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Implementing a code-competition with timeout [closed]

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I would like to create a code-competition in Java. The basic plan is:
Each competitor submits a class that implements the Function interface.
I apply the function of each submitted class on a set of pre-made inputs.
The grade of each submission is its number of correct outputs.
Now, I want to add a timeout: each class is allowed to run for at most 1 second on each input. If a class runs for more than one second, it should be stopped and graded 0 on that input.
My initial idea was to run each test in a separate thread, and stop the thread after a second. However, to stop a thread in Java, it is required to change its code. Here, the code is submitted by other people and I do not want to read all the submissions to verify that they allow interruption.
How can I implement such a competition?

Threads are not guaranteed to share resources fairly. Therefore, wall clock time in a "online judge" should be suspect, especially with the upper limit set in the second or minute range.
If you want to determine if people are using optimized solutions, perhaps you could set the limit a lot higher and add a few test cases with data sets that assured one was using a reasonable algorithm. With a ten minutes to compete, the odds of small scheduling differences is average out in ways that obliterate the need for more sophisticated CPU time measurements.
As for the Thread safety, you'd probably want to not use Threads in this case. Spawning a process would offload the online judge, prevent one contestant from possibly inspecting / interfering with another, provide an obvious means of termination (by the kill signal), and permit better bench marking of time (akin to the Unix command "time").
When something goes wrong in a threaded environment, it has the potential to destabilize the program, by using processes, extra barriers will prevent any such destabilization from impacting your online judge.

Using Junit? You could give this a try:
https://github.com/junit-team/junit4/wiki/timeout-for-tests

So One way that you could implement this would be to use two separate threads for 1 competitor. A ThreadTimer and A ThreadHelper
public class ThreadTimer extends Thread {
public ThreadTimer() {
}
#Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
Logger.getLogger(ThreadTimer.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
And ThreadHelper Which runs the function
public class ThreadHelper extends Thread {
Calculator c;
public ThreadHelper(Calculator c) {
this.c = c;
}
public Calculator getC() {
return c;
}
public void setC(Calculator c) {
this.c = c;
}
#Override
public void run() {
long startTime = System.nanoTime();
long plus = c.add();
long endTime = System.nanoTime();
long duration = (endTime - startTime);
long seconds = duration / 1000000000;
System.out.println("Add Time: " + seconds);
}
}
Your interface you created I am calling Calculator in my code.
This is calculating how long add takes and outputs the duration. I am sure the calculations are much more complex, but a potential answer to your question would come in the startup class:
public class Competition {
public static void main(String[] args) throws InterruptedException, Exception {
Calculator jim = new JimSmithsCalculator();
Calculator john = new JohnDoesCalculator();
ThreadHelper jimsThread = new ThreadHelper(jim);
ThreadTimer time1 = new ThreadTimer();
ThreadHelper JohnsThread = new ThreadHelper(john);
ThreadTimer time2 = new ThreadTimer();
time1.start();
jimsThread.start();
//This will run a loop ensuring both of the above threads are terminated...
checkSeconds(time1, jimsThread);//This also does the time check
//...Before moving on to these threads.
time2.start();
JohnsThread.start();
checkSeconds(time2, JohnsThread);
}
public static void checkSeconds(ThreadTimer time, ThreadHelper t) throws Exception {
while (t.isAlive()) {
if (time.getState() == Thread.State.TERMINATED) {
throw new Exception(t.getName() + " >> " + t.getClass() + " Failed!!!");
}
}
}
}
Since You can not use the stop() method anymore, you could throw an exception if ThreadTimer completes before ThreadHelper does.
This will output an exception and continue the program. You could then see that a competitors thread failed with the exception.
The main point to all of this random code and my answer to your question is this method :
public static void checkSeconds(ThreadTimer time, ThreadHelper t) throws Exception {
while (t.isAlive()) {
if (time.getState() == Thread.State.TERMINATED) {
throw new Exception(t.getName() + " >> " + t.getClass() + " Failed!!!");
}
}
}
I don't know if this would work exactly as you would want it.
I hope this at least sparks an idea.

java threads common questions [closed]

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I have some questions about java threads:
if I have a class that runs a number of threads when one thread reaches to exit(1) command line will it exit the thread or the whole application including the other threads?
2.how can I make one thread notify all other threads that run from the same class to stop?
3.does using synchronized make only one thread ,regardless his source in the code, to do this part of code?
4.if I am running a thread in java using run() if I call a method from inside the run() does this still running as a thread or it is not allowed?

1) Only if thread are daemons, application will not wait to close even if they are running.
Example: comment or not the setDaemon(true)
public class Test {
public static void main(final String[] args) {
new Launcher(parent).start();
}
public static class Launcher extends Thread {
public Launcher(final String name) {
this.setName(name);
}
#Override
public void run() {
System.err.println(getName() + is starting);
for (int i = 0; i < 10; i++) {
new Waiter(child + i).start();
}
try {
Thread.sleep(1000);
System.err.println(getName() + is no more sleeping);
} catch (final InterruptedException e) {
e.printStackTrace();
}
System.err.println(getName() + is stopping);
}
}
public static class Waiter extends Thread {
public Waiter(final String name) {
super(name);
super.setDaemon(true);
}
#Override
public void run() {
System.err.println(getName() + is starting);
try {
Thread.sleep(12000);
System.err.println(getName() + is no more sleeping);
} catch (final InterruptedException e) {
e.printStackTrace();
}
}
}
}
2) To force the stop, you can use different approach, different patterns. Sorry to not have THE solution. You can take a look at : http://docs.oracle.com/javase/7/docs/api/java/lang/ThreadGroup.html
3) Yeah, it's the goal of synchronized.
4) It depends what you do but basically, calling a method do not launch another thread, except if the methods does it.

if I have a class that runs a number of threads when one thread reaches to exit(1) command line will it exit the thread or the whole application including the other threads?
A call to System.exit(1) will terminate the calling thread and all other threads. It is the most brutal way of ending a Java program, namely completely shutting down the JVM, and should rarely be used at all.
how can I make one thread notify all other threads that run from the same class to stop?
Not at all. If you have references to the other Thread objects, you can call Thread#interrupt(), but whether or not the other threads check their interrupted status or reach a place where they may throw an InterruptedException is left to them.
does using synchronized make only one thread ,regardless his source in the code, to do this part of code?
Intuitively yes. However, when there is a wait() in this block, then the corresponding lock (that is, the object that was synchronized on) will be released and can be acquired by other threads.
if I am running a thread in java using run() if I call a method from inside the run() does this still running as a thread or it is not allowed?
When a thread calls a method, then the same thread also executes this method (silently wondering how else it should be...)

Core concepts of java multithreading [closed]

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I know some basic concepts in java multithreading.But now i want to create 5 threads that should work concurrently.How can i get execution time of a thread?...Somebody please help me with the deep concepts of threads including methods and purposes.

Your question is really unclear. What do you mean by execution time of a thread? When it started vs. when it stopped (wall time) Or how long it was actually running, not including times it was on hold (i.e., CPU time)?
Take a look at Monitor cpu usage per thread in java?
BTW, Threading isn't something you can simply learn from a StackOverflow answer.
The official guide to Java explains concurrency quite well:
http://docs.oracle.com/javase/tutorial/essential/concurrency/
The book "Java Concurrency in Practice" is even better.

Make a proxy
class Proxy implements Runnable {
final Runnable target;
Proxy(Runnable target) {
this.target = target;
}
public void run() {
long t0 = System.currentTimeMillis();
try {
target.run();
} finally {
System.out.println(Thread.currentThread() + " execution time = " + (System.currentTimeMillis() - t0));
}
}
}
and use it
new Thread(new Proxy(task)).start();

You can use the methods of
ThreadMxBean interface
you can get the instance using
ManagementFactory.getThreadMXBean();
after that you can call a method
getThreadCpuTime(Thread.currentThread().getId());
so your code will look like
ManagementFactory.getThreadMXBean.getThreadCpuTime(Thread.currentThread().getId());
for more details see Docs

Something like this code could be useful http://blog.sheidaei.com/2013/06/simple-thread-example-in-java.html.
You can use System.currentTimeMillis() instead of System.out.println() to get the execution time of the threads.
/**
* Created with IntelliJ IDEA.
* User: shahin
* Date: 6/5/13
* Time: 11:32 PM
* To change this template use File | Settings | File Templates.
*/
public class SimpleThread implements Runnable{
public SimpleThread(String simpleName) {
this.simpleName = simpleName;
System.out.println(">>> Constructor for " + getSimpleName());
}
public String getSimpleName() {
return simpleName;
}
public void setSimpleName(String simpleName) {
this.simpleName = simpleName;
}
private String simpleName;
#Override
public void run() {
System.out.println(" >> "+getSimpleName() + " started.");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
}
System.out.println(" >> "+getSimpleName() + " stopped.");
}
public static void main(String args[])
{
System.out.println("Main Thread started.");
SimpleWaitNotifyThread simpleThread;
Thread thread;
for(int i=0;i<5;i++)
{
simpleThread = new SimpleWaitNotifyThread("Thread "+(i+1));
thread = new Thread(simpleThread,"Thread "+(i+1));
thread.start();
}
System.out.println("Main Thread finished.");
}
}

Thread-launched running processes won't destroy (Java)

Starting multiple threads and having each exec() then destroy() a running java process result in some of the process not being destroyed and still running after program exit. Here is some code that reproduce the issue. I noticed the more threads you start, the more processes stay alive. And the more sleep before destroy(), the less processes stay alive. (I used InfiniteLoop as an example. Any running process will do the trick.)
EDIT : Bug has been reported to Oracle, waiting for an answer. Feel free to share any knowledge/experiments on the subject.
for(int i = 0; i < 100; i++)
{
new Thread(new Runnable()
{
public void run()
{
try
{
Process p = Runtime.getRuntime().exec(new String[]{"java", "InfiniteLoop"});
Thread.sleep(1);
p.destroy();
}catch(IOException | InterruptedException e){e.printStackTrace();}
}
}).start();
}

Use a p.waitFor(); before p.destroy(); ,
this will ensure the completion of the previous process. I think you p.destroy command gets invoked sooner than the exec() command performs the action. Therefore it becomes useless.

If subprocesses write anything to stdout or stderr (intentionally or not), that could cause trouble:
"Because some native platforms only provide limited buffer size for
standard input and output streams, failure to promptly write the input
stream or read the output stream of the subprocess may cause the
subprocess to block, and even deadlock."
Source: http://www.javaworld.com/jw-12-2000/jw-1229-traps.html
The whole article is IMO worth reading if you need to use Runtime.exec().

This is simply because before the threads execute the destroy call, your main program terminates and all the associated threads leaving the started processes running. To verify this, simply add a System.out call after the destroy and you will find it is not executed. To overcome this add a Thread.sleep at the end of your main method and you will not have the orphaned processes. The below does not leave any process running.
public class ProcessTest {
public static final void main (String[] args) throws Exception {
for(int i = 0; i < 100; i++) {
new Thread(new Runnable()
{
public void run() {
try {
Process p = Runtime.getRuntime().exec(new String[]{"java", "InfiniteLoop"});
Thread.sleep(1);
p.destroy();
System.out.println("Destroyed");
}catch(IOException e) {
System.err.println("exception: " + e.getMessage());
} catch(InterruptedException e){
System.err.println("exception: " + e.getMessage());
}
}
}).start();
}
Thread.sleep(1000);
}
}

You should close the input/output/error streams to the process. We saw some issues in the past where the forked process was not completing properly due to those streams not being closed (even if they weren't being used).
An exemplary solution:
p.destroy();
p.getInputStream().close();
p.getOutputStream().close();
p.getErrorStream().close();

I believe that according to link, a distinct process is spawned by the operating system in response to this call. This process has a lifetime independent of your Java program and threads within it so you would expect it to continue running after your program has exited. I just tried it on my machine and it appeared to work as expected:
import java.io.*;
class Mp {
public static void main(String []args) {
for(int i = 0; i < 100; i++) {
new Thread(new Runnable() {
public void run() {
try {
System.out.println("1");
Process p = Runtime.getRuntime().exec
(new String[]{"notepad", ""});
System.out.println("2");
Thread.sleep(5);
System.out.println("3");
p.destroy();
System.out.println("4");
}
catch(IOException | InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
}
}

This is not an answer; I am posting complete source for my own attempt at recreating this problem as per discussion in question comments.
I cannot reproduce this problem on Ubuntu 12.04; OpenJDK 6b_27 (however, see below).
ProcessTest.java:
import java.io.*;
public class ProcessTest {
public static final void main (String[] args) throws Exception {
for(int i = 0; i < 100; i++) {
new Thread(new Runnable()
{
public void run() {
try {
Process p = Runtime.getRuntime().exec(new String[]{"java", "InfiniteLoop"});
Thread.sleep(1);
p.destroy();
}catch(IOException e) {
System.err.println("exception: " + e.getMessage());
} catch(InterruptedException e){
System.err.println("exception: " + e.getMessage());
}
}
}).start();
}
}
}
InfiniteLoop.java
public class InfiniteLoop {
public static final void main (String[] args) {
while (true) ;
}
}
I cannot reproduce the issue where processes remaining running after the JVM terminates. However, if I add a long delay in the main thread after starting the threads but before returning from main, I do see roughly a dozen running java processes that stick around (although they are terminated when the main program terminates).
Update:
I just had it leave about 5 processes running after it terminated. It doesn't always happen. Weird. I want to know more about this too. I have a hunch that it has something to do with destroying the process too quickly or some kind of race condition; maybe java forks something off or does something to create a new process that destroy() doesn't take care of if called too quickly / at the wrong time.
I found an old bug (but it is not mark resolved) stating that if a process spawns subprocesses they may not be killed by destroy(). bugs.sun.com/bugdatabase/view_bug.do?bug_id=4770092 What version of the JDK are you using.
Here's another reference to what looks like a similar issue: Java tool/method to force-kill a child process And I want to apologize if I've only added confusion to your life, I don't actually use Process that much and am not familiar with the quirks. Hopefully somebody else will step in with a definitive answer. It seems like it doesn't handle subprocesses well, and I'm presuming java forks something off. That's all I got.

There is a race condition between the time Runtime.exec kicks off a new thread to start a Process and when you tell that process to destroy itself.
I'm on a linux machine so I will use the UNIXProcess.class file to illustrate.
Runtime.exec(...) will create a new ProcessBuilder and start it which on a unix machine creates a new UNIXProcess instance. In the constructor of UNIXProcess there is this block of code which actually executes the process in a background (forked) thread:
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction() {
public Object run() {
Thread t = new Thread("process reaper") {
public void run() {
try {
pid = forkAndExec(prog,
argBlock, argc,
envBlock, envc,
dir,
redirectErrorStream,
stdin_fd, stdout_fd, stderr_fd);
} catch (IOException e) {
gate.setException(e); /*remember to rethrow later*/
gate.exit();
return;
}
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction() {
public Object run() {
stdin_stream = new BufferedOutputStream(new
FileOutputStream(stdin_fd));
stdout_stream = new BufferedInputStream(new
FileInputStream(stdout_fd));
stderr_stream = new FileInputStream(stderr_fd);
return null;
}
});
gate.exit(); /* exit from constructor */
int res = waitForProcessExit(pid);
synchronized (UNIXProcess.this) {
hasExited = true;
exitcode = res;
UNIXProcess.this.notifyAll();
}
}
};
t.setDaemon(true);
t.start();
return null;
}
});
Notice that the background thread sets the field pid which is the UNIX process id. This will be used by destroy() to tell the OS which process to kill.
Because there is no way to make sure that this background thread has run when destroy() is called, we may try to kill the process before it has run OR we may try to kill the process before pid field has been set; pid is uninitialized and therefore is 0. So I think calling destroy too early will do the equivalent of a kill -9 0
There is even a comment in the UNIXProcess destroy() that alludes to this but only considers calling destroy after the process has already finished, not before it has started:
// There is a risk that pid will be recycled, causing us to
// kill the wrong process! So we only terminate processes
// that appear to still be running. Even with this check,
// there is an unavoidable race condition here, but the window
// is very small, and OSes try hard to not recycle pids too
// soon, so this is quite safe.
The pid field is not even marked as volatile so we may not even see the most recent value all the time.

I had a very similar issue and the problem with destroy() not working was manifesting even with a single thread.
Process process = processBuilder(ForeverRunningMain.class).start()
long endTime = System.currentTimeMillis() + TIMEOUT_MS;
while (System.currentTimeMillis() < endTime) {
sleep(50);
}
process.destroy();
The process was not always destroyed if TIMEOUT_MS was too low. Adding an additional sleep() before destroy() fixed it (even though I don't have an explanation why):
Thread.sleep(300);
process.destroy();

Java make thread start simultaneously

I got a code here, when it runs it creats and starts a new thread that prints a word per sec, after 5 sec the main method stops the thread. So the program will print 5 words and it does....but not on my homecomputer only on my laptop. On my home computer it prints 6 times, why?
public class Main {
public static void main (String [] args){
try {
T1 t1 = new T1();
System.out.println("Creating and staring thread 1\n");
Thread.sleep(5000);
t1.stopThread();
} catch(InterruptedException ie) {}
}
}
public class T1 extends Thread{
private boolean alive = true;
public T1(){
start();
}
public void run(){
while(alive){
try {
System.out.println("Tråd T1: Tråd 1");
Thread.sleep(1000);
} catch(InterruptedException ie) {}
}
}
public void stopThread(){
alive = false;
}
}

Both results are correct. Sleep times are approximate.

You are lucky that your program stops printing at all. You have a program that has undefined behavior and it could run forever on some machines. You must make alive volatile, otherwise there is no guarantee that your secondary thread will ever see the change made to alive in the main thread.
Look at the end of the Java language specification chapter on memory; they basically give your example as something that must not be done.
That being said, you might still get 6 printed lines instead of 5 from the inaccuracy of Thread.sleep.