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Java - Terminate a thread

I'd like to terminate a thread in java without using the deprecated stop() method.
The run() method in my thread calls to another method which performs a task which can take a long period of time, and I want to be able to stop it before it returns.
My problem is that I don't want to handle this in the method which is called from run, so for instance I can't use a global boolean variable in order to terminate the thread cause i can't check it within the method.
Is there a way I can use the interrupt() method in order to achieve this goal ?

Short answer: you have to rewrite the long-running method so that it checks, for instance, some global boolean or the interrupted flag. As per assylias suggestion in the comments below, you could for instance do while (!isInterrupted()) { ... }.
If your long-running method is in the middle of some (well behaved) blocking call, then you will want to interrupt the thread, to make it return early from the blocking call, so that you can check the condition to see if it's time to terminate.

Kill an infinite-loop thread in java

Is there any way I can stop a thread without using deprecated stop()?
The thread invokes some methods from a potentially badly written class which may contain infinite loops. Since I can't modify this class, I have no control over the loop condition and therefore can't exit normally using interrupt() or any other variable. Is it justified in this case to use stop()?

If you cannot use stop() and cannot fix the code, your only other option is to run the code in another process and kill the process.
In your case, the simplest option is to use Thread.stop()

The problem with the stop() method is that it can leave synchronized objects in an inconsistent state (i.e. if you stop the thread while you're halfway through a synchronized object update, then only part of the object may be updated). If your thread isn't using any synchronized methods / blocks then it's safe to stop it.
If your thread might be using a synchronized block/method, then try to cleanly shut it down before calling stop, e.g. call interrupt on it and then call stop X minutes later if the thread still hasn't stopped (operating on the assumption that the thread is calling a broken method). Just be sure that the thread leaves everything in a consistent state before calling a potentially broken method.

Shut down a single thread running

Suppose during my running I would like to shutdown a single thread gracefully!
I don't want to use Thread.stop() nor Thread.destroy() due to their unsafe behavior.
Note: I'm familiar with using ExecutorService.shutdown() option.
But I would like to know the other way to implement.

The standard way to stop a thread is to call thread.interrupt();. To make it work, you need to make sure you thread responds to interruption, for example:
Thread t = new Thread(new Runnable() { public void run {
while(!Thread.currentThread().isInterrupted()) {
//your code here
}
}});
t.start();
t.interrupt();
This only works if the condition is checked regularly. Note that you can delegate the interruption mechanism to interruptible methods (typically I/O, blocking queues, sleep/wait provide methods that can block until they are interrupted).
Note: In this example, you can also use:
while(!interrupted()) {
//your code here
}
interrupted() does the same thing as Thread.currentThread().isInterrupted() except that the interrupted flag is reset. Since it is your thread, it does not matter.

You could have isStopped()flag in your code. And the running thread should regularly check this flag to see if it should stop. Note that stopping a thread gracefully requires the running code to be written in a way that allows stopping.
You can take a look at this question for some more detailed answers

You have to make the run() method of the thread terminate for some reason. How you achieve this depends on what the thread does.
If the thread is looping, you can stop it by raising a flag (checked by the condition of the loop).
If the thread is waiting over a Socket or any other stream, just close the stream.
If the thread is blocked on a call that can throw an InterruptedException, you can interrupt() the thread and ignore the exception.
If the thread is consuming the elements of a blocking queue, use the poison pill method, which means putting on the queue an element that just means "stop looping".

If you have a loop inside your run() method of your Thread then one option would be that your loop checks for the value of a flag on every iteration.
You can set the flag from outside the code, such as your thread would stop executing before starting the next iteration.

End execution of Thread without using its stop() method

I have a Swing form with a button, which is when clicked starts SocketServer for listening to incoming requests in a separate thread. Following is structure of classes I have.
MainForm : This is my main class which launches Swing Form. It has two buttons, "start server" and "stop buttons".
MySocketServer : This class is where SocketServer object exists, it has methods startServer() and stopServer().
Following is Start button's Click Event Body.
t = new Thread(new Runnable() //Object t is created globally in this main class.
{
public void run()
{
myss = new MySocketServer(); //Object myss has similar accessibility as t.
myss.startServer();
}
});
t.start();
And Following is Stop Button's Click Event Body
myss.stopServer();
if(t.isAlive());
System.out.println("Thread is still alive!!");
Though I can toggle SockeServer "start" and "stop" as my times I want, but I realize that everytime I start the server, new thread gets created and it remains active even though server is stopped using MySocketServer's method.
I can use stop() of Thread and stop the thread execution but since it is depreciated and I have studied that threads get ended once their run() method has executed completely, but I have startServer() method separated so that it can handle connected clients separately.
Please note that startServer() has While-Listen loop, so essentially run() method of thread is in infinite state of execution, until I explicitly call stopServer() and halt the loop.
what can be done here?

Firstly, you are right to not try to use Thread.stop(). It is potentially dangerous.
So what should you do?
One possibility might to be to write your server thread like this:
....
ServerSocket ss = new ServerSocket(...);
try {
while (keepGoing) {
Socket s = ss.accept(...);
try {
// do stuff
} finally {
// close socket
}
}
} finally {
// close the server socket
}
and have stopServer clear the keepGoing flag. But the problem is that the stop typically will come while the thread is blocked in the accept call, and there's nothing to unblock it.
Another possibility might be to call Thread.interrupt() on the thread. That causes some things to unblock, and throw an exception, but I don't think it will unblock the accept() call though. (However, this is still better than setting a flag if the "do stuff" part needs interrupting.)
The real solution (I think) is to close the ServerSocket. This will cause the ss.accept() call to unblock and throw an exception, which you need to handle in the server thread.

In your MySocketServer class in the while loop you need a flag which will test if it should keep running or not.
In your newly added shutdown method set the flag which the loop in the thread will test. Once the loop breaks and run() returns the thread will end.

You shouldn't use stop(). Take a look at this http://docs.oracle.com/javase/1.4.2/docs/guide/misc/threadPrimitiveDeprecation.html
The solution with infinite loop and start/stop flags is straightforward but leads to ineffective using of CPU time. The better way is to use wait/notify approach. The way you operate with MySocketServer gives me feeling that you have other infinite loop inside the startServer(). That's why you have to stop it. It would be better to wrap this loop into it's own thread internally and make start/stop methods operate with this thread state in wait/notify manner.
Forgot to mention, as your GUI runs in its own thread you can't escape start/stop flag inside the MySocketServer because using wait() in GUI thread will make it hanged.

How to start/stop/restart a thread in Java?

I am having a real hard time finding a way to start, stop, and restart a thread in Java.
Specifically, I have a class Task (currently implements Runnable) in a file Task.java. My main application needs to be able to START this task on a thread, STOP (kill) the thread when it needs to, and sometimes KILL & RESTART the thread...
My first attempt was with ExecutorService but I can't seem to find a way for it restart a task. When I use .shutdownnow() any future call to .execute() fails because the ExecutorService is "shutdown"...
So, how could I accomplish this?

Once a thread stops you cannot restart it. However, there is nothing stopping you from creating and starting a new thread.
Option 1: Create a new thread rather than trying to restart.
Option 2: Instead of letting the thread stop, have it wait and then when it receives notification you can allow it to do work again. This way the thread never stops and will never need to be restarted.
Edit based on comment:
To "kill" the thread you can do something like the following.
yourThread.setIsTerminating(true); // tell the thread to stop
yourThread.join(); // wait for the thread to stop

Review java.lang.Thread.
To start or restart (once a thread is stopped, you can't restart that same thread, but it doesn't matter; just create a new Thread instance):
// Create your Runnable instance
Task task = new Task(...);
// Start a thread and run your Runnable
Thread t = new Thread(task);
To stop it, have a method on your Task instance that sets a flag to tell the run method to exit; returning from run exits the thread. If your calling code needs to know the thread really has stopped before it returns, you can use join:
// Tell Task to stop
task.setStopFlag(true);
// Wait for it to do so
t.join();
Regarding restarting: Even though a Thread can't be restarted, you can reuse your Runnable instance with a new thread if it has state and such you want to keep; that comes to the same thing. Just make sure your Runnable is designed to allow multiple calls to run.

It is impossible to terminate a thread unless the code running in that thread checks for and allows termination.
You said: "Sadly I must kill/restart it ... I don't have complete control over the contents of the thread and for my situation it requires a restart"
If the contents of the thread does not allow for termination of its exectuion then you can not terminate that thread.
In your post you said: "My first attempt was with ExecutorService but I can't seem to find a way for it restart a task. When I use .shutdownnow()..."
If you look at the source of "shutdownnow" it just runs through and interrupts the currently running threads. This will not stop their execution unless the code in those threads checks to see if it has been ineterrupted and, if so, stops execution itself. So shutdownnow is probably not doing what you think.
Let me illustrate what I mean when I say that the contents of the thread must allow for that thread to be terminated:
myExecutor.execute(new Runnable() {
public void run() {
while (true) {
System.out.println("running");
}
}
});
myExecutor.shutdownnow();
That thread will continue to run forever, even though shutdownnow was called, because it never checks to see if it has been terminated or not. This thread, however, will shut down:
myExecutor.execute(new Runnable() {
public void run() {
while (!Thread.interrupted()) {
System.out.println("running");
}
}
});
myExecutor.shutdownnow();
Since this thread checks to see whether or not it has been interrupted / shut down / terminated.
So if you want a thread that you can shut down, you need to make sure it checks to see if it has been interrupted. If you want a thread that you can "shut down" and "restart" you can make a runnable that can take new tasks as was mentioned before.
Why can you not shut down a running thread? Well I actually lied, you can call "yourThread.stop()" but why is this a bad idea? The thread could be in a synchronized (or other critical section, but we will limit ourselves to setions guarded by the syncrhonized key word here) section of code when you stop it. synch blocks are supposed to be executed in their entirity and only by one thread before being accessed by some other thread. If you stop a thread in the middle of a synch block, the protection put into place by the synch block is invalidated and your program will get into an unknown state. Developers make put stuff in synch blocks to keep things in synch, if you use threadInstance.stop() you destroy the meaning of synchronize, what the developer of that code was trying to accomplish and how the developer of that code expected his synchronized blocks to behave.

You can't restart a thread so your best option is to save the current state of the object at the time the thread was stopped and when operations need to continue on that object you can recreate that object using the saved and then start the new thread.
These two articles Swing Worker and Concurrency may help you determine the best solution for your problem.

As stated by Taylor L, you can't just "stop" a thread (by calling a simple method) due to the fact that it could leave your system in an unstable state as the external calling thread may not know what is going on inside your thread.
With this said, the best way to "stop" a thread is to have the thread keep an eye on itself and to have it know and understand when it should stop.

If your task is performing some kind of action in a loop there is a way to pause/restart processing, but I think it would have to be outside what the Thread API currently offers. If its a single shot process I am not aware of any way to suspend/restart without running into API that has been deprecated or is no longer allowed.
As for looped processes, the easiest way I could think of is that the code that spawns the Task instantiates a ReentrantLock and passes it to the task, as well as keeping a reference itself. Every time the Task enters its loop it attempts a lock on the ReentrantLock instance and when the loop completes it should unlock. You may want to encapsulate all this try/finally, making sure you let go of the lock at the end of the loop, even if an exception is thrown.
If you want to pause the task simply attempt a lock from the main code (since you kept a reference handy). What this will do is wait for the loop to complete and not let it start another iteration (since the main thread is holding a lock). To restart the thread simply unlock from the main code, this will allow the task to resume its loops.
To permanently stop the thread I would use the normal API or leave a flag in the Task and a setter for the flag (something like stopImmediately). When the loop encountered a true value for this flag it stops processing and completes the run method.

Sometimes if a Thread was started and it loaded a downside dynamic class which is processing with lots of Thread/currentThread sleep while ignoring interrupted Exception catch(es), one interrupt might not be enough to completely exit execution.
In that case, we can supply these loop-based interrupts:
while(th.isAlive()){
log.trace("Still processing Internally; Sending Interrupt;");
th.interrupt();
try {
Thread.currentThread().sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}

There's a difference between pausing a thread and stopping/killing it. If stopping for you mean killing the thread, then a restart simply means creating a new thread and launching.
There are methods for killing threads from a different thread (e.g., your spawner), but they are unsafe in general. It might be safer if your thread constantly checks some flag to see if it should continue (I assume there is some loop in your thread), and have the external "controller" change the state of that flag.
You can see a little more in:
http://java.sun.com/j2se/1.4.2/docs/guide/misc/threadPrimitiveDeprecation.html
May I ask why you want to kill the thread and restart it? Why not just have it wait until its services are needed again? Java has synchronization mechanisms exactly for that purpose. The thread will be sleeping until the controller notifies it to continue executing.

You can start a thread like:
Thread thread=new Thread(new Runnable() {
#Override
public void run() {
try {
//Do you task
}catch (Exception ex){
ex.printStackTrace();}
}
});
thread.start();
To stop a Thread:
thread.join();//it will kill you thread
//if you want to know whether your thread is alive or dead you can use
System.out.println("Thread is "+thread.isAlive());
Its advisable to create a new thread rather than restarting it.