java - how Futures.addCallback guarantee the callback to be executed at the thread executing the future task? -

code shown below:

final listeningexecutorservice executor = moreexecutors.listeningdecorator(executors.newfixedthreadpool(4));  final callable<string> asynctask = new callable<string>() {     @override     public string call() throws exception {     return testfuture.computeresult();     } };  final int listsize = 10;  final list<listenablefuture<string>> listenablefutures = lists.newarraylistwithexpectedsize(listsize); (int = 0; < listsize; i++) {     listenablefutures.add(executor.submit(asynctask)); }  (final listenablefuture<string> listenablefuture2 : listenablefutures) {     futures.addcallback(listenablefuture2, new futurecallback<string>() {     @override     public void onsuccess(final string result) {         system.out.println("callback success " + result + " @ " + thread.currentthread().getname());     }      @override     public void onfailure(final throwable thrown) {         system.out.println("callback failed " + thrown.getmessage());     }     }); } 

i cannot figure out how callback scheduled executed @ thread executing asynctask?

i can see futures.addcallback calls addcallback(future, callback, moreexecutors.samethreadexecutor());, donnot know how moreexecutors.samethreadexecutor guarantee this?

samethreadexecutor() executes runnable that's passed execute(runnable) method inline: is, calls run() on it. nothing related threads @ all. , callback runnables passed associated executor on same thread executes asynctask because listeningexecutorservice wraps asynctask in futuretask calls callbacks when done() method called.

note: not guaranteed of callbacks executed on thread executes asynctask. example, if add callback after asynctask has completed, it'll executed on thread added callback on (assuming don't provide executor other samethreadexecutor() callback use).