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ScheduledExecutorService waits for task to complete, does pending tasks pile up to ultimately interrupting main thred?

I was curious for my new implementation using ScheduledExecutorService in which the task is expected to finish within 100ms period and 0ms delay. But in case if there is system load and its taking say 550 ms, would there be a queue maintained by ScheduledExecutorService for those pending 4 tasks? And then run as soon as (0ms delay) first one is finished. And what if second execution takes 560 ms , would that add another 4 threads to its queue?
There is not documentation around that, or I might be overlooking it. But I want to make sure that the pile up of such executions would trigger to leak or overflow.
For example: below code, could main thread ever fail?
private static ScheduledExecutorService consumerThreadPool = Executors.newSingleThreadScheduledExecutor();
public static void main(String[] args) throws Exception {
consumerThreadPool.scheduleAtFixedRate(() -> performTask(), 0, 1, TimeUnit.MILLISECONDS);
}
private static void performTask () {
try {
Thread.sleep(550);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

Your tasks will be skipped if they overrun the next scheduled time, you can verify easily with System.out.println and alter the sleep under 500ms to 5000ms:
public static void main(final String[] args) throws InterruptedException, ExecutionException
{
var executor = Executors.newScheduledThreadPool(1);
var count = new AtomicInteger();
Runnable task = () -> {
String desc = "run("+((System.currentTimeMillis() / 1000) % 60)+") "+Thread.currentThread().getName()+" count "+count.incrementAndGet();
System.out.println(desc);
if(count.get() == 50)
throw new RuntimeException("I give up!");
try
{
Thread.sleep(2500);
}
catch (InterruptedException e)
{
System.out.println("Thread "+Thread.currentThread().getName()+" INTERRUPTED");
}
};
var future = executor.scheduleAtFixedRate(task, 5000, 1000, TimeUnit.MILLISECONDS);
System.out.println("Calling future.get() ...");
try {
var res = future.get();
System.out.println("future.get()="+res);
}
catch(Exception e)
{
System.err.println("There was an exception:" +e);
// Decide between "continue" or "throw e" here
// ...
}
executor.shutdownNow();
System.out.println("shutdown complete");
}

My program is not returning/ending with Executor

Given the following Java example, which uses multithreading:
import java.util.concurrent.*;
public class SquareCalculator {
private ExecutorService executor = Executors.newSingleThreadExecutor();
public Future<Integer> calculate(Integer input) {
return executor.submit( () -> {
Thread.sleep(1000);
return input * input;
});
}
public static void main(String[] args) {
try {
Future<Integer> future = new SquareCalculator().calculate(10);
while (!future.isDone()){
System.out.println("Calculating...");
Thread.sleep(300);
}
Integer result = future.get();
System.out.println("we got: " + result);
} catch(InterruptedException | ExecutionException e) {
System.out.println("had exception");
}
}
}
It produces:
java SquareCalculator
Calculating...
Calculating...
Calculating...
Calculating...
we got: 100
But the application is never terminating.
Am I suppose to join the thread or something?

Should be in comment but not enough reputation.
You should call shutdown on executor. You can get more details from below link:
Reason for calling shutdown() on ExecutorService

I bet you want to add something like this:
finally {
if (executor != null) {
executor.shutdown();
}
}

You need to shut down the executor framework towards the end of the program and wait until it gracefully terminates..
executor.shutdown();
try {
executor.awaitTermination(4 * 3600, TimeUnit.SECONDS);
} catch (Exception e) {
e.printStackTrace();
}

java simple thread, but it can't run

it's a simple java thread.
there are 2 threads, and it should be printed every 5 second.
there's no error.
but i just can't run.
please help me finding out what's wrong...
class MyThread extends Thread {
String message;
int delay;
public MyThread(String s, int d) {
message = s;
delay = d;
}
public void run() {
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
System.out.println(message + "+" + delay );
}
}
}
public class applet {
public static void main(String[] args){
MyThread mt1, mt2;
mt1 = new MyThread("Hello", 5000);
mt2 = new MyThread("Bye ", 10000);
mt1.start();
mt2.start();
}
}

You are printing ONLY ONCE if an exception occurs...
Modify the run method to:
public void run() {
try {
Thread.sleep(delay);
System.out.println(message + "+" + delay );
} catch (InterruptedException e) {
System.err.println("error here" );
}
}

There are actually 2 mistakes in your program:
1. Print your message in the normal flow
So far you only print your message when you get an InterruptedException which is an exception that is thrown when your thread has been interrupted while sleeping. So instead of printing your message in the catch block, you are supposed to print it after the sleep as next:
try {
Thread.sleep(delay);
System.out.println(message + "+" + delay );
} catch (InterruptedException e) {
// Re-set the interrupted flag
Thread.currentThread().interrupt();
}
2. Add an infinite loop
As you wish to print your message every 5 seconds, you need to call sleep in an infinite loop otherwise your thread will print it only once and die, so the code should be at the end:
try {
while (true) {
Thread.sleep(delay);
System.out.printf("%s+%d%n", message, delay);
}
} catch (InterruptedException e) {
// Re-set the interrupted flag
Thread.currentThread().interrupt();
}
3. Bonus
Alternatively, you can use a ScheduledExecutorService to schedule your tasks with a fixed rate using the method scheduleAtFixedRate, your code would then be:
// Create a ScheduledExecutorService of 2 threads
ScheduledExecutorService executor = Executors.newScheduledThreadPool(2);
// Schedule my task every 5 seconds starting in 5 seconds
executor.scheduleAtFixedRate(new MyTask("Hello", 5000), 5L, 5L, TimeUnit.SECONDS);
// Schedule my task every 10 seconds starting in 10 seconds
executor.scheduleAtFixedRate(new MyTask("Bye ", 10000), 10L, 10L, TimeUnit.SECONDS);
The class MyTask would simply be:
class MyTask implements Runnable {
String message;
int delay;
public MyTask(String s, int d) {
message = s;
delay = d;
}
public void run() {
System.out.printf("%s+%d%n", message, delay);
}
}

Java execution shutdown does not terminate all threads?

I have following sample created to mimic the situation i am encountering related to ExecutionService shutdown process. It seems that it terminates only one thread out of 3 something... and i get error messages on tomcat server.
public class Test {
static final ExecutorService threadExecutor = Executors.newCachedThreadPool();
static Runnable getTask(final String name) {
return new Thread() {
#Override
public void run() {
this.setName("Thread-" + name);
while (true) {
try {
System.out.println(name + " running...[" + this.getName() + "]");
Thread.sleep(10000);
} catch (InterruptedException e) {
System.out.println("InterruptedException..." + this.getName());
throw new Exception(e);
}
}
}
};
}
public static void main(String... strings) {
threadExecutor.submit(getTask("Task-1"));
threadExecutor.submit(getTask("Task-2"));
threadExecutor.submit(getTask("Task-3"));
//--
Runtime.getRuntime().addShutdownHook(new Thread() {
#Override
public void run() {
ThreadPoolExecutor tpe = (ThreadPoolExecutor) threadExecutor;
System.out.println("Active Threads=====>" + tpe.getActiveCount());
tpe.shutdown();
try {
if (!threadExecutor.awaitTermination(1000, TimeUnit.MILLISECONDS)) {
System.out.println("Executor did not terminate in the specified time.");
List<Runnable> droppedTasks = tpe.shutdownNow();
System.out.println("Shutdown thread pool forecibly. " + droppedTasks.size() + " tasks will not be executed.");
}
System.out.println("Active Threads=====>" + tpe.getActiveCount());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
}

shutdown() initiates the shutdown process within the thread pool but allows current running tasks to finish. In your example the task does not finish because of while(true).
shutdownNow() initiates the shutdown and also interrupts the currently running threads. But again your task is handling that interrupted exception and running the while(true) loop.
I think you can simply share a common boolean between your tasks and caller code from where you are calling the threadPoolExecuror.shutdown(). Use that boolean in task instead of while(true).

In Java: how can I make thread watch over another thread?

Sorry if the question is quite simple. I am a beginner.
I have to create thread that calulates something, while the first thread works the other one have to measure if the first thread calculate the function in specified time. If not, it has to throw exception. Else it returns the answer.

I'd take the java.util.concurrent components - simple example
public void myMethod() {
// select some executor strategy
ExecutorService executor = Executors.newFixedThreadPool(1);
Future f = executor.submit(new Runnable() {
#Override
public void run() {
heresTheMethodToBeExecuted();
}
});
try {
f.get(1000, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {
// do something clever
} catch (ExecutionException e) {
// do something clever
} catch (TimeoutException e) {
// do something clever
}
}

Have your thread notify a synchronization object when it is done and have your other thread wait x number of milliseconds for it to finish.
public class Main {
private static final Object mThreadLock = new Object();
static class DoTaskThread extends Thread {
public void run() {
try {
int wait = new Random().nextInt(10000);
System.out.println("Waiting " + wait + " ms");
Thread.sleep(wait);
} catch (InterruptedException ex) {
}
synchronized (mThreadLock) {
mThreadLock.notifyAll();
}
}
}
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
synchronized (mThreadLock) {
DoTaskThread thread = new DoTaskThread();
thread.start();
try {
// Only wait 2 seconds for the thread to finish
mThreadLock.wait(2000);
} catch (InterruptedException ex) {
}
if (thread.isAlive()) {
throw new RuntimeException("thread took too long");
} else {
System.out.println("Thread finished in time");
}
}
}
}

join is a lot simpler than using a lock.
join (millis)
Waits at most millis milliseconds
for this thread to die. A timeout of 0
means to wait forever.
Example code:
Thread calcThread = new Thread(new Runnable(){
#Override
public void run() {
//some calculation
}
});
calcThread.start();
//wait at most 2secs for the calcThread to finish.
calcThread.join(2000);
//throw an exception if the calcThread hasn't completed.
if(calcThread.isAlive()){
throw new SomeException("calcThread is still running!");
}

Have a look at http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html#awaitTermination(long,%20java.util.concurrent.TimeUnit) which allows you to handle this without dealing with thread synchronization yourself.