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Java - Allow one thread in a method without waiting

I've a situation where I need to implement a thread safe method, The method must be executed by only one thread at a time, And while the method is being executed by a thread, all other threads trying to execute the same method shouldn't wait and must exit the method.
Synchronization won't help here since threads will be waiting to execute the method sequentially.
I thought I would achieve this by making use of ConcurrentHashMap using below code, but not sure if this is the perfect way to implement it.
Class Test {
private ConcurrentHashMap<String, Object> map = new ConcurrentHashMap<>();
public void execute() {
if (map.putIfApsent("key", new Object()) != null) { // map has value for key which means a thread has already entered.
return; // early exit
}
threadSafeMethod();
map.remove("key");
}
private void threadSafeMethod() {
// my code
}
}

You can do this without synchronization, with compare-and-swap using a boolean:
private AtomicBoolean entered = new AtomicBoolean(false);
public void execute() {
if(entered.compareAndSet(false,true) {
try {
method()
} finally {
entered.set(false)
}
}
}

You could use a ReentrantLock and specify a negative value for waiting time. In that case the scheduler will not try to wait if there is a thread already executing the code.
// define the lock somewhere as an instance variable
Lock lock = new ReentrantLock();
try {
var isAvailable = lock.tryLock(-1, TimeUnit.NANOSECONDS);
if(isAvailable) {
System.out.println("do work");
lock.unlock();
}
} catch (InterruptedException e) {
e.printStackTrace();
}

Interrupting unknown thread

Consider the following (simplified) class, designed to allow my entire component to enter some interim state before completely stopping. (The purpose of the interim state is to allow the component to complete its existing tasks, but reject any new ones).
The component might be started and stopped multiple times from any number of threads.
class StopHandler {
boolean isStarted = false;
synchronized void start() {isStarted = true;}
//synchronized as I do want the client code to block until the component is stopped.
//I might add some async method as well, but let's concentrate on the sync version only.
synchronized void stop(boolean isUrgent) {
if (isStarted) {
if (!isUrgent) {
setGlobalState(PREPARING_TO_STOP); //assume it is implemented
try {Thread.sleep(10_000L);} catch (InterruptedException ignored) {}
}
isStarted = false;
}
}
The problem with the current implementation is that if some client code needs to urgently stop the component while it is in the interim state, it will still have to wait.
For example:
//one thread
stopHandler.stop(false); //not urgent => it is sleeping
//another thread, after 1 millisecond:
stopHandler.stop(true); //it's urgent, "please stop now", but it will wait for 10 seconds
How would you implement it?
I might need to interrupt the sleeping thread, but I don't have the sleeping thread object on which to call 'interrupt()'.

How about storing a reference to current Thread (returned by Thread.currentThread()) in a field of StopHandler directly before you call sleep? That would allow you you to interrupt it in the subsequent urgent call in case the Thread is still alive.

Couldn't find a better solution than the one suggested by Lars.
Just need to encapsulate the sleep management for completeness.
class SleepHandler {
private final ReentrantLock sleepingThreadLock;
private volatile Thread sleepingThread;
SleepHandler() {
sleepingThreadLock = new ReentrantLock();
}
void sleep(long millis) throws InterruptedException {
setSleepingThread(Thread.currentThread());
Thread.sleep(millis);
setSleepingThread(null);
}
void interruptIfSleeping() {
doWithinSleepingThreadLock(() -> {
if (sleepingThread != null) {
sleepingThread.interrupt();
}
});
}
private void setSleepingThread(#Nullable Thread sleepingThread) {
doWithinSleepingThreadLock(() -> this.sleepingThread = sleepingThread);
}
private void doWithinSleepingThreadLock(Runnable runnable) {
sleepingThreadLock.lock();
try {
runnable.run();
} finally {
sleepingThreadLock.unlock();
}
}
}
With this helper class, handling of the original problem is trivial:
void stop(boolean isUrgent) throws InterruptedException {
if (isUrgent) {sleepHandler.interruptIfSleeping();} //harmless if not sleeping
try {
doStop(isUrgent); //all the stuff in the original 'stop(...)' method
} catch (InteruptedException ignored) {
} finally {
Thread.interrupted(); //just in case, clearing the 'interrupt' flag as no need to propagate it futher
}

Returning/Stopping the execution of a function on a keypress in Java

I have a certain function in my program that I want to stop on the press of a key. I have a native keyboard hook set up for that purpose. Right now, I call System.exit(0) when that key is detected. However, I don't want to exit the program, just stop that operation and return to where it was called. An example is given below.
public class Main {
public static void main(String[] args) {
System.out.println("Calling function that can be stopped with CTRL+C");
foo(); // Should return when CTRL+C is pressed
System.out.println("Function has returned");
}
}
I've tried putting the call to foo() in a thread so I could call Thread.interrupt() but I want the function call to be blocking, not non-blocking. Also there are blocking IO calls in foo() so I'd rather not deal with interrupts unless it's necessary, because I'd have to deal with ClosedByInterruptException exceptions and that has caused problems before.
Also the body of foo() is very long and has many function calls inside it, so writing if (stop == true) return; in the function is not an option.
Is there a better way to do this than making a blocking thread? If so, how? If not, how would I make a blocking thread?

How about this?
// Create and start the thread
MyThread thread = new MyThread();
thread.start();
while (true) {
// Do work
// Pause the thread
synchronized (thread) {
thread.pleaseWait = true;
}
// Do work
// Resume the thread
synchronized (thread) {
thread.pleaseWait = false;
thread.notify();
}
// Do work
}
class MyThread extends Thread {
boolean pleaseWait = false;
// This method is called when the thread runs
public void run() {
while (true) {
// Do work
// Check if should wait
synchronized (this) {
while (pleaseWait) {
try {
wait();
} catch (Exception e) {
}
}
}
// Do work
}
}
}
(taken from http://www.exampledepot.com/egs/java.lang/PauseThread.html not my own work)

Using boolean var for stopping threads

I have a Java book I'm learning from and in one of the examples, I saw something suspicious.
public class ThreadExample extends MIDlet {
boolean threadsRunning = true; // Flag stopping the threads
ThreadTest thr1;
ThreadTest thr2;
private class ThreadTest extends Thread {
int loops;
public ThreadTest(int waitingTime) {
loops = waitTime;
}
public void run() {
for (int i = 1; i <= loops; i++) {
if (threadsRunning != true) { // here threadsRunning is tested
return;
}
try {
Thread.sleep(1000);
} catch(InterruptedException e) {
System.out.println(e);
}
}
}
}
public ThreadExample() {
thr1 = new ThreadTest(2);
thr2 = new ThreadTest(6);
}
public void startApp() throws MIDletStateChangeException {
thr1.start();
thr2.start();
try {
Thread.sleep(4000); // we wait 4 secs before stopping the threads -
// this way one of the threads is supposed to finish by itself
} catch(InterruptedException e) {
System.out.println(e);
}
destroyApp();
}
public void destroyApp() {
threadsRunning = false;
try {
thr1.join();
thr2.join();
} catch(InterruptedException e) {
System.out.println(e);
}
notifyDestroyed();
}
}
As it is a MIDlet app, when it's started, the startApp method is executed. To keep it simple, the startApp method itself calls destroyApp and so the program destroys, stopping the threads and notifying the destruction.
The question is, is it safe to use this 'threadsRunning' variable and would its use inside both threads and in the destroyApp method cause any trouble at some point? Would 'volatile' keyword put in front of the declaration help to synchronize it?

Setting a boolean value is atomic, and there is no "read then modify" logic in this example, so access to the variable doesn't need to be synchronised in this particular case.
However, the variable should at least be marked volatile.
Marking the variable volatile does not synchronise the threads' access to it; it makes sure that a thread doesn't miss another thread's update to the variable due to code optimisation or value caching. For example, without volatile, the code inside run() may read the threadsRunning value just once at the beginning, cache the value, and then use this cached value in the if statement every time, rather than reading the variable again from main memory. If the threadsRunning value gets changed by another thread, it might not get picked up.
In general, if you use a variable from multiple threads, and its access is not synchronised, you should mark it volatile.

Java 1.4 synchronization: only allow one instance of method to run (non blocking)?

I have a class proposing translations utilities. The translations themselves should be reloaded every 30 minutes. I use Spring Timer support for that. Basically, my class looks like :
public interface Translator {
public void loadTranslations();
public String getTranslation(String key);
}
loadTranslations() can be pretty long to run, so while it is running the old translations are still available. This is done by loading the translations in a local Map and just changing the reference when all translations are loaded.
My problem is : how do I make sure that when a thread is already loading translations, is a second one also tries to run, it detects that and returns immediately, without starting a second update.
A synchronized method will only queue the loads ... I'm still on Java 1.4, so no java.util.concurrent.
Thanks for your help !

Use some form of locking mechanism to only perform the task if it is not already in progress. Acquiring the locking token must be a one-step process. See:
/**
* #author McDowell
*/
public abstract class NonconcurrentTask implements Runnable {
private boolean token = true;
private synchronized boolean acquire() {
boolean ret = token;
token = false;
return ret;
}
private synchronized void release() {
token = true;
}
public final void run() {
if (acquire()) {
try {
doTask();
} finally {
release();
}
}
}
protected abstract void doTask();
}
Test code that will throw an exception if the task runs concurrently:
public class Test {
public static void main(String[] args) {
final NonconcurrentTask shared = new NonconcurrentTask() {
private boolean working = false;
protected void doTask() {
System.out.println("Working: "
+ Thread.currentThread().getName());
if (working) {
throw new IllegalStateException();
}
working = true;
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
if (!working) {
throw new IllegalStateException();
}
working = false;
}
};
Runnable taskWrapper = new Runnable() {
public void run() {
while (true) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
shared.run();
}
}
};
for (int i = 0; i < 100; i++) {
new Thread(taskWrapper).start();
}
}
}

I am from a .net background(no java experience at all), but you could try a simple static flag of some sort that checks at the beginning of the method if its alrady running. Then all you need to do is make sure any read/write of that flag is synchronized. So at beginning check the flag, if its not set, set it, if it is set, return. If its not set, run the rest of the method, and after its complete, unset it. Just make sure to put the code in a try/finally and the flag iunsetting in the finally so it always gets unset in case of error. Very simplified but may be all you need.
Edit: This actually probably works better than synchronizing the method. Because do you really need a new translation immediately after the one before it finishes? And you may not want to lock up a thread for too long if it has to wait a while.

Keep a handle on the load thread to see if it's running?
Or can't you just use a synchronized flag to indicate if a load is in progress?

This is actually identical to the code that is required to manage the construction of a Singleton (gasp!) when done the classical way:
if (instance == null) {
synchronized {
if (instance == null) {
instance = new SomeClass();
}
}
}
The inner test is identical to the outer test. The outer test is so that we dont routinely enter a synchronised block, the inner test is to confirm that the situation has not changed since we last made the test (the thread could have been preempted before entering Synchronized).
In your case:
if (translationsNeedLoading()) {
synchronized {
if (translationsNeedLoading()) {
loadTranslations();
}
}
}
UPDATE: This way of constructing a singleton will not work reliably under your JDK1.4. For explanation see here. However I think you are you will be OK in this scenario.