opwvhk commented on code in PR #2900:
URL: https://github.com/apache/avro/pull/2900#discussion_r1622618815
##########
lang/java/avro/src/main/java/org/apache/avro/reflect/ReflectData.java:
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@@ -379,12 +379,14 @@ private ClassAccessorData(Class<?> c) {
* Return the field accessors as an array, indexed by the field index of
the
* given schema.
*/
- private synchronized FieldAccessor[] getAccessorsFor(Schema schema) {
- // if synchronized is removed from this method, adjust bySchema
appropriately
+ private FieldAccessor[] getAccessorsFor(Schema schema) {
+ // to avoid synchronization, we replace the map for each modification
FieldAccessor[] result = bySchema.get(schema);
if (result == null) {
result = createAccessorsFor(schema);
+ Map<Schema, FieldAccessor[]> bySchema = new
WeakHashMap<>(this.bySchema);
bySchema.put(schema, result);
+ this.bySchema = bySchema;
Review Comment:
The remark about `volatile` is valid: that's exactly what it does.
Also a few notes about `WeakHashMap`:
1. it has weak keys, so it'll only cleanup entries that can no longer be
looked up
2. its internal hash table is a singly-linked list, and the update to remove
an entry is an atomic operation (that never yields an invalid data structure)
3. the update happens without a memory barrier, which means that the worst
case scenario is that the update is not yet available to the other thread,
which means that the entry to be removed is read & skipped over in the
concurrent call to `get`
In practical terms, this means that because the we update a *copy* of the
`WeakHashMap` before it becomes available, and it's only modified to expunge
stale entries after it becomes available, the result is effectively thread safe.
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