建造者模式

用过lombok的人可能对这个创建型模式很熟悉了，通常我们使用@Builder注解的时候，就是lombok自动帮我们实现了建造者模式。

例如用户类举例：

import lombok.Builder;

@Builder
public class User {

	private String username;
	
	private String password;
}

使用：

public static void main(String[] args) {
    User user = User.builder().username("zhangsan").password("123456").build();
}

实际编译后的代码：

public class User {
    private String username;
    private String password;

    User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public static User.UserBuilder builder() {
        return new User.UserBuilder();
    }

    public static class UserBuilder {
        private String username;
        private String password;

        UserBuilder() {
        }

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

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

        public User build() {
            return new User(this.username, this.password);
        }

        public String toString() {
            return "User.UserBuilder(username=" + this.username + ", password=" + this.password + ")";
        }
    }
}