Spring Boot微服务调用出现NullPointerException问题求助

核心问题 #

报错明确指向UserRating.getUserRating()返回null，调用stream()方法时触发java.lang.NullPointerException。给Rating类添加无参构造无法解决问题，因为根源是UserRating的列表字段未初始化，或远程服务返回的响应未正确映射到该字段。

修复步骤 #

1. 初始化UserRating的列表字段 #

修改UserRating类，直接初始化列表字段，从根源避免null：

public class UserRating {
    private List<Rating> userRating = new ArrayList<>();

    public UserRating() {}

    public UserRating(List<Rating> userRating) {
        this.userRating = userRating;
    }

    public List<Rating> getUserRating() {
        return userRating;
    }

    public void setUserRating(List<Rating> userRating) {
        this.userRating = userRating;
    }
}

2. 校验远程服务响应格式 #

确保评分服务返回的JSON结构与UserRating类匹配，正确格式示例：

{
  "userRating": [
    {"movieId":"1","rating":4},
    {"movieId":"2","rating":3}
  ]
}

若远程服务直接返回列表（而非包裹在userRating字段的对象），需调整接收类型为List<Rating>，而非UserRating。

3. 添加空值防护逻辑 #

在调用远程服务后，增加空值检查，避免意外null导致报错：

// 调用评分服务
UserRating userRating = restTemplate.getForObject(
    "http://rating-service/ratings/users/{userId}", 
    UserRating.class, 
    userId
);

// 空值防护
if (userRating == null) {
    userRating = new UserRating();
}
if (userRating.getUserRating() == null) {
    userRating.setUserRating(new ArrayList<>());
}

// 安全执行流操作
List<CatalogItem> catalogItems = userRating.getUserRating().stream()
    .map(rating -> {
        Movie movie = restTemplate.getForObject(
            "http://movie-service/movies/{movieId}", 
            Movie.class, 
            rating.getMovieId()
        );
        return new CatalogItem(movie.getName(), movie.getDesc(), rating.getRating());
    })
    .collect(Collectors.toList());

4. 验证服务可用性 #

确认评分服务已正常启动，直接访问其接口（如http://localhost:端口/ratings/users/foo），检查返回的JSON结构正确且非空。

验证结果 #

修复后重启服务，访问http://localhost:8080/catalog/foo，应返回预期响应：

[{"name":"Test","desc":"TestDesc","rating":4},{"name":"Test","desc":"TestDesc","rating":3}]

内容的提问来源于stack exchange，提问作者sandeep seervi