原文标题：请谨慎使用 @Builder 注解！
原文作者：阿里云开发者
冷月清谈：
怜星夜思：
2、使用@Builder注解时，如何避免默认值失效问题？
3、@Builder注解的适用场景有哪些？
原文内容
阿里妹导读
本文主要指出 @Builder 存在的一些问题，指出它并不是链式编程的最佳实践。
一、前言
二、为什么？
可以看第三部分的例子， @Builder 注解产生的 Builder 类的构造方法默认并不能限定必传参数。
如果非要使用 @Builder ，那么不要用 @Data ，要用 @Getter。相对来说，反而 @Accessors 的行为更符合这个要求。
实际使用中经常发现 @Builder 滥用的情况，有些仅仅一两个属性的类也都要用 @Builder，真的没必要用，直接用全参的构造方法都比这更简洁。
三、怎么做？
3.1 不使用 @Builder
package io.gitrebase.demo;import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;@Data
@NoArgsConstructor
@AllArgsConstructor
public class APIResponse<T> {private T payload;
private Status status;
}
使用示例：
package io.gitrebase.demo;import lombok.extern.slf4j.Slf4j;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestControllerAdvice;@Slf4j
@RestControllerAdvice(assignableTypes = io.gitrebase.demo.RestApplication.class)
public class ApplicationExceptionHandler {@ResponseStatus(code = HttpStatus.INTERNAL_SERVER_ERROR)
public APIResponse handleException(Exception exception) {
log.error(“Unhandled Exception”, exception);
Status status = new Status();
status.setResponseCode(“RESPONSE_CODE_IDENTIFIER”);
status.setDescription(“Bla Bla Bla”);
APIResponse response = new APIResponse();
response.setStatus(status);
return response;
}
}
3.2 使用 @Builder
package io.gitrebase.demo;import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;@Builder
@Data
@NoArgsConstructor
@AllArgsConstructor
public class APIResponse<T> {private T payload;
private Status status;
}
等价代码：
package io.gitrebase.demo;import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Getter;@Getter
// Make constructor private so object cannot be created outside the class
// Allowing only PersonBuilder.build to create an instance of the class
@AllArgsConstructor(access = AccessLevel.PRIVATE)
public class Person {private String firstName;
private String lastName;
private String email;
private String mobile;// PersonBuilder can only be created with firstName, lastName
// which implies that Person object ll always be instantiated with firstName & lastName
public static PersonBuilder builder(String firstName, String lastName) {
return new PersonBuilder(firstName, lastName);
}public static class PersonBuilder {
private final String firstName;
private final String lastName;
private String email;
private String mobile;// Make constructor private so object cannot be created outside the class
// Allowing only Person.builder to create an instance of PersonBuilder
private PersonBuilder(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}public PersonBuilder email(String email) {
this.email = email;
return this;
}public PersonBuilder mobile(String mobile) {
this.mobile = mobile;
return this;
}public Person build() {
return new Person(firstName, lastName, email, mobile);
}}
}
使用示例：
package io.gitrebase.demo;import lombok.extern.slf4j.Slf4j;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestControllerAdvice;@Slf4j
@RestControllerAdvice(basePackageClasses = io.gitrebase.demo.RestApplication.class)
public class ApplicationExceptionHandler {@ResponseStatus(code = HttpStatus.INTERNAL_SERVER_ERROR)
public APIResponse handleException(Exception exception) {
log.error(“Unhandled Exception”, exception);
return APIResponse.builder().status(Status.builder()
.responseCode(“RESPONSE_CODE_IDENTIFIER”)
.description(“Bla Bla Bla”)
.build())
.build();
}
}
3.3 使用 @Accessors 注解
package io.gitrebase.demo;import lombok.Data;
import lombok.experimental.Accessors;@Data
@Accessors(chain = true)
public class APIResponse<T> {private T payload;
private Status status;
}
等价代码：
package io.gitrebase.demo;import lombok.experimental.Accessors;
@Accessors(chain = true)
public class APIResponse<T> {private T payload;
private Status status;
public T getPayload() {
return this.payload;
}public APIResponse<T> setPayload(T payload) {
this.payload = payload;
return this;
}public Status getStatus() {
return this.status;
}
public APIResponse<T> setStatus(Status status) {
this.status = status;
return this;
}
}
使用示例：
package io.gitrebase.demo;import lombok.extern.slf4j.Slf4j;
import lombok.var;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestControllerAdvice;@Slf4j
@RestControllerAdvice(basePackageClasses = io.gitrebase.demo.RestApplication.class)
public class ApplicationExceptionHandler {@ResponseStatus(code = HttpStatus.INTERNAL_SERVER_ERROR)
public APIResponse handleException(Exception exception) {
log.error(“Unhandled Exception”, exception);
var status = new Status().setResponseCode(“RESPONSE_CODE_IDENTIFIER”).setDescription(“Bla Bla Bla”);
return new APIResponse().setStatus(status);
}
}
原文中给的示例相对简单，实际运用时，如果属性较多，且分为必传属性和选填属性时，可以将必传参数定义在构造方法中，加上 @Accessors 注解，这样就可以实现必传参数的传入，又可以实现选填参数的链式调用。
import lombok.Accessors;// 使用 @Accessors 注解，设置 chain = true，生成返回 this 的 setter 方法
@Accessors(chain = true)
public class Student {
// 定义必传属性，使用 final 修饰，不提供 setter 方法
private final int studentId; // 学生ID
private final int grade; // 年级
private final int classNum; // 班级// 定义选填属性，提供 setter 方法
private String name; // 姓名
private String gender; // 性别
private String address; // 住址// 定义构造方法，接收必传参数
public Student(int studentId, int grade, int classNum) {
this.studentId = studentId;
this.grade = grade;
this.classNum = classNum;
}// 省略 getter 和 setter 方法
}
// 使用示例
Student student = new Student(1001, 3, 8) // 创建一个学生对象，传入必传参数
.setName(“张三”) // 设置姓名
.setGender(“男”) // 设置性别
.setAddress(“北京市朝阳区”); // 设置住址
另外该注解还有很多设置项，感兴趣可以进一步研究：
/** * A container for settings for the generation of getters, setters and "with"-ers. * <p> * Complete documentation is found at <a href="https://projectlombok.org/features/experimental/Accessors">the project lombok features page for @Accessors</a>. * <p> * Using this annotation does nothing by itself; an annotation that makes lombok generate getters, setters, or "with"-ers * such as {@link lombok.Setter} or {@link lombok.Data} is also required. */ @Target({ElementType.TYPE, ElementType.FIELD}) @Retention(RetentionPolicy.SOURCE) public @interface Accessors { /** * If true, accessors will be named after the field and not include a {@code get} or {@code set} * prefix. If true and {@code chain} is omitted, {@code chain} defaults to {@code true}.<br> * NB: This setting has no effect on {@code @With}; they always get a "with" prefix.<br> * <strong>default: false</strong> * * @return Whether or not to make fluent methods (named {@code fieldName()}, not for example {@code setFieldName}). */ boolean fluent() default false;/**
- If true, setters return {@code this} instead of {@code void}.
- <strong>default: false</strong>, unless {@code fluent=true}, then <strong>default: true</strong>
- @return Whether or not setters should return themselves (chaining) or {@code void} (no chaining).
*/
boolean chain() default false;/**
- If true, generated accessors will be marked {@code final}.
- <strong>default: false</strong>
- @return Whether or not accessors should be marked {@code final}.
*/
boolean makeFinal() default false;/**
- If present, only fields with any of the stated prefixes are given the getter/setter treatment.
- Note that a prefix only counts if the next character is NOT a lowercase character or the last
- letter of the prefix is not a letter (for instance an underscore). If multiple fields
- all turn into the same name when the prefix is stripped, an error will be generated.
@return If you are in the habit of prefixing your fields (for example, you name them {@code fFieldName}, specify such prefixes here).
*/
String prefix() default {};
}
3.4 使用 IDEA 默认 Setter 生成功能
四、启发
参考链接：
[1]https://medium.com/gitrebase/oh-stop-using-builder-9061a5911d8c
[2]https://www.baeldung.com/lombok-builder-inheritance
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