延迟加载线程安全的单例模式

public class ThreadSafeSingleton {
        /**
         *volatile 具有synchronized的可见特性，
         *也就是说线程能够自动发现volatitle修饰属性的最新值
         */
	private volatile static ThreadSafeSingleton instance = null;

	private ThreadSafeSingleton() {

	}

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

另外一种写法

public class LazyLoadedSingelton {
	private LazyLoadedSingelton() {

	}

	private static class LazyHolder {
		private final static LazyLoadedSingelton instance = new LazyLoadedSingelton();
	}

	public static LazyLoadedSingelton getInstance() {
		return LazyHolder.instance;
	}
}