Java设计模式-Builder构造者模式

介绍:
构造者模式,又称之为建造者模式,建造者模式,单例模式以及工厂模式都属于创建型模式
1
应用场景

今天学mybatis的时候，知道了SQLSessionFactory使用的是builder模式来生成的。再次整理一下什么是builder模式以及应用场景。
当一个bean类重载了多个构造方法时,并且参数随机使用时,考虑使用构造者模式,

builder模式也叫建造者模式，builder模式的作用将一个复杂对象的构建与他的表示分离，使用者可以一步一步的构建一个比较复杂的对象。

2.3 builder模式

package com.wangjun.designPattern.builder;

public class Product3 {
    
    private final int id;
    private final String name;
    private final int type;
    private final float price;
    
    private Product3(Builder builder) {
        this.id = builder.id;
        this.name = builder.name;
        this.type = builder.type;
        this.price = builder.price;
    }
    
    public static class Builder {
        private int id;
        private String name;
        private int type;
        private float price;
        
        public Builder id(int id) {
            this.id = id;
            return this;
        }
        public Builder name(String name) {
            this.name = name;
            return this;
        }
        public Builder type(int type) {
            this.type = type;
            return this;
        }
        public Builder price(float price) {
            this.price = price;
            return this;
        }
        
        public Product3 build() {
            return new Product3(this);
        }
    }

}

可以看到builder模式将属性定义为不可变的，然后定义一个内部静态类Builder来构建属性，再通过一个只有Builder参数的构造器来生成Product对象。Builder的setter方法返回builder本身，以便可以将属性连接起来。我们就可以像下面这样使用了。

Product3 p3 = new Product3.Builder()
                            .id(10)
                            .name("phone")
                            .price(100)
                            .type(1)
                            .build();

当然具体使用builder的情况肯定没有这么简单，但是思路大致一样：先通过某种方式取得构造对象需要的所有参数，再通过这些参数一次性构建这个对象。比如MyBatis中SqlSessionFactoryBuilder就是通过读取MyBatis的xml配置文件来获取构造SqlSessionFactory所需要的参数的。

1
Demo

package com.yunsuibi;

public class User {
private final String firstName; // required
private final String lastName; // required
private final int age; // optional
private final String phone; // optional
private final String address; // optional

private User(UserBuilder builder) {
this.firstName = builder.firstName;
this.lastName = builder.lastName;
this.age = builder.age;
this.phone = builder.phone;
this.address = builder.address;
}

public String getFirstName() {
return firstName;
}

public String getLastName() {
return lastName;
}

public int getAge() {
return age;
}

public String getPhone() {
return phone;
}

public String getAddress() {
return address;
}

@Override
public String toString() {
return "User [firstName=" + firstName + ", lastName=" + lastName + ", age=" + age + ", phone=" + phone
+ ", address=" + address + "]";
}

public static class UserBuilder {
private final String firstName;
private final String lastName;
private int age;
private String phone;
private String address;

public UserBuilder(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}

public UserBuilder age(int age) {
this.age = age;
return this;
}

public UserBuilder phone(String phone) {
this.phone = phone;
return this;
}

public UserBuilder address(String address) {
this.address = address;
return this;
}

public User build() {
return new User(this);
}

}
}

运行

public static void main(String[] args) {
User build = new User.UserBuilder("Jhon", "Doe").address("北京").build();
System.out.println(build);
User build2 = new User.UserBuilder("李四", "哈哈").address("12").build();
System.out.println(build2);
}

参考：java之构造者模式

参考：Java设计模式之builder模式