无法编写JSON ：（是java.lang.NullPointerException）；嵌套的异常是com.fasterxml.jackson.databind.JsonMappingException

Question

在我最近的项目中，我需要发送分页列表。我在较早的项目中做了分页。然后我没有遇到任何错误。但是现在我无法在最近的项目中进行分页。我不知道怎么了。我正在逐步提供我的代码。

这是我的控制器类，它返回一个自定义页面类

@GetMapping(value = "")
public CustomerResponsePage getCustomerList(@RequestParam Long agentId,
                                            @RequestParam(value = "page", defaultValue = "0") Integer page) {
    if (agentId == null)
        throw new NullValueException("Null value received for agentId");
    return customerService.getCustomersOfAgent(agentId, page);
}

我的服务方式是

 @Override
    public CustomerResponsePage getCustomersOfAgent(Long agentId, Integer page) {
        User agent = userService.read(agentId);
        if (agent == null)
            throw new EntityNotFoundException("Agent with this id is not found");
        if (agent.getReferenceNo() == null)
            throw new EntityNotFoundException("No reference found for this agent");
        Page<User> customers = userRepository.findByReferenceNo(agent.getReferenceNo(),
                new PageRequest(page, PageAttr.PAGE_SIZE));
        List<UserDTO> dtoList = new ArrayList<>();
        for (User user : customers) {
            dtoList.add(customerMapper.customerResponseMapper(user));
        }
        System.out.println("Page is " + customers.getContent());
        System.out.println("List is " + dtoList);
        return new CustomerResponsePage(dtoList, customers);
    }

我的自定义页面类是

public class CustomerResponsePage implements Page {


    private List<UserDTO> content;
    private Pageable pageable;
    private boolean last;
    private Integer totalPages;
    private Long totalElements;
    private Integer size;
    private Integer number;
    private Integer numberOfElements;
    private boolean first;
    private Sort sort;

    public CustomerResponsePage(List<UserDTO> content, Page page) {
        this.content = content;
        this.pageable = page.getPageable();
        this.last = page.isLast();
        this.totalPages = page.getTotalPages();
        this.totalElements = page.getTotalElements();
        this.size = page.getSize();
        this.number = page.getNumber();
        this.numberOfElements = page.getNumberOfElements();
        this.first = page.isFirst();
        this.sort = page.getSort();

        System.out.println("content is " + content.toString());
        System.out.println("Page value is " + page.getTotalElements());
        System.out.println("Content is " + page.getContent());
    }

    public void setContent(List<UserDTO> content) {
        this.content = content;
    }

    @Override
    public Pageable getPageable() {
        return pageable;
    }

    @Override
    public int getTotalPages() {
        return this.totalPages;
    }

    @Override
    public long getTotalElements() {
        return this.totalElements;
    }

    @Override
    public Page map(Function function) {
        return null;
    }

    @Override
    public int getNumber() {
        return this.number;
    }

    @Override
    public int getSize() {
        return this.size;
    }

    @Override
    public int getNumberOfElements() {
        return this.numberOfElements;
    }

    @Override
    public List getContent() {
        return this.content;
    }

    @Override
    public boolean hasContent() {
        return this.content.size() > 0;
    }

    @Override
    public Sort getSort() {
        return this.sort;
    }

    @Override
    public boolean isFirst() {
        return this.first;
    }

    @Override
    public boolean isLast() {
        return this.last;
    }

    @Override
    public boolean hasNext() {
        return !this.isLast();
    }

    @Override
    public boolean hasPrevious() {
        return !this.isFirst();
    }

    @Override
    public Pageable nextPageable() {
        return null;
    }

    @Override
    public Pageable previousPageable() {
        return null;
    }

    @Override
    public Iterator iterator() {
        return null;
    }
}

当我打邮递员时，我会收到类似

在我的控制台中，出现了

之类的错误

Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: com.sweetitech.capitalexpress.pageable.CustomerResponsePage["empty"])]

请注意，CustomerResponsePage内的打印方法有效。

Answer 1

堆栈跟踪引用CustomerResponsePage中名为empty的字段。问题可能与该字段的序列化或其getter方法的结果有关。

如果您不需要在JSON输出中使用该字段，则可以使用@JsonIgnore来跳过它。

[编辑]

在您的代码段中，使用CustomerResponsePage手动实例化了new CustomerResponsePage(dtoList, customers)。通过手动实例化，Spring将无法像往常一样处理分页。

我将使用map提供的Page函数，而不是手动遍历User对象来创建对象。然后，您将不需要CustomerResponsePage作为对象的包装，而可以返回Page<UserDto>。

Page<User> customers = userRepository.findByReferenceNo(agent.getReferenceNo(),
            new PageRequest(page, PageAttr.PAGE_SIZE));

Page<UserDto> userDtos = customers.map(c -> customerMapper.customerResponseMapper(c));

return userDtos;