java - ElementType.CONSTRUCTOR 有什么用？

Question

我正在学习注释和注释处理器。

我遇到了 javax.validation.constraints.NotNull声明

@Target(value={METHOD,FIELD,ANNOTATION_TYPE,CONSTRUCTOR,PARAMETER})
@Retention(value=RUNTIME)
@Documented
@Constraint(validatedBy={})
public @interface NotNull

我已经对这个注解有点熟悉了。

@NotNull
public Integer getAge() {
    return age;
}

@NotNull
private String name;

private int age;

@NotNull 和 ElementType.CONSTRUCTOR 有什么用法？

Answer 1

ElementType.CONSTRUCTOR 指出注释可以在构造函数级别分类(当在注释声明中指定 @Target 时)。

例子:

@Entity
@Table(name = "Request")
public class Request implements Serializable {

    @NotNull
    public Request() {

    }
}

---

Beans Validation Specification (1.0 final) 声明如下(在 2.1 约束注释 章):

Constraint annotations can target any of the following ElementTypes:

• FIELD for constrained attributes
• METHOD for constrained getters
• TYPE for constrained beans
• ANNOTATION_TYPE for constraints composing other constraints.

While other ElementTypes are not forbidden, the provider does not have to recognize and process constraints placed on such types. Built-in types do support PARAMETER and CONSTRUCTOR to allow Bean Validation provider specific extensions. It is considered good practice to follow the same approach for custom annotations.

本质上，虽然还没有建设性/有用的示例在构造函数级别使用@NotNull，但规范声明包含ElementType 注释(因为内置注释已经包含它们)。

希望对您有所帮助。