将超类向下转换为子类通过 instanceof 但遇到 ClassCastException

Question

我有两个对象，User 和 CustomUserDetails，它们扩展了 User 并实现了 org.springframework.security.core.userdetails.UserDetails；

在我的存储库层中，我调用我的内存数据库并检索用户。

public User findByUsername(String username) {
    CustomUserDetails customUserDetails = new CustomUserDetails();
    User user = em.createQuery("from User where username = :username", User.class)
            .setParameter("username", username).getSingleResult();

    System.out.println(user.toString());

    if(customUserDetails instanceof User) {
        System.out.println("CustomUserDetails instance of User");
        customUserDetails = (CustomUserDetails) user;
        return customUserDetails;
    } else {
        System.out.println("CustomUserDetails is not instance of User");
        throw new ClassCastException();
    }
}

这是我的控制台输出

ID: 1, USERNAME : joe.bloggs@example.com, PASSWORD: gojoe, LIST<ROLE>: ROLE_ADMIN
CustomUserDetails instance of User
java.lang.ClassCastException: model.User cannot be cast to model.CustomUserDetails

为什么我已经通过了 instanceof 检查，却无法从 User 向下转换为 CustomUserDetails？ UserDetails 接口 class 是否妨碍我成功投射？

Answer 1

您正在尝试投射

customUserDetails = (CustomUserDetails) user;

但是用户不是那个的实例，用户是用户的实例。尝试向用户 class 添加 getter 以获取 CustomUserDetails 或使用 if(user instanceof CustomUserDetails){...

将超类向下转换为子类通过 instanceof 但遇到 ClassCastException

Downcasting Superclass to Subclass passes instanceof but encounters ClassCastException

java

spring-security

casting

downcast