and do one of two things.

First you can store all results in a Future and when you want to know if an Exception occurs just invoke Future.get() and handle any exception.

Second you can create the ExecutorService with a ThreadFactory in which you set it a knew UncaughtExceptionHandler like

       final UncaughtExceptionHandler handler = new UncaughtExceptionHandler() {
            public void uncaughtException(Thread t, Throwable e) {
                // notify error
            }
        });
        Executor executor = Executors.newFixedThreadPool(1, new ThreadFactory() {
            public Thread newThread(Runnable r) {
                Thread thread = new Thread(r);
                thread.setUncaughtExceptionHandler(handler);
                return thread;
            }
        });