guava/src/com/google/common/collect/Multimaps.java
public static <K, V> ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
    return Synchronized.listMultimap(multimap, null);
  }
public static <K, V> ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
    return Synchronized.listMultimap(multimap, null);
  }
Returns a synchronized (thread-safe) {@code ListMultimap} backed by the specified multimap.

<p>You must follow the warnings described in {@link #synchronizedMultimap}.

@param multimap the multimap to be wrapped
@return a synchronized view of the specified multimap