ToMany: ensure double-checked locking works (e.g. use volatile).

Also performance improvement by only reading listFactory field once in most cases.

Author: greenrobot-team

objectbox-java/src/main/java/io/objectbox/relation/ToMany.java

@@ -68,14 +68,14 @@ public class ToMany<TARGET> implements List<TARGET>, Serializable {
  private final Object entity;
  private final RelationInfo<Object, TARGET> relationInfo;
 
- private ListFactory listFactory;
+ private volatile ListFactory listFactory;
  private List<TARGET> entities;
 
  /** Counts of all entities in the list ({@link #entities}). */
  private Map<TARGET, Integer> entityCounts;
 
  /** Entities added since last put/sync. Map is used as a set (value is always Boolean.TRUE). */
- private Map<TARGET, Boolean> entitiesAdded;
+ private volatile Map<TARGET, Boolean> entitiesAdded;
 
  /** Entities removed since last put/sync. Map is used as a set (value is always Boolean.TRUE). */
  private Map<TARGET, Boolean> entitiesRemoved;
@@ -129,14 +129,16 @@ public synchronized void setRemoveFromTargetBox(boolean removeFromTargetBox) {
  }
 
  public ListFactory getListFactory() {
- if (listFactory == null) {
+ ListFactory result = listFactory;
+ if (result == null) {
  synchronized (this) {
- if (listFactory == null) {
- listFactory = new CopyOnWriteArrayListFactory();
+ result = listFactory;
+ if (result == null) {
+ listFactory = result = new CopyOnWriteArrayListFactory();
  }
  }
  }
- return listFactory;
+ return result;
  }
 
  private void ensureBoxes() {