设计模式—原型模式

技术分享 4年前 (2022-09-22) 0 999+

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简述

类型：创建型
目标：通过拷贝快速创建相同或相似对象。

接下来我们看一个需要改进的案例。

优化案例

话不多说，先来看一个创建相同或相似对象的传统写法。

原版v0

public class Department {     private String name;     private String country;     private String province;     private String city;     private List<Employee> employees;     public String getName() {         return name;     }     public String getCountry() {         return country;     }     public String getProvince() {         return province;     }     public String getCity() {         return city;     }     public List<Employee> getEmployees() {         return employees;     }     public Department(String name, String country, String province,                       String city, List<Employee> employees) {         this.name = name;         this.country = country;         this.province = province;         this.city = city;         this.employees = employees;     } } class Employee {     private String name;     private String sex;     private int age;     private String country;     private String province;     private String city;     private String post;     public String getName() {         return name;     }     public String getSex() {         return sex;     }     public int getAge() {         return age;     }     public String getCountry() {         return country;     }     public String getProvince() {         return province;     }     public String getCity() {         return city;     }     public String getPost() {         return post;     }     public Employee(String name, String sex, int age,                     String country, String province,                     String city, String post) {         this.name = name;         this.sex = sex;         this.age = age;         this.country = country;         this.province = province;         this.city = city;         this.post = post;     } }

已知一个Department类型的对象，我们想构造一个相似的对象。

public static void main(String[] args) {     Employee emp = new Employee("张三", "男", 15, "中国", "江西省", "南昌市", "124-1241-1353");     Department department = new Department("开发部", "中国", "江西省", "南昌市", List.of(e)); // 已知对象     Department department1 = new Department(department.getName(), department.getCountry(), department.getProvince(), department.getCity(), department.getPost()); // 拷贝对象 }

可以感受到，对象拷贝的朴素写法非常的麻烦。而且想到每一处对象拷贝都需要这样写就感觉头皮发麻。

为了解决这个问题，我们引入原型模式。请看以下样例。

修改版v1(浅拷贝)

public class Department {     private String name;     private String country;     private String province;     private String city;     private List<Employee> employees;     public Department(String name, String country, String province,                       String city, List<Employee> employees) {         this.name = name;         this.country = country;         this.province = province;         this.city = city;         this.employees = employees;     } } class Employee {     private String name;     private String sex;     private int age;     private String country;     private String province;     private String city;     private String post;     public Employee(String name, String sex, int age,                     String country, String province,                     String city, String post) {         this.name = name;         this.sex = sex;         this.age = age;         this.country = country;         this.province = province;         this.city = city;         this.post = post;     } }

使用clone()方法拷贝目标对象。

public static void main(String[] args) throws CloneNotSupportedException {     Employee e = new Employee("张三", "男", 15, "中国", "江西省", "南昌市", "124-1241-1353");     Department department = new Department("开发部", "中国", "江西省", "南昌市", List.of(e));     Department department1 = (Department)department.clone();     System.out.println(department == department1); // false     System.out.println(department.employees == department1.employees); // true }

我们发现第8行输出true，这说明两个对象的employees的引用相同，这会导致修改其中一个employees的元素会影响到另一个，这并不好。

如何解决属性相同引用的问题？看以下样例。

修改版v2(深拷贝)

public class Department implements Cloneable {     private String name;     private String country;     private String province;     private String city;     private List<Employee> employees;     public Department(String name, String country, String province,                       String city, List<Employee> employees) {         this.name = name;         this.country = country;         this.province = province;         this.city = city;         this.employees = employees;     }     @Override     public Object clone() throws CloneNotSupportedException {         Department department = (Department)super.clone();         List<Employee> emps = new ArrayList<>();         for (int i = 0; i < department.employees.size(); i ++) {             emps.add((Employee) employees.get(i).clone());         }         department.employees = emps;         return department;     } } class Employee implements Cloneable {     private String name;     private String sex;     private int age;     private String country;     private String province;     private String city;     private String post;     public Employee(String name, String sex, int age,                     String country, String province,                     String city, String post) {         this.name = name;         this.sex = sex;         this.age = age;         this.country = country;         this.province = province;         this.city = city;         this.post = post;     }     @Override     public Object clone() throws CloneNotSupportedException {         return super.clone();     } }

使用clone() 拷贝对象，因为类以及类中的属性也重写了clone()。

public static void main(String[] args) throws CloneNotSupportedException {     Employee e = new Employee("张三", "男", 15, "中国", "江西省", "南昌市", "124-1241-1353");     Department department = new Department("开发部", "中国", "江西省", "南昌市", List.of(e));     Department department1 = (Department)department.clone();     System.out.println(department == department1); // false     System.out.println(department.employees == department1.employees); // false }

虽然这种方式可以深拷贝，但是这会让代码量激增。

序列化与反序列化可以解决这个问题。

修改版v3(序列化与反序列化)(推荐使用)

public class Department {     private String name;     private String country;     private String province;     private String city;     private List<Employee> employees;     public Department(String name, String country, String province,                       String city, List<Employee> employees) {         this.name = name;         this.country = country;         this.province = province;         this.city = city;         this.employees = employees;     } } class Employee {     private String name;     private String sex;     private int age;     private String country;     private String province;     private String city;     private String post;     public Employee(String name, String sex, int age,                     String country, String province,                     String city, String post) {         this.name = name;         this.sex = sex;         this.age = age;         this.country = country;         this.province = province;         this.city = city;         this.post = post;     } }

序列化与反序列化的实现方式有很多种，本文使用Gson来实现。以下是样例。

public static void main(String[] args) throws CloneNotSupportedException {     Employee e = new Employee("张三", "男", 15, "中国", "江西省", "南昌市", "124-1241-1353");     Department department = new Department("开发部", "中国", "江西省", "南昌市", List.of(e));     Gson gson = new Gson();     String s = gson.toJson(department);     Department department1 = s.fromJson(s, Department.class);     System.out.println(department == department1); // false     System.out.println(department.employees == department1.employees); // false }

基于序列化和反序列化实现的克隆不仅仅是深度克隆，更重要的是通过泛型限定，可以检查出要克隆的对象是否支持序列化，这项检查是编译器完成的，不是在运行时抛出异常，这种是方案明显优于使用Object类的clone方法克隆对象。让问题在编译的时候暴露出来总是优于把问题留到运行时。