跟着JHipster学做项目 (12) 分页处理（上）后端利用Pageable进行分页查询

JHipster的后端分页处理最大的特点是保持DTO类不受分页信息污染，不必包含分页相关属性，如页数，总数，排序等。

Controller类如下：

    @GetMapping("/users")
    public ResponseEntity<List<UserDTO>> getAllUsers(Pageable pageable) {
        final Page<UserDTO> page = userService.getAllManagedUsers(pageable);
        HttpHeaders headers = PaginationUtil.generatePaginationHttpHeaders(ServletUriComponentsBuilder.fromCurrentRequest(), page);
        return new ResponseEntity<>(page.getContent(), headers, HttpStatus.OK);
    }

Service类如下：我们可以看到通过利用Pageable接口代码变得十分简洁。

    public Page<UserDTO> getAllManagedUsers(Pageable pageable) {
        return userRepository.findAllByLoginNot(pageable, Constants.ANONYMOUS_USER).map(UserDTO::new);
    }

其中pageable参数通过简单对象属性匹配方式从前端传递过来：

/api/users?sort=id,desc&page=0&size=20

最核心的思路在于关于pageable的信息通过response的header传递回前端，以下是示例代码：

PaginationUtil.class

    public static <T> HttpHeaders generatePaginationHttpHeaders(UriComponentsBuilder uriBuilder, Page<T> page) {
        HttpHeaders headers = new HttpHeaders();
        headers.add(HEADER_X_TOTAL_COUNT, Long.toString(page.getTotalElements()));
        int pageNumber = page.getNumber();
        int pageSize = page.getSize();
        StringBuilder link = new StringBuilder();
        if (pageNumber < page.getTotalPages() - 1) {
            link.append(prepareLink(uriBuilder, pageNumber + 1, pageSize, "next"))
                .append(",");
        }
        if (pageNumber > 0) {
            link.append(prepareLink(uriBuilder, pageNumber - 1, pageSize, "prev"))
                .append(",");
        }
        link.append(prepareLink(uriBuilder, page.getTotalPages() - 1, pageSize, "last"))
            .append(",")
            .append(prepareLink(uriBuilder, 0, pageSize, "first"));
        headers.add(HttpHeaders.LINK, link.toString());
        return headers;
    }


    private static String prepareLink(UriComponentsBuilder uriBuilder, int pageNumber, int pageSize, String relType) {
        return MessageFormat.format(HEADER_LINK_FORMAT, preparePageUri(uriBuilder, pageNumber, pageSize), relType);
    }

    private static String preparePageUri(UriComponentsBuilder uriBuilder, int pageNumber, int pageSize) {
        return uriBuilder.replaceQueryParam("page", Integer.toString(pageNumber))
            .replaceQueryParam("size", Integer.toString(pageSize))
            .toUriString()
            .replace(",", "%2C")
            .replace(";", "%3B");
    }

下篇会讲述前端如何组装查询链接，以及如何处理后端传回的response.

Good Luck,

Cheers!