Annotation Type LazyInit


@IncompatibleModifiers(modifier=FINAL)
@Retention(RUNTIME)
@Target(FIELD)
public @interface LazyInit
Use this annotation on any static or field that will be initialized lazily, where races yield no semantic difference in the code (as, for example, is the case with String.hashCode()). Note that lazily initializing a non-volatile field is hard to do correctly, and one should rarely use this. It should also only be done by developers who clearly understand the potential issues, and then, always using the pattern as presented in the getData method of this sample code below:

 private final String source;
 {@literal @}LazyInit private String data;

 public String getData() {
   String local = data;
   if (local == null) {
     data = local = expensiveCalculation(source);
   }
   return local;
  }

 private static String expensiveCalculation(String string) {
   return string.replaceAll(" ", "_");
 }
 

The need for using the local variable is detailed in http://jeremymanson.blogspot.com/2008/12/benign-data-races-in-java.html (see, particularly, the part after "Now, let's break the code").

Also note that LazyInit must not be used on 64-bit primitives (longs and doubles), because the Java Language Specification does not guarantee that writing to these is atomic. Furthermore, when used for non-primitives, the non-primitive must be either truly immutable or at least thread safe (in the Java memory model sense). And callers must accommodate the fact that different calls to something like the above getData() method may return different (though identically computed) objects, with different identityHashCode() values. Again, unless you really understand this and you really need the performance benefits of introducing the data race, do not use this construct.