Chapter 6. Request Bindings

Chapter 6. Request Bindings
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Bindings associated with the request returned by the send-*/(suspend|forward) functions, or passed as parameters to procedures that are either published or the target of a dispatch, can be extracted and associated to language-level variables.

Bindings, as returned by the function get-bindings are contained in an opaque object. This object can be converted to an association list via bindings->alist, or queried in constant time using the extract-bindings or extract-single-binding functions. The object is guaranteed to be serializable, but code should not rely on its specific implementation.

Extracting Bindings

Requires: (import siscweb/bindings)
Located in: siscweb.jar

This module assists developers in obtaining values of parameters passed in a request.

procedure: (get-bindings [char-set] request) => bindings
Given a request object, this procedure returns an opaque object representing the request parameters as key-value(s) pairs.
The optional char-set parameter can be used to override the character encoding of the request.

procedure: (exists-binding? name bindings) => #t/#f
Returns a boolean indicating whether a parameter with the given name contained in the request object. name can be either a string or a symbol.

procedure: (extract-single-binding name bindings) => string/#f
Returns the value associated to the given name or #f if name is not present in the request object. If more than one value is bound to name, only the first one is returned.

procedure: (extract-bindings name bindings) => list
Returns the values bound to the given name in a list. Even if only one value is bound to the request parameter name, a list will still be returned.

procedure: (bindings->alist bindings) => ((name . value) ...)
Returns an association list corresponding to the bindings. The name returned is always a string to maintain its casing, while the value returned is an atom or a list if name is associated to multiple values.

syntax: (let-bindings ((variable name) ... ) bindings body ...)) => value
Behaves like (let ...), but variables are set to the values corresponding to the names in the bindings object. The value returned is that of the last expression in the body. A typical usage scenario could be:

(let-bindings ((name "name")) (get-bindings (send-html/suspend (lambda (k-url) `(html (body (form (@ (action ,k-url)) (input (@ (type "text") (name "name"))))))))) (send-html/back `(html (body (p "You typed: " ,name)))))

Extracting Bindings from Java

Bindings specified in the send-forward/* functions are assigned to request attributes. This method supercedes the previous request.getBinding*() API, which is now deprecated and will be removed in the next release.

Multi-valued bindings, as in '((messages "hello" "there")), are turned into a java.util.List and can be easily scanned using such tools as JSTL's <c:forEach>. Single-valued bindings, such as '((message . "hello")) are simply assigned to the attribute.

Multi-valued bindings originating from a Scheme list are also marked as such, so that they can be converted back to a list of values rather than to an object of type java.util.List.

A minimal amount of type conversion is performed. Scheme values are passed as SISC objects, except for Scheme strings, which are converted to Java strings. Java objects are left untouched. This also applies to the individual values of multi-valued bindings.

For instance, below is the Polyglot Hello World example using JSP/JSTL as the presentation layer. This example is not included in the distribution because of its external dependencies on the JSTL libraries.

        
;; file: hello.scm
(require-library 'siscweb/forward)

(module examples/hello-world
  (hello-world)

  (import siscweb/forward)

  (define messages '("Hello, world!" "Salve, mundo!" "Hallo, Welt!" "Salve, mondo!" "Bonjour, monde!" "Hola, mundo!"))

  (define (hello-world req)
    (let loop ()
      (for-each
       (lambda (message)
         (send-forward/suspend "/jsp/hello.jsp" `((message . ,message))))
       messages)
      (loop)))
  )


<%-- File: jsp/hello.jsp
--%><%@ page contentType="text/html" %><%--
--%><%@ page isELIgnored="false" %><%-- just for servlet v2.3 containers
--%><%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%><%--
--%><%@ taglib prefix="x" uri="http://java.sun.com/jsp/jstl/xml"%><%--
--%><%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt"%><%--
--%><%@ taglib prefix="sql" uri="http://java.sun.com/jsp/jstl/sql"%><%--
--%><%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions"%><%--
--%><?xml version="1.0"?>

<!DOCTYPE html
     PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
    <title><c:out value="${requestScope.message}"/></title>

    <c:url var="cssUrl" value="/css/default.css"/>
    <link href="${cssUrl}"
          rel="stylesheet"
          type="text/css"/>
  </head>

  <body>
      <h3>Polyglot hello world</h3>
      <p>${requestScope.message}</p>

    <p><a href="${requestScope["siscweb.kURL"]}">Next language &gt;</a></p>
    <p><a href="${requestScope["siscweb.kURL"]}" target="_blank">Next language in new window</a></p>
    <c:url var="homeUrl" value="/"/>
    <p><a href="${homeUrl}">^ Home</a></p>
  </body>
</html>

Bindings and Security

At the lower level, bindings are looked up in the request scope first (i.e. from request attributes) and then from request parameters. This behavior allows a more natural integration with standard J2EE components such as JSPs, but can be unsafe during forwards.

Specifically, if the application performs a server-side forward to Scheme code that relies on the absence of a binding to determine a course of action, a malicious user can inject a parameter by the same name into the request (provided they can guess the name), and upset the application's behavior. The best options in this case are:

When forwarding from Scheme, pass bindings to the forward/* and send-forward/* functions even if they are #f or the empty list. SISCweb will add the name to a per-request blacklist, and will not look up the binding among the request parameters.
When forwarding from Java to Scheme, set the attributes to null. Again, the name will be blacklisted.
If the Java code that forwards to the Scheme code cannot be trusted or modified for some reason, the Scheme side can always use the java/get-request-attribute function described in the section called “Request Procedures”.

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