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Queue print jobs in a separate single Thread for JavaFX

currently I am experimenting with Concurrency in Java/JavaFX. Printing must run in a different thread otherwise it will make the JavaFX main thread freeze for a couple seconds. Right now my printing is done with this simplified example.
public void print(PrintContent pt) {
setPrintContent(pt);
Thread thread = new Thread(this);
thread.start();
}
#Override
public void run() {
// send content to printer
}
With this code I am sending many print jobs parallel to my printer. Therefore I get the error telling me that my printer can only handle 1 print job at a time. Since I know that Threads cannot be reused, I would like to know if there is a possibility to queue up Threads, so that my printer only handles one print job at a time.
Thank you very much for your effort and your time.

Use a single threaded executor to execute the print jobs. It will create one (and only one) background thread and queue the jobs:
// it might be better not to make this static; but you need to ensure there is
// only one instance of this executor:
private static final Executor PRINT_QUEUE = Executors.newSingleThreadExecutor();
// ...
public void print(PrintContent pt) {
PRINT_QUEUE.execute(() -> {
// send content to printer
});
}

~~> WAY 1
You can implement your own BlockingQueue read this is very useful or use a default implementation from Java libraries tutorial
So after reading the above links,you add a method in your class like
public void addJob(Object job){
queue.put(job);
}
Secondly you implement a Thread that is running into an infinite while loop.Inside it you call the method
queue.take();
When the queue is empty this Thread is blocked waiting until a new Object is added,so you dont have to worry about spending cpu time.
Finally you can set some upper bounds so for example queue can contain .. 27 items.
Mention that in case of Thread failure you have to recreate it manually.
~~>WAY 2 Better Approach
You can use an Executors Interface:
ExecutorService executorService1 = Executors.newSingleThreadExecutor();
From documentation:
Creates an Executor that uses a single worker thread operating off an
unbounded queue. (Note however that if this single thread terminates
due to a failure during execution prior to shutdown, a new one will
take its place if needed to execute subsequent tasks.) Tasks are
guaranteed to execute sequentially, and no more than one task will be
active at any given time.
With the method below you retrieve a result if the job has successfully done.
Future future = executorService.submit(new Callable(){ public Object call() throws Exception { System.out.println("Asynchronous Callable"); return "Callable Result"; } });
System.out.println("future.get() = " + future.get());
If future.get() returns null, the job has been done successfully.
Remember to call
executorService.shutdown(); because the active threads inside this ExecutorService may prevent the JVM from shutting down.
Full tutorial here

java ThreadPool race condition

I have the following piece of code, I have a large loop that I want to run in parallel. Unfortunately a race condition exists, in some cases (not all, not predictable) I get blocked at s.awaitTermination. There is no thread synchronization except at the end where I remove the finished thread from the set, and if the set is empty call shutdown. Where am I going wrong? FYI thousands of tasks get added to the queue, I don't want them all trying to run at once, is there a better pattern than this?
When I check the queue, it has tasks left, and the pool threads are "parked at unsafe...." according to Netbeans debug.
EDIT: updating Thread to Runnable as suggested - did not fix the problem
ExecutorService s = Executors.newFixedThreadPool(8);
final Set<Runnable> threads = new HashSet<>();
for(/*lots of loops*/){
Runnable t = new Runnable(){
public void run(){
//some long task...
synchronized(threads){
threads.remove(this);
if(threads.isEmpty()){
s.shutdown();
}
}
}
}
}
synchronized(threads){
for(Runnable t : threads){
s.submit(t);
}
}
s.awaitTermination(1000, TimeUnit.SECONDS);

This is not is a solution for your question, but it might help you to do the same task easily. Take a look at the ExecutorCompletionService it can execute multiple tasks and it will return you a future that you can use to wait. Internally it uses a queue for the completed tasks so basically accomplished what you are trying to do here.

Running threads in background and passing data to thread as needed

I have a requirement to run a thread in background in java and may need to pass some information to the thread before it starts doing its actual function.
How to implement this?
Secondly, is it possible to start a thread which does further operation and exit from request context. Please note that I have to implement in a web application.

An example to answer your first question:
class NewThread extends Thread
{
public String someInformation;
#Override
public void run()
{
System.out.println(someInformation);
}
}
public class YourClass
{
public void startANewThread()
{
NewThread newThread = new NewThread();
newThread.someInformation = "hello";
newThread.start();
}
}
As for your second question: This code should run in a web application, but you must be extremely careful to make sure, that these background Threads will finish at some point, or else that would be a nice little memory leak.
Please note, that if you want to share information when both Threads are running, you need to synchronize information access.
Depending on what the background Thread does it might make sense to use a framework like Quartz

is it possible to start a thread which does further operation and exit from request context. Please note that I have to implement in a web application.
You can use an ExecutorService to pass any number of tasks to and shutdown when you have finished with it.

Try this block of code, with two threads, where one thread will run in background and another thread will start executing the task. And here you can set a return type as well unlike thread.run() method.
FutureTask<Response> future = new FutureTask<>(newCallable<Response>() {
public yourreturntype call() {
doSomething(Thread1);
}
});
ExecutorService executor = Executors.newSingleThreadExecutor();
executor.execute(future);
doSomethingelse(Thread2)
//This thread doesnot wait for thread1 to complete

Terminated Thread Revival

I am storing a bunch of threads objects in an arraylist. I want to be able to start these threads at random. Same thread can be started more than once. Before I start a thread object, I check on whether the thread is alive, and if they have either of NEW or TERMINATED status. This restriction because, I don't want to disturb the 'busy' threads. Now, for NEW threads, this works fine. But for TERMINATED thread, I get an exception.
When a thread ends, shouldn't it go back to being 'new'? Or are threads 'disposable' - like use once and done?

As it says in the documentation for Thread.start(), "It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution."
It is better for you to keep hold of Runnable instances and implement your own logic for keeping track of when the execution of each one of them finishes. Using an Executor is probably the simplest way to run the Runnables.

You should probably be using the awesome stuff provided in java.util.concurrent. Based on your description, ThreadPoolExecutor sounds like a good thing to check out.

This is the way I did it
class GarbageDisposalThread extends Thread {
public void start() {
try {
super.start();
} catch( IllegalThreadStateException e ) {
this.arrayList.remove(this);
this.arrayList.add( new GarbageDisposalThread( this.arrayList ));
}
}
private GarbageDisposalThread() {
}
public GarbageDisposalThread( ArrayList<Whatever> arrayList ) {
this.arrayList = arrayList;
this.start();
}
public void run() {
// whatever the code
}
private ArrayList<Whatever> arrayList = null;
}
that's it!
you can change the code according to your needs :P

Java threads cannot be restarted.
From the javadoc:
It is never legal to start a thread
more than once. In particular, a
thread may not be restarted once it
has completed execution.
See the Thread.start() javadoc for more information.
There are other ways to accomplish what you are trying to do. For example, you could use new Threads that continue the work that was done in the Thread that has finished execution. You may also want to investigate the java.util.concurrent package.

From another post...
You could use ThreadPoolExecutor, which would allow you to pass in tasks and let the service assign a thread to a task. When the task is finished, the thread goes idle until it gets the next task.
So, you don't restart a thread, but you would redo/resume a task.

How does one stop a thread without a stop() method?

I have question about the Java threads. Here is my scenario:
I have a thread calling a method that could take while. The thread keeps itself on that method until I get the result. If I send another request to that method in the same way, now there are two threads running (provided the first did not return the result yet). But I want to give the priority to the last thread and don't want to get the results from the previously started threads. So how could I get rid of earlier threads when I do not have a stop method?

The standard design pattern is to use a local variable in the thread that can be set to stop it:
public class MyThread extends Thread {
private volatile boolean running = true;
public void stop() {
running = false;
}
public void run() {
while (running) {
// do your things
}
}
}
This way you can greacefully terminate the thread, i.e. without throwing an InterruptedException.

The best way really depends on what that method does. If it waits on something, chances are an interrupt will result in an InterruptedException which you handle and cleanly exit. If it's doing something busy, it won't:
class Scratchpad {
public static void main(String[] a) {
Thread t = new Thread(new Runnable() {
public void run() {doWork();}
});
t.start();
try {
Thread.sleep(50);
} catch (InterruptedException ie) {}
t.interrupt();
}
private static void doWork() {
for ( long i = 1; i != 0; i *=5 );
}
}
In the case above, the only viable solution really is a flag variable to break out of the loop early on a cancel, ala #inflagranti.
Another option for event-driven architectures is the poison-pill: if your method is waiting on a blocking queue for a new item, then you can have a global constant item called the "poison-pill" that when consumed (dequeued) you kill the thread:
try {
while(true) {
SomeType next = queue.take();
if ( next == POISON_PILL ) {
return;
}
consume(next);
}
} catch //...
EDIT:
It looks like what you really want is an executor service. When you submit a job to an executor service, you get back a Future which you can use to track results and cancel the job.

You can interrupt a Thread, its execution chain will throw an InterruptedException most of the time (see special cases in the documentation).

If you just want to slow down the other thread and not have it exit, you can take some other approach...
For one thing, just like exiting you can have a de-prioritize variable that, when set, puts your thread to sleep for 100ms on each iteration. This would effectively stop it while your other thread searched, then when you re-prioritize it it would go back to full speed.
However, this is a little sloppy. Since you only ever want one thing running but you want to have it remember to process others when the priority one is done, you may want to place your processing into a class with a .process() method that is called repeatedly. When you wish to suspend processing of that request you simply stop calling .process on that object for a while.
In this way you can implement a stack of such objects and your thread would just execute stack.peek().process(); every iteration, so pushing a new, more important task onto the stack would automatically stop any previous task from operating.
This leads to much more flexible scheduling--for instance you could have process() return false if there is nothing for it to do at which point your scheduler might go to the next item on the stack and try its' process() method, giving you some serious multi-tasking ability in a single thread without overtaxing your resources (network, I'm guessing)

There is a setPriority(int) method for Thread. You can set the first thread its priority like this:
Thread t = new Thread(yourRunnable);
t.start();
t.setPriority(Thread.MIN_PRIORITY); // The range goes from 1 to 10, I think
But this won't kill your thread. If you have only two threads using your runnable, then this is a good solution. But if you create threads in a loop and you always sets the priority of the last thread to minimum, you will get a lot of threads.
If this is what is application is going to do, take a look at a ThreadPool. This isn't an existing class in the Java API. You will have create one by yourself.
A ThreadPool is another Thread that manages all your other Threads the way you want. You can set a maximum number of running Threads. And in that ThreadPool, you can implement a system that manages the Thread priority automatically. Eg: You can make that older threads gain more priority, so you can properly end them.
So, if you know how to work with a ThreadPool, it can be very interesting.

According to java.lang.Thread API, you should use interrupt() method and check for isInterrupted() flag while you're doing some time-consuming cancelable operation. This approach allows to deal with different kind of "waiting situations":
1. wait(), join() and sleep() methods will throw InterruptedExcetion after you invoke interrupt() method
2. If thread blocked by java.nio.channels.Selector it will finish selector operation
3. If you're waiting for I/O thread will receive ClosedByInterruptException, but in this case your I/O facility must implement InterruptibleChannel interface.
If it's not possible to interrupt this action in a generic way, you could simply abandon previous thread and get results from a new one. You could do it by means of java.util.concurrent.Future and java.util.concurrent.ExecutorService.
Cosider following code snippet:
public class RequestService<Result> {
private ExecutorService executor = Executors.newFixedThreadPool(3);
private Future<Result> result;
public Future<Result> doRequest(){
if(result !=null){
result.cancel(true);
}
result = executor.submit(new Callable<Result>() {
public Result call() throws Exception {
// do your long-running service call here
}
});
return result;
}
}
Future object here represents a results of service call. If you invoke doRequest method one more time, it attempts to cancel previous task and then try to submit new request. As far as thread pool contain more than one thread, you won't have to wait until previous request is cancelled. New request is submitted immediately and method returns you a new result of request.