code: public static <K, V> Multimap<K, V> synchronizedMultimap(Multimap<K, V> multimap) {
      return Synchronized.multimap(multimap, null);
    }
docs: Returns a synchronized (thread-safe) multimap backed by the specified multimap. In order to
  guarantee serial access, it is critical that <b>all</b> access to the backing multimap is
  accomplished through the returned multimap.
  
  <p>It is imperative that the user manually synchronize on the returned multimap when accessing
  any of its collection views:
  
  <pre>{@code
  Multimap<K, V> multimap = Multimaps.synchronizedMultimap(
  HashMultimap.<K, V>create());
  ...
  Collection<V> values = multimap.get(key);  // Needn't be in synchronized block
  ...
  synchronized (multimap) {  // Synchronizing on multimap, not values!
  Iterator<V> i = values.iterator(); // Must be in synchronized block
  while (i.hasNext()) {
  foo(i.next());
  }
  }
  }</pre>
  
  <p>Failure to follow this advice may result in non-deterministic behavior.
  
  <p>Note that the generated multimap's {@link Multimap#removeAll} and {@link
  Multimap#replaceValues} methods return collections that aren't synchronized.
  
  <p>The returned multimap will be serializable if the specified multimap is serializable.
  
  @param multimap the multimap to be wrapped in a synchronized view
  @return a synchronized view of the specified multimap