建造者模式

> 简单的记录下lombok中的建造者模式及使用。不对该模式本身发表意见看法。

### 一、Lombok中的建造者

添加@Builder注解后，可直接使用建造者模式创建对象并为其属性赋值。

```java
package com.example.demo.pojo;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

/**
 * @author: HanXu
 * on 2021/11/10
 * Class description: Builder建造者模式的链式实现
 */
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class MessageLogEntity {

private int passagewayType;
    private String chatbotId;
    private String messageInfo;

public static void main(String[] args) {
        MessageLogEntity messageLog = MessageLogEntity.builder()
                .chatbotId("chatbotId123")
                .messageInfo("messageInfo123")
                .passagewayType(1).build();
        System.out.println(messageLog);
        //MessageLogEntity(passagewayType=1, chatbotId=chatbotId123, messageInfo=messageInfo123)
    }
}
```

从中很容易看出建造者模式的好处：当某个类属性很多时，使用一般的方法创建对象（构造函数）太复杂，容易搞混是为哪个属性赋了哪些值。而使用建造者模式能够很清爽的完成这一过程。

```java
	MessageLogEntity messageLog = new MessageLogEntity(1, "chatbotId123", "messageInfo123");
```

这仅仅是三个属性，试想一下，当一个类有多个属性时，且某些属性是必选的，某些是可选的，业务再复杂些，就会出现很多不同的构造函数：

```java
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import org.springframework.http.HttpMethod;
import org.springframework.http.MediaType;
import org.springframework.web.multipart.MultipartFile;

import java.util.HashMap;
import java.util.Map;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class HttpRequestDTO {

/**
     * 请求url
     */
    private String url;
    /**
     * 请求contentType，默认application/json;charset=UTF-8
     */
    private String contentType = MediaType.APPLICATION_JSON_UTF8_VALUE;
    /**
     * 请求方式，默认post
     */
    private String httpMethod = HttpMethod.POST.name();
    /**
     * 请求超时时间，默认5s
     */
    private int connTimeout = 5;
    /**
     * 会话超时时间，默认5s
     */
    private int socketTimeout = 5;
    /**
     * 连接池获取连接/创建连接超时时间，默认1s
     */
    private int connReqTimeout = 1;
    //结果化参数
    /**
     * 请求body 相当于postman中的raw json
     */
    private String bodyJson;
    private Map<String, Object> head = new HashMap<>();
    private Map<String, Object> query = new HashMap<>();
    private Map<String, Object> body = new HashMap<>();
    private Map<String, Object> template = new HashMap<>();
    private Map<String, Object> paths = new HashMap<>();

// 随机数
    private String boundary;

private MultipartFile file;

private long startTime = System.currentTimeMillis();

public HttpRequestDTO(){

}

public HttpRequestDTO(String httpMethod, String url, Map<String, Object> head){
        this.httpMethod = httpMethod;
        this.url=url;
        this.head = head;
    }

public HttpRequestDTO(String url, Map<String, Object> head, MultipartFile file){
        this.url=url;
        this.file = file;
        this.head = head;
    }

public HttpRequestDTO(String url, Map<String, Object> head, String bodyJson){
        this.url=url;
        this.head = head;
        this.bodyJson = bodyJson;
    }

public HttpRequestDTO(String url, Map<String, Object> head, Map<String, Object> body){
        this.url=url;
        this.head = head;
        this.body = body;
    }
}
```

面对这么多构造函数，使用时难免弄混，若许多参数类型一样，那就更烦了。

### 二、如何实现？

Lombok是通过直接操纵抽象语法树（AST）实现的，一个注解帮我们解决了很多事情，那如果自己写建造过程，该怎么写呢？

```java
package com.example.demo.pojo;

/**
 * @author: HanXu
 * on 2021/11/10
 * Class description: Builder建造者模式的链式实现
 */
public class MessageLogDownEntity {

private int passagewayType;
    private String chatbotId;
    private String messageInfo;

@Override
    public String toString() {
        return "MessageLogDownEntity{" +
                "passagewayType=" + passagewayType +
                ", chatbotId='" + chatbotId + '\'' +
                ", messageInfo='" + messageInfo + '\'' +
                '}';
    }
    
    // TODO: 2021/11/22 省略get set方法

/**
     * 供Builder类内部builder，为主体类赋值
     * @param messageLogDownEntityBuilder
     */
    private MessageLogDownEntity(MessageLogDownEntityBuilder messageLogDownEntityBuilder) {
        this.passagewayType = messageLogDownEntityBuilder.passagewayType;
        this.chatbotId = messageLogDownEntityBuilder.chatbotId;
        this.messageInfo = messageLogDownEntityBuilder.messageInfo;
    }

/**
     * 供外界直接使用 Class.builder()
     * @return
     */
    public static MessageLogDownEntityBuilder builder(){
        return new MessageLogDownEntityBuilder();
    }

/**
     * 内部Builder类，主要功能是提供给外部.chatbotId()的方法
     */
    public static class MessageLogDownEntityBuilder {
        private int passagewayType;
        private String chatbotId;
        private String messageInfo;

public MessageLogDownEntityBuilder chatbotId(String chatbotId) {
            this.chatbotId = chatbotId;
            return this;
        }

public MessageLogDownEntityBuilder messageInfo(String messageInfo) {
            this.messageInfo = messageInfo;
            return this;
        }

public MessageLogDownEntityBuilder passagewayType(int passagewayType) {
            this.passagewayType = passagewayType;
            return this;
        }

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

public static void main(String[] args) {
        MessageLogDownEntity messageDown = MessageLogDownEntity.builder()
                .chatbotId("chatbotId123")
                .messageInfo("messageInfo123")
                .passagewayType(1).build();
        System.out.println(messageDown);
        //MessageLogDownEntity{passagewayType=1, chatbotId='chatbotId123', messageInfo='messageInfo123'}
    }
}
```
以上就是链式创建者模式的实现。相较于传统的4角色创建者模式，省去了许多不必要的繁琐步骤。

扩展：
为什么不直接在外部主体类中添加chatbotId()这些属性的赋值方法呢？

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