注释反射(使用 getAnnotation)不起作用 [英] Annotation reflection (using getAnnotation) does not work

问题描述

我必须按照代码来检查我的 model 中的实体是否在字段上具有 nullable=false 或类似的注释.

I have to following code to check whether the entity in my model has a nullable=false or similar annotation on a field.

import javax.persistence.Column;
import .....

private boolean isRequired(Item item, Object propertyId) {
        Class<?> property = getPropertyClass(item, propertyId);

        final JoinColumn joinAnnotation = property.getAnnotation(JoinColumn.class);
        if (null != joinAnnotation) {
            return !joinAnnotation.nullable();
        }

        final Column columnAnnotation = property.getAnnotation(Column.class);
        if (null != columnAnnotation) {
            return !columnAnnotation.nullable();
        }

        ....
        return false;
    }

这是我的模型的一个片段.

Here's a snippet from my model.

import javax.persistence.*;
import .....

@Entity
@Table(name="m_contact_details")
public class MContactDetail extends AbstractMasterEntity implements Serializable {

    @Column(length=60, nullable=false)
    private String address1;

对于那些不熟悉 @Column 注释的人，这里是标题:

For those people unfamiliar with the @Column annotation, here's the header:

@Target({METHOD, FIELD})
@Retention(RUNTIME)
public @interface Column {

我希望 isRequired 时不时地返回 true ，但它从来没有.我已经在我的项目中完成了 mvn clean 和 mvn install ，但这没有帮助.

I'd expect the isRequired to return true every now and again, but instead it never does. I've already done a mvn clean and mvn install on my project, but that does not help.

Q1:我做错了什么?

Q2:是否有更简洁的方式来编写 isRequired(也许更好地利用泛型)?

Q2: is there a cleaner way to code isRequired (perhaps making better use of generics)?

推荐答案

1. property 代表一个类(它是一个Class)
2. @Column 和 @JoinColumn 只能注释字段/方法.

1. property represents a class (it's a Class<?>)
2. @Column and @JoinColumn can only annotate fields/methods.

因此，您永远不会在 property 上找到这些注释.

Consequently you will never find these annotations on property.

稍微修改过的代码版本，打印出是否需​​要 Employee 实体的 email 属性:

A slightly modified version of your code that prints out whether the email property of the Employee entity is required:

public static void main(String[] args) throws NoSuchFieldException {
      System.out.println(isRequired(Employee.class, "email"));
}

private static boolean isRequired(Class<?> entity, String propertyName) throws NoSuchFieldException {
    Field property = entity.getDeclaredField(propertyName);

    final JoinColumn joinAnnotation = property.getAnnotation(JoinColumn.class);
    if (null != joinAnnotation) {
        return !joinAnnotation.nullable();
    }

    final Column columnAnnotation = property.getAnnotation(Column.class);
    if (null != columnAnnotation) {
        return !columnAnnotation.nullable();
    }

    return false;
}

请注意，这是一个半生不熟的解决方案，因为 JPA 注释既可以在字段上，也可以在方法上.还要注意 getFiled()/getDeclaredField() 等反射方法之间的区别.前者也返回继承的字段，而后者只返回特定类的字段，忽略从其父类继承的字段.

Note that this is a half-baked solution, because JPA annotations can either be on a field or on a method. Also be aware of the difference between the reflection methods like getFiled()/getDeclaredField(). The former returns inherited fields too, while the latter returns only fields of the specific class ignoring what's inherited from its parents.

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