如果您发现此页面是因为您试图使用JsonGenerator在Spring的JsonSerializer中序列化对象。write（String/Number）Field函数并看到com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value错误，即使您已经提供了一个值：
确保在调用writeField之前调用了JsonGenerator.writeStartObject();

把这个换了

@JsonSubTypes(value = {
            @JsonSubTypes.Type(value = SimpleGrantedAuthority.class)
        }
)

对此，

@JsonSubTypes({
@JsonSubTypes.Type(value = SimpleGrantedAuthority.class, name = "ROLE_ADMIN"),
})

这里，JsonSubTypes的name参数应该是您在JSON对象中用来标识该子类的名称。

我最近遇到了同样的问题，但我成功地解决了它。如果您还想序列化List<GrantedAuthority> authorities，可以参考我的方法。
我还尝试分别序列化和反序列化List<GrantedAuthority> authorities字段，但是序列化失败，反序列化成功。这是因为在序列化过程中，我们必须使用gen.writeStartObject()序列化对象，否则将抛出一个异常，说明“无法写入字段名，期待一个值”。然而，这种方法不会给予我们预期的数组格式，相反，它看起来像这样：{ ["role1","role2"]}。

UserDetails

@Data
@JsonSerialize(using = AuthoritySerializer.class)
public class SysUser implements Serializable, UserDetails,
        JSONSerializer<JSONObject, SysUser>, JSONDeserializer<SysUser> {

    @TableId(type = IdType.AUTO)
    private Long id;

    private String username;

    private String password;

    @TableField(exist = false)
    @JsonDeserialize(using = AuthorityDeserializer.class)
    private List<GrantedAuthority> authorities = new ArrayList<>();

    @TableField(exist = false)
    private String codeKey;

    @TableField(exist = false)
    private String Code;

    private String avatar;

    private String email;

    private String city;

    private LocalDateTime createTime;

    private LocalDateTime updateTime;

    private Integer status;

    private LocalDateTime lastLogin;

    @TableField(exist = false)
    private List<SysRole> roles;

    public SysUser() {
    }

    public SysUser(ArrayList<GrantedAuthority> authorities) {
        this.authorities = authorities;
    }

    @Override
    public Collection<? extends GrantedAuthority> getAuthorities() {
        return this.authorities;
    }

    @Override
    public boolean isAccountNonExpired() {
        return true;
    }

    @Override
    public boolean isAccountNonLocked() {
        return true;
    }

    @Override
    public boolean isCredentialsNonExpired() {
        return true;
    }

    @Override
    public boolean isEnabled() {
        return this.status == 1;
    } 
}

序列化器

public class AuthoritySerializer extends JsonSerializer<SysUser> {

    @Override
    public void serialize(SysUser sysUser, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {      
       List<String> authorities = sysUser.getAuthorities().stream().map(GrantedAuthority::getAuthority).collect(Collectors.toList());
        jsonGenerator.writeStartObject();
        jsonGenerator.writeObjectField("id", sysUser.getId());
        jsonGenerator.writeStringField("username", sysUser.getUsername());
        jsonGenerator.writeStringField("password", sysUser.getPassword());
        jsonGenerator.writeStringField("avtar", sysUser.getAvatar());
        jsonGenerator.writeStringField("city", sysUser.getCity());
        jsonGenerator.writeStringField("email", sysUser.getEmail());
        jsonGenerator.writeObjectField("roles", sysUser.getRoles() == null ? "" : sysUser.getRoles());
        jsonGenerator.writeObjectField("authorities", authorities.toString());
        jsonGenerator.writeObjectField("status", sysUser.getStatus());
        jsonGenerator.writeObjectField("createTime", sysUser.getCreateTime());
        jsonGenerator.writeObjectField("updateTime", sysUser.getUpdateTime());
        jsonGenerator.writeObjectField("lastLogin", sysUser.getLastLogin());
    }
}

Serialize测试

@Test
    void testSerialize() throws JsonProcessingException {
        UserDetails admin = userDetailsService.loadUserByUsername("admin");
        JacksonObjectMapper objectMapper = new JacksonObjectMapper();
        String s = objectMapper.writeValueAsString(admin);
        stringRedisTemplate.opsForValue().set("user:admin",s);

下面是序列化的JSON结果。

AuthorityDeserializer

public class AuthorityDeserializer extends StdDeserializer<List<GrantedAuthority>> {
    public AuthorityDeserializer() {
        this(null);
    }

    protected AuthorityDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public List<GrantedAuthority> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JacksonException {
        String authorities = jsonParser.getCodec().readValue(jsonParser, String.class);
        List<GrantedAuthority> grantedAuthorities = AuthorityUtils.commaSeparatedStringToAuthorityList(authorities);
        return grantedAuthorities;
    }
}

测试DeSerialize

@Test
    void testDeSerialize() throws JsonProcessingException {
        String json = stringRedisTemplate.opsForValue().get("user:admin");
        JacksonObjectMapper objectMapper = new JacksonObjectMapper();
        SysUser sysUser = objectMapper.readValue(json, SysUser.class);
        System.out.println(sysUser);
    }

下面是DeSerialize结果。