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Using Lock to lock a position in an array instead of using synchronized for concurrency (Fine Grained Locking)

I have been asked to implement fine grained locking on a hashlist. I have done this using synchronized but the questions tells me to use Lock instead.
I have created a hashlist of objects in the constructor
private LinkedList<E> data[];;
private Lock lock[];
private Lock lockR = new ReentrantLock();
// The constructors ensure that both the data and the dataLock are the same size
#SuppressWarnings("unchecked")
public ConcurrentHashList(int n){
if(n > 1000) {
data = (LinkedList<E>[])(new LinkedList[n/10]);
lock = new Lock [n/10];
}
else {
data = (LinkedList<E>[])(new LinkedList[100]);
lock = new Lock [100]; ;
}
for(int j = 0; j < data.length;j++) {
data[j] = new LinkedList<E>();
lock[j] = new ReentrantLock();// Adding a lock to each bucket index
}
}
The original method
public void add(E x){
if(x != null){
lock.lock();
try{
int index = hashC(x);
if(!data[index].contains(x))
data[index].add(x);
}finally{lock.unlock();}
}
}
Using synchronization to grab a handle on the object hashlist to allow mutable Threads to work on mutable indexes concurrently.
public void add(E x){
if(x != null){
int index = hashC(x);
synchronized (dataLock[index]) { // Getting handle before adding
if(!data[index].contains(x))
data[index].add(x);
}
}
}
I do not know how to implement it using Lock though I can not lock a single element in a array only the whole method which means it is not coarse grained.
Using an array of ReentrantLock
public void add(E x){
if(x != null){
int index = hashC(x);
dataLock[index].lock();
try {
// Getting handle before adding
if(!data[index].contains(x))
data[index].add(x);
}finally {dataLock[index].unlock();}
}
}
The hash function
private int hashC(E x){
int k = x.hashCode();
int h = Math.abs(k % data.length);
return(h);
}

Presumably, hashC() is a function that is highly likely to produce unique numbers. As in, you have no guarantee that the hashes are unique, but the incidence of non-unique hashes is extremely low. For a data structure with a few million entries, you have a literal handful of collisions, and any given collision always consists of only a pair or maybe 3 conflicts (2 to 3 objects in your data structure have the same hash, but not 'thousands').
Also, assumption: the hash for a given object is constant. hashC(x) will produce the same value no matter how many times you call it, assuming you provide the same x.
Then, you get some fun conclusions:
The 'bucket' (The LinkedList instance found at array slot hashC(x) in data) that your object should go into, is always the same - you know which one it should be based solely on the result of hashC.
Calculating hashC does not require a lock of any sort. It has no side effects whatsoever.
Thus, knowing which bucket you need for a given operation on a single value (Be it add, remove, or check-if-in-collection) can be done without locking anything.
Now, once you know which bucket you need to look at / mutate, okay, now locking is involved.
So, just have 1 lock for each bucket. Not a List<Object> locks[];, that's a whole list worth of locks per bucket. Just Object[] locks is all you need, or ReentrantLock[] locks if you prefer to use lock/unlock instead of synchronized (lock[bucketIdx]) { ... }.
This is effectively fine-grained: After all, the odds that one operation needs to twiddle its thumbs because another thread is doing something, even though that other thread is operating on a different object, is very low; it would require the two different objects to have a colliding hash, which is possible, but extremely rare - as per assumption #1.
NB: Note that therefore lock can go away entirely, you don't need it, unless you want to build into your code that the code may completely re-design its bucket structure. For example, 1000 buckets feels a bit meh if you end up with a billion objects. I don't think 'rebucket everything' is part of the task here, though.

Concurrent hashmap simultaneous insertion

I have read that in concurrent hashmap in Java, simultaneous insertions are possible because it is divided into segments and separate lock is taken for each segment.
But if two insertions are going to happen on same segment, then these simultaneous will not happen.
My question is what will happen in such a case? Will second insertion waits till first one gets completed or what?

In general you don't need be too concerned how ConcurrentHashMap is implemented. It simply complies to the the contract of ConcurrentMap which ensures that concurrent modifications are possible.
But to answer your question: yes, one insertion may wait for completion of the other one. Internally, it uses locks which ensure that one thread is waiting until the other one releases the lock. Class Segment used internally actually inherits from ReentrantLock. Here is a shortened version of Segmenet.put():
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value);
V oldValue;
try {
// modifications
} finally {
unlock();
}
return oldValue;
}
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
// ...
int retries = -1; // negative while locating node
while (!tryLock()) {
if (retries < 0) {
// ...
}
else if (++retries > MAX_SCAN_RETRIES) {
lock();
break;
}
else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) {
e = first = f; // re-traverse if entry changed
retries = -1;
}
}
return node;
}
This could give you an idea.

ConcurrentHashMap does not block when performing retrieval operations, and there is no locking for the usual operations.
The heuristic with most Concurrent Data Structures is that there's a backing data structure that gets modified first, with a front-facing data structure that's visible to outside methods. Then, when the modification is complete, the backing data structure is made the public data structure and the public data structure is pushed to the back. There's way more to it than that, but that's the typical contract.

If 2 updates try to happen on the same segment they will go into contention with each other and one of them will have to wait. You can optimise this by choosing a concurrencyLevel value which takes into account the number of threads which will be concurrently updating the hashmap.
You can find all the details in the javadoc for the class

ConcurrentHashMap contains array of Segment which in turn holds array of HashEntry. Each HashEntry holds a key, a value, and a pointer to it's next adjacent entry.
But it acquires the lock in segment level. Hence you are correct. i.e second insertion waits till first one gets completed

Take a look at the javadoc for ConcurrentMap. It describes the extra methods available to deal with concurrent map mutations.

Cache using ConcurrentHashMap

I have the following code:
public class Cache {
private final Map map = new ConcurrentHashMap();
public Object get(Object key) {
Object value = map.get(key);
if (value == null) {
value = new SomeObject();
map.put(key, value);
}
return value;
}
}
My question is:
The put and get methods of the map are thread safe, but since the whole block in not synchronized - could multiple threads add a the same key twice?

put and get are thread safe in the sense that calling them from different threads cannot corrupt the data structure (as, e.g., is possible with a normal java.util.HashMap).
However, since the block is not synchronized, you may still have multiple threads adding the same key:
Both threads may pass the null check, one adds the key and returns its value, and then the second will override that value with a new one and returns it.

As of Java 8, you can also prevent this addition of duplicate keys with:
public class Cache {
private final Map map = new ConcurrentHashMap();
public Object get(Object key) {
Object value = map.computeIfAbsent(key, (key) -> {
return new SomeObject();
});
return value;
}
}
The API docs state:
If the specified key is not already associated with a value, attempts
to compute its value using the given mapping function and enters it
into this map unless null. The entire method invocation is performed
atomically, so the function is applied at most once per key. Some
attempted update operations on this map by other threads may be
blocked while computation is in progress, so the computation should be
short and simple, and must not attempt to update any other mappings of
this map.

could multiple threads add a the same key twice?
Yes, they could. To fix this problem you can:
1) Use putIfAbsent method instead of put. It very fast but unnecessary SomeObject instances can be created.
2) Use double checked locking:
Object value = map.get(key);
if (value == null) {
synchronized (map) {
value = map.get(key);
if (value == null) {
value = new SomeObject();
map.put(key, value);
}
}
}
return value;
Lock is much slower, but only necessary objects will be created

you could also combine checking and putIfAbsent such as:
Object value = map.get(key);
if (value == null) {
return map.putIfAbsent(key, new SomeObject());
}
return value;
thereby reducing the unneccessary new objects to cases where new entries are introduced in the short time between the check and the putIfAbsent.
If you are feeling lucky and reads vastly outnumber writes to your map, you can also create your own copy-on-write map similar to CopyOnWriteArrayList.

Synchronizing a Map of Sets/Lists

I would like to implement a variation on the "Map of Sets" collection that will be constantly accessed by multiple threads. I am wondering whether the synchronization I am doing is sufficient to guarantee that no issues will manifest.
So given the following code, where Map, HashMap, and Set are the Java implementations, and Key and Value are some arbitrary Objects:
public class MapOfSets {
private Map<Key, Set<Value>> map;
public MapOfLists() {
map = Collections.synchronizedMap(new HashMap<Key, Set<Value>());
}
//adds value to the set mapped to key
public void add(Key key, Value value) {
Set<Value> old = map.get(key);
//if no previous set exists on this key, create it and add value to it
if(old == null) {
old = new Set<Value>();
old.add(value);
map.put(old);
}
//otherwise simply insert the value to the existing set
else {
old.add(value);
}
}
//similar to add
public void remove(Key key, Value value) {...}
//perform some operation on all elements in the set mapped to key
public void foo(Key key) {
Set<Value> set = map.get(key);
for(Value v : set)
v.bar();
}
}
The idea here is that because I've synchronized the Map itself, the get() and put() method should be atomic right? So there should be no need to do additional synchronization on the Map or the Sets contained in it. So will this work?
Alternatively, would the above code be advantageous over another possible synchronization solution:
public class MapOfSets {
private Map<Key, Set<Value>> map;
public MapOfLists() {
map = new HashMap<Key, Set<Value>();
}
public synchronized void add(Key key, Value value) {
Set<Value> old = map.get(key);
//if no previous set exists on this key, create it and add value to it
if(old == null) {
old = new Set<Value>();
old.add(value);
map.put(old);
}
//otherwise simply insert the value to the existing set
else {
old.add(value);
}
}
//similar to add
public synchronized void remove(Key key, Value value) {...}
//perform some operation on all elements in the set mapped to key
public synchronized void foo(Key key) {
Set<Value> set = map.get(key);
for(Value v : set)
v.bar();
}
}
Where I leave the data structures unsynchronized but synchronize all the possible public methods instead. So which ones will work, and which one is better?

The first implementation you posted is not thread safe. Consider what happens when the add method is accessed by two concurrent threads with the same key:
thread A executes line 1 of the method, and gets a null reference because no item with the given key is present
thread B executes line 1 of the method, and gets a null reference because no item with the given key is present — this will happen after A returns from the first call, as the map is synchronized
thread A evaluates the if condition to false
thread B evaluates the if condition to false
From that point on, the two threads will carry on with execution of the true branch of the if statement, and you will lose one of the two value objects.
The second variant of the method you posted looks safer.
However, if you can use third party libraries, I would suggest you to check out Google Guava, as they offer concurrent multimaps (docs).

The second one is correct, but the first one isn't.
Think about it a minute, and suppose two threads are calling add() in parallel. Here's what could occur:
Thread 1 calls add("foo", bar");
Thread 2 calls add("foo", baz");
Thread 1 gets the set for "foo" : null
Thread 2 gets the set for "foo" : null
Thread 1 creates a new set and adds "bar" in it
Thread 2 creates a new set and adds "baz" in it
Thread 1 puts its set in the map
Thread 2 puts its set in the map
At the end of the story, the map contains one value for "foo" instead of two.
Synchronizing the map makes sure that its internal state is coherent, and that each method you call on the map is thread-safe. but it doesn't make the get-then-put operation atomic.
Consider using one of Guava's SetMultiMap implementations, which does everything for you. Wrap it into a call to Multimaps.synchronizedSetMultimap(SetMultimap) to make it thread-safe.

Your second implementation will work, but it holds locks for longer than it needs to (an inevitable problem with using synchronized methods rather than synchronized blocks), which will reduce concurrency. If you find that the limit on concurrency here is a bottleneck, you could shrink the locked regions a bit.
Alternatively, you could use some of the lock-free collections providded by java.util.concurrent. Here's my attempt at that; this isn't tested, and it requires Key to be comparable, but it should not perform any locking ever:
public class MapOfSets {
private final ConcurrentMap<Key, Set<Value>> map;
public MapOfSets() {
map = new ConcurrentSkipListMap<Key, Set<Value>>();
}
private static ThreadLocal<Set<Value>> freshSets = new ThreadLocal<Set<Value>>() {
#Override
protected Set<Value> initialValue() {
return new ConcurrentSkipListSet<Value>();
}
};
public void add(Key key, Value value) {
Set<Value> freshSet = freshSets.get();
Set<Value> set = map.putIfAbsent(key, freshSet);
if (set == null) {
set = freshSet;
freshSets.remove();
}
set.add(value);
}
public void remove(Key key, Value value) {
Set<Value> set = map.get(key);
if (set != null) {
set.remove(value);
}
}
//perform some operation on all elements in the set mapped to key
public void foo(Key key) {
Set<Value> set = map.get(key);
if (set != null) {
for (Value v: set) {
v.bar();
}
}
}
}

For your Map implementation you could just use a ConcurrentHashMap - You wouldn't have to worry about ensuring thread safety for access, whether it's input or retrieval, as the implemenation takes care of that for you.
And if you really want to use a Set, you could call
Collections.newSetFromMap(new ConcurrentHashMap<Object,Boolean>())
on your ConcurrentHashMap.

Java synchronized block using method call to get synch object

We are writing some locking code and have run into a peculiar question. We use a ConcurrentHashMap for fetching instances of Object that we lock on. So our synchronized blocks look like this
synchronized(locks.get(key)) { ... }
We have overridden the get method of ConcurrentHashMap to make it always return a new object if it did not contain one for the key.
#Override
public Object get(Object key) {
Object o = super.get(key);
if (null == o) {
Object no = new Object();
o = putIfAbsent((K) key, no);
if (null == o) {
o = no;
}
}
return o;
}
But is there a state in which the get-method has returned the object, but the thread has not yet entered the synchronized block. Allowing other threads to get the same object and lock on it.
We have a potential race condition were
thread 1: gets the object with key A, but does not enter the synchronized block
thread 2: gets the object with key A, enters a synchronized block
thread 2: removes the object from the map, exits synchronized block
thread 1: enters the synchronized block with the object that is no longer in the map
thread 3: gets a new object for key A (not the same object as thread 1 got)
thread 3: enters a synchronized block, while thread 1 also is in its synchronized block both using key A
This situation would not be possible if java entered the synchronized block directly after the call to get has returned. If not, does anyone have any input on how we could remove keys without having to worry about this race condition?

As I see it, the problem originates from the fact that you lock on map values, while in fact you need to lock on the key (or some derivation of it). If I understand correctly, you want to avoid 2 threads from running the critical section using the same key.
Is it possible for you to lock on the keys? can you guarantee that you always use the same instance of the key?
A nice alternative:
Don't delete the locks at all. Use a ReferenceMap with weak values. This way, a map entry is removed only if it is not currently in use by any thread.
Note:
1) Now you will have to synchronize this map (using Collections.synchronizedMap(..)).
2) You also need to synchronize the code that generates/returns a value for a given key.

you have 2 options:
a. you could check the map once inside the synchronized block.
Object o = map.get(k);
synchronized(o) {
if(map.get(k) != o) {
// object removed, handle...
}
}
b. you could extend your values to contain a flag indicating their status. when a value is removed from the map, you set a flag indicating that it was removed (within the sync block).
CacheValue v = map.get(k);
sychronized(v) {
if(v.isRemoved()) {
// object removed, handle...
}
}

The code as is, is thread safe. That being said, if you are removing from the CHM then any type of assumptions that are made when synchronizing on an object returned from the collection will be lost.
But is there a state in which the
get-method has returned the object,
but the thread has not yet entered the
synchronized block. Allowing other
threads to get the same object and
lock on it.
Yes, but that happens any time you synchronize on an Object. What is garunteed is that the other thread will not enter the synchronized block until the other exists.
If not, does anyone have any input on
how we could remove keys without
having to worry about this race
condition?
The only real way of ensuring this atomicity is to either synchronize on the CHM or another object (shared by all threads). The best way is to not remove from the CHM.

Thanks for all the great suggestions and ideas, really appreciate it! Eventually this discussion made me come up with a solution that does not use objects for locking.
Just a brief description of what we're actually doing.
We have a cache that receives data continuously from our environment. The cache has several 'buckets' for each key and aggregated events into the buckets as they come in. The events coming in have a key that determines the cache entry to be used, and a timestamp determining the bucket in the cache entry that should be incremented.
The cache also has an internal flush task that runs periodically. It will iterate all cache entries and flushes all buckets but the current one to database.
Now the timestamps of the incoming data can be for any time in the past, but the majority of them are for very recent timestamps. So the current bucket will get more hits than buckets for previous time intervals.
Knowing this, I can demonstrate the race condition we had. All this code is for one single cache entry, since the issue was isolated to concurrent writing and flushing of single cache elements.
// buckets :: ConcurrentMap<Long, AtomicLong>
void incrementBucket(long timestamp, long value) {
long key = bucketKey(timestamp, LOG_BUCKET_INTERVAL);
AtomicLong bucket = buckets.get(key);
if (null == bucket) {
AtomicLong newBucket = new AtomicLong(0);
bucket = buckets.putIfAbsent(key, newBucket);
if (null == bucket) {
bucket = newBucket;
}
}
bucket.addAndGet(value);
}
Map<Long, Long> flush() {
long now = System.currentTimeMillis();
long nowKey = bucketKey(now, LOG_BUCKET_INTERVAL);
Map<Long, Long> flushedValues = new HashMap<Long, Long>();
for (Long key : new TreeSet<Long>(buckets.keySet())) {
if (key != nowKey) {
AtomicLong bucket = buckets.remove(key);
if (null != bucket) {
long databaseKey = databaseKey(key);
long n = bucket.get()
if (!flushedValues.containsKey(databaseKey)) {
flushedValues.put(databaseKey, n);
} else {
long sum = flushedValues.get(databaseKey) + n;
flushedValues.put(databaseKey, sum);
}
}
}
}
return flushedValues;
}
What could happen was: (fl = flush thread, it = increment thread)
it: enters incrementBucket, executes until just before the call to addAndGet(value)
fl: enters flush and iterates the buckets
fl: reaches the bucket that is being incremented
fl: removes it and calls bucket.get() and stores the value to the flushed values
it: increments the bucket (which will be lost now, because the bucket has been flushed and removed)
The solution:
void incrementBucket(long timestamp, long value) {
long key = bucketKey(timestamp, LOG_BUCKET_INTERVAL);
boolean done = false;
while (!done) {
AtomicLong bucket = buckets.get(key);
if (null == bucket) {
AtomicLong newBucket = new AtomicLong(0);
bucket = buckets.putIfAbsent(key, newBucket);
if (null == bucket) {
bucket = newBucket;
}
}
synchronized (bucket) {
// double check if the bucket still is the same
if (buckets.get(key) != bucket) {
continue;
}
done = true;
bucket.addAndGet(value);
}
}
}
Map<Long, Long> flush() {
long now = System.currentTimeMillis();
long nowKey = bucketKey(now, LOG_BUCKET_INTERVAL);
Map<Long, Long> flushedValues = new HashMap<Long, Long>();
for (Long key : new TreeSet<Long>(buckets.keySet())) {
if (key != nowKey) {
AtomicLong bucket = buckets.get(key);
if (null != value) {
synchronized(bucket) {
buckets.remove(key);
long databaseKey = databaseKey(key);
long n = bucket.get()
if (!flushedValues.containsKey(databaseKey)) {
flushedValues.put(databaseKey, n);
} else {
long sum = flushedValues.get(databaseKey) + n;
flushedValues.put(databaseKey, sum);
}
}
}
}
}
return flushedValues;
}
I hope this will be useful for others that might run in to the same problem.

The two code snippets you've provided are fine, as they are. What you've done is similar to how lazy instantiation with Guava's MapMaker.makeComputingMap() might work, but I see no problems with the way that the keys are lazily created.
You're right by the way that it's entirely possible for a thread to be prempted after the get() lookup of a lock object, but before entering sychronized.
My problem is with the third bullet point in your race condition description. You say:
thread 2: removes the object from the map, exits synchronized block
Which object, and which map? In general, I presumed that you were looking up a key to lock on, and then would be performing some other operations on other data structures, within the synchronized block. If you're talking about removing the lock object from the ConcurrentHashMap mentioned at the start, that's a massive difference.
And the real question is whether this is necessary at all. In a general purpose environment, I don't think there will be any memory issues with just remembering all of the lock objects for all the keys that have ever been looked up (even if those keys no longer represent live objects). It is much harder to come up with some way of safely disposing of an object that may be stored in a local variable of some other thread at any time, and if you do want to go down this route I have a feeling that performance will degrade to that of a single coarse lock around the key lookup.
If I've misunderstood what's going on there then feel free to correct me.
Edit: OK - in which case I stand by my above claim that the easiest way to do this is not remove the keys; this might not actually be as problematic as you think, since the rate at which the space grows will be very small. By my calculations (which may well be off, I'm not an expert in space calculations and your JVM may vary) the map grows by about 14Kb/hour. You'd have to have a year of continuous uptime before this map used up 100MB of heap space.
But let's assume that the keys really do need to be removed. This poses the problem that you can't remove a key until you know that no threads are using it. This leads to the chicken-and-egg problem that you'll require all threads to synchronize on something else in order to get atomicity (of checking) and visibility across threads, which then means that you can't do much else than slap a single synchronized block around the whole thing, completely subverting your lock striping strategy.
Let's revisit the constraints. The main thing here is that things get cleared up eventually. It's not a correctness constraint but just a memory issue. Hence what we really want to do is identify some point at which the key could definitely no longer be used, and then use this as the trigger to remove it from the map. There are two cases here:
You can identify such a condition, and logically test for it. In which case you can remove the keys from the map with (in the worst case) some kind of timer thread, or hopefully some logic that's more cleanly integrated with your application.
You cannot identify any condition by which you know that a key will no longer be used. In this case, by definition, there is no point at which it's safe to remove the keys from the map. So in fact, for correctness' sake, you must leave them in.
In any case, this effectively boils down to manual garbage collection. Remove the keys from the map when you can lazily determine that they're no longer going to be used. Your current solution is too eager here since (as you point out) it's doing the removal before this situation holds.