Field annotation to simplify lazy initialization.

Example usage without any special modifiers just defers initialization until the first call but is not thread-safe:

@Lazy T x

becomes

private T $x

T getX() {
   if ($x != null)
      return $x
   else {
      $x = new T()
      return $x
   }
}

If the field is declared volatile then initialization will be synchronized using the double-checked locking pattern as shown here:

@Lazy volatile T x

becomes

private volatile T $x

T getX() {
   T $x_local = $x
   if ($x_local != null)
      return $x_local
   else {
      synchronized(this) {
         if ($x == null) {
            $x = new T()
         }
         return $x
      }
   }
}

By default a field will be initialized by calling its default constructor. If the field has an initial value expression then this expression will be used instead of calling the default constructor. In particular, it is possible to use closure { ... } () syntax as follows:

@Lazy T x = { [1, 2, 3] } ()

becomes

private T $x

T getX() {
   T $x_local = $x
   if ($x_local != null)
      return $x_local
   else {
      synchronized(this) {
         if ($x == null) {
            $x = { [1, 2, 3] } ()
         }
         return $x
      }
   }
}

@Lazy(soft=true) will use a soft reference instead of the field and use the above rules each time re-initialization is required.

If the soft flag for the annotation is not set but the field is static, then the initialization on demand holder idiom is used as follows:

@Lazy static FieldType field
@Lazy static Date date1
@Lazy static Date date2 = { new Date().copyWith(year: 2000) }()
@Lazy static Date date3 = new GregorianCalendar(2009, Calendar.JANUARY, 1).time

becomes these methods and inners classes within the class containing the above definitions:

private static class FieldTypeHolder_field {
    private static final FieldType INSTANCE = new FieldType()
}

private static class DateHolder_date1 {
    private static final Date INSTANCE = new Date()
}

private static class DateHolder_date2 {
    private static final Date INSTANCE = { new Date().copyWith(year: 2000) }()
}

private static class DateHolder_date3 {
    private static final Date INSTANCE = new GregorianCalendar(2009, Calendar.JANUARY, 1).time
}

static FieldType getField() {
    return FieldTypeHolder_field.INSTANCE
}

static Date getDate1() {
    return DateHolder_date1.INSTANCE
}

static Date getDate2() {
    return DateHolder_date2.INSTANCE
}

static Date getDate3() {
    return DateHolder_date3.INSTANCE
}