Authentication Provider详解

一、Authentication Provider概述

Authentication Provider是Spring Security中最核心的概念之一。简单来说，它是处理用户认证的组件，负责验证用户身份是否合法，并在验证通过后返回身份信息给Spring Security。Authentication Provider对于Spring Security来说相当于是身份验证的入口口。通过向Authentication Provider提供认证请求，我们可以得到认证结果，进而提供其他权限控制服务。

在Spring Security中，Authentication Provider是一个接口，其实现类需要覆盖authenticate(Authentication authentication)方法。当用户请求认证时，Authentication Provider就会尝试对用户提供的信息进行认证评估，并返回Authentication对象。

二、Authentication Provider的使用方式

除了以前常用的XML方式进行配置，Spring Security 5开始提供了一种更为方便的Java Config方式进行Authentication Provider的配置。如果我们要使用这种方式，需要创建一个继承了WebSecurityConfigurerAdapter的配置类，并在该类中重写configure(AuthenticationManagerBuilder auth)方法，然后在该方法中注册Authentication Provider。下面是一个简单的示例代码：

import org.springframework.context.annotation.Configuration;
import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.core.userdetails.UsernameNotFoundException;

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
    
    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
        auth.authenticationProvider(new CustomAuthenticationProvider());
    }
    
    private static class CustomUserDetailsService implements UserDetailsService {

        @Override
        public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
            return User.builder()
                    .username(username)
                    .password("{noop}password")
                    .roles("USER")
                    .build();
        }
    }

    private static class CustomAuthenticationProvider implements AuthenticationProvider {

        private final UserDetailsService userDetailsService;

        public CustomAuthenticationProvider() {
            this.userDetailsService = new CustomUserDetailsService();
        }

        @Override
        public Authentication authenticate(Authentication authentication) throws AuthenticationException {
            String username = authentication.getName();
            String password = authentication.getCredentials().toString();

            UserDetails user = userDetailsService.loadUserByUsername(username);

            if (password.equals(user.getPassword())) {
                return new UsernamePasswordAuthenticationToken(username, password, user.getAuthorities());
            } else {
                throw new BadCredentialsException("Password is incorrect");
            }
        }

        @Override
        public boolean supports(Class authentication) {
            return authentication.equals(UsernamePasswordAuthenticationToken.class);
        }
    }
}

三、Authentication Provider的自定义

如果默认实现的Authentication Provider不能满足我们的需求，我们也可以自己实现一个定制化的Authentication Provider。下面是一个自定义的Authentication Provider的示例代码：

import org.springframework.security.authentication.AuthenticationProvider;
import org.springframework.security.authentication.BadCredentialsException;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.AuthenticationException;
import org.springframework.stereotype.Component;

@Component
public class CustomAuthenticationProvider implements AuthenticationProvider {

    @Override
    public Authentication authenticate(Authentication authentication) throws AuthenticationException {
        String username = authentication.getName();
        String password = authentication.getCredentials().toString();
        // 在此处根据用户名和密码进行自定义认证
        if ("admin".equals(username) && "password".equals(password)) {
            return new UsernamePasswordAuthenticationToken(username, password, new ArrayList());
        } else {
            throw new BadCredentialsException("Authentication failed");
        }
    }

    @Override
    public boolean supports(Class authentication) {
        return UsernamePasswordAuthenticationToken.class.isAssignableFrom(authentication);
    }
}

四、Authentication Provider的扩展

在Spring Security中，我们还可以通过继承AbstractUserDetailsAuthenticationProvider类来扩展Authentication Provider的功能。AbstractUserDetailsAuthenticationProvider提供了一些模板方法，可以让我们方便地自定义扩展自己的Authentication Provider。下面是一个自定义扩展了AbstractUserDetailsAuthenticationProvider的示例代码：

import org.springframework.security.authentication.dao.AbstractUserDetailsAuthenticationProvider;
import org.springframework.security.core.Authentication;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.stereotype.Component;

@Component
public class CustomAuthenticationProvider extends AbstractUserDetailsAuthenticationProvider {

    @Override
    protected UserDetails retrieveUser(String username, UsernamePasswordAuthenticationToken authentication) throws AuthenticationException {
        //在此处实现根据用户名和密码查找用户信息的逻辑
    }

    @Override
    protected void additionalAuthenticationChecks(UserDetails userDetails, UsernamePasswordAuthenticationToken authentication) throws AuthenticationException {
        //在此处实现额外的认证检查逻辑
    }
}