Develop Reference

Lock List when in use

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.

Java - concurrent clear of the list

I am trying to find a good way to achieve the following API:
void add(Object o);
void processAndClear();
The class would store the objects and upon calling processAndClear would iterate through the currently stored ones, process them somehow, and then clear the store. This class should be thread safe.
the obvious approach is to use locking, but I wanted to be more "concurrent". This is the approach which I would use:
class Store{
private AtomicReference<CopyOnWriteArrayList<Object>> store = new AtomicReference<>(new CopyOnWriteArrayList <>());
void add(Object o){
store.get().add(o);
}
void processAndClear(){
CopyOnWriteArrayList<Object> objects = store.get();
store.compareAndSet(objects, new CopyOnWriteArrayList<>());
for (Object object : objects) {
//do sth
}
}
}
This would allow threads that try to add objects to proceed almost immediately without any locking/waiting for the xlearing to complete. Is this the more or less correct approach?

Your above code is not thread-safe. Imagine the following:
Thread A is put on hold at add() right after store.get()
Thread B is in processAndClear(), replaces the list, processes all elements of the old one, then returns.
Thread A resumes and adds a new item to the now obsolete list that will never be processed.
The probably easiest solution here would be to use a LinkedBlockingQueue, which would as well simplify the task a lot:
class Store{
final LinkedBlockingQueue<Object> queue = new LinkedBlockingQueue<>();
void add(final Object o){
queue.put(o); // blocks until there is free space in the optionally bounded queue
}
void processAndClear(){
Object element;
while ((element = queue.poll()) != null) { // does not block on empty list but returns null instead
doSomething(element);
}
}
}
Edit: How to do this with synchronized:
class Store{
final LinkedList<Object> queue = new LinkedList<>(); // has to be final for synchronized to work
void add(final Object o){
synchronized(queue) { // on the queue as this is the shared object in question
queue.add(o);
}
}
void processAndClear() {
final LinkedList<Object> elements = new LinkedList<>(); // temporary local list
synchronized(queue) { // here as well, as every access needs to be properly synchronized
elements.addAll(queue);
queue.clear();
}
for (Object e : elements) {
doSomething(e); // this is thread-safe as only this thread can access these now local elements
}
}
}
Why this is not a good idea
Although this is thread-safe, it is much slower if compared to the concurrent version. Assume that you have a system with 100 threads that frequently call add, while one thread calls processAndClear. Then the following performance bottle-necks will occur:
If one thread calls add the other 99 are put on hold in the meantime.
During the first part of processAndClear all 100 threads are put on hold.
If you assume that those 100 adding threads have nothing else to do, you can easily show, that the application runs at the same speed as a single-threaded application minus the cost for synchronization. That means: adding will effectively be slower with 100 threads than with 1. This is not the case if you use a concurrent list as in the first example.
There will however be a minor performance gain with the processing thread, as doSomething can be run on the old elements while new ones are added. But again the concurrent example could be faster, as you could have multiple threads do the processing simultaneously.
Effectively synchronized can be used as well, but you will automatically introduce performance bottle-necks, potentially causing the application to run slower as single-threaded, forcing you to do complicated performance tests. In addition extending the functionality always contains a risk of introducing threading issues, as locking needs to be done manually.A concurrent list in contrast solves all these problems without additional code and the code can easily changed or extended later on.

The class would store the objects and upon calling processAndClear would iterate through the currently stored ones, process them somehow, and then clear the store.
This seems like you should use a BlockingQueue for this task. Your add(...) method would add to the queue and your consumer would call take() which blocks waiting for the next item. The BlockingQueue (ArrayBlockingQueue is a typical implementation) takes care of all of the synchronization and signaling for you.
This means that you don't have to have a CopyOnWriteArrayList nor an AtomicReference. What you would lose is a collection and you can iterate through for other reasons than your post articulates currently.

achieving synchronized addAll to a list in java

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.

Understanding the behaviour of Synchronized

I'm trying to improve my understanding of the scope of the lock issued during a synchronized call.
Eg:
class CopyOnReadList<T> {
private final List<T> items = new ArrayList<T>();
public void add(T item) {
items.add(item);
}
public List<T> makeSnapshot() {
List<T> copy = new ArrayList<T>();
synchronized (items) {
// Make a copy while holding the lock.
for (T t : items) copy.add(t);
}
return copy;
}
}
(Code lovingly borrowed from this excellent answer)
In this code snippet, can one thread call add while another is calling makeSnapshot?. Ie., does the lock created by synchronized (items) affect all attempted reads to items, or only those attempted through the makeSnapshot() method?
The original post actually used a synchonized lock in the add method:
public void add(T item) {
synchronized (items) {
// Add item while holding the lock.
items.add(item);
}
}
What is the side effect of removing this?

It affects only those attempted in makeSnapshot() or, more generally, any other method that has synchronized(items) block (it means that it will try to aquire lock on items object and block until it's possible).
The side effect of removing synchronized block from add() method is that add() will not try to synchronize on items object, and therefore will allow concurrent modifications, including while makeSnapshot() is executing.
Without synchronize in add() you can have other threads add elements to items collection WHILE the snapshot is being made.

In this code snippet, can one thread call add while another is calling
makeSnapshot?
Certainly - and either one of the methods can then fail with a ConcurrentModificationException, or the content of the list may be corrupted.
does the lock created by synchronized (items) affect all attempted
reads to items, or only those attempted through the makeSnapshot()
method?
Neither. The lock has not effect whatsoever on the behaviour of the object items, only on blocks or methods that snychronize on it - namely to ensure that no two threads can execute any of those blocks or methods at the same time.

can one thread call add while another is calling makeSnapshot?
Yes. synchronized makes sure that any other thread can't enter another block of code which is also synchronized, on the same object (the CopyOnReadList, in this case). Since you have not synchronized the add method, several threads can call add concurrently, even if one thread is executing makeSnapshot.
By removing the synchronized on the add method, you've made the code non-threadsafe, since ArrayList is not thread-safe.
The rule of thumb is: every access (read or write) to a shared mutable state must be synchronized on the same lock.

Using synchronized lists

This is my first time using the synchronized keyword, so I am still unsure of how it exactly works. I have a list that I want to be accessed by multiple threads so I do this:
players = Collections.synchronizedList(new ArrayList<Player>(maxPlayers));
Now, I want to make sure that I am not calling players.add() at the same time as players.get(), so I think i should use synchronized statements (methods A and B could be called at the same time):
public void A() {
synchronized(players) {
players.add(new Player());
}
}
public void B(String msg) {
synchronized(players) {
for(int i = 0;i<players.size();i++) {
players.get(i).out.println(msg);
}
}
}
Is this the correct procedure? If not, what should I do instead?

Provided you only access the list through the object returned by synchronizedList then access should be thread-safe, though note that you may need to used synchronized blocks for compound actions like iterating through the list or making actions and decisions based on multiple calls into the list (for example, getting a value making a decision then adding a value).
So in your example A() doesn't need the synchronized block, but B() might if you don't want the list to be changed or be read by some other thread during the iteration. (In fact, by using the counter to iterate it is needed to prevent a race condition between the loop termination condition and another thread removing an item; other ways of iterating might not have this issue though).