懒汉式的三种线程安全的写法

双重检查锁

public class Student {
	private static Student student=null;
	private Student() {
		
	}
	public static Student getSingletonInstance() {
		if (student==null) {
			synchronized (Student.class) {
				if (student==null) {
					student=new Student();
				}
			}
		}
		return student;
	}
}

静态内部类的单例写法

public class Student {
	private Student() {
		
	}
	private static class StaticStudentFactory {
		private static Student student=new Student();
	}
	public static Student getSingletonInstance() {
		return StaticStudentFactory.student;
	}
}

如何破坏单例？（导致单例对象不单例）

• 反射攻击

• public class SingletonAttack {
      public static void main(String[] args) throws Exception {
  	reflectionAttack();
      }
  
      public static void reflectionAttack() throws Exception {
  	//通过反射，获取单例类的私有构造器
  	Constructor constructor = DoubleCheckLockSingleton.class.getDeclaredConstructor();
          //设置私有成员的暴力破解
  	constructor.setAccessible(true);
  	// 通过反射去创建单例类的多个不同的实例
  	DoubleCheckLockSingleton s1 = (DoubleCheckLockSingleton)constructor.newInstance();
          // 通过反射去创建单例类的多个不同的实例
  	DoubleCheckLockSingleton s2 = (DoubleCheckLockSingleton)constructor.newInstance();
  	s1.tellEveryone();
  	s2.tellEveryone();
  	System.out.println(s1 == s2);
      }
  }

   

• 序列化攻击

• public class SingletonAttack {
      public static void main(String[] args) throws Exception {
  	serializationAttack();
      }
  
      public static void serializationAttack() throws Exception {
  	// 对象序列化流去对对象进行操作
  	ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("serFile"));
  	//通过单例代码获取一个对象
  	DoubleCheckLockSingleton s1 = DoubleCheckLockSingleton.getInstance();
  	//将单例对象，通过序列化流，序列化到文件中
  	outputStream.writeObject(s1);
  					
  	// 通过序列化流，将文件中序列化的对象信息读取到内存中
  	ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream(new File("serFile")));
  	//通过序列化流，去创建对象
  	DoubleCheckLockSingleton s2 = (DoubleCheckLockSingleton)inputStream.readObject();
  	s1.tellEveryone();
  	s2.tellEveryone();
  					
  	System.out.println(s1 == s2);
      }
  }

   

枚举单例（最简单最安全）==枚举没有默认的无参构造器

• public enum EnumSingleton {
      INSTANCE;
  
      public void tellEveryone() {
  	System.out.println("This is an EnumSingleton " + this.hashCode());
      }
  }