Develop Reference

Producer consumer code with wait/notify doesn't work on second produce

This is a follow-up question from my previous question asked here.
I am using a PriorityBlockingQueue now. I changed my producer to the following:
synchronized(Manager.queue) {
Manager.queue.add(new Job());
Manager.queue.notify();
}
And changed Consumer to the following. Full code skeleton is here:
//my consumer thread run()
public void run() {
synchronized(Manager.queue) {
while (Manager.queue.peek() == null) {
System.out.println("111111111111111");
try {
Manager.queue.wait();
} catch (InterruptedException e) {
}
}
Job job=Manager.queue.peek();
if (job != null) {
submitJob(job);
if (job.SubmissionFailed.equals("false")) {
// successful submission. Remove from queue. Add to another.
Manager.queue.poll();
Manager.submissionQueue.put(job.uniqueid, job);
}
}
}
My code only works for the first time (first produce and first consume), but it doesn't work for the second time. Somewhere the wait/notify logic fails I guess. The producer pushes new jobs to the queue, but the consumer doesn't peek any more items. In fact, it doesn't even go to the while loop and no more 111111111111111 printing.
What is the problem? How to fix it?

You could simplify all this code to just:
In the Producer:
Manager.queue.add(new Job());
and in the Consumer:
while (true) {
try {
submitJob(Manager.queue.take()); //or do something else with the Job
//your code here, then remove the break
break;
} catch (InterruptedException ex) {
//usually no need to do anything, simply live on unless you
//caused that
}
}
//or your code here, then you need an surrounding while and the break
When using an PriorityBlockingQueue, you don't need any syncronized statements, since they're inside the PriorityBlockingQueue already. And according to the documentation take() wait for an element to be added if necessary and than polls it. See https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/PriorityBlockingQueue.html#take() for reference.
And for the InterruptedException you might want to take a look here: Handling InterruptedException in Java
Edit: added missing try{} catch()

How to terminate a task and continue the next one after a specified time limit? [duplicate]

I have a method that I would like to call. However, I'm looking for a clean, simple way to kill it or force it to return if it is taking too long to execute.
I'm using Java.
to illustrate:
logger.info("sequentially executing all batches...");
for (TestExecutor executor : builder.getExecutors()) {
logger.info("executing batch...");
executor.execute();
}
I figure the TestExecutor class should implement Callable and continue in that direction.
But all i want to be able to do is stop executor.execute() if it's taking too long.
Suggestions...?
EDIT
Many of the suggestions received assume that the method being executed that takes a long time contains some kind of loop and that a variable could periodically be checked.
However, this is not the case. So something that won't necessarily be clean and that will just stop the execution whereever it is is acceptable.

You should take a look at these classes :
FutureTask, Callable, Executors
Here is an example :
public class TimeoutExample {
public static Object myMethod() {
// does your thing and taking a long time to execute
return someResult;
}
public static void main(final String[] args) {
Callable<Object> callable = new Callable<Object>() {
public Object call() throws Exception {
return myMethod();
}
};
ExecutorService executorService = Executors.newCachedThreadPool();
Future<Object> task = executorService.submit(callable);
try {
// ok, wait for 30 seconds max
Object result = task.get(30, TimeUnit.SECONDS);
System.out.println("Finished with result: " + result);
} catch (ExecutionException e) {
throw new RuntimeException(e);
} catch (TimeoutException e) {
System.out.println("timeout...");
} catch (InterruptedException e) {
System.out.println("interrupted");
}
}
}

Java's interruption mechanism is intended for this kind of scenario. If the method that you wish to abort is executing a loop, just have it check the thread's interrupted status on every iteration. If it's interrupted, throw an InterruptedException.
Then, when you want to abort, you just have to invoke interrupt on the appropriate thread.
Alternatively, you can use the approach Sun suggest as an alternative to the deprecated stop method. This doesn't involve throwing any exceptions, the method would just return normally.

I'm assuming the use of multiple threads in the following statements.
I've done some reading in this area and most authors say that it's a bad idea to kill another thread.
If the function that you want to kill can be designed to periodically check a variable or synchronization primitive, and then terminate cleanly if that variable or synchronization primitive is set, that would be pretty clean. Then some sort of monitor thread can sleep for a number of milliseconds and then set the variable or synchronization primitive.

Really, you can't... The only way to do it is to either use thread.stop, agree on a 'cooperative' method (e.g. occassionally check for Thread.isInterrupted or call a method which throws an InterruptedException, e.g. Thread.sleep()), or somehow invoke the method in another JVM entirely.
For certain kinds of tests, calling stop() is okay, but it will probably damage the state of your test suite, so you'll have to relaunch the JVM after each call to stop() if you want to avoid interaction effects.
For a good description of how to implement the cooperative approach, check out Sun's FAQ on the deprecated Thread methods.
For an example of this approach in real life, Eclipse RCP's Job API's 'IProgressMonitor' object allows some management service to signal sub-processes (via the 'cancel' method) that they should stop. Of course, that relies on the methods to actually check the isCancelled method regularly, which they often fail to do.
A hybrid approach might be to ask the thread nicely with interrupt, then insist a couple of seconds later with stop. Again, you shouldn't use stop in production code, but it might be fine in this case, esp. if you exit the JVM soon after.
To test this approach, I wrote a simple harness, which takes a runnable and tries to execute it. Feel free to comment/edit.
public void testStop(Runnable r) {
Thread t = new Thread(r);
t.start();
try {
t.join(2000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
if (!t.isAlive()) {
System.err.println("Finished on time.");
return;
}
try {
t.interrupt();
t.join(2000);
if (!t.isAlive()) {
System.err.println("cooperative stop");
return;
}
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.err.println("non-cooperative stop");
StackTraceElement[] trace = Thread.getAllStackTraces().get(t);
if (null != trace) {
Throwable temp = new Throwable();
temp.setStackTrace(trace);
temp.printStackTrace();
}
t.stop();
System.err.println("stopped non-cooperative thread");
}
To test it, I wrote two competing infinite loops, one cooperative, and one that never checks its thread's interrupted bit.
public void cooperative() {
try {
for (;;) {
Thread.sleep(500);
}
} catch (InterruptedException e) {
System.err.println("cooperative() interrupted");
} finally {
System.err.println("cooperative() finally");
}
}
public void noncooperative() {
try {
for (;;) {
Thread.yield();
}
} finally {
System.err.println("noncooperative() finally");
}
}
Finally, I wrote the tests (JUnit 4) to exercise them:
#Test
public void testStopCooperative() {
testStop(new Runnable() {
#Override
public void run() {
cooperative();
}
});
}
#Test
public void testStopNoncooperative() {
testStop(new Runnable() {
#Override
public void run() {
noncooperative();
}
});
}
I had never used Thread.stop() before, so I was unaware of its operation. It works by throwing a ThreadDeath object from whereever the target thread is currently running. This extends Error. So, while it doesn't always work cleanly, it will usually leave simple programs with a fairly reasonable program state. For example, any finally blocks are called. If you wanted to be a real jerk, you could catch ThreadDeath (or Error), and keep running, anyway!
If nothing else, this really makes me wish more code followed the IProgressMonitor approach - adding another parameter to methods that might take a while, and encouraging the implementor of the method to occasionally poll the Monitor object to see if the user wants the system to give up. I'll try to follow this pattern in the future, especially methods that might be interactive. Of course, you don't necessarily know in advance which methods will be used this way, but that is what Profilers are for, I guess.
As for the 'start another JVM entirely' method, that will take more work. I don't know if anyone has written a delegating class loader, or if one is included in the JVM, but that would be required for this approach.

Nobody answered it directly, so here's the closest thing i can give you in a short amount of psuedo code:
wrap the method in a runnable/callable. The method itself is going to have to check for interrupted status if you want it to stop (for example, if this method is a loop, inside the loop check for Thread.currentThread().isInterrupted and if so, stop the loop (don't check on every iteration though, or you'll just slow stuff down.
in the wrapping method, use thread.join(timeout) to wait the time you want to let the method run. or, inside a loop there, call join repeatedly with a smaller timeout if you need to do other things while waiting. if the method doesn't finish, after joining, use the above recommendations for aborting fast/clean.
so code wise, old code:
void myMethod()
{
methodTakingAllTheTime();
}
new code:
void myMethod()
{
Thread t = new Thread(new Runnable()
{
public void run()
{
methodTakingAllTheTime(); // modify the internals of this method to check for interruption
}
});
t.join(5000); // 5 seconds
t.interrupt();
}
but again, for this to work well, you'll still have to modify methodTakingAllTheTime or that thread will just continue to run after you've called interrupt.

The correct answer is, I believe, to create a Runnable to execute the sub-program, and run this in a separate Thread. THe Runnable may be a FutureTask, which you can run with a timeout ("get" method). If it times out, you'll get a TimeoutException, in which I suggest you
call thread.interrupt() to attempt to end it in a semi-cooperative manner (many library calls seem to be sensitive to this, so it will probably work)
wait a little (Thread.sleep(300))
and then, if the thread is still active (thread.isActive()), call thread.stop(). This is a deprecated method, but apparently the only game in town short of running a separate process with all that this entails.
In my application, where I run untrusted, uncooperative code written by my beginner students, I do the above, ensuring that the killed thread never has (write) access to any objects that survive its death. This includes the object that houses the called method, which is discarded if a timeout occurs. (I tell my students to avoid timeouts, because their agent will be disqualified.) I am unsure about memory leaks...
I distinguish between long runtimes (method terminates) and hard timeouts - the hard timeouts are longer and meant to catch the case when code does not terminate at all, as opposed to being slow.
From my research, Java does not seem to have a non-deprecated provision for running non-cooperative code, which, in a way, is a gaping hole in the security model. Either I can run foreign code and control the permissions it has (SecurityManager), or I cannot run foreign code, because it might end up taking up a whole CPU with no non-deprecated means to stop it.
double x = 2.0;
while(true) {x = x*x}; // do not terminate
System.out.print(x); // prevent optimization

I can think of a not so great way to do this. If you can detect when it is taking too much time, you can have the method check for a boolean in every step. Have the program change the value of the boolean tooMuchTime to true if it is taking too much time (I can't help with this). Then use something like this:
Method(){
//task1
if (tooMuchTime == true) return;
//task2
if (tooMuchTime == true) return;
//task3
if (tooMuchTime == true) return;
//task4
if (tooMuchTime == true) return;
//task5
if (tooMuchTime == true) return;
//final task
}

How can I "retry" a try-catch?

In my code, I will attempt a certain task in a try-block. If exceptions are thrown, the catch-blocks will rectify the errors encountered (usually prompting for correct input). After that, the program is supposed to try again repeatedly until success is achieved. My current code is as follows:
for (bool isSuccess = false; !isSuccess;) {
try {
...
isSuccess = true;
}
// catch blocks here to correct any exceptions thrown
}
As can be seen, I'm currently using a loop structure to make the try-catch block start over if the try block fail, until the try block succeeds completely. Is there a more elegant solution?

I'd prefer to see a do loop that exits by default (then there is less chance of encountering an infinite loop). Your use of the boolean type only really mimics the behaviour of break and continue. As an alternative, perhaps more natural way, consider
do {
try {
// some code
} catch (/*whatever*/){
continue; // go again
}
break;
} while (true);

Just to give you an alternative, and if your code allows you to do so, then you could also use recursion. But in my eyes this is less favorable compared to the other ways to solve this.
public void doSomething() {
try {
...
} catch(Exception e) {
doSomething();
}
}

You can simple use a while loop.
while(!isSuccess){
try{
//DO Stuff
}
}

I think you should define a maximum of attempts. Otherwise in a worst scenario when all failures occur you app will be frozen.

You can do it with while loop
boolean isSuccess = false;
while(!isSuccess) { // loop till to isSuccess become true
try {
....
isSuccess = true;
} catch (Exception e) {
// Some Exception accrued, you can have 2 options
// 1. break the loop with break;
// 2. continue the process
}
}

You can also use recursion to do this. But I don't think it would be efficient. Do ... while would be the best approach.

Resume code in try block after exception is caught

I am fairly new to using try/catch blocks, so I don't know how exactly to perform this command.
If I catch an error, I would like to wait a period of time(10 seconds or so) then try to run the same line of code to attempt to continue in my try block. My program is written in Java. I have looked at both these pages: Page1, Page2, but neither of them are in Java. I have also looked at this, but they are not solving in the using the catch block. Is this possible to do, and how would I implement this in my catch block instead of just printing the error?

99% of time, you want to re-run a code-block after a try-catch and not the line with exception.
If you need to run from that line, than that is an indication for you to take your code in another method that encapsulates only that code (maybe move the try-catch there too).
What i would advice is something like this:
void method(){
try{
codeline1;
codeline2;
codeline3;
codeline4;
}
catch(Exception ex)
{
restorClassStateBeforeCodeLine1();
method();
}
}
By that snipped i propose to have your entire try-catch in a separate method.
Waiting random intervals is bad practice also. You never know if 10 seconds is right every time or at all.
Another way that I advise against would be:
label: {
try {
...
if (condition)
break label;
...
} catch (Exception e) {
...
}
}
It uses java labels to retry that part. I never tried but the break could be moved in the catch and the label in the try.

I don't think it is possible to return to a certain line in your try-block from inside a catch-block. Because when the throwis executed, the runtime system is going to pop frames from the call stack, looking for an exception handler to match the thrown exception and once the frame is popped from the stack, it's gone. More info about this can be found here
What you can do is call the method that caused the throw from within the catch-block. But that means it is going to execute your method from the beginning, so maybe you want to try to rearrange your code so that this does not cause any other problems. EDIT: The other answer demonstrates exactly what I mean.

This simple program loops through array values, testing each until it finds a value that doesn't generate an exception
public static void main(String[] args) {
int[] array=new int[]{0,0,0,0,5};
for(int i=0; i<array.length;i++) {
try {
System.out.println(10/array[i]);
break;
} catch(Exception e) {
try { Thread.sleep(1000); } catch(Exception ignore){}
}
}
}

while(true){
try{
//actions where some exception can be thrown
break;//executed when no exceptions appeared only
}
catch(YourException e){
Thread.sleep(10_000);
}
}
This cycle will be repeated while you instructions haven't executed. When code in try-block executed succesfully break helps you leave this cycle

Since you are saying that it is only 2 lines of code that you experience the intermittent error with, try something similar to this.
public static void main(String... args)
{
try
{
//Some Logic
//Error throwing logic in method
while(!doLogic())
{
Thread.sleep(1000);//Sleep here or in doLogic catch
}
//Continuing other logic!
}
catch(Exception e)
{
e.printStackTrace();
}
}
static Integer i = null;
public static boolean doLogic()
{
try
{
//Lines that throw error
System.out.println(i.toString());//NPE First run
}
catch (Exception e)
{
i = 1;
return false;
}
return true;
}

Which is the Best way to exit a method

I was looking for the ways to exit a method,
i found two methods
System.exit();
Return;
System.exit() - Exits the full program
Return exits current method and returns an error that remaining code are unreachable.
class myclass
{
public static void myfunc()
{
return;
System.out.println("Function ");
}
}
public class method_test
{
public static void main(String args[])
{
myclass mc= new myclass();
mc.myfunc();
System.out.println("Main");
}
}

There is no best way, it depends on situation.
Ideally, there is no need to exit at all, it will just return.
int a() {
return 2;
}
If there is a real need to exit, use return, there are no penalties for doing so.
void insertElementIntoStructure(Element e, Structure s) {
if (s.contains(e)) {
return; // redundant work;
}
insert(s, e); // insert the element
}
this is best avoided as much as possible as this is impossible to test for failure in voids
Avoid system.exit in functions, it is a major side effect that should be left to be used only in main.
void not_a_nice_function() {
if (errorDetected()) {
System.exit(-1);
}
print("hello, world!");
}
this pseudocode is evil because if you try to reuse this code, it will be hard to find what made it exit prematurely.

The best and proper way to exit from method is adding return statement.
System.exit() will shutdown your programm.
if you use system.exit once a thread goes there, it won't come back.
system.exit is part of Design of the Shutdown Hooks API

first of all your code will kill good programmers imagine this code Which is the Best way to exit a method this code example that how a return comes before a System.out.print(); as it becomes unreachable after the return statement lols
the command
System.exit(int status); (status=0 for Normal Exit && status=-1 for abnormal exit
is only used if you want to exactly quit your whole app whereas
the command
return;
is used to get out/return from a method
these two are different in their operations