Java反射 - 通过反射获取注解的值

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一个ORM例子

定义注解

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@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Table {
    String value();
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Field {
    String colName();
    String type();
    int length();
}

实体类

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@Table("user")
class User {
    @Field(colName = "user_id", type = "int", length = 10)
    private int id;
    @Field(colName = "user_name", type = "varchar", length = 10)
    private String name;
    @Field(colName = "user_password", type = "varchar", length = 64)
    private String password;
}

获取注解进行处理

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public class ReflectAnno {
    public static void main(String[] args) throws NoSuchFieldException, NoSuchMethodException {
        Class<User> c = User.class;

        // 获得类上的注解
        Annotation[] annotations = c.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println("annotation = " + annotation);
        }

        // 获取注解的值
        Table target = c.getAnnotation(Table.class);
        System.out.println(target.value());

        // 获取类的Field
        java.lang.reflect.Field name = c.getDeclaredField("name");
        // 获取Field上的注解
        Field nameAnnotation = name.getAnnotation(Field.class);
        // 获取注解的值
        System.out.println("nameAnnotation.colName() = " + nameAnnotation.colName());
        System.out.println("nameAnnotation.type() = " + nameAnnotation.type());
        System.out.println("nameAnnotation.length() = " + nameAnnotation.length());

        // 获取类的Method
        Method setPassword = c.getDeclaredMethod("setPassword", String.class);
        // 获取Method上的注解
        setPassword.getAnnotations();
    }
}