设计模式之原型模式

原型模式（Prototype Pattern） 是用于创建重复的对象，同时又能保证性能。这种类型的设计模式属于创建型模式，它提供了一种创建对象的最佳方式。

这种模式是实现了一个原型接口，该接口用于创建当前对象的克隆。当直接创建对象的代价比较大时，则采用这种模式。例如，一个对象需要在一个高代价的数据库操作之后被创建。我们可以缓存该对象，在下一个请求时返回它的克隆，在需要的时候更新数据库，以此来减少数据库调用。

浅拷贝

/**
 * 浅克隆
 */
public class Test {
    public static void main(String[] args) throws Exception {
        Person p1 = new Person();
        Person p2 = (Person)p1.clone();
        System.out.println(p2.age + " " + p2.score); // 8 100
        System.out.println(p2.loc); // Location{street='bj', roomNo=22}

        System.out.println(p1.loc == p2.loc); // true
        p1.loc.street = "sh";
        System.out.println(p2.loc); // Location{street='sh', roomNo=22}
        // 修改的是p1的loc，p2的loc却跟着改变
    }
}

class Person implements Cloneable {
    int age = 8;
    int score = 100;
    Location loc = new Location("bj", 22);

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

class Location {
    String street;
    int roomNo;

    @Override
    public String toString() {
        return "Location{" +
                "street='" + street + '\'' +
                ", roomNo=" + roomNo +
                '}';
    }

    public Location(String street, int roomNo) {
        this.street = street;
        this.roomNo = roomNo;
    }
}

深拷贝

/**
 * 深克隆的处理
 */
public class Test {
    public static void main(String[] args) throws Exception {
        Person p1 = new Person();
        Person p2 = (Person)p1.clone();
        System.out.println(p2.age + " " + p2.score); // 8 100
        System.out.println(p2.loc); // Location{street='bj', roomNo=22}

        System.out.println(p1.loc == p2.loc); // false
        p1.loc.street = "sh";
        System.out.println(p2.loc); // Location{street='bj', roomNo=22}
    }
}

class Person implements Cloneable {
    int age = 8;
    int score = 100;
    Location loc = new Location("bj", 22);

    @Override
    public Object clone() throws CloneNotSupportedException {
        Person p = (Person)super.clone();
        p.loc = (Location)loc.clone();
        return p;
    }
}

class Location implements Cloneable {
    String street;
    int roomNo;

    @Override
    public String toString() {
        return "Location{" +
                "street='" + street + '\'' +
                ", roomNo=" + roomNo +
                '}';
    }

    public Location(String street, int roomNo) {
        this.street = street;
        this.roomNo = roomNo;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

String需要进一步深克隆吗？不需要

/**
 * String需要进一步深克隆吗？不需要
 */
public class Test {
    public static void main(String[] args) throws Exception {
        Person p1 = new Person();
        Person p2 = (Person)p1.clone();
        System.out.println(p2.age + " " + p2.score); // 8 100
        System.out.println(p2.loc); // Location{street='bj', roomNo=22}

        System.out.println(p1.loc == p2.loc); // false
        p1.loc.street = "sh";
        System.out.println(p2.loc); // Location{street='bj', roomNo=22}

        p1.loc.street.replace("sh", "sz");
        System.out.println(p2.loc.street); // bj
    }
}

class Person implements Cloneable {
    int age = 8;
    int score = 100;
    Location loc = new Location("bj", 22);

    @Override
    public Object clone() throws CloneNotSupportedException {
        Person p = (Person)super.clone();
        p.loc = (Location)loc.clone();
        return p;
    }
}

class Location implements Cloneable {
    String street;
    int roomNo;

    @Override
    public String toString() {
        return "Location{" +
                "street='" + street + '\'' +
                ", roomNo=" + roomNo +
                '}';
    }

    public Location(String street, int roomNo) {
        this.street = street;
        this.roomNo = roomNo;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

String需要进一步深克隆吗？其实这里是StringBuilder，所以需要

/**
 * String需要进一步深克隆吗？其实这里是StringBuilder，所以需要
 */
public class Test {
    public static void main(String[] args) throws Exception {
        Person p1 = new Person();
        Person p2 = (Person)p1.clone();
        System.out.println("p1.loc == p2.loc? " + (p1.loc == p2.loc)); // p1.loc == p2.loc? false

        p1.loc.street.reverse();
        System.out.println(p2.loc.street); // jb
    }
}

class Person implements Cloneable {
    int age = 8;
    int score = 100;
    Location loc = new Location(new StringBuilder("bj"), 22);

    @Override
    public Object clone() throws CloneNotSupportedException {
        Person p = (Person)super.clone();
        p.loc = (Location)loc.clone();
        return p;
    }
}

class Location implements Cloneable {
    StringBuilder street; // 这里是StringBuilder，不是String
    int roomNo;

    @Override
    public String toString() {
        return "Location{" +
                "street='" + street + '\'' +
                ", roomNo=" + roomNo +
                '}';
    }

    public Location(StringBuilder street, int roomNo) {
        this.street = street;
        this.roomNo = roomNo;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}