Stop thread using return statement - 【StackMirror】|android|multithreading|return

Stop a thread like:

new Thread ( new Runnable() { 

 public void run(){ 
    if ( condition ) return; // this will stop the thread. 

} 
}).start();

is correct/safe?

2012-04-04 17:32
by amp

Yes... it is correct and safe. Didn't you try yourself - Cristian 2012-04-04 17:33

Yes, I tried and it works, but I was not really sure if this is the best solution.. - amp 2012-04-04 17:37

It is safe, but this code doesn't make any sense. the run() method will return period. if you are referencing a boolean value inside your thread and depending on it (as in the case of looping until it is met), condition should be synchronized. So while this example will always safely return, the use of condition is not necessarily safe - sethro 2012-04-04 17:54

@sethro - It's probably enough to declare condition to be volatile if it's a variable being set from another thread. However, there's no indication in OP's question that condition is being changed from another thread - Ted Hopp 2012-04-04 18:48

@sethro, In my case, just this thread can change the condition (before the if statement), so I think the synchronization is not needed.. - amp 2012-04-04 20:31

A thread stops when it's run() method returns. It doesn't really matter what logic is used inside run() to decide when or how to return. Your code is perfectly correct and safe.

2012-04-04 17:35
by Ted Hopp

It can certainly matter what logic is used inside run() to decide when to return. For example, if the logic inside run() was, while(!condition){ // do something } return; run() method may never return even if condition was set to true (on a different thread), unless there was some synchronization on condition - sethro 2012-04-04 18:01

@sethro - That doesn't change the fact that the thread stops when the run() method returns. It's certainly possible to write infinite loops or run afoul of inter-thread synchronization issues like you describe, but that just means that the run() method may never return (and hence the thread will never stop). My answer does need an amendment, however: the thread will stop when the run() method exits, even if it exits by throwing an exception instead of returning - Ted Hopp 2012-04-04 18:45

I really don't mean to be pedantic, but the way this question is asked implies that the OP is trying to explicitly stop a Thread, which is different than letting a Thread complete its task and end. If that's what OP wants to do, then why the condition? Maybe I'm just reading it incorrectly. I am truthfully trying to be helpful as I have seen this kind of asynchronous thread termination many times - sethro 2012-04-04 19:39

@sethro - Your point about synchronization is a good one and I wasn't trying to dismiss it. I just wanted to clarify that two different things are being discussed: the rule that a thread exits when the run() method returns or abnormally ends (which is a direct answer to OP's question as I understood it) and the rules for correctly sharing data between threads - Ted Hopp 2012-04-04 20:33

Certainly. The Thread will then finish and can be joined with. The thread would also exit if you threw a RuntimeException or just let the code run off the end of the run() method obviously.

2012-04-04 17:33
by Gray

Yes, it is correct and safe...

2012-04-04 17:34
by Win Myo Htet