单例模式

1、饿汉模式

public class HungrySingleton {

    private static HungrySingleton hungrySingleton = new HungrySingleton();

    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }
}

 2、懒汉模式

public class HoonSyncSingleton {

    private static HoonSyncSingleton hoonSingleton = null;

    private HoonSyncSingleton() {

    }

    public synchronized static HoonSyncSingleton getInstance() {
        if(hoonSingleton == null) {
            hoonSingleton = new HoonSyncSingleton();
        }
        return hoonSingleton;
    }

}

 3、DCL模式

public class DCL {

    private static DCL instance = null;

    private DCL() {

    }

    public static DCL getInstance() {
        if(instance == null) {
            synchronized (DCL.class) {
                if(instance == null) {
                    instance = new DCL();
                }
            }
        }
        return instance;
    }
}

 4、内部类模式：

public class HolderDemo {

    private HolderDemo() {

    }

    private static class Holder {
        private static HolderDemo instance = new HolderDemo();
    }

    public static HolderDemo getInstance() {
        return Holder.instance;
    }
}

 5、枚举模式：

public class EnumSingletonDemo {

    private EnumSingletonDemo() {

    }

    private enum EnumSingleton {

        INSTANCE;

        private EnumSingletonDemo instance;

        EnumSingleton() {
            instance = new EnumSingletonDemo();
        }

        public EnumSingletonDemo getInstance() {
            return instance;
        }
    }

    public EnumSingletonDemo getInstance() {
        return EnumSingleton.INSTANCE.getInstance();
    }

}