如何获取成员变量的注释？

Question

我想知道一个类的一些成员变量的注解，我用BeanInfo beanInfo = Introspector.getBeanInfo(User.class)自省一个类，用BeanInfo.getPropertyDescriptors()找到具体的属性，用Classtype = propertyDescriptor.getPropertyType()得到属性的类。

但是我不知道如何将注解添加到成员变量中？

我尝试了 type.getAnnotations() 和 type.getDeclaredAnnotations() ，但都返回了类的注释，而不是我想要的。例如：

class User 
{
  @Id
  private Long id;

  @Column(name="ADDRESS_ID")
  private Address address;

  // getters , setters
}

@Entity
@Table(name = "Address")
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
class Address 
{
  ...
}

我想获取地址的注释：@Column，而不是类地址的注释（@Entity、@Table、@Cache）。如何实现呢？谢谢。

Answer 1

for(Field field : cls.getDeclaredFields()){
  Class type = field.getType();
  String name = field.getName();
  Annotation[] annotations = field.getDeclaredAnnotations();
}

另请参阅：http://docs.oracle.com/javase/tutorial/reflect/class/classMembers.html

Answer 2

我的方式

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;

public class ReadAnnotation {
    private static final Logger LOGGER = LoggerFactory.getLogger(ReadAnnotation.class);

    public static boolean hasIgnoreAnnotation(String fieldName, Class entity) throws NoSuchFieldException {
        return entity.getDeclaredField(fieldName).isAnnotationPresent(IgnoreAnnotation.class);
    }

    public static boolean isSkip(PropertyDescriptor propertyDescriptor, Class entity) {
        boolean isIgnoreField;
        try {
            isIgnoreField = hasIgnoreAnnotation(propertyDescriptor.getName(), entity);
        } catch (NoSuchFieldException e) {
            LOGGER.error("Can not check IgnoreAnnotation", e);
            isIgnoreField = true;
        }
        return isIgnoreField;
    }

    public void testIsSkip() throws Exception {
        Class<TestClass> entity = TestClass.class;
        BeanInfo beanInfo = Introspector.getBeanInfo(entity);

        for (PropertyDescriptor propertyDescriptor : beanInfo.getPropertyDescriptors()) {
            System.out.printf("Field %s, has annotation %b", propertyDescriptor.getName(),  isSkip(propertyDescriptor, entity));
        }
    }

}

Answer 3

你必须使用反射来获取User类的所有成员字段，遍历它们并找到它们的注解

类似这样的：

public void getAnnotations(Class clazz){
    for(Field field : clazz.getDeclaredFields()){
        Class type = field.getType();
        String name = field.getName();
        field.getDeclaredAnnotations(); //do something to these
    }
}

Answer 4

如果您需要知道是否存在特定注释。你可以这样做：

    Field[] fieldList = obj.getClass().getDeclaredFields();

        boolean isAnnotationNotNull, isAnnotationSize, isAnnotationNotEmpty;

        for (Field field : fieldList) {

            //Return the boolean value
            isAnnotationNotNull = field.isAnnotationPresent(NotNull.class);
            isAnnotationSize = field.isAnnotationPresent(Size.class);
            isAnnotationNotEmpty = field.isAnnotationPresent(NotEmpty.class);

        }

其他注解以此类推...

我希望能帮助别人。

Answer 5

或者你可以试试这个

try {
    BeanInfo bi = Introspector.getBeanInfo(User.getClass());
    PropertyDescriptor[] properties = bi.getPropertyDescriptors();
    for(PropertyDescriptor property : properties) {
        //One way
        for(Annotation annotation : property.getAnnotations()){
            if(annotation instanceof Column) {
                String string = annotation.name();
            }
        }
        //Other way
        Annotation annotation = property.getAnnotation(Column.class);
        String string = annotation.name();
    }
}catch (IntrospectonException ie) {
    ie.printStackTrace();
}

希望这会有所帮助。

Answer 6

每个人都描述了获取注释的问题，但问题在于注释的定义。您应该在注释定义中添加@Retention(RetentionPolicy.RUNTIME):

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface MyAnnotation{
    int id();
}

Answer 7

package be.fery.annotation;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.PrePersist;

@Entity
public class User {
    @Id
    private Long id;

    @Column(name = "ADDRESS_ID")
    private Address address;

    @PrePersist
    public void doStuff(){

    }
}

还有一个测试类：

    package be.fery.annotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class AnnotationIntrospector {

    public AnnotationIntrospector() {
        super();
    }

    public Annotation[] findClassAnnotation(Class<?> clazz) {
        return clazz.getAnnotations();
    }

    public Annotation[] findMethodAnnotation(Class<?> clazz, String methodName) {

        Annotation[] annotations = null;
        try {
            Class<?>[] params = null;
            Method method = clazz.getDeclaredMethod(methodName, params);
            if (method != null) {
                annotations = method.getAnnotations();
            }
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        return annotations;
    }

    public Annotation[] findFieldAnnotation(Class<?> clazz, String fieldName) {
        Annotation[] annotations = null;
        try {
            Field field = clazz.getDeclaredField(fieldName);
            if (field != null) {
                annotations = field.getAnnotations();
            }
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        return annotations;
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        AnnotationIntrospector ai = new AnnotationIntrospector();
        Annotation[] annotations;
        Class<User> userClass = User.class;
        String methodDoStuff = "doStuff";
        String fieldId = "id";
        String fieldAddress = "address";

        // Find class annotations
        annotations = ai.findClassAnnotation(be.fery.annotation.User.class);
        System.out.println("Annotation on class '" + userClass.getName()
                + "' are:");
        showAnnotations(annotations);

        // Find method annotations
        annotations = ai.findMethodAnnotation(User.class, methodDoStuff);
        System.out.println("Annotation on method '" + methodDoStuff + "' are:");
        showAnnotations(annotations);

        // Find field annotations
        annotations = ai.findFieldAnnotation(User.class, fieldId);
        System.out.println("Annotation on field '" + fieldId + "' are:");
        showAnnotations(annotations);

        annotations = ai.findFieldAnnotation(User.class, fieldAddress);
        System.out.println("Annotation on field '" + fieldAddress + "' are:");
        showAnnotations(annotations);

    }

    public static void showAnnotations(Annotation[] ann) {
        if (ann == null)
            return;
        for (Annotation a : ann) {
            System.out.println(a.toString());
        }
    }

}

希望对你有帮助...

;-)

Answer 8

可以在getter方法上获取注解：

propertyDescriptor.getReadMethod().getDeclaredAnnotations();

获取私有字段的注释似乎是个坏主意...如果该属性甚至没有由字段支持，或者由具有不同名称的字段支持怎么办？即使忽略这些情况，您也会通过查看私有内容来打破抽象。