关于Lombok使用的一些问题

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Lombok 是一款非常实用的工具，通过注解能消除冗长的 Java 代码，工程中需要导入 lombok.jar，相应 IDE 的安装方式可参考 官网。项目中使用到的注解主要是 @Data 和 @Builder，前一个注解用在类上，相当于同时使用了 @ToString、@EqualsAndHashCode、@Getter、@Setter 和 @RequiredArgsConstrutor；后一个主要用在类、构造器、方法上，能非常方便的实现建造者模式、链式编程，相关文档可参考 这里。

@Builder 注解

在有继承（Inheritance）关系的实体类中使用 @Builder 时，遇到子类的 Builder 对象无法使用到父类的属性，以下方例子来说明

如果是使用 Parent p = new Parent(); p.setParentAge(123);... 方式赋值，那么以下内容可忽略！

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@Data
//@Builder
public class Parent {
    private String parentName;
    private int parentAge;
}
 
@Data
@Builder
public class Child extends Parent {
    private String childName;
    private int childAge;
}

在使用 Child.builder().parentAge(18).parentName("zhangsan") 时编译失败，查看 Child 对应的 class 文件如下

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public class Child extends Parent {
    private String childName;
    private int childAge;

    Child(String childName, int childAge) {
        this.childName = childName;
        this.childAge = childAge;
    }

    public static Child.ChildBuilder builder() {
        return new Child.ChildBuilder();
    }

    public static class ChildBuilder {
        private String childName;
        private int childAge;

        ChildBuilder() {
        }

        public Child.ChildBuilder childName(String childName) {
            this.childName = childName;
            return this;
        }

        public Child.ChildBuilder childAge(int childAge) {
            this.childAge = childAge;
            return this;
        }

        public Child build() {
            return new Child(this.childName, this.childAge);
        }

        public String toString() {
            return "Child.ChildBuilder(childName=" + this.childName + ", childAge=" + this.childAge + ")";
        }
    }
}

并未发现有父类的属性存在。若是在父类中加 @Builder 注解而子类不加，则会报如下错误

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Error:(14, 8) java: 无法将类 com.leaface.Parent中的构造器 Paren t应用到给定类型;
  需要: java.lang.String,int
  找到: 没有参数
  原因: 实际参数列表和形式参数列表长度不同

之后参考了有关文档，将代码改成

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@Data
@NoArgsConstructor
@AllArgsConstructor
public class Parent {
    private final String parentName;
    private final int parentAge;
}
 
@Data
public class Child extends Parent {
    private final String childName;
    private final int childAge;
 
    @Builder
    public Child(String parentName, int parentAge, String childName, int childAge) {
        super(parentName, parentAge);
        this.childName = childName;
        this.childAge = childAge;
    }
}

PS：父类中的 @NoArgsConstructor 是在使用了 @Data 注解后，为解决子类出现 Lombok needs a default constructor in the base class 这个错误而加的

这样配置后，子类就能设置父类的属性了，注意：此时父类是没有加 @Builder 的，而我们项目中又需要这个注解，若是都加上 @Builder，子类编译报错

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Error:(17, 5) java: com.leaface.Child 中的 builder() 无法隐藏 com.leaface.Parent 中的 builder()
  返回类型com.leaface.Child.ChildBuilder与com.leaface.Parent.ParentBuilder不兼容

以下是 @Builder 注解源码

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@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR})
@Retention(RetentionPolicy.SOURCE)
public @interface Builder {
    String builderMethodName() default "builder";

    String buildMethodName() default "build";

    String builderClassName() default "";

    boolean toBuilder() default false;

    @Target({ElementType.FIELD})
    @Retention(RetentionPolicy.SOURCE)
    public @interface Default {
    }

    @Target({ElementType.FIELD, ElementType.PARAMETER})
    @Retention(RetentionPolicy.SOURCE)
    public @interface ObtainVia {
        String field() default "";

        String method() default "";

        boolean isStatic() default false;
    }
}

其中的 builderMethodName 默认为 builder，子类试图公开和父类具有相同名称的构建器，所以报错。

解决方案：为父类和子类分别指定不同名称的构建器！！！

例如在子类中加 builderMethodName = "leafChildBuilder"，修改后代码如下

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@Data
public class Child extends Parent{
    private String childName;
    private int childAge;

    @Builder(builderMethodName = "leafChildBuilder")
    public Child(String parentName, int parentAge, String childName, int childAge) {
        super(parentName, parentAge);
        this.childName = childName;
        this.childAge = childAge;
    }
}

查看子类的 class 文件，发现已生成其对应的构建器 leafChildBuilder() 和父类的属性

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public static Child.ChildBuilder leafChildBuilder() {
    return new Child.ChildBuilder();
}

public Child(String parentName, int parentAge, String childName, int childAge) {
    super(parentName, parentAge);
    this.childName = childName;
    this.childAge = childAge;
}

public static class ChildBuilder {
    private String parentName;
    private int parentAge;
    private String childName;
    private int childAge;

    ChildBuilder() {
    }

    public Child.ChildBuilder parentName(String parentName) {
        this.parentName = parentName;
        return this;
    }

    public Child.ChildBuilder parentAge(int parentAge) {
        this.parentAge = parentAge;
        return this;
    }
    
    public Child.ChildBuilder childName(String childName) {
        this.childName = childName;
        return this;
    }

    public Child.ChildBuilder childAge(int childAge) {
        this.childAge = childAge;
        return this;
    }
    
    public Child build() {
        return new Child(this.parentName, this.parentAge, this.childName, this.childAge);
    }
}

这里没有改父类的构建器名称，所以还是默认的 builder()，两者已不存在冲突

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public static Parent.ParentBuilder builder() {
    return new Parent.ParentBuilder();
}

使用时，子类直接调 leafChildBuilder() 即可，父类还是使用默认的 builder()

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Child child = Child.leafChildBuilder().parentAge(88).childAge(18).childName("zhangsan").build();

Parent parent = Parent.builder().parentName("admin").parentAge(99).build();

@SuperBuilder 注解（推荐）

Lombok v1.18.2 推出了 @SuperBuilder 注解，这样就不用再定义特殊的构建器了，比如这样

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@Data
@NoArgsConstructor
@SuperBuilder
public class Parent {
    private String parentName;
    private int parentAge;
}
 
@Data
@SuperBuilder
public class Child extends Parent {
    private String childName;
    private int childAge;
}

使用默认的 builder() 调用即可

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Child child = Child.builder().parentAge(88).childAge(18).childName("zhangsan").build();

Parent parent = Parent.builder().parentName("admin").parentAge(99).build();

以上两种方式，赋值后均能通过 @Data 提供的 get 方法取到值。

@SuperBuilder 和 @Builder 不能在同一继承体系中混用，否则会编译失败。多层级继承中这两者的使用方式同上！

至此，Lombok @Builder 继承问题解决！

附录

最后分别附上使用了 @Builder 和 @SuperBuilder 后的父类、子类 class 文件（省略了 @Data 注解）

@Builder class

Parent.class ======>>>

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public class Parent {
    private String parentName;
    private int parentAge;

    public static Parent.ParentBuilder builder() { return new Parent.ParentBuilder();}

    public Parent(String parentName, int parentAge) { this.parentName = parentName; this.parentAge = parentAge;}

    public static class ParentBuilder {
        private String parentName;
        private int parentAge;

        ParentBuilder() {}

        public Parent.ParentBuilder parentName(String parentName) { this.parentName = parentName; return this; }

        public Parent.ParentBuilder parentAge(int parentAge) { this.parentAge = parentAge; return this; }

        public Parent build() { return new Parent(this.parentName, this.parentAge); }

        public String toString() {
            return "Parent.ParentBuilder(parentName=" + this.parentName + ", parentAge=" + this.parentAge + ")"; }
    }
}

Child.class ======>>>

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public class Child extends Parent {
    private String childName;
    private int childAge;

    public Child(String parentName, int parentAge, String childName, int childAge) {
        super(parentName, parentAge);
        this.childName = childName;
        this.childAge = childAge;
    }

    public static Child.ChildBuilder leafChildBuilder() { return new Child.ChildBuilder(); }

    public static class ChildBuilder {
        private String parentName;
        private int parentAge;
        private String childName;
        private int childAge;

        ChildBuilder() {}

        public Child.ChildBuilder parentName(String parentName) { this.parentName = parentName; return this; }

        public Child.ChildBuilder parentAge(int parentAge) { this.parentAge = parentAge; return this; }

        public Child.ChildBuilder childName(String childName) { this.childName = childName; return this; }

        public Child.ChildBuilder childAge(int childAge) { this.childAge = childAge; return this; }

        public Child build() { return new Child(this.parentName, this.parentAge, this.childName, this.childAge); }

        public String toString() {
            return "Child.ChildBuilder(parentName=" + this.parentName + ", parentAge=" + this.parentAge + ", childName=" + this.childName + ", childAge=" + this.childAge + ")"; }
    }
}

@SuperBuilder class

Parent.class ======>>>

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public class Parent {
    private String parentName;
    private int parentAge;

    protected Parent(Parent.ParentBuilder<?, ?> b) { this.parentName = b.parentName; this.parentAge = b.parentAge; }

    public static Parent.ParentBuilder<?, ?> builder() { return new Parent.ParentBuilderImpl(); }

    private static final class ParentBuilderImpl extends Parent.ParentBuilder<Parent, Parent.ParentBuilderImpl> {

        private ParentBuilderImpl() {}

        protected Parent.ParentBuilderImpl self() { return this; }

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

    public abstract static class ParentBuilder<C extends Parent, B extends Parent.ParentBuilder<C, B>> {
        private String parentName;
        private int parentAge;

        public ParentBuilder() {}

        protected abstract B self();

        public abstract C build();

        public B parentName(String parentName) { this.parentName = parentName; return this.self(); }

        public B parentAge(int parentAge) { this.parentAge = parentAge; return this.self(); }

        public String toString() {
            return "Parent.ParentBuilder(parentName=" + this.parentName + ", parentAge=" + this.parentAge + ")"; }
    }
}

Child.class ======>>>

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public class Child extends Parent {
    private String childName;
    private int childAge;

    protected Child(Child.ChildBuilder<?, ?> b) {
        super(b);
        this.childName = b.childName;
        this.childAge = b.childAge; 
    }

    public static Child.ChildBuilder<?, ?> builder() { return new Child.ChildBuilderImpl(); }

    private static final class ChildBuilderImpl extends Child.ChildBuilder<Child, Child.ChildBuilderImpl> {

        private ChildBuilderImpl() {}

        protected Child.ChildBuilderImpl self() { return this; }

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

    public abstract static class ChildBuilder<C extends Child, B extends Child.ChildBuilder<C, B>> extends ParentBuilder<C, B> {
        private String childName;
        private int childAge;

        public ChildBuilder() {}

        protected abstract B self();

        public abstract C build();

        public B childName(String childName) { this.childName = childName; return this.self(); }

        public B childAge(int childAge) { this.childAge = childAge; return this.self(); }

        public String toString() {
            return "Child.ChildBuilder(super=" + super.toString() + ", childName=" + this.childName + ", childAge=" + this.childAge + ")"; }
    }
}

参考资料

Lombok @Builder
Lombok @SuperBuilder
Project Lombok
Lombok @Builder with Inheritance

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本文标题:关于Lombok使用的一些问题

文章作者:北宸

发布时间:2019年12月28日 - 14:19:25

最后更新:2023年08月19日 - 13:26:00

原始链接:https://www.liaofuzhan.com/posts/4067495121.html

许可协议: 署名-非商业性使用-禁止演绎 4.0 国际 转载请保留原文链接及作者。

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