如何访问Java自定义注释中定义的字段

Question

我有以下Java代码-

import java.lang.reflect.Field;

public class AnnotationTest
{
    public @interface Size
    {
        int size();
        int location();
    }

    @Size(size = 40, location = 85)
    private String firstName;

    @Size(size = 1, location = 21)
    private String middleInitial;

    @Size(size = 50, location = 115)
    private String lastName;

    public static void main(String[] args)
    {
        AnnotationTest t = new AnnotationTest();

        Class<? extends AnnotationTest> classInstance = t.getClass();

        for (Field f : classInstance.getDeclaredFields())
        {
            Size s = f.getAnnotation(Size.class); 
            int size = s.size(); // this is line 29
            int location = s.location();

            System.out.println("size = "+ size);
            System.out.println("location = "+location);
        }

    }
}

我得到的错误是

Exception in thread "main" java.lang.NullPointerException
    at com.stackoverflowx.AnnotationTest.main(Demo.java:125

如何正确访问注释字段？

Answer 1

默认情况下，注释在运行时不可用。您需要添加@Retention(RetentionPolicy.RUNTIME)到注释定义以使其可用于运行时处理。例如：

@Retention(RetentionPolicy.RUNTIME)
public @interface Size {

在实践中，还应Field.isAnnotationPresent在实际尝试从字段中删除注释之前，检查该字段是否确实具有给定的注释。

另外，指定注释所属的元素类型也是一种好习惯@Target。因此，您的示例将是：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Size {