Lombok features - @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

import lombok.Synchronized; public class SynchronizedExample {   private final Object readLock = new Object();      @Synchronized   public static void hello() {     System.out.println("world");   }      @Synchronized   public int answerToLife() {     return 42;   }      @Synchronized("readLock")   public void foo() {     System.out.println("bar");   } }

Vanilla Java

public class SynchronizedExample {   private static final Object $LOCK = new Object[0];   private final Object $lock = new Object[0];   private final Object readLock = new Object();      public static void hello() {     synchronized($LOCK) {       System.out.println("world");     }   }      public int answerToLife() {     synchronized($lock) {       return 42;     }   }      public void foo() {     synchronized(readLock) {       System.out.println("bar");     }   } }

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.