java.util.util.optional＆lt; t＆gt; gt;不能应用于给定类型

Question

我的代码在flatmap中给出了错误。

public Optional<User> getUserWithAuthorities() {
    return SecurityUtils.getCurrentUserLogin().flatMap(UserDao::findOneWithAuthoritiesByLogin);
}

以下是错误：

controller/AuthenticationController.java:[77,51] method flatMap in class java.util.Optional<T> cannot be applied to given types;
       [ERROR]   required: java.util.function.Function<? super java.lang.String,java.util.Optional<U>>
       [ERROR]   found: UserDao::f[...]Login
       [ERROR]   reason: cannot infer type-variable(s) U
       [ERROR]     (argument mismatch; invalid method reference
       [ERROR]       cannot find symbol
       [ERROR]         symbol:   method findOneWithAuthoritiesByLogin(java.lang.String)
       [ERROR]         location: interface net.javaguides.springboot.dao.UserDao)

这是UserDao代码：

@Repository
public interface UserDao extends JpaRepository<User, Long> {

    User findByUsername(String username);

    Optional<User> findOneWithAuthoritiesByLogin(String login);

}

这是SecurityUtils代码：

public final class SecurityUtils {

    private SecurityUtils() {}

    /**
     * Get the login of the current user.
     *
     * @return the login of the current user.
     */
    public static Optional<String> getCurrentUserLogin() {
        SecurityContext securityContext = SecurityContextHolder.getContext();
        return Optional.ofNullable(extractPrincipal(securityContext.getAuthentication()));
    }

    private static String extractPrincipal(Authentication authentication) {
        if (authentication == null) {
            return null;
        } else if (authentication.getPrincipal() instanceof UserDetails) {
            UserDetails springSecurityUser = (UserDetails) authentication.getPrincipal();
            return springSecurityUser.getUsername();
        } else if (authentication.getPrincipal() instanceof String) {
            return (String) authentication.getPrincipal();
        }
        return null;
    }

问题是什么？

Answer 1

错误是由对接口类userdao :: findOneWithAuthoritiesBylogin引起的。 Java不能单独使用接口调用该方法。

要解决它，而是请参考类型userdao的实例化对象。

这是接口的示例实现：

    class UserDaoImplementation implements UserDao {
        @Override
        public Optional<User> findOneWithAuthoritiesByLogin(String login) {
            User user = User.builder().name("Tom").build();
            return Optional.of(user);
        }
    }

然后创建一个实例并引用它而不是接口类：

    public static Optional<User> getUserWithAuthorities() {
        UserDao userDao = new UserDaoImplementation();
        return SecurityUtils.getCurrentUserLogin().flatMap(userDao::findOneWithAuthoritiesByLogin);
    }

在这种情况下，getUserWithAuthorities方法现在返回可选包含>用户对象。