单例，枚举，反射，序列化--effectiveJava读书笔记

旧城等待， 2022-08-06 03:22 142阅读 0赞

## 先看一个单例： ##

public class Singleton{
    	private final static Singleton INSTANCE  = new Singleton();
    	private Singleton(){};
    	public static Singleton getInstance(){return INSTANCE;}
    }

## 我们用序列化来打破单例 ##

public class Singleton implements Serializable{
    	private final static Singleton INSTANCE  = new Singleton();
    	private Singleton(){};
    	public static Singleton getInstance(){return INSTANCE;}
    	
    	public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
    		Singleton s1 = Singleton.getInstance();
    		File objectF = new File("/object");
    		ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(objectF));
    		out.writeObject(s1);
    		out.close();
    		ObjectInputStream in = new ObjectInputStream(new FileInputStream(objectF));
    		Singleton s2 = (Singleton) in.readObject();
    		in.close();
    		System.out.println("是单例么？" + (s1 == s2));
    	}
    }

将会打印：

是单例么？false。

可见我们可以这样破坏其单例属性。要保持应该怎么办呢？需要增加readResolve方法，Java反序列化的时候会用这个方法的返回值直接代替序列化得到的对象

public class Singleton implements Serializable{
    	private final static Singleton INSTANCE  = new Singleton();
    	private Singleton(){};
    	public static Singleton getInstance(){return INSTANCE;}
    		
    	private Object readResolve() {
    		return INSTANCE;
    	}
    	
    	public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
    		Singleton s1 = Singleton.getInstance();
    		File objectF = new File("/object");
    		ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(objectF));
    		out.writeObject(s1);
    		out.close();
    		ObjectInputStream in = new ObjectInputStream(new FileInputStream(objectF));
    		Singleton s2 = (Singleton) in.readObject();
    		in.close();
    		System.out.println("是单例么？" + (s1 == s2));
    	}
    }

打印：

是单例么？true

## 我们再通过反射来打破其的单例性： ##

public class Singleton{
    	private final static Singleton INSTANCE  = new Singleton();
    	private Singleton(){};
    	public static Singleton getInstance(){return INSTANCE;}
    	
    	public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException {
    		Singleton s1 = Singleton.getInstance();
    		Constructor<Singleton> c = Singleton.class.getDeclaredConstructor();
    		c.setAccessible(true);
    		Singleton s2 = c.newInstance();
    		System.out.println("是单例么？" + (s1 == s2));
    	}
    }

将会打印：

是单例么？false。

说明使用反射调用私有构造器也是可以破坏单例的,解决的办法是如下：

public class Singleton{
    	private final static Singleton INSTANCE  = new Singleton();
    	private Singleton(){
    		if(++COUNT > 1){
    			throw new RuntimeException("can not be construt more than once");
    		}
    	};
    	private static int COUNT = 0;
    	public static Singleton getInstance(){
    		return INSTANCE;
    	}
    }

这样当使用反射调用的时候，就会抛出异常。

再用clone来破坏单例性

public class Singleton implements Cloneable{
    	private final static Singleton INSTANCE  = new Singleton();
    	public static Singleton getInstance(){
    		return INSTANCE;
    	}
    	
    	public static void main(String[] args) {
    		Singleton s1 = Singleton.getInstance();
    		Singleton s2 = (Singleton) s1.clone();
    		System.out.println("是单例么？" + (s1 == s2));
    	}
    }

这样也会发现不是单例了，办法是重新clone方法。

public class Singleton implements Cloneable{
    	private final static Singleton INSTANCE  = new Singleton();
    	public static Singleton getInstance(){
    		return INSTANCE;
    	}
    	
    	@Override
    	protected Object clone() throws CloneNotSupportedException {
    		return INSTANCE;
    	}
    	
    	public static void main(String[] args) {
    		Singleton s1 = Singleton.getInstance();
    		Singleton s2 = (Singleton) s1.clone();
    		System.out.println("是单例么？" + (s1 == s2));
    	}
    }

## 如果想要比较简便的避免上诉的问题，最好的方式是使用枚举： ##

public enum SingleEnum {
    	INSTANCE;
    	
    	public static SingleEnum getInstance(){
    		return INSTANCE;
    	}
    }

其通过反射会抛出如下异常：

java.lang.NoSuchMethodException: com.price.effective.create.SingleEnum.<init>()

通过反序列化其也会返回INSTANCE对象。

其没有clone方法

综上，Enum可以作为想用单例时的第一选择。