ExecutorService - 并行运行任务并保存结果

Question

I want to send a ping for up to 10 users at the same time, and update the user object with the result once the ping is done.

In order to do this, I am trying to use ExecutorService .

I started with a code like this:

private void pingUsers(List<User> userList) throws ExecutionException, InterruptedException {
    final int NUM_THREADS = 10;
    ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);

    for (User user : userList) {
        SnmpPingDevice pingUser = new PingUser(user);
        Future<Boolean> isUserActive = executor.submit(pingUser);
        user.isActive = isUserActive.get() ; // -- I guess it will block other pings and i'm back to my starting point where I need to run the pings in parallel.
    }

    executor.shutdown();
    try {
        executor.awaitTermination(30, TimeUnit.SECONDS);


    } catch (InterruptedException e) {
        logger.error("Failed to terminate executor");
    }
}

This is how my PingUser class look like:

@Override
    public Boolean call() {
        ping = new CmdRunner(toolDir,outDir,
                new UserOidWorkerPing(version,community,ip,logger));

        return this.isActive();
    }

public boolean isActive(){
        String cmd = ping.getCmdNoRedirect(); 
        String rc = this.cmdRunner.runShellCmd(cmd,this.outDir +"/dummy",false);
        logger.debug("PING was sent with cmd:" + cmd + ",rc:" + rc);
        return rc != null && !rc.contains("Timeout:") && !rc.isEmpty();
    }

And back to the same issue, that the pings won't run in parallel (as soon as the loop waiting for the isUserActive.get() to end)

Any idea what I'm missing? How I can make those pings run in parallel and save the result for each user in my List<User> userList ?

Answer 1

Future::get is a blocking operation so the invoking thread will be blocked until the call is completed. So you submit a new task only after the previous was finished.

Consider using ExecutorService::invokeAll which will return a list of Future s :

List<PingUser> pingUsers = userList.stream().map(PingUser::new).collect(Collectors.toList());
List<Future<Boolean>> results = executor.invokeAll(pingUsers);

Answer 2

You are blocking your execution for each call, with this line:

user.isActive = isUserActive.get() ;

This effectively waits for the call to end, and does this for each call, on at a time.

You should rather submit all tasks, and build a list of Future s, to only wait for results when all tasks have been submitted. Something like this:

List<Future<Boolean>> tasks = new ArrayList<>();
for (User user : userList) {
    SnmpPingDevice pingUser = new PingUser(user);
    tasks.add(executor.submit(pingUser));
}

for(Future<Boolean> task: tasks) {
    //use the result... OK to get() here.
}

Answer 3

What you can do is add the user.isActive = future.get() into the Runnable you submit.

for (User user : userList) {
    SnmpPingDevice pingUser = new PingUser(user);
    executor.submit(() -> user.isActive = pingUser.call());
}