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