设计模式---原型模式
简述
- 类型：创建型
- 目的：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。
最初版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; }} public 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 class Client { 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; }} public 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 class Client { 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; }} public 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 class Client { 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; }} public 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 Client { 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。
总结
优点
- 由于是直接从内存中读取对象进行克隆，所以性能卓越。
- 代码量不论是相较于传统写法要精简很多，尤其是序列化与反序列化的方式。
缺点
- 代码的理解难度增加。尤其是深拷贝的理解较为复杂。
适用场景
- 适用于只有细微参数变动的对象创建。
- 适用于需要备份的场景。如，当业务执行过程中，某种情况下需要数据回滚的时候，提前备份可以使用。
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如有侵权，请联系 cloudcommunity@tencent.com 删除。
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