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。