3. 用私有构造器或枚举类型强化singleton属性

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Effective Java 中提出实现单例的3中方法，详细内容如下：

1.将公有静态成员变量做成final域

package com.jason.effectivejava.rule3.one;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Elvis {

	public static final Elvis INSTANCE = new Elvis();

	private Elvis() {
               //it is very import for this type
		if(INSTANCE != null){
			throw new IllegalArgumentException("No exist the second instance");
		}
	}

	public void leaveTheBuilding() {

		System.out.println("Whoa baby, I'm outta here!");

	}

	public static void main(String[] args) {

		Elvis singleton = Elvis.INSTANCE;
		singleton.leaveTheBuilding();
		Constructor[] arrayConstructor = singleton.getClass().getDeclaredConstructors();
		arrayConstructor[0].setAccessible(true);
		
		try {
			arrayConstructor[0].newInstance();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

 注意：为了防止特殊用户通过AccessibleObject.setAccessible方法，利用反射机制访问私有构造函数，创建新的实例，应在私有构造函数中添加判断，阻止创建新的实例。

测试结果如下：

Whoa baby, I'm outta here!
java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
	at com.jason.effectivejava.rule3.Elvis.main(Elvis.java:30)
Caused by: java.lang.IllegalArgumentException: No exist the second instance
	at com.jason.effectivejava.rule3.Elvis.(Elvis.java:12)
	... 5 more

2.将公有成员变量做成静态工厂方法

package com.jason.effectivejava.rule3.two;

public class Elvis {

	private static final Elvis INSTANCE = new Elvis();

	private Elvis() {
	}

	public static Elvis getInstance() {
		return INSTANCE;
	}

	public void leaveTheBuilding() {

		System.out.println("Whoa baby, I'm outta here!");
	}

	// 必须提供该方法，以便重新指定反序列化得到的对象.
	private Object readResolve(){

		return INSTANCE;
	}

	public static void main(String[] args) {

		Elvis elvis = Elvis.getInstance();
		elvis.leaveTheBuilding();
	}
}

3.单元素枚举类型

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
/**
*
*枚举实现单例
*
目前最好的方式，避免了反射的攻击和序列化的问题
*
*反射调用枚举私有构造函数测试结果:
* Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.book.chap2.singleton.Singleton3.main(Singleton3.java:34)
*
*/

public enum Elvis {

	INSTANCE;

	public void leaveTheBuilding() {

		System.out.println("Whoa baby, I'm outta here!");
	}

	public static void main(String[] args) {

		Elvis elvis = Elvis.INSTANCE;
		elvis.leaveTheBuilding();
		
		//测试，是否可以反射生成枚举,利用反射调用私有构造器
		Constructor[] arrayConstructor=Elvis.INSTANCE.getClass().getDeclaredConstructors();

		arrayConstructor[0].setAccessible(true);
		try {
			arrayConstructor[0].newInstance();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}
}

 测试结果：

Whoa baby, I'm outta here!
java.lang.IllegalArgumentException: Cannot reflectively create enum objects
	at java.lang.reflect.Constructor.newInstance(Constructor.java:511)
	at com.jason.effectivejava.rule3.two.Elvis.main(Elvis.java:36)

 总结：

在三种方法中，单元素枚举类型的单例，代码简洁，无偿提供序列化机制，同时可防止多次实例化，在实际中应该是最佳选择。