我正在尝试为Racket Web Server制作一个servlet，允许用户将图片上传到网站，并将已上传的文件作为图像显示在同一页面上。我想直接将图片流入和流出PostgreSQL数据库，而不是将它们保存到磁盘或内存中的临时文件中。可能吗？如果是这样，最好的方法是什么？可以用无状态servlet完成吗？非常感谢任何帮助！
答案 0 :(得分：4)
应该是。我推荐PLaneT的db package（因为我写了它）。您可以在线阅读the docs。
PostgreSQL表应该有一个bytea字段用于图像内容;在Racket一侧，它将表示为字节串。
在您的servlet中，您应该返回带有图像内容的response/full结构。你必须自己处理返回码，MIME类型等。 （参见文档中的示例。）
答案 1 :(得分：1)
以科学的名义，我发布了自己问题答案的一半。此页面将显示数据库中已有的图像。上传页面仍然是一个悬而未决的问题。
Ryan Culpepper帮助我进行私人通信，超出了此处发布的内容。我感谢他的帮助。所有看起来像黑魔法的东西都来自他，所有笨拙的傻瓜都是我的。我将非常感谢有关如何改进代码的所有建议。
#lang racket
#|
================================================================================================================
We are assuming that the PostgreSQL database we are connecting to
has a table "person" with columns
"id", "firstname", "lastname" and "portrait".
The "portrait" column contains the OID of a BLOB
that stores the image file we want to display.
Suppose further that the table "person" has a legitimate entry with
id=22, firstname="John", lastname="Doe"
Then the page
http://127.0.0.1/page/22
should display greetings "Hello, John Doe!"
and show the portrait of the person below the greeting.
The portrait itself should be at
http://127.0.0.1/portrait/22.jpg
The program should be run via Racket -t "<filename>"
after defining the environment variables
"DB_USER", "DB_NAME", "DB_PORT", "DB_PASSWORD".
================================================================================================================
|#
(require
web-server/servlet
web-server/servlet-env
web-server/dispatch
web-server/stuffers/hmac-sha1
web-server/http
web-server/http/response-structs
(planet ryanc/db:1:4)
(planet ryanc/db:1:4/util/connect)
net/base64)
;---------------------------------------------------------------------------------------------------------------
; response
;---------------------------------------------------------------------------------------------------------------
(define (start given-request)
(site-dispatch given-request))
(define-values (site-dispatch given-request)
(dispatch-rules
[("page" (integer-arg)) show-page]
[("portrait" (string-arg)) show-portrait]))
(define (show-page given-request given-person-id)
(let* ( [db-person_firstname_lastname
(query-maybe-row my-connection
"SELECT firstname, lastname FROM person WHERE id = $1"
given-person-id)]
[my-firstname (vector-ref db-person_firstname_lastname 0)]
[my-lastname (vector-ref db-person_firstname_lastname 1)])
(response/xexpr
`(html ([xmlns "http://www.w3.org/1999/xhtml"])
(head
(title "Page with a portrait"))
(body
(div ([id "greetings"])
,(string-append
"Hello, " my-firstname " " my-lastname "! "))
(img ( [src ,(string-append "/portrait/"
(number->string given-person-id) ".jpg")])))))))
(define (show-portrait given-request given-portrait-file)
(let* ( [my-user-id (car (regexp-match #rx"^([0-9]+)"
given-portrait-file))]
[my-portrait-oid (query-value my-connection
"SELECT portrait FROM person WHERE id = $1"
(string->number my-user-id))]
[STREAMOUT_CHUNK_SIZE 1000]
[INV_READ #x00040000])
(response
200 ; code
#"Okay" ; message
(current-seconds) ; seconds
#"image/jpeg" ; mime type
empty ; headers
(lambda (given-output-stream) ; body generator
(start-transaction my-connection)
(define object-descriptor
(query-value my-connection
"SELECT LO_OPEN( $1, $2 )" my-portrait-oid INV_READ))
(define (stream-next-chunk)
(begin
(define my-next-chunk
(query-value my-connection
"SELECT LOREAD( $1, $2 )"
object-descriptor STREAMOUT_CHUNK_SIZE))
(if (> (bytes-length my-next-chunk) 0)
(begin
(write-bytes my-next-chunk given-output-stream)
(stream-next-chunk)
#t)
#f)))
(stream-next-chunk)
(commit-transaction my-connection)))))
;---------------------------------------------------------------------------------------------------------------
; database connection
;---------------------------------------------------------------------------------------------------------------
(define my-connection
(virtual-connection
(connection-pool
(lambda ()
(eprintf "(Re)establishing database connection...\n")
(postgresql-connect
#:user (getenv "DB_USER")
#:database (getenv "DB_NAME")
#:port (string->number (getenv "DB_PORT"))
#:socket #f
#:password (getenv "DB_PASSWORD")
#:allow-cleartext-password? #f
#:ssl 'optional ; other choices: 'yes 'no
)))))
;---------------------------------------------------------------------------------------------------------------
; servlet parameters
;---------------------------------------------------------------------------------------------------------------
(serve/servlet start
#:command-line? #t ; #t to use serve/servlet in a start up script for a Web application, and don't want a browser opened or the DrRacket banner printed
#:connection-close? #f ; #t to close every connection after one request. (Otherwise, the client decides based on what HTTP version it uses.)
#:launch-browser? #f
#:quit? #f ; #t makes the URL "/quit" end the server
#:banner? #t ; #t to print an informative banner
#:listen-ip #f ; give an IP to accept connections from external machines
#:port 80 ; 443 is the default for SSL, 80 - for open connections
#:servlet-regexp #rx"" ; #rx"" captures top-level requests
#:stateless? #t
#:server-root-path ; where the server files are rooted, default=(the distribution root)
(build-path ".")
#:ssl? #f
#:log-file (build-path "server.log"))