面试题——单例模式与反射、序列化

ゝ一世哀愁。 2021-08-31 20:30 347阅读 0赞

#### 饿汉式 ####

立即加载，线程安全。

public class Singleton { 
        private static Singleton INSTANCE = new Singleton();//立即加载
        private Singleton(){ };
        public static Singleton getInstance() { 
            return INSTANCE;
        }
    }

#### 懒汉式 ####

延迟加载，线程不安全。

public class Singleton { 
        private static Singleton INSTANCE;//延迟加载
        private Singleton(){ };
        public static Singleton getInstance() { 
            if (INSTANCE == null) { 
                INSTANCE = new Singleton();
            } 
            return INSTANCE;
        }
    }

##### 双检锁（DCL） #####

延迟加载，线程安全。

public class Singleton { 
        private volatile static Singleton INSTANCE;//延迟加载
        private Singleton(){ };
        public static Singleton getInstance() { 
            if (INSTANCE == null) { 
                synchronized (Singleton.class) { 
                    if (INSTANCE == null) { 
                        INSTANCE = new Singleton();
                    }
                }
            }
            return INSTANCE;
        }
    }

#### 静态内部类 ####

延迟加载，线程安全。  
静态内部类属于被动引用，只有在调用时才加载。而JVM的类加载机制会保证只有一个线程成功执行`<clinit>`方法，初始化类变量。

#### 枚举 ####

延迟加载，线程安全。

public class Singleton { 
        private Singleton(){ }
        
        //静态枚举类
        static enum SingletonHolder { 
            INSTANCE;
            private Singleton singleton;
    
            private SingletonHolder() { 
                singleton = new Singleton();
            }
    
            public Singleton getInstance() { 
                return singleton;
            }
        }
    
        //对外暴露的获取方法
        public static Singleton getInstance() { 
            return SingletonHolder.INSTANCE.getInstance();
        }
        
    }

#### 单例与反射 ####

反射破坏单例：

public class Singleton { 
        private Singleton(){ }
        private static class SingletonHolder { 
            private static Singleton INSTANCE = new Singleton();
        }
        public static Singleton getInstance() { 
            return SingletonHolder.INSTANCE;
        }
    
    
        public static void main(String[] args) throws Exception { 
            Class clazz = Singleton.class;
            Constructor constructor = clazz.getDeclaredConstructor(null);
            constructor.setAccessible(true);
            Singleton o1 = (Singleton) constructor.newInstance();
            Singleton o2 = (Singleton) constructor.newInstance();
            System.out.println(o1);
            System.out.println(o2);
            System.out.println(o1 == o2);//false
        }
    }

一般而言有两种解决方案：

一：在私有构造中设置标志位，进行访问次数的判断，限制只能访问一次构造函数。

public class Singleton { 
        private static boolean flag = false;
        private Singleton(){ 
            if (flag  == false) { 
                flag = true;
            } else { 
                throw new RuntimeException("反射尝试破坏...");
            }
        }
        private static class SingletonHolder { 
            private static Singleton INSTANCE = new Singleton();
        }
        public static Singleton getInstance() { 
            return SingletonHolder.INSTANCE;
        }
    
    
        public static void main(String[] args) throws Exception { 
            Class clazz = Singleton.class;
            Constructor constructor = clazz.getDeclaredConstructor(null);
            constructor.setAccessible(true);
            Singleton o1 = (Singleton) constructor.newInstance();
            Singleton o2 = (Singleton) constructor.newInstance();
            System.out.println(o1);
            System.out.println(o2);
            System.out.println(o1 == o2);
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]

二：直接使用枚举式单例，枚举没有无参构造，天然单例防反射。

#### 单例与序列化 ####

序列化单例对象，再反序列化也可以破坏单例模式：

public class Singleton implements Serializable{ 
        private static boolean flag = false;
        private Singleton(){ 
            if (flag  == false) { 
                flag = true;
            } else { 
                throw new RuntimeException("反射尝试破坏...");
            }
        }
        private static class SingletonHolder { 
            private static Singleton INSTANCE = new Singleton();
        }
        public static Singleton getInstance() { 
            return SingletonHolder.INSTANCE;
        }
    
    
        public static void main(String[] args) throws Exception { 
            Singleton source = Singleton.getInstance();
            writeObject(source);//序列化
            Singleton target = (Singleton) readObjcet();//反序列化
            System.out.println(source);
            System.out.println(target);
            System.out.println(source == target);
        }
    
        public static void writeObject(Object o) { 
            FileOutputStream fileOut = null;
            ObjectOutputStream out = null;
            try { 
                fileOut = new FileOutputStream("test.txt");
                out = new ObjectOutputStream(fileOut);
                out.writeObject(o);
                System.out.println("Serialized data is saved");
            } catch (FileNotFoundException e) { 
                e.printStackTrace();
            } catch (IOException e) { 
                e.printStackTrace();
            }finally { 
                try { 
                    out.close();
                } catch (IOException e) { 
                    e.printStackTrace();
                }
                try { 
                    fileOut.close();
                } catch (IOException e) { 
                    e.printStackTrace();
                }
            }
        }
    
    
        public static Object readObjcet() { 
            Object temp = null;
            FileInputStream fileIn = null;
            ObjectInputStream in = null;
            try { 
                fileIn = new FileInputStream("test.txt");
                in = new ObjectInputStream(fileIn);
                temp  =  in.readObject();
                System.out.println("Deserialized Object...");
                return temp;
            } catch (Exception e) { 
                e.printStackTrace();
                return null;
            }finally { 
                try { 
                    in.close();
                } catch (IOException e) { 
                    e.printStackTrace();
                }
                try { 
                    fileIn.close();
                } catch (IOException e) { 
                    e.printStackTrace();
                }
            }
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 1]

解决方案：添加`readResolve()方法`，在方法中返回单例

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 2]

原理：在`ObjectInputStream`类的`readOrdinaryObject()`方法中，对`readResolve()`方法的存在做了判断，如果存在，则调用该方法获取返回值作为最终返回。

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