Databases
StackOverflow2013
Postsdbo
POSynchronizing a Map of Sets/Lists
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POSynchronizing a Map of Sets/Lists
    primarykey
    data
    text
    <p>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.</p> <p>So given the following code, where Map, HashMap, and Set are the Java implementations, and Key and Value are some arbitrary Objects:</p> <pre><code>public class MapOfSets { private Map&lt;Key, Set&lt;Value&gt;&gt; map; public MapOfLists() { map = Collections.synchronizedMap(new HashMap&lt;Key, Set&lt;Value&gt;()); } //adds value to the set mapped to key public void add(Key key, Value value) { Set&lt;Value&gt; 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&lt;Value&gt;(); 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&lt;Value&gt; set = map.get(key); for(Value v : set) v.bar(); } } </code></pre> <p>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?</p> <p>Alternatively, would the above code be advantageous over another possible synchronization solution:</p> <pre><code>public class MapOfSets { private Map&lt;Key, Set&lt;Value&gt;&gt; map; public MapOfLists() { map = new HashMap&lt;Key, Set&lt;Value&gt;(); } public synchronized void add(Key key, Value value) { Set&lt;Value&gt; 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&lt;Value&gt;(); 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&lt;Value&gt; set = map.get(key); for(Value v : set) v.bar(); } } </code></pre> <p>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?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USdonnyton
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POSynchronizing a Map of Sets/Lists
      1. This table or related slice is empty.
      1. VTUpMod
    1. CO`old = new Set<Value>();` - `Set` is an interface, what class are you using? There might be problems if you use non-concurrent class
      singulars
      1. POSynchronizing a Map of Sets/Lists
      1. USamit
    2. CODoes it matter? I was considering using the CopyOnWriteArraySet, which is guaranteed thread-safe, but doesn't synchronization of either the map or accessors already guarantee that only one thread will be looking at a Set at a time?
      singulars
      1. POSynchronizing a Map of Sets/Lists
      1. USdonnyton
    3. COWhy does the first implementation guarantees only one thread will be looking at `Set` at a time? I don't think this statement is true, and you might have concurrent issues when 2 threads try to add/del element from the set concurrently, if the set is not thread-safe.
      singulars
      1. POSynchronizing a Map of Sets/Lists
      1. USamit
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload