@Synchronized

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

01 import lombok.Synchronized;
02 
03 public class SynchronizedExample {
04   private final Object readLock = new Object();
05   
06   @Synchronized
07   public static void hello() {
08     System.out.println("world");
09   }
10   
11   @Synchronized
12   public int answerToLife() {
13     return 42;
14   }
15   
16   @Synchronized("readLock")
17   public void foo() {
18     System.out.println("bar");
19   }
20 }

Vanilla Java

01 public class SynchronizedExample {
02   private static final Object $LOCK = new Object[0];
03   private final Object $lock = new Object[0];
04   private final Object readLock = new Object();
05   
06   public static void hello() {
07     synchronized($LOCK) {
08       System.out.println("world");
09     }
10   }
11   
12   public int answerToLife() {
13     synchronized($lock) {
14       return 42;
15     }
16   }
17   
18   public void foo() {
19     synchronized(readLock) {
20       System.out.println("bar");
21     }
22   }
23 }

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!