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@Synchronized

synchronized done right: Don't expose your locks.

Overview

- @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. -

With Lombok

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Vanilla Java

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Supported configuration keys:

lombok.synchronized.flagUsage = [warning | error] (default: not set): Lombok will flag any usage of @Synchronized as a warning or error if configured.

Small print

- 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. -

- Having at least one @Synchronized method in your class means there will be a lock field, but if you later remove all such methods, - there will no longer be a lock field. That means your predetermined serialVersionUID changes. We suggest you always add - a serialVersionUID to your classes if you intend to store them long-term via java's serialization mechanism. If you do so, removing - all @Synchronized annotations from your method will not break serialization. -

- 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! -

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