Singleton

• Singleton
• Muster:_Singleton
• Java_Standard:_Muster_Singleton#Hier_die_Thread-sichere_Version

Threadsafe Java Code (for Java 1.5 or later)

// Works with acquire/release semantics for volatile
// Broken under current semantics for volatile
  class Foo {
        private volatile Helper helper = null;
        public Helper getHelper() {
            if (helper == null) {
                synchronized(this) {
                    if (helper == null)
                        helper = new Helper();
                }
            }
            return helper;
        }
    }

Q: http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

• schlecht testbar (Mocken)
• nur über andere Singletons konfigurierbar
• entspricht (leider) einem globalem Objekt
• nicht vollständig thread-safe in C++
  • http://www.bombaydigital.com/arenared/2005/10/25/1
  • C++ and the Perils of Double-Checked Locking

• Dependency injection