Vanilla Java
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@Synchronized is a safer variant of the synchronized method modifier. Like synchronized, the
annotation can be used on static and instance methods only. It operates similarly to the synchronized keyword, but it locks
on different objects. The keyword locks on this, but the annotation locks on a field named $lock, which is private.
If the field does not exist, it is created for you. If you annotate a static method, the annotation locks on a static field
named $LOCK instead.
If you want, you can create these locks yourself. The $lock and $LOCK fields will of course not be generated if you
already created them yourself. You can also choose to lock on another field, by specifying it as parameter to the @Synchronized
annotation. In this usage variant, the fields will not be created automatically, and you must explicitly create them yourself, or an error will be emitted.
Locking on this or your own class object can have unfortunate side-effects, as other code not under your control can lock on these
objects as well, which can cause race conditions and other nasty threading-related bugs.
If $lock and/or $LOCK are auto-generated, the fields are initialized with an empty Object[] array, and not
just a new Object() as most snippets showing this pattern in action use. Lombok does this because a new object is NOT
serializable, but 0-size array is. Therefore, using @Synchronized will not prevent your object from being serialized.
If you'd like to know why a field is not automatically generated when you choose your own name for the lock object: Because otherwise making a typo in the field name will result in a very hard to find bug!