防止单例模式被攻击

public class SingletonDemo {
	private SingletonDemo singletonDemo;

	private SingletonDemo() {
		if (singletonDemo != null) {
			throw new IllegalThreadStateException("An example already exists");
		}
	}

	public SingletonDemo getInstance() {
		if (singletonDemo == null) {
			singletonDemo = new SingletonDemo();
		}
		return singletonDemo;
	}
}

防止通过AccessibleObjet.setAccessible方法，通过反射机制调用私有构造器