What is the recommended / best way to implement a blocking function call in Java, that can be later unblocked by a call from another thread?
Basically I want to have two methods on an object, where the first call blocks any calling thread until the second method is run by another thread:
public class Blocker {
/* Any thread that calls this function will get blocked */
public static SomeResultObject blockingCall() {
// ...
}
/* when this function is called all blocked threads will continue */
public void unblockAll() {
// ...
}
}
The intention BTW is not just to get blocking behaviour, but to write a method that blocks until some future point when it is possible to compute the required result.
You can use a CountDownLatch.
latch = new CountDownLatch(1);
To block, call:
latch.await();
To unblock, call:
latch.countDown();
If you're waiting on a specific object, you can call myObject.wait() with one thread, and then wake it up with myObject.notify() or myObject.notifyAll(). You may need to be inside a synchronized block:
class Example {
List list = new ArrayList();
// Wait for the list to have an item and return it
public Object getFromList() {
synchronized(list) {
// Do this inside a while loop -- wait() is
// not guaranteed to only return if the condition
// is satisfied -- it can return any time
while(list.empty()) {
// Wait until list.notify() is called
// Note: The lock will be released until
// this call returns.
list.wait();
}
return list.remove(0);
}
}
// Add an object to the list and wake up
// anyone waiting
public void addToList(Object item) {
synchronized(list) {
list.add(item);
// Wake up anything blocking on list.wait()
// Note that we know that only one waiting
// call can complete (since we only added one
// item to process. If we wanted to wake them
// all up, we'd use list.notifyAll()
list.notify();
}
}
}
There are a couple of different approaches and primitives available, but the most appropriate sounds like a CyclicBarrier or a CountDownLatch.
Related
public class MyClass {
private List<Integer> resources = new ArrayList<>();
public synchronized Integer getAndRemoveResourceOrWait(Integer requestedResource) throws InterruptedException {
while(resources.stream().anyMatch((r) -> { return r >= requestedResource; })) {
wait();
}
Integer found = resources.stream().findFirst((r) -> {
return r >= requestedResource;
}).get();
resources.remove(found);
return found;
}
public void addResource(Integer resource) {
resources.add(resource);
notifyAll();
}
}
Thread "A" episodically invokes addResource with random value.
A few another threads actively invokes getAndRemoveResourceOrWait.
What I need to do to let method getAndRemoveResourceOrWait work concurrently?
For example, thread "X" invokes getAndRemoveResourceOrWait with variable 128 which does not exists in resources collection. So, it become waiting for it. While it is waiting, thread "Y" invokes getAndRemoveResourceOrWait with variable 64 and it exists in resources collection. Thread "Y" should not wait for thread "X" to complete.
What I need to do to let method getAndRemoveResourceOrWait work concurrently?
It simply needs to run on a different thread to the one that calls addResource(resource).
Note that getAndRemoveResource is a blocking (synchronous) operation in the sense that the thread making the call is blocked until it gets the answer. However one thread that is calling getAndRemoveResource does not block another thread calling getAndRemoveResource. The key is that the wait() call releases the mutex, and then reacquires it when the mutex is notified. What will happen here is that a notifyAll will cause all waiting threads to way up, one at a time.
However, there is a bug on your addResource method. The method needs to be declared as synchronized. If you don't call notifyAll() while the current thread holds the mutex for on this, you will get an exception. (And this is also necessary to ensure that the updates to the shared resources object are visible ... in both directions.)
Also, this implementation is not going to scale well:
Each waiting thread will scan the entire resource list on every update; i.e. on every call to addResource.
When a waiting thread finds a resource, it will scan the list twice more to remove it.
All of this is done while holding the mutex on the shared MyClass instance ... which blocks addResource as well.
UPDATE - Assuming that the Resource values are unique, a better solution would be to use replace ArrayList with TreeSet. This should work:
public class MyClass {
private TreetSet<Integer> resources = new TreeSet<>();
public synchronized Integer getAndRemoveResourceOrWait(
Integer resource) throws InterruptedException {
while (true) {
Integer found = resources.tailSet(resource, true).pollFirst();
if (found != null) {
return found;
}
wait();
}
}
public synchronized void addResource(Integer resource) {
resources.add(resource);
notifyAll();
}
}
(I also tried ConcurrentSkipListSet but I couldn't figure out a way to avoid using a mutex while adding and removing. If you were trying to remove an equal resource, it could be done ...)
I've got pretty complex method, with few loops and other method calls. I'd like to make it possible to interrupt this method. The only solution I've found to do so, is by checking if Thread.currentThread().isInterrupted(). The problem is I'd like to check it in every iteration of every loop and in few other places. After doing that the code does not really look so nice.
So there are two questions really.
1. Is there any other way to stop the method when thread was interrupted than checking the same flag over and over again?
2. Is it better - mostly in case of performance - to just add !Thread.currentThread().isInterrupted() condition in every loop or use some method like the one below?
void checkIfInterrupted() {
if (Thread.interrupted()) {
throw new InterruptedException();
}
}
Preferred way is to check Thread.currentThread().isInterrupted() every loop in your thread. Viz. Java Concurrency In Practice - Listening 7.5:
class PrimeProducer extends Thread {
private final BlockingQueue<BigInteger> queue;
PrimeProducer(BlockingQueue<BigInteger> queue) {
this.queue = queue;
}
public void run() {
try {
BigInteger p = BigInteger.ONE;
while (!Thread.currentThread().isInterrupted())
queue.put(p = p.nextProbablePrime());
} catch (InterruptedException consumed) {
/* Allow thread to exit */
}
}
public void cancel() { interrupt(); }
}
There are two points in each loop iteration where interruption may be
detected: in the blocking put call, and by explicitly polling the
interrupted status in the loop header. The explicit test is not
strictly necessary here because of the blocking put call, but it makes
PrimeProducer more responsive to interruption because it checks for
interruption before starting the lengthy task of searching for a
prime, rather than after. When calls to interruptible blocking methods
are not frequent enough to deliver the desired responsiveness,
explicitly testing the interrupted status can help.
I'm having a code in Java where two objects wait and notify each other when one finished processing. I'll keep my code simple with the following example and assuming there are no syntax error (I just want you to know the logic is more important here rather than the syntax).
Assuming I have object A which is a thread having this pseudo code
class A is Thread {
run() {
while(true) {
wait(); // wait for signal from B
// then do something if signal received
B.signal(); // let B know that we're done and wait again
}
}
}
Then we have here B which is also a thread having this pseudo code
class B is Thread {
run() {
while(true) {
// Do something
A.signal(); // Let A know to continue processing
wait(); // Wait for signal from A before doing something again
}
}
}
So as you can see there's a cycle. The problem is I am having a dead-lock and the reason here is because when A is finished processing, it signals B to work before it waits.. But by the time B is notified, there are chances that A still haven't reached the wait() code and B is already calling A.signal() and leads to a dead lock.
How do I properly solve this problem? The solution I have in mind is that when B is notified to work, I will let the thread of B sleep for a number of milliseconds but I don't think this is ever a good idea. Any help is appreciated, thanks in advance.
When you use notify() this should be associated with a state change.
When you use wait() this should be associated with a check for a state change.
In real code, you should only wait when you are waiting for something.
Note: wait() can wake spuriously, it doesn't mean notify() was called. As you noticed, notify() does nothing if nothing is wait()ing.
Instead of using this pattern, you can use a BlockingQueue to pass work/messages between threads. This has the wait/notify and the object containing work built in.
However, since you normally need a thread to do the work, there is an ExecutorService builtin to do this. This allows you to pass work to a pool of threads and collect the results.
In short, you should be using an ExecutorService.
If A is using the result of B, then maybe you can consider a BlockingQueue.
As you can find described in the Javadoc, you need to put your wait calls inside a loop that checks for a condition. Otherwise, if you don't have a condition variable or expression that you can check, it is possible that you miss the notify because you were not waiting at that point.
Also, as others have pointed out, you need to hold the monitor of the object you are calling the wait or notify method on; that's what the synchronized keyword is for.
In the below fix, the condition is very simple; it's a variable called notified in classes A and B.
Also, to get this right, A and B need to know about each other. In your code you seemed to be invoking static methods; but the notify method needs to be called on an instance, so you need to keep references to the instances of A and B in B and A, respectively.
This fixes the problems:
class A is Thread {
private B b;
private boolean notified;
public void run() {
while(true) {
synchronized(this) {
while (!notified) {
try {
wait(); // wait for signal from B
} catch (InterruptedException e) {}
}
notified = false;
}
synchronized(b) {
// then do something if signal received
b.notified = true;
b.notify(); // let B know that we're done and wait again
}
}
}
}
class B is Thread {
private A a;
private boolean notified;
public void run() {
while(true) {
synchronized(a) {
// Do something
a.notified = true;
a.notify(); // Let A know to continue processing
}
synchronized(this) {
while (!notified) {
try {
wait(); // Wait for signal from A before doing something again
} catch (InterruptedException e) {}
}
notified = false;
}
}
}
}
I'm new to Java so I have a simple question that I don't know where to start from -
I need to write a function that accepts an Action, at a multi-threads program , and only the first thread that enter the function do the action, and all the other threads wait for him to finish, and then return from the function without doing anything.
As I said - I don't know where to begin because,
first - there isn't a static var at the function (static like as in c / c++ ) so how do I make it that only the first thread would start the action, and the others do nothing ?
second - for the threads to wait, should I use
public synchronized void lala(Action doThis)
{....}
or should i write something like that inside the function
synchronized (this)
{
...
notify();
}
Thanks !
If you want all threads arriving at a method to wait for the first, then they must synchronize on a common object. It could be the same instance (this) on which the methods are invoked, or it could be any other object (an explicit lock object).
If you want to ensure that the first thread is the only one that will perform the action, then you must store this fact somewhere, for all other threads to read, for they will execute the same instructions.
Going by the previous two points, one could lock on this 'fact' variable to achieve the desired outcome
static final AtomicBoolean flag = new AtomicBoolean(false); // synchronize on this, and also store the fact. It is static so that if this is in a Runnable instance will not appear to reset the fact. Don't use the Boolean wrapper, for the value of the flag might be different in certain cases.
public void lala(Action doThis)
{
synchronized (flag) // synchronize on the flag so that other threads arriving here, will be forced to wait
{
if(!flag.get()) // This condition is true only for the first thread.
{
doX();
flag.set(true); //set the flag so that other threads will not invoke doX.
}
}
...
doCommonWork();
...
}
If you're doing threading in any recent version of Java, you really should be using the java.util.concurrent package instead of using Threads directly.
Here's one way you could do it:
private final ExecutorService executor = Executors.newCachedThreadPool();
private final Map<Runnable, Future<?>> submitted
= new HashMap<Runnable, Future<?>>();
public void executeOnlyOnce(Runnable action) {
Future<?> future = null;
// NOTE: I was tempted to use a ConcurrentHashMap here, but we don't want to
// get into a possible race with two threads both seeing that a value hasn't
// been computed yet and both starting a computation, so the synchronized
// block ensures that no other thread can be submitting the runnable to the
// executor while we are checking the map. If, on the other hand, it's not
// a problem for two threads to both create the same value (that is, this
// behavior is only intended for caching performance, not for correctness),
// then it should be safe to use a ConcurrentHashMap and use its
// putIfAbsent() method instead.
synchronized(submitted) {
future = submitted.get(action);
if(future == null) {
future = executor.submit(action);
submitted.put(action, future);
}
}
future.get(); // ignore return value because the runnable returns void
}
Note that this assumes that your Action class (I'm assuming you don't mean javax.swing.Action, right?) implements Runnable and also has a reasonable implementation of equals() and hashCode(). Otherwise, you may need to use a different Map implementation (for example, IdentityHashMap).
Also, this assumes that you may have multiple different actions that you want to execute only once. If that's not the case, then you can drop the Map entirely and do something like this:
private final ExecutorService executor = Executors.newCachedThreadPool();
private final Object lock = new Object();
private volatile Runnable action;
private volatile Future<?> future = null;
public void executeOnlyOnce(Runnable action) {
synchronized(lock) {
if(this.action == null) {
this.action = action;
this.future = executor.submit(action);
} else if(!this.action.equals(action)) {
throw new IllegalArgumentException("Unexpected action");
}
}
future.get();
}
public synchronized void foo()
{
...
}
is equivalent to
public void foo()
{
synchronized(this)
{
...
}
}
so either of the two options should work. I personally like the synchronized method option.
Synchronizing the whole method can sometimes be overkill if there is only a certain part of the code that deals with shared data (for example, a common variable that each thread is updating).
Best approach for performance is to only use the synchronized keyword just around the shared data. If you synchronized the whole method when it is not entirely necessarily then a lot of threads will be waiting when they can still do work within their own local scope.
When a thread enters the synchronize it acquires a lock (if you use the this object it locks on the object itself), the other will wait till the lock-acquiring thread has exited. You actually don't need a notify statement in this situation as the threads will release the lock when they exit the synchronize statement.
The Following class DoStuff starts a thread and syncs to protect the listener object from being accessed when null.
Now when accessing the DoStuff class function setOnProgressListener() externally I'm having issues because the call is getting held for a long time before it exits the function call. I'm not sure why this happens? I seems as if the synchronization has queued up a lot of calls? Any input on this would help!
I'm essentially passing null to the listener because I no longer wish to get updated for this status. I do this as part of my process to kill the DoStuff Thread.
Thanks!
public class DoStuff extends Runnable
{
Object MUTEX = new Object();
private OnProgressListener mOnProgressListener = null;
public DoStuff()
{
new Thread(this).start();
}
public void setOnProgressListener( OnProgressListener onProgressListener )
{
synchronized (MUTEX)
{
mOnProgressListener = onProgressListener;
}
}
private void reportStatus( int statusId )
{
synchronized (MUTEX)
{
if (null != mOnStatusListener)
{
mOnStatusListener.setStatusMessage(new OnStatusEvent(this, statusId));
}
}
}
// this is the run of a thread
public void run()
{
int status = 0;
do
{
// do some work and report the current work status
status = doWork();
reportStatus( status );
} while(true);
}
}
You should use wait/notify. here is sample;
public class DoStuff {
Object MUTEX = new Object();
String data = null;
public void setData(String data) {
synchronized (MUTEX) {
this.data = data;
System.out.println(Thread.currentThread());
MUTEX.notifyAll();
}
}
public void run() {
do {
synchronized (MUTEX) {
if (null == data) {
return;
} else {
System.out.println(data);
}
try {
MUTEX.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} while (true);
}
}
The trouble with this code is that your while() loop is constantly trying to grab the monitor for MUTEX immediately after releasing it or even yield()-ing to help the scheduler put another thread in. So there's a very good chance that anyone else trying to obtain that monitor will be starved, because your while() loop will consume most of your CPU time and even when other threads could run, they might not get the monitor they're waiting for.
Ideally a wait()/notify() pair should be used or failing that, you should at least call a Thread.yield() in your while loop, outside the synchronized block. (But I this second "solution" really isn't a very good one, you should consider using the first one instead.)
UPDATE: I read the code again and I think I believe to see what you wanted to achieve: printing the value of data every time you set a new value. If that's true, you should definitely go for the wait/notify solution, although if you want to absolutely guarantee that every single value is printed, you need to do even more work, possibly using a queue.
I'm a little confused about your code, can you provide the full listing?
First, where does DoStuff start a thread? Why are you quitting if your data is still null? (you might actually be out of the thread before setData even executes).
But the main thing here is that you're doing essentially a busy-waiting loop, in which you synchronize on the mutex. This is pretty wasteful and will generally block cores of your CPU.
Depending on what you are trying to do, you might want to use a wait-notify scheme, in which the thread goes to sleep until something happens.
Thanks all for your help. I was able to determine why the indefinite lock. Something important and obvious is that once I run the reportStatus() function call it will hold the lock MUTEX until it is completely done executing the callback. My fault was that at the registered callback I was calling setOnProgressListener(null) by mistake. Yes, I admit didn't post enough code, and most likely all of you would have catched the bug... So calling setOnProgressListener(null) would wait until the MUTEX object has been released, and the reportStatus() was held waiting to call setOnProgressListener(null), therefore I was in a deadlock!
Again the main point I learned is to remember that triggering a callback message will hold until the registered callback function is done processing it's call.
Thanks all!