不考慮反射這種極端情況，還有一種情況會破壞單例模式。
序列化，反序列化！

package cp2;

import java.io.Serializable;
import java.util.Objects;

/**
 * Created by dubby on 16/3/25.
 */
public class SerSingleton implements Serializable {
    String name;
    private SerSingleton(){
        System.out.println("Singleton is creating");
    }

    private
 
 static SerSingleton instance = new SerSingleton();

    public static SerSingleton getInstance(){
        return instance;
    }

    public static void createString(){
        System.out.println("create string in singleton");
    }
}

測試程式碼：

import cp2.SerSingleton;
import junit.framework.Assert;
import
 
 org.junit.Test;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

/**
 * Created by dubby on 16/3/25.
 */
public class SerSingletonTest {

    @Test
    public void test() throws Exception{
        SerSingleton s1 = null
 
;
        SerSingleton s = SerSingleton.getInstance();

        FileOutputStream fos = new FileOutputStream("a.txt");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(s);
        oos.flush();
        oos.close();

        FileInputStream fis = new FileInputStream("a.txt");
        ObjectInputStream ois = new ObjectInputStream(fis);
        s1 = (SerSingleton) ois.readObject();

        Assert.assertEquals(s,s1);
    }
}

結果：

junit.framework.AssertionFailedError: 
Expected :cp2.SerSingleton@7ab2bfe1
Actual   :cp2.SerSingleton@497470ed
   <Click to see difference>

問題來了，怎麼解決呢？jdk其實預料到這種情況了。
解決方法：加入readResolve()

在jdk中ObjectInputStream的類中有readUnshared（）方法，上面詳細解釋了原因。我簡單描述一下，那就是如果被反序列化的物件的類存在readResolve這個方法，他會呼叫這個方法來返回一個“array”（我也不明白），然後淺拷貝一份，作為返回值，並且無視掉反序列化的值，即使那個位元組碼已經被解析。
所以，完整的單例模式是：

package cp2;

import java.io.Serializable;
import java.util.Objects;

/**
 * Created by dubby on 16/3/25.
 */
public class SerSingleton implements Serializable {
    String name;
    private SerSingleton(){
        System.out.println("Singleton is creating");
    }

    private static SerSingleton instance = new SerSingleton();

    public static SerSingleton getInstance(){
        return instance;
    }

    public static void createString(){
        System.out.println("create string in singleton");
    }

    private Object readResolve(){
        System.out.println("read resolve");
        return instance;
    }
}