序

最近闲着无聊，研究了一下Mybatis的源码，发现有如下的设计方式。

package org.apache.ibatis.mapping;

import javax.sql.DataSource;
import org.apache.ibatis.transaction.TransactionFactory;

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;

  public Environment(String id, TransactionFactory transactionFactory, DataSource dataSource) {
    if (id == null) {
      throw new IllegalArgumentException("Parameter 'id' must not be null");
    }
    if (transactionFactory == null) {
        throw new IllegalArgumentException("Parameter 'transactionFactory' must not be null");
    }
    this.id = id;
    if (dataSource == null) {
      throw new IllegalArgumentException("Parameter 'dataSource' must not be null");
    }
    this.transactionFactory = transactionFactory;
    this.dataSource = dataSource;
  }

  public static class Builder {
      private String id;
      private TransactionFactory transactionFactory;
      private DataSource dataSource;

    public Builder(String id) {
      this.id = id;
    }

    public Builder transactionFactory(TransactionFactory transactionFactory) {
      this.transactionFactory = transactionFactory;
      return this;
    }

    public Builder dataSource(DataSource dataSource) {
      this.dataSource = dataSource;
      return this;
    }

    public String id() {
      return this.id;
    }

    public Environment build() {
      return new Environment(this.id, this.transactionFactory, this.dataSource);
    }

  }

  public String getId() {
    return this.id;
  }

  public TransactionFactory getTransactionFactory() {
    return this.transactionFactory;
  }

  public DataSource getDataSource() {
    return this.dataSource;
  }
}

疑惑

1. 为什么这个Environment类定义了一个内部的Builder类？
2. 为什么通过这个内部Builder类对外部类的赋值？
3. 这么做是不是多此一举？

解惑

1. 之前看到一篇提问，提问者问，为什么建议使用静态的工厂类创建对象？
2. 而这个回答者，正是Effective Java的作者，java集合框架的架构师Joshua Bloch。
3. 于是，我就京东买了这本书。
4. 正巧第一章第二条命中了我的疑问。

何为构建器

1. 第二条是这么说的，多个构造器参数时考虑使用构建器。

Why

重载构造函数

1. 通常我们会创建多个重载的构造函数来初始化对象。
2. 然而当参数个数不断膨胀，客户端代码就会变得异常难写。

问题

1. 怎么用？
2. 可读性？
3. 赋值顺序手误了，consturctor(a, b)，极有可能复制成b, a。

JavaBean模式

1. 私有域。
2. setter和getter赋值。

问题

1. 你可能在好几个地方赋值。
2. 这就导致参数可能不一致。
3. 而且不能声明immutable的类型，因为setter强制私有域不能为final。

怎么办

构建器：既能保持一致性，又能将类定义为期待的样子

1. 定义一个共有的内部构建器。
2. 构建器对参数进行赋值。

package com.lucas;

public class LucasLuo {
    private final String name;
    private final String sex;
    private final int age;
    private final int height;
    private final int weight;
    private final boolean single;
    private final String mobile;
    private final String qq;
    private final String wechat;
    private final String email;
    
    public static class Builder {
        private String name;
        private String sex;
        private int age;
        private int height;
        private int weight;
        private boolean single;
        private String mobile;
        private String qq;
        private String wechat;
        private String email;
        
        public Builder name(String name) {
            this.name = name;
            return this;
        }
        
        public Builder sex(String sex) {
            this.sex = sex;
            return this;
        }
        
        public Builder age(int age) {
            this.age = age;
            return this;
        }
        
        public Builder height(int height) {
            this.height = height;
            return this;
        }
        
        public Builder weight(int weight) {
            this.weight = weight;
            return this;
        }
        
        public Builder single(boolean single) {
            this.single = single;
            return this;
        }
        
        public Builder mobile(String mobile) {
            this.mobile = mobile;
            return this;
        }
        
        public Builder qq(String qq) {
            this.qq = qq;
            return this;
        }
        
        public Builder wechat(String wechat) {
            this.wechat = wechat;
            return this;
        }
        
        public Builder email(String email) {
            this.email = email;
            return this;
        }
        
        public LucasLuo build() {
            return new LucasLuo(this);
        }
    }
    
    private LucasLuo(Builder builder) {
        this.name = builder.name;
        this.sex = builder.sex;
        this.age = builder.age;
        this.height = builder.height;
        this.weight = builder.weight;
        this.single = builder.single;
        this.mobile = builder.mobile;
        this.qq = builder.qq;
        this.wechat = builder.wechat;
        this.email = builder.email;
    }
    
    public String getName() {
        return name;
    }
    
    public String getSex() {
        return sex;
    }
    
    public int getAge() {
        return age;
    }
    
    public int getHeight() {
        return height;
    }
    
    public int getWeight() {
        return weight;
    }
    
    public boolean getSingle() {
        return single;
    }
    
    public String getMobile() {
        return mobile;
    }
    
    public String getQq() {
        return qq;
    }
    
    public String getWechat() {
        return wechat;
    }
    
    public String getEmail() {
        return email;
    }
}

package com.lucas;

public class LucasMain {

    public static void main(String[] args) {
        LucasLuo lucasluo =
            new LucasLuo
            .Builder()
            .name("Lucas Luo")
            .age(20)
            .sex("男")
            .height(175)
            .weight(70)
            .single(true)
            .qq("123456")
            .wechat("lucasluo")
            .email("lucasluo@somesite.com")
            .build();
        
        System.out.println(lucasluo.getName());
    }
}

总结

当参数有很多个的时候，使用构建器替代静态工厂或重载构造函数是一个不错的办法。

彩蛋

电脑里钻进了几只小蟑螂，怎么抠也抠不出来。
还好，写了一段代码，起了四个线程一直跑。
终于把蟑螂热出来了，碾死。

Done