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can subsequent sleep() be skipped if the thread actually slept too long?

Let's assume that I have a following loop in a Thread (Let's call it Thread-A).
while (threadCondition) {
System.out.println(new Date());
Thread.sleep(1000);
}
and assume that other Thread-B will cause that the application is hanging up of the time >=2sec (because of some other Thread (Let's call it Thread-B, because of not enough CPU resources, low available memory, etc.)
Is it possible that when the Thread-A will come into action after those mentioned >=2 sec then the System.out.println(new Date()) will be executed twice one after another ('instantly', without the sleep), and will print the equal date (with the same number of millis) twice?

JVM does not guarantee the precision of time slept. It may wake after 996ms, 1003ms, or 2000ms - all are valid times
However, these sleep times are not guaranteed to be precise, because
they are limited by the facilities provided by the underlying OS.
Also, the sleep period can be terminated by interrupts, as we'll see
in a later section. In any case, you cannot assume that invoking sleep
will suspend the thread for precisely the time period specified --
source
No track is kept of the amount of time the thread really slept.
When another sleep() is encountered, the thread will wait another (and again, more or less) 1000ms before waking.
Since the sleep time is not precise, it is possible that at time 10.000 your app printed the date once, and at 10.981 it printed the same date again, evn though almost once second has passed.
Also: remember, that the sleep() method may be interrupted. If the exception handling code is in this loop, the sleep may get interrupted, exception swallowed and two dates printed.

Two calls to new Date() can give the same time if
if is in the same milli-second
if they are within the resolution of the clock. e.g. on Windows XP it is ~16 ms.
if time is changed by NTP or similar. e.g. time can go backwards and repeat.
if you use have byte code instrumentation to override the time. e.g. because you want a test driven clock.
if two thread start 2 seconds apart but finish at the same time, they can print the same time.
if thread B starts first and thread A starts after, thread B can print a time after thread A. Just because it starts earlier doesn't mean it will finish first.
new Date() will print the same date if they occur in the same second by default.

No, Thread.sleep(...) delays execution from when it is called for the specified amount of time. There's no internal counter keeping track of "current time" which can become out of sync due to the thread being pushed to the background temporarily, which seems to be what you're thinking.

I don't believe it is possible.
When application is hanging up because of any of the reasons you mentioned, it resumes executing from the line of code it stopped.
The fact that the code cause a hang that now may be redundant is irrelevant. You don't really expect the JVM to analyze your code...

Why not use System.nanoTime() ( ie the high-resolution performance counter in the PC) in place of date(). And if you put the sleep in an if and write the condition correctly it won't sleep the thread.

How can I brutally and mercilessly abort a task in Java?

I programmed a sudoku solver in Java for a homework, and I am currently trying to figure out the problematic inputs it can face to make it better. I have generated a few thousand sudoku grids with David Bau's sudoku generator, and now I am running my program against them.
The problem is that while most of them complete in very reasonable times, some of them prove to be problematic and make my algorithm search like crazy until I run out of heap space. So I thought I should offshore the solving job to a secondary thread and run it with a timeout. Right now, I'm using a thread 'pool' of one thread (in the form of an ExecutorService) and I'm submitting Callables to it. I then try to get the value with a timeout:
Callable<Long> solveAndReturnTime = new Callable<Long>() { /* snip */ };
Future<Long> time = executor.submit(solveAndReturnTime);
try
{
long result = time.get(10, TimeUnit.SECONDS);
System.out.printf("%d millis\n", result);
}
catch (TimeoutException e)
{
System.err.println("timed out");
time.cancel(true);
}
My problem is that apparently, one does not simply cancel a Future in Java. Future<T>.cancel(boolean) apparently doesn't interrupt the task right away. Because of that, the pool is stuck with carrying an undying task, and the subsequent attempts timeout because they never get a chance to run.
Adding more threads to the pool is not an option because I run on limited cores and if too many tasks obstinately run, the legitimate ones will be unfairly slowed down. I also don't want the overhead of frequently checking if the task was aborted from my main algorithm.
How can I abruptly, mercilessly and brutally terminate a task? I'm open to anything that will let me recover on the main thread.
EDIT My algorithm is completely sequential, uses no global object, and contains no lock. As far as I can tell, nothing will go wrong if the task is cancelled at a random moment; and even if it does, it's not production code. I'm ready to walk the dangerous and treacherous walk for this one.

Just as in any other language methods to mercifully terminate a thread are Deprecated or not recommended. Because such methods may cause deadlocks (a thread being terminated will not release the locks it's holding).
The correct solution to the problem is having an additional check for Thread.currentThread ().isInterrupted () on every iteration of the main cycle in you Callable. So when the thread is being interrupted it would see it and gracefully shut down.
And since it's you code running in another thread it shouldn't be difficult for you to modify it.

In addition to Andrei's answer, which is correct, you should be aware that doing this work in a thread will not protect your application from running out of memory via an OOM. If your worker thread consumes the entire heap, the main thread can very well die too.

I believe my case was 'special' enough to use Thread.stop, so here is my solution to the people who believe their case is special enough too. (I would take extreme care using that somewhere it could actually matter, though.)
As mostly everyone points out, there's no clean way to stop a task without having that task check if it should stop itself. I created a class that implements Runnable to carry out in such a way that it won't be dramatic if it's killed. The result field (milliseconds) is an AtomicLong because writes on regular long variables are not guaranteed to be atomic.
class SolveTimer implements Runnable
{
private String buildData;
private AtomicLong milliseconds = new AtomicLong(-1);
public SolveTimer(String buildData)
{
assert buildData != null;
this.buildData = buildData;
}
public void run()
{
long time = System.currentTimeMillis();
// create the grid, solve the grid
milliseconds.set(System.currentTimeMillis() - time);
}
public long getDuration() throws ContradictionException
{
return milliseconds.get();
}
}
My code creates a thread on each iteration and runs a SolveTimer. It then attempts to join within 10 seconds. After join returns, the main thread calls getDuration on the run timer; if it returns -1, then the task is taking too long and the thread is killed.
SolveTimer timer = new SolveTimer(buildData);
Thread worker = new Thread(timer);
worker.start();
worker.join(10000);
long result = timer.getDuration();
if (result == -1)
{
System.err.println("Unable to solve");
worker.stop();
}
It should be noted that this makes worker threads harder to debug: when the thread is suspended by the debugger, it can still be killed by Thread.stop(). On my machine, this writes a short error message about ThreadDeath in the console and crashes the Java process.
There is a possible race condition where the worker thread completes exactly (or right after) getDuration is called, and because of that result will be -1 even if the task actually succeeded. However, that's something I can live with: 10 seconds is already far too long, so at that point I don't really care anymore if it's nearly good enough.

What are the main uses of yield(), and how does it differ from join() and interrupt()?

I am a little bit confused about the use of Thread.yield() method in Java, specifically in the example code below. I've also read that yield() is 'used to prevent execution of a thread'.
My questions are:
I believe the code below result in the same output both when using yield() and when not using it. Is this correct?
What are, in fact, the main uses of yield()?
In what ways is yield() different from the join() and interrupt() methods?
The code example:
public class MyRunnable implements Runnable {
public static void main(String[] args) {
Thread t = new Thread(new MyRunnable());
t.start();
for(int i=0; i<5; i++) {
System.out.println("Inside main");
}
}
public void run() {
for(int i=0; i<5; i++) {
System.out.println("Inside run");
Thread.yield();
}
}
}
I obtain the same output using the code above both with and without using yield():
Inside main
Inside main
Inside main
Inside main
Inside main
Inside run
Inside run
Inside run
Inside run
Inside run

Source: http://www.javamex.com/tutorials/threads/yield.shtml
Windows
In the Hotspot implementation, the way that Thread.yield() works has
changed between Java 5 and Java 6.
In Java 5, Thread.yield() calls the Windows API call Sleep(0). This
has the special effect of clearing the current thread's quantum and
putting it to the end of the queue for its priority level. In other
words, all runnable threads of the same priority (and those of greater
priority) will get a chance to run before the yielded thread is next
given CPU time. When it is eventually re-scheduled, it will come back
with a full full quantum, but doesn't "carry over" any of the
remaining quantum from the time of yielding. This behaviour is a
little different from a non-zero sleep where the sleeping thread
generally loses 1 quantum value (in effect, 1/3 of a 10 or 15ms tick).
In Java 6, this behaviour was changed. The Hotspot VM now implements
Thread.yield() using the Windows SwitchToThread() API call. This call
makes the current thread give up its current timeslice, but not its
entire quantum. This means that depending on the priorities of other
threads, the yielding thread can be scheduled back in one interrupt
period later. (See the section on thread scheduling for more
information on timeslices.)
Linux
Under Linux, Hotspot simply calls sched_yield(). The consequences of
this call are a little different, and possibly more severe than under
Windows:
a yielded thread will not get another slice of CPU until all other threads have had a slice of CPU;
(at least in kernel 2.6.8 onwards), the fact that the thread has yielded is implicitly taken into account by the scheduler's heuristics
on its recent CPU allocation— thus, implicitly, a thread that has
yielded could be given more CPU when scheduled in the future.
(See the section on thread scheduling for more details on priorities
and scheduling algorithms.)
When to use yield()?
I would say practically never. Its behaviour isn't standardly defined
and there are generally better ways to perform the tasks that you
might want to perform with yield():
if you're trying to use only a portion of the CPU, you can do this in a more controllable way by estimating how much CPU the thread
has used in its last chunk of processing, then sleeping for some
amount of time to compensate: see the sleep() method;
if you're waiting for a process or resource to complete or become available, there are more efficient ways to accomplish this,
such as by using join() to wait for another thread to complete, using
the wait/notify mechanism to allow one thread to signal to another
that a task is complete, or ideally by using one of the Java 5
concurrency constructs such as a Semaphore or blocking queue.

I see the question has been reactivated with a bounty, now asking what the practical uses for yield are. I'll give an example from my experience.
As we know, yield forces the calling thread to give up the processor that it's running on so that another thread can be scheduled to run. This is useful when the current thread has finished its work for now but wants to quickly return to the front of the queue and check whether some condition has changed. How is this different from a condition variable? yield enables the thread to return much quicker to a running state. When waiting on a condition variable the thread is suspended and needs to wait for a different thread to signal that it should continue. yield basically says "allow a different thread to run, but allow me to get back to work very soon as I expect something to change in my state very very quickly". This hints towards busy spinning, where a condition can change rapidly but suspending the thread would incur a large performance hit.
But enough babbling, here's a concrete example: the wavefront parallel pattern. A basic instance of this problem is computing the individual "islands" of 1s in a bidimensional array filled with 0s and 1s. An "island" is a group of cells that are adjacent to eachother either vertically or horizontally:
1 0 0 0
1 1 0 0
0 0 0 1
0 0 1 1
0 0 1 1
Here we have two islands of 1s: top-left and bottom-right.
A simple solution is to make a first pass over the entire array and replace the 1 values with an incrementing counter such that by the end each 1 was replaced with its sequence number in row major order:
1 0 0 0
2 3 0 0
0 0 0 4
0 0 5 6
0 0 7 8
In the next step, each value is replaced by the minimum between itself and its neighbours' values:
1 0 0 0
1 1 0 0
0 0 0 4
0 0 4 4
0 0 4 4
We can now easily determine that we have two islands.
The part we want to run in parallel is the the step where we compute the minimums. Without going into too much detail, each thread gets rows in an interleaved manner and relies on the values computed by the thread processing the row above. Thus, each thread needs to slightly lag behind the thread processing the previous line, but must also keep up within reasonable time. More details and an implementation are presented by myself in this document. Note the usage of sleep(0) which is more or less the C equivalent of yield.
In this case yield was used in order to force each thread in turn to pause, but since the thread processing the adjacent row would advance very quickly in the meantime, a condition variable would prove a disastrous choice.
As you can see, yield is quite a fine-grain optimization. Using it in the wrong place e.g. waiting on a condition that changes seldomly, will cause excessive use of the CPU.
Sorry for the long babble, hope I made myself clear.

About the differences between yield(), interrupt() and join() - in general, not just in Java:
yielding: Literally, to 'yield' means to let go, to give up, to surrender. A yielding thread tells the operating system (or the virtual machine, or what not) it's willing to let other threads be scheduled in its stead. This indicates it's not doing something too critical. It's only a hint, though, and not guaranteed to have any effect.
joining: When multiple threads 'join' on some handle, or token, or entity, all of them wait until all other relevant threads have completed execution (entirely or upto their own corresponding join). That means a bunch of threads have all completed their tasks. Then each one of these threads can be scheduled to continue other work, being able to assume all those tasks are indeed complete. (Not to be confused with SQL Joins!)
interruption: Used by one thread to 'poke' another thread which is sleeping, or waiting, or joining - so that it is scheduled to continue running again, perhaps with an indication it has been interrupted. (Not to be confused with hardware interrupts!)
For Java specifically, see
Joining:
How to use Thread.join? (here on StackOverflow)
When to join threads?
Yielding:
Interrupting:
Is Thread.interrupt() evil? (here on StackOverflow)

First, the actual description is
Causes the currently executing thread object to temporarily pause and
allow other threads to execute.
Now, it is very likely that your main thread will execute the loop five times before the run method of the new thread is being executed, so all the calls to yield will happen only after the loop in the main thread is executed.
join will stop the current thread until the thread being called with join() is done executing.
interrupt will interrupt the thread it is being called on, causing InterruptedException.
yield allows a context switch to other threads, so this thread will not consume the entire CPU usage of the process.

The current answer(s) are out-of-date and require revision given recent changes.
There is no practical difference of Thread.yield() between Java versions since 6 to 9.
TL;DR;
Conclusions based on OpenJDK source code (http://hg.openjdk.java.net/).
If not to take into account HotSpot support of USDT probes (system tracing information is described in dtrace guide) and JVM property ConvertYieldToSleep then source code of yield() is almost the same. See explanation below.
Java 9:
Thread.yield() calls OS-specific method os::naked_yield():
On Linux:
void os::naked_yield() {
sched_yield();
}
On Windows:
void os::naked_yield() {
SwitchToThread();
}
Java 8 and earlier:
Thread.yield() calls OS-specific method os::yield():
On Linux:
void os::yield() {
sched_yield();
}
On Windows:
void os::yield() { os::NakedYield(); }
As you can see, Thread.yeald() on Linux is identical for all Java versions.
Let's see Windows's os::NakedYield() from JDK 8:
os::YieldResult os::NakedYield() {
// Use either SwitchToThread() or Sleep(0)
// Consider passing back the return value from SwitchToThread().
if (os::Kernel32Dll::SwitchToThreadAvailable()) {
return SwitchToThread() ? os::YIELD_SWITCHED : os::YIELD_NONEREADY ;
} else {
Sleep(0);
}
return os::YIELD_UNKNOWN ;
}
The difference between Java 9 and Java 8 in the additional check of the existence of the Win32 API's SwitchToThread() method. The same code is present for Java 6.
Source code of os::NakedYield() in JDK 7 is slightly different but it has the same behavior:
os::YieldResult os::NakedYield() {
// Use either SwitchToThread() or Sleep(0)
// Consider passing back the return value from SwitchToThread().
// We use GetProcAddress() as ancient Win9X versions of windows doen't support SwitchToThread.
// In that case we revert to Sleep(0).
static volatile STTSignature stt = (STTSignature) 1 ;
if (stt == ((STTSignature) 1)) {
stt = (STTSignature) ::GetProcAddress (LoadLibrary ("Kernel32.dll"), "SwitchToThread") ;
// It's OK if threads race during initialization as the operation above is idempotent.
}
if (stt != NULL) {
return (*stt)() ? os::YIELD_SWITCHED : os::YIELD_NONEREADY ;
} else {
Sleep (0) ;
}
return os::YIELD_UNKNOWN ;
}
The additional check has been dropped due to SwitchToThread() method are available since Windows XP and Windows Server 2003 (see msdn notes).

What are, in fact, the main uses of yield()?
Yield suggests to the CPU that you may stop the current thread and start executing threads with higher priority. In other words, assigning a low priority value to the current thread to leave room for more critical threads.
I believe the code below result in the same output both when using yield() and when not using it. Is this correct?
NO, the two will produce different results. Without a yield(), once the thread gets control it will execute the 'Inside run' loop in one go. However, with a yield(), once the thread gets control it will print the 'Inside run' once and then will hand over control to other thread if any. If no thread in pending, this thread will be resumed again. So every time "Inside run' is executed it will look for other threads to execute and if no thread is available, the current thread will keep on executing.
In what ways is yield() different from the join() and interrupt() methods?
yield() is for giving room to other important threads, join() is for waiting for another thread to complete its execution, and interrupt() is for interrupting a currently executing thread to do something else.

Thread.yield() causes thread to go from "Running" state to "Runnable" state.
Note: It doesn't cause thread to go "Waiting" state.

Thread.yield(); frees the bottom thread.
Thread is using OS threads, so Thread.yield(); might free the hardware thread.
Bad implementation for sleep(millis)
public class MySleep {
public static void sleep(long millis) throws InterruptedException {
long start = System.currentTimeMillis();
do {
Thread.yield();
if (Thread.interrupted()) {
throw new InterruptedException();
}
} while (System.currentTimeMillis() - start < millis);
}
}
and join()
public class MyJoin {
public static void join(Thread t) throws InterruptedException {
while (t.getState() != Thread.State.TERMINATED) {
Thread.yield();
if (Thread.interrupted()) {
throw new InterruptedException();
}
}
}
public static void main(String[] args) {
Thread thread = new Thread(()-> {
try {
Thread.sleep(2000);
} catch (Exception e) {
}
});
thread.start();
System.out.println("before");
try {
join(thread);
} catch (Exception e) {
}
System.out.println("after");
}
}
This should work even if there is only one hardware thread, unless Thread.yield(); is removed.

Thread.yield()
When we invoke Thread.yield() method, the thread scheduler keep the currently running thread to Runnable state and picks another thread of equal priority or higher priority. If there is no equal and higher priority thread then it reschedule the calling yield() thread. Remember yield method does not make the thread to go to Wait or Blocked state. It can only make a thread from Running State to Runnable State.
join()
When join is invoked by a thread instance, this thread will tell currently executing thread to wait till the Joining thread completes. Join is used in the situations when a task which should be completed before the current task is going to finish.

yield() main use is for putting a multi-threading application on hold.
all these methods differences are yield() puts thread on hold while executing another thread and returning back after the completion of that thread, join() will bring the beginning of threads together executing until the end and of another thread to run after that thread has ended, interrupt() will stop the execution of a thread for a while.

Java Timer and scheduleAtFixedRate + System Suspend

I am working on a Java program and using Timer objects to run tasks every few minutes or hours. This works fine in normal operations, but I am running into a problem with "Sleep mode" on Mac (maybe on other OSes, but I haven't tried yet).
Consider this code sample:
//Setup the timer to fire the ping worker (every 3 minutes)
_PingTimer.scheduleAtFixedRate(new TimerTask(){
public void run(){
Program.PingThread = new PingWorker(Settings.Username, Settings.UserHash, true, true);
Program.PingThread.CheckOpenPort = true;
Program.SwingExecutor.execute(Program.PingThread);
}
}, 0, 180000);
In normal operation this would fire every 3 minutes with enough accuracy (I'm not concerned about the exact second or anything). The problem with this is after sleeping the computer for a few hours or so it seems to just BLAST the system with backlogged timer requests.
It seems to be running all of the missed timer hits during sleep at once trying to make up for lost time.
Is there a way i can prevent this? I tried using synchronized and some other thread techniques, but this only ensures that they aren't all running at the same time. They still continue to run one after another until the backlog is passed.
Thanks for any help you can provide!

Have you looked at the API? It clearly states the following:
In fixed-rate execution, each
execution is scheduled relative to the
scheduled execution time of the
initial execution. If an execution is
delayed for any reason (such as
garbage collection or other background
activity), two or more executions will
occur in rapid succession to "catch
up." In the long run, the frequency of
execution will be exactly the
reciprocal of the specified period
(assuming the system clock underlying
Object.wait(long) is accurate).
This is one reason why you should consider using a ScheduledExecutorService. This link may also prove useful.

Use schedule instead of scheduleAtFixedRate.

Java Thread.sleep waking up "much" earlier than the scheduled time

I have a requirement of writing a daemon which initiates number of threads that wakes up at different times. The daemon is written in Java using commons apache library and is run on a Linux machine (Fedora 13 ).
One thread wakes up everyday to perform task A which happens as scheduled.
But there is another thread which is scheduled to wake up every Monday at 6 am to perform some task which does not happen as scheduled. The problem is that this thread wakes up much before the actual scheduled time. It runs 2 days after the completion of the previous run though it should run only after a week. The waiting time is calculated correctly using our own timer class and because this reuses existing code i do not see a problem in this.
What could be the problem here?
Thanks

Thread.sleep() doesn't make any guarantees, and it might wake up earlier than expected. you should always use it in the loop:
long curTime = System.currentTimeMillis();
while (wakeUpTime - curTime > 0) {
try {
Thread.sleep(wakeUpTime - curTime);
} catch (InterruptedException ex) { }
curTime = System.currentTimeMillis();
}