三种Singleton的实现方式

来源：http://melin.iteye.com/blog/838258

三种Singleton的实现方式，一种是用大家熟悉的DCL，另外两种使用cas特性来实现。

    public class LazySingleton {

        private static volatile LazySingleton instance;     

        public static LazySingleton getInstantce() {

            if (instance == null) {

                synchronized (LazySingleton.class) {

                    if (instance == null) {

                        instance = new LazySingleton();

                    }

                }

            }

            return instance;

        }

    }

    /**

     * 利用putIfAbsent线程安全操作，实现单例模式

     *

     * @author Administrator

     *

     */

    public class ConcurrentSingleton {

        private static final ConcurrentMap<String, ConcurrentSingleton> map = new ConcurrentHashMap<String, ConcurrentSingleton>();

        private static volatile ConcurrentSingleton instance;  

        public static ConcurrentSingleton getInstance() {

            if (instance == null) {

                instance = map.putIfAbsent("INSTANCE", new ConcurrentSingleton());

            }

            return instance;

        }

    }

    public class AtomicBooleanSingleton {

        private static AtomicBoolean initialized = new AtomicBoolean(false);

        private static volatile AtomicBooleanSingleton instance;  

        public static AtomicBooleanSingleton getInstantce() {

            checkInitialized();

            return instance;

        }  

        private static void checkInitialized() {

            if(instance == null && initialized.compareAndSet(false, true)) {

                instance = new AtomicBooleanSingleton();

            }

        }

    }

三种Singleton的实现方式的相关教程结束。