Method annotation to make a method call synchronized for concurrency handling with some useful baked-in conventions.

@Synchronized is a safer variant of the synchronized method modifier. The annotation can only be used on static and instance methods. It operates similarly to the synchronized keyword, but it locks on different objects. When used with an instance method, the synchronized keyword locks on this, but the annotation locks on a (by default automatically generated) field named $lock. 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 not be generated if you create them yourself. You can also choose to lock on another field, by specifying its name as parameter to the @Synchronized annotation. In this usage variant, the lock field will not be created automatically, and you must explicitly create it yourself.

Rationale: 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.

Example usage:

 class SynchronizedExample {
   private final myLock = new Object()

   @Synchronized
   static void greet() {
     println "world"
   }

   @Synchronized
   int answerToEverything() {
     return 42
   }

   @Synchronized("myLock")
   void foo() {
     println "bar"
   }
 }
 

 class SynchronizedExample {
   private static final $LOCK = new Object[0]
   private final $lock = new Object[0]
   private final myLock = new Object()

   static void greet() {
     synchronized($LOCK) {
       println "world"
     }
   }

   int answerToEverything() {
     synchronized($lock) {
       return 42
     }
   }

   void foo() {
     synchronized(myLock) {
       println "bar"
     }
   }
 }
 

Details: If $lock and/or $LOCK are auto-generated, the fields are initialized with an empty Object[] array, and not just a new Object() as many snippets using this pattern tend to use. This is because a new Object is NOT serializable, but a 0-size array is. Therefore, using @Synchronized will not prevent your object from being serialized.

More examples:

 import groovy.transform.Synchronized

 class Util {
     private counter = 0

     private def list = ['Groovy']

     private Object listLock = new Object[0]

     @Synchronized
     void workOnCounter() {
         assert 0 == counter
         counter++
         assert 1 == counter
         counter --
         assert 0 == counter
     }

     @Synchronized('listLock')
     void workOnList() {
         assert 'Groovy' == list[0]
         list << 'Grails'
         assert 2 == list.size()
         list = list - 'Grails'
         assert 'Groovy' == list[0]
     }
 }

 def util = new Util()
 def tc1 = Thread.start {
     100.times {
         util.workOnCounter()
         sleep 20 
         util.workOnList()
         sleep 10
     }
 }
 def tc2 = Thread.start {
     100.times {
         util.workOnCounter()
         sleep 10 
         util.workOnList()
         sleep 15
     }
 }
 tc1.join()
 tc2.join()
 

Since:

1.7.3