How do I lock a data structure (such as List) when someone is iterating over it?
For example, let's say I have this class with a list in it:
class A{
private List<Integer> list = new ArrayList<>();
public MyList() {
// initialize this.list
}
public List<Integer> getList() {
return list;
}
}
And I run this code:
public static void main(String[] args) {
A a = new A();
Thread t1 = new Thread(()->{
a.getList().forEach(System.out::println);
});
Thread t2 = new Thread(()->{
a.getList().removeIf(e->e==1);
});
t1.start();
t2.start();
}
I don't have a single block of code that uses the list, so I can't use synchronized().
I was thinking of locking the getList() method after it has been called but how can I know if the caller has finished using it so I could unlock it?
And I don't want to use CopyOnWriteArrayList because of I care about my performance;
after it has been called but how can I know if the caller has finished using it so I could unlock it?
That's impossible. The iterator API fundamentally doesn't require that you explicitly 'close' them, so, this is simply not something you can make happen. You have a problem here:
Iterating over the same list from multiple threads is an issue if anybody modifies that list in between. Actually, threads are immaterial; if you modify a list then interact with an iterator created before the modification, you get ConcurrentModificationException guaranteed. Involve threads, and you merely usually get a CoModEx; you may get bizarre behaviour if you haven't set up your locking properly.
Your chosen solution is "I shall lock the list.. but how do I do that? Better ask SO". But that's not the correct solution.
You have a few options:
Use a lock
It's not specifically the iteration that you need to lock, it's "whatever interacts with this list". Make an actual lock object, and define that any interaction of any kind with this list must occur in the confines of this lock.
Thread t1 = new Thread(() -> {
a.acquireLock();
try {
a.getList().forEach(System.out::println);
} finally {
a.releaseLock();
}
});
t1.start();
Where acquireLock and releaseLock are methods you write that use a ReadWriteLock to do their thing.
Use CopyOnWriteArrayList
COWList is an implementation of java.util.List with the property that it copies the backing store anytime you change anything about it. This has the benefit that any iterator you made is guaranteed to never throw ConcurrentModificationException: When you start iterating over it, you will end up iterating each value that was there as the list was when you began the iteration. Even if your code, or any other thread, starts modifying that list halfway through. The downside is, of course, that it is making lots of copies if you make lots of modifications, so this is not a good idea if the list is large and you're modifying it a lot.
Get rid of the getList() method, move the tasks into the object itself.
I don't know what a is (the object you call .getList() on, but apparently one of the functions that whatever this is should expose is some job that you really can't do with a getList() call: It's not just that you want the contents, you want to get the contents in a stable fashion (perhaps the method should instead have a method that gives you a copy of the list), or perhaps you want to do a thing to each element inside it (e.g. instead of getting the list and calling .forEach(System.out::println) on it, instead pass System.out::println to a and let it do the work. You can then focus your locks or other solutions to avoid clashes in that code, and not in callers of a.
Make a copy yourself
This doesn't actually work, even though it seems like it: Immediately clone the list after you receive it. This doesn't work, because cloning the list is itself an operation that iterates, just like .forEach(System.out::println) does, so if another thread interacts with the list while you are making your clone, it fails. Use one of the above 3 solutions instead.
Related
I am not sure how to properly use the Collections.synchronizedList() implementation.
I have these two:
public synchronized static List<CurrencyBox> getOrderList() {
return Collections.synchronizedList(orderList);
}
and
public static List<CurrencyBox> getOrderList() {
return Collections.synchronizedList(orderList);
}
So as far as I understood, synchronizedList really returns the orderList and not a copy, correct?
So If I want to gurantee atomic operations, like add and remove, which of the implementation above is correct?
And does something maybe changes with Java9? Or is it still the way to go or have you any other suggestion?
Thank you
Without context it's a bit hard to tell, from the snippets provided neither give you guaranteed atomic operations.
The documentation states:
Returns a synchronized (thread-safe) list backed by the specified
list. In order to guarantee serial access, it is critical that all
access to the backing list is accomplished through the returned list.
So even if you synchronize the method the best you'll get is a guarantee that no two objects are creating the synchronized list at the same time.
You need to wrap the original orderList with Collections.synchronizedList to begin with and return the stored result of that each time.
private static List<CurrencyBox> orderList = Collections.synchronizedList(new ArrayList<CurrencyBox>());
public static List<CurrencyBox> getOrderList() {
return orderList
}
A synchronized list only synchronized methods of this list.
It means a thread won't be able to modify the list while another thread is currently running a method from this list. The object is locked while processing method.
As an example, Let's say two threads run addAllon your list, with 2 different lists (A=A1,A2,A3, B=B1,B2,B3) as parameter.
As the method is synchronized, you can be sure those lists won't be merged randomly like A1,B1,A2,A3,B2,B3
You don't decide when a thread handover the process to the other thread so you can either get A1,A2,A3,B1,B2,B3 or B1,B2,B3,A1,A2,A3.
Credit : jhamon
In thread A, an ArrayList is created. It is managed from thread A only.
In thread B, I want to copy that to a new instance.
The requirement is that copyList should not fail and should return a consistent version of the list (= existed at some time at least during the copying process).
My approach is this:
public static <T> ArrayList<T> copyList(ArrayList<? extends T> list) {
List<? extends T> unmodifiableList = Collections.unmodifiableList(list);
return new ArrayList<T>(unmodifiableList);
}
Q1: Does that satisfy the requirements?
Q2: How can I do the same without Collections.unmodifiableList with proably iterators and try-catch blocks?
UPD. That is an interview question I was asked a year ago. I understand this a bad idea to use non-thread-safe collections like ArrayList in multithreaded environment
No. ArrayList is not thread safe and you are not using an explicit synchronization.
While you are executing the method unmodifiableList the first thread can modify the original list and you will have a not valid unmodifiable list.
The simplest way I think is the following:
Replace the List with a synchronized version of it.
On the copy list synchronize on the arrayList and make a copy
For example, something like:
List<T> l = Collections.synchronizedList(new ArrayList<T>());
...
public static <T> List<T> copyList(List<? extends T> list) {
List<T> copyList = null;
synchronized(list) {
copyList = new ArrayList<T>(list);
}
return copyList;
}
You should synchronize access to the ArrayList, or replace ArrayList with a concurrent collection like CopyOnWriteArrayList.
Without doing that you might end up with a copy that is inconsistent.
There is absolutely no way to create a copy of a plain ArrayList if the "owning" thread does not offer some protocol to do so.
Without any protocol, thread A can modify the list potentially at any time, meaning thread B never gets a chance to ensure that is sees a consistent state of the list.
To actually allow a consistent copy to be made, thread A must ensure that any modifications it has made are written to memory and are visible to other threads.
Normally, the VM is allowed to reorder instructions, reads and writes as it sees fit, provided no difference can be observed from within the thread executing the program. This includes, for example, delaying writes by holding values in CPU registers or on the local stack.
The only way to ensure that everything is consistently written to main menory, is for thread A to execute an instruction that presents a reordering barrier to the VM (e.g. synchronized block or volatile field access).
So without some cooperation from thread A, there is no way to ensure above conditions are guaranteed to be fulfilled.
Common methods of circumventing this are to synchronize access to the List by only using it in a safely wrapped form (Collections.synchronizedCollection), or use of a List implementation that has these guarantees built in (any type of concurrent list implementation).
The javadoc for Collections.unmodifiableList(...) says, "Returns an unmodifiable view of the specified list."
The key word there is "view". That means it does not copy the data. All it does is create a wrapper for the given list with mutators that all throw exceptions rather than modify the base list.
Yes, but I acually create new ArrayList(Collections.unmodif...), wouldn't this work?
Oops! I missed that. If you're going to copy the list, then there's no point in calling unmodifiableList(). The only code that will ever access the unmodifiable view is the code that's right there in the same method where it's created. You don't have to worry about that code modifying the list contents because you wrote it.
On the other hand, if you're going to copy the list when other threads could be updating the list, then you're going to need synchronized all around. Every place where code could update the list needs to be in a synchronized block, as does the code that makes the copy. Of course, all of those synchronized blocks must synchronize on the same object.
Some programmers will use the list object itself as the lock object. Others will prefer to use a separate object.
Q1: Does that satisfy the requirements?
If the provided list is modified while copying it using new ArrayList<T>(unmodifiableList), you will get a ConcurrentModificationException even if you wrapped it using Collections.unmodifiableList because the Iterator of an UnmodifiableList simply calls the Iterator of the wrapped list and here as it is a non thread safe list you can still get a ConcurrentModificationException.
What you could do is indeed use CopyOnWriteArrayList instead as it is a thread safe list implementation that provides consistent snapshots of the List when you try to iterate over it. Another way could be to make the Thread A push for other threads regularly a safe copy of it using new ArrayList<T>(myList) as it is the only thread that modifies it we know that while creating the copy no other thread will modify it so it would be safe.
Q2: How can I do the same without Collections.unmodifiableList with
probably iterators and try-catch blocks?
As mentioned above Collections.unmodifiableList is not helping here to make it thread safe, for me the only thing that could make sense is actually the opposite: the thread A (the only thread that can modify the list) creates a safe copy of your ArrayList using new ArrayList<T>(list) then it pushes to other threads an unmodified list of it using Collections.unmodifiableList(list).
Generally speaking you should avoid specifying implementations in your method's definition especially public ones, you should only use interfaces or abstract classes because otherwise you would provide an implementation details to the users of your API which is not expected. So here it should be List or Collection not ArrayList.
My program looks like this:
public class Main {
private static ArrayList<T> list;
public static void main(String[] args) {
new DataListener().start();
new DataUpdater().start();
}
static class DataListener extends Thread {
#Override
public void run() {
while(true){
//Reading the ArrayList and displaying the updated data
Thread.sleep(5000);
}
}
}
static class DataUpdater extends Thread{
#Override
public void run() {
//Continuously receive data and update ArrayList;
}
}
}
In order to use this ArrayList in both threads, I know two options:
To make the ArrayList volatile. However I read in this article that making variables volatile is only allowed if it "Writes to the variable do not depend on its current value." which I think in this case it does (because for example when you do an add operation on an ArrayList, the contents of the ArrayList after this operation depend on the current contents of the ArrayList, or doesn't it?). Also the DataUpdater has to remove some elements from the list every now and then, and I also read that editing a volatile variable from different threads is not possible.
To make this ArrayList a synchronized variable. However, my DataUpdater will continuously update the ArrayList, so won't this block the DataListener from reading the ArrayList?
Did I misunderstand any concepts here or is there another option to make this possible?
Volatile won't help you at all. The meaning of volatile is that changes made by thread A to a shared variable are visible to thread B immediately. Usually such changes may be in some cache visible only to the thread that made them, and volatile just tells the JVM not to do any caching or optimization that will result in the value update being delayed.
So it is not a means of synchronization. It's just a means of ensuring visibility of change. Moreover, it's change to the variable, not to the object referenced by that variable. That is, if you mark list as volatile, it will only make any difference if you assign a new list to list, not if you change the content of the list!
Your other suggestion was to make the ArrayList a synchronized variable. There is a misconception here. Variables can't be synchronized. The only thing that can be synchronized is code - either an entire method or a specific block inside it. You use an object as the synchronization monitor.
The monitor is the object itself (actually, it's a logical part of the object that is the monitor), not the variable. If you assign a different object to the same variable after synchronizing on the old value, then you won't have your old monitor available.
But in any case, it's not the object that's synchronized, it's code that you decided to synchronize using that object.
You can therefore use the list as the monitor for synchronizing the operations on it. But you can not have list synchronized.
Suppose you want to synchronize your operations using the list as a monitor, you should design it so that the writer thread doesn't hold the lock all the time. That is, it just grabs it for a single read-update, insert, etc., and then releases it. Grabs it again for the next operation, then releases it. If you synchronize the whole method or the whole update loop, the other thread will never be able to read it.
In the reading thread, you should probably do something like:
List<T> listCopy;
synchronized (list) {
listCopy = new ArrayList(list);
}
// Use listCopy for displaying the value rather than list
This is because displaying is potentially slow - it may involve I/O, updating GUI etc. So to minimize the lock time, you just copy the values from the list, and then release the monitor so that the updating thread can do its work.
Other than that, there are many types of objects in the java.util.concurrent package etc. that are designed to help in situations like this, where one side is writing and the other is reading. Check the documentation - perhaps a ConcurrentLinkedDeque will work for you.
Indeed, none of the two solutions is sufficient. You actually need to synchronize the complete iteration on the arraylist, and every write access to the arraylist:
synchronized(list) {
for (T t : list) {
...
}
}
and
synchronized(list) {
// read/add/modify the list
}
make the ArrayList volatile.
You can't make an ArrayList volatile. You can't make any object volatile. The only things in Java that can be volatile are fields.
In your example, list is not an ArrayList.
private static ArrayList<T> list;
list is a static field of the Main class.
The volatile keyword only matters when one thread updates the field, and another thread subsequently accesses the field.
This line updates the list, but does not update the volatile field:
list.add(e);
After executing that line, the list has changed, but the field still refers to the same list object.
I want to create a static arrayList for communication between 3 threads:
1. thread will add some data to list;
2. thread will remove data from list;
3. thread (timer every 3 sec) will check if size of list equals 0, and if no print all object of this list.
I'm wondereing what would happen if more than 1 thread will try to access this list at same time (for example timer will check size of list at same time when second thread will be removeing object from list).
I guess that i have to synchronize this list, but have no idea how to do that. Can you give me some advise?
I'm wondereing what would happen if more than 1 thread will try to
access this list at same time
Firstly ArrayList is not thread-safe, so you should not use it directly.
One possibility is to use Collections.synchronizedList(...); which provides you thread safety but will hamper concurrency.
Your requirement seems to be like a Queue, so you should use BlockingQueue for that, e.g LinkedBlockingQueue which is thread-safe.
NOTE:
Compound operations on list will still be unsafe if you use Collections.synchronizedList(...).
You can use
List myList = Collections.synchronizedList(new ArrayList());
ArrayList is not thread-safe. You will need to handle the synchronization by yourself or the results will be unpredictable.
As suggested in the related question: How do I make my arraylist thread safe you could use Collections.synchronizedList(...) to obtain a thread-safe version of your List
The javadoc gives an example of how to utilize the List returned by that method.
List list = Collections.synchronizedList(new ArrayList());
...
synchronized (list) {
Iterator i = list.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
Other solutions have already suggested this, but using this approach has a potential performance penalty since the synchronization blocks concurrent access to the List. This will ultimately limit the rate at which you are able to add & remove data from the backing List.
This code snippet will help you in the right direction
//have a lock Object
private static final Object lock = new Object();
// your list
private static List list = null
//synchronized on lock object. this will allow only one thread at a time
synchronized(lock){
//access,add to, remove from list
}
By this way, you can make sure there is only one thread at a time accessing your list
Better yet, using concurrency primitives (wait, notify, and synchronized) you can have the printing thread wait, and only wake up after one of the other threads actually changed the ArrayList. This has the advantage of not printing when there's no change, and being woken up immediately when a change happens. You may still get false wake-ups, but can be coded around if it's a problem.
In printing thread:
synchronized(lock)
{
while(true/*or some boolean to indicate you're done*/)
{
lock.wait();
//print list
}
}
In a thread that edits the list:
synchronized(lock)
{
//modify list
lock.notify();
}
You can also do lock.wait(3000); to print after 3 seconds anyway even if no change happens.
Updated the question.. please check secodn part of question
I need to build up a master list of book ids. I have multiple threaded tasks which brings up a subset of book ids. As soon as each task execution is completed, I need to add them to the super list of book ids. Hence I am planning to pass below aggregator class instance to all of my execution tasks and have them call the updateBookIds() method. To ensure it's thread safe, I have kept the addAll code in synchronized block.
Can any one suggest is this same as Synchronized list? Can I just say Collections.newSynchronizedList and call addAll to that list from all thread tasks? Please clarify.
public class SynchronizedBookIdsAggregator {
private List<String> bookIds;
public SynchronizedBookIdsAggregator(){
bookIds = new ArrayList<String>();
}
public void updateBookIds(List<String> ids){
synchronized (this) {
bookIds.addAll(ids);
}
}
public List<String> getBookIds() {
return bookIds;
}
public void setBookIds(List<String> bookIds) {
this.bookIds = bookIds;
}
}
Thanks,
Harish
Second Approach
So after below discussions, I am currently planning to go with below approach. Please let me know if I am doing anything wrong here:-
public class BooksManager{
private static Logger logger = LoggerFactory.getLogger();
private List<String> fetchMasterListOfBookIds(){
List<String> masterBookIds = Collections.synchronizedList(new ArrayList<String>());
List<String> libraryCodes = getAllLibraries();
ExecutorService libraryBookIdsExecutor = Executors.newFixedThreadPool(BookManagerConstants.LIBRARY_BOOK_IDS_EXECUTOR_POOL_SIZE);
for(String libraryCode : libraryCodes){
LibraryBookIdsCollectionTask libraryTask = new LibraryBookIdsCollectionTask(libraryCode, masterBookIds);
libraryBookIdsExecutor.execute(libraryTask);
}
libraryBookIdsExecutor.shutdown();
//Now the fetching of master list is complete.
//So I will just continue my processing of the master list
}
}
public class LibraryBookIdsCollectionTask implements Runnable {
private String libraryCode;
private List<String> masterBookIds;
public LibraryBookIdsCollectionTask(String libraryCode,List<String> masterBookIds){
this.libraryCode = libraryCode;
this.masterBookIds = masterBookIds;
}
public void run(){
List<String> bookids = new ArrayList<String>();//TODO get this list from iconnect call
synchronized (masterBookIds) {
masterBookIds.addAll(bookids);
}
}
}
Thanks,
Harish
Can I just say Collections.newSynchronizedList and call addAll to that list from all thread tasks?
If you're referring to Collections.synchronizedList, then yes, that would work fine. That will give you a object that implements the List interface where all of the methods from that interface are synchronized, including addAll.
Consider sticking with what you have, though, since it's arguably a cleaner design. If you pass the raw List to your tasks, then they get access to all of the methods on that interface, whereas all they really need to know is that there's an addAll method. Using your SynchronizedBookIdsAggregator keeps your tasks decoupled from design dependence on the List interface, and removes the temptation for them to call something other than addAll.
In cases like this, I tend to look for a Sink interface of some sort, but there never seems to be one around when I need it...
The code you have implemented does not create a synchronization point for someone who accesses the list via getBookIds(), which means they could see inconsistent data. Furthermore, someone who has retrieved the list via getBookIds() must perform external synchronization before accessing the list. Your question also doesn't show how you are actually using the SynchronizedBookIdsAggregator class, which leaves us with not enough information to fully answer your question.
Below would be a safer version of the class:
public class SynchronizedBookIdsAggregator {
private List<String> bookIds;
public SynchronizedBookIdsAggregator() {
bookIds = new ArrayList<String>();
}
public void updateBookIds(List<String> ids){
synchronized (this) {
bookIds.addAll(ids);
}
}
public List<String> getBookIds() {
// synchronized here for memory visibility of the bookIds field
synchronized(this) {
return bookIds;
}
}
public void setBookIds(List<String> bookIds) {
// synchronized here for memory visibility of the bookIds field
synchronized(this) {
this.bookIds = bookIds;
}
}
}
As alluded to earlier, the above code still has a potential problem with some thread accessing the ArrayList after it has been retrieved by getBookIds(). Since the ArrayList itself is not synchronized, accessing it after retrieving it should be synchronized on the chosen guard object:
public class SomeOtherClass {
public void run() {
SynchronizedBookIdsAggregator aggregator = getAggregator();
List<String> bookIds = aggregator.getBookIds();
// Access to the bookIds list must happen while synchronized on the
// chosen guard object -- in this case, aggregator
synchronized(aggregator) {
<work with the bookIds list>
}
}
}
I can imagine using Collections.newSynchronizedList as part of the design of this aggregator, but it is not a panacea. Concurrency design really requires an understanding of the underlying concerns, more than "picking the right tool / collection for the job" (although the latter is not unimportant).
Another potential option to look at is CopyOnWriteArrayList.
As skaffman alluded to, it might be better to not allow direct access to the bookIds list at all (e.g., remove the getter and setter). If you enforce that all access to the list must run through methods written in SynchronizedBookIdsAggregator, then SynchronizedBookIdsAggregator can enforce all concurrency control of the list. As my answer above indicates, allowing consumers of the aggregator to use a "getter" to get the list creates a problem for the user of that list: to write correct code they must have knowledge of the synchronization strategy / guard object, and furthermore they must also use that knowledge to actively synchronize externally and correctly.
Regarding your second approach. What you have shown looks technically correct (good!).
But, presumably you are going to read from masterBookIds at some point, too? And you don't show or describe that part of the program! So when you start thinking about when and how you are going to read masterBookIds (i.e. the return value of fetchMasterListOfBookIds()), just remember to consider concurrency concerns there too! :)
If you make sure all tasks/worker threads have finished before you start reading masterBookIds, you shouldn't have to do anything special.
But, at least in the code you have shown, you aren't ensuring that.
Note that libraryBookIdsExecutor.shutdown() returns immediately. So if you start using the masterBookIds list immediately after fetchMasterListOfBookIds() returns, you will be reading masterBookIds while your worker threads are actively writing data to it, and this entails some extra considerations.
Maybe this is what you want -- maybe you want to read the collection while it is being written to, to show realtime results or something. But then you must consider synchronizing properly on the collection if you want to iterate over it while it is being written to.
If you would just like to make sure all writes to masterBookIds by worker threads have completed before fetchMasterListOfBookIds() returns, you could use ExecutorService.awaitTermination (in combination with .shutdown(), which you are already calling).
Collections.SynchronizedList (which is the wrapper type you'd get) would synchronize almost every method on either itself or a mutex object you pass to the constructor (or Collections.synchronizedList(...) ). Thus it would basically be the same as your approach.
All the methods called using the wrapper returned by Collections.synchronizedList() will be synchronized. This means that the addAll method of normal List when called by this wrapper will be something like this :-
synchronized public static <T> boolean addAll(Collection<? super T> c, T... elements)
So, every method call for the list (using the reference returned and not the original reference) will be synchronized.
However, there is no synchronization between different method calls.
Consider following code snippet :-
List<String> l = Collections.synchronizedList(new ArrayList<String>);
l.add("Hello");
l.add("World");
While multiple threads are accessing the same code, it is quite possible that after Thread A has added "Hello", Thread B will start and again add "Hello" and "World" both to list and then Thread A resumes. So, list would have ["hello", "hello", "world", "world"] instead of ["hello", "world", hello", "world"] as was expected. This is just an example to show that list is not thread-safe between different method calls of the list. If we want the above code to have desired result, then it should be inside synchronized block with lock on list (or this).
However, with your design there is only one method call. SO IT IS SAME AS USING Collections.synchronizedList().
Moreover, as Mike Clark rightly pointed out, you should also synchronized getBookIds() and setBookIds(). And synchronizing it over List itself would be more clear since it is like locking the list before operating on it and unlocking it after operating. So that nothing in-between can use the List.