JWT（JSON Web Token）是一种轻量级的认证和授权解决方案，它将用户身份信息和访问权限信息存储在一个安全的令牌中，并且该令牌是不可篡改的。下面介绍如何使用JWT实现用户认证和授权。

1. 用户登录时，服务器验证用户名和密码是否正确，如果正确则生成一个JWT并返回给客户端。
2. 客户端收到JWT后，每次请求都需要在请求头中添加Authorization字段，并将JWT放入其中。
3. 服务器通过验证JWT的签名来确定用户身份和访问权限。

实现步骤：

1. 添加依赖

在pom.xml文件中添加以下依赖：

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.11.2</version>
</dependency>

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.11.2</version>
    <scope>runtime</scope>
</dependency>

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-jackson</artifactId>
    <version>0.11.2</version>
    <scope>runtime</scope>
</dependency>

1. 编写工具类

创建一个JwtUtils工具类，用于生成、解析、校验JWT。

@Component
public class JwtUtils {

	private static final Logger logger = LoggerFactory.getLogger(JwtUtils.class);

	@Value("${jwt.secret}")
	private String secret;

	@Value("${jwt.expiration}")
	private int expiration;

	public String generateToken(UserDetails userDetails) {
		Map<String, Object> claims = new HashMap<>();
		return createToken(claims, userDetails.getUsername());
	}

	private String createToken(Map<String, Object> claims, String subject) {
		Date now = new Date();
		Date expirationDate = new Date(now.getTime() + expiration * 1000);

		return Jwts.builder()
				.setClaims(claims)
				.setSubject(subject)
				.setIssuedAt(now)
				.setExpiration(expirationDate)
				.signWith(SignatureAlgorithm.HS512, secret)
				.compact();
    }
    
    public boolean validateToken(String token, UserDetails userDetails) {
        final String username = getUsernameFromToken(token);
        return (username.equals(userDetails.getUsername()) && !isTokenExpired(token));
    }

    private boolean isTokenExpired(String token) {
        final Date expiration = getExpirationDateFromToken(token);
        return expiration.before(new Date());
    }

    public String getUsernameFromToken(String token) {
        return getClaimFromToken(token, Claims::getSubject);
    }

    public Date getExpirationDateFromToken(String token) {
        return getClaimFromToken(token, Claims::getExpiration);
    }

    private <T> T getClaimFromToken(String token, Function<Claims, T> claimsResolver) {
        final Claims claims = getAllClaimsFromToken(token);
        return claimsResolver.apply(claims);
    }

    private Claims getAllClaimsFromToken(String token) {
        return Jwts.parser().setSigningKey(secret).parseClaimsJws(token).getBody();
    }
}

1. 添加配置

在application.properties中添加以下配置：

# JWT
jwt.secret=your_secret_key_here
jwt.expiration=86400

1. 创建认证接口

创建一个/authenticate接口，用于用户登录。

@RestController
public class AuthenticationController {

	@Autowired
	private AuthenticationManager authenticationManager;

	@Autowired
	private JwtUtils jwtUtils;

	@PostMapping("/authenticate")
	public ResponseEntity<?> createAuthenticationToken(@RequestBody AuthenticationRequest authenticationRequest) throws Exception {
		try {
			authenticationManager.authenticate(
					new UsernamePasswordAuthenticationToken(authenticationRequest.getUsername(), authenticationRequest.getPassword())
			);
		} catch (BadCredentialsException e) {
			throw new Exception("Incorrect username or password", e);
		}

		final UserDetails userDetails = userDetailsService.loadUserByUsername(authenticationRequest.getUsername());
        final String token = jwtUtils.generateToken(userDetails);

        return ResponseEntity.ok(new AuthenticationResponse(token));
    }
}

1. 创建授权拦截器

创建一个JwtAuthorizationFilter类，用于对请求进行授权验证。

public class JwtAuthorizationFilter extends BasicAuthenticationFilter {

	private final UserDetailsService userDetailsService;
	private final JwtUtils jwtUtils;

	public JwtAuthorizationFilter(AuthenticationManager authenticationManager, UserDetailsService userDetailsService, JwtUtils jwtUtils) {
	    super(authenticationManager);
	    this.userDetailsService = userDetailsService;
	    this.jwtUtils = jwtUtils;
    }

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException {
        String header = request.getHeader(HttpHeaders.AUTHORIZATION);

        if (header == null || !header.startsWith("Bearer ")) {
            chain.doFilter(request, response);
            return;
        }

        String token = header.substring(7);

        if (!jwtUtils.validateToken(token)) {
            throw new BadCredentialsException("Invalid JWT token");
        }

        String username = jwtUtils.getUsernameFromToken(token);
        UserDetails userDetails = userDetailsService.loadUserByUsername(username);

        UsernamePasswordAuthenticationToken authentication = new UsernamePasswordAuthenticationToken(userDetails, null, userDetails.getAuthorities());
        authentication.setDetails(new WebAuthenticationDetailsSource().buildDetails(request));
        SecurityContextHolder.getContext().setAuthentication(authentication);

        chain.doFilter(request, response);
    }
}

1. 配置安全

在WebSecurityConfigurerAdapter中配置安全，添加认证和授权拦截器。

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

	@Autowired
	private UserDetailsService userDetailsService;

	@Autowired
	private JwtUtils jwtUtils;

	@Override
	protected void configure(HttpSecurity http) throws Exception {
	    http.csrf().disable()
	            .authorizeRequests()
	            .antMatchers("/authenticate").permitAll()
	            .anyRequest().authenticated()
	            .and()
	            .sessionManagement().sessionCreationPolicy(SessionCreationPolicy.STATELESS)
	            .and()
	            .addFilterBefore(new JwtAuthorizationFilter(authenticationManager(), userDetailsService, jwtUtils), UsernamePasswordAuthenticationFilter.class);
	}

	@Override
	public void configure(AuthenticationManagerBuilder auth) throws Exception {
	    auth.userDetailsService(userDetailsService).passwordEncoder(passwordEncoder());
    }

    @Bean
	public PasswordEncoder passwordEncoder() {
	    return new BCryptPasswordEncoder();
    }
}

以上就是使用JWT实现用户认证和授权的完整流程。