设计模式---原型模式
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。
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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