Requesting web services using URL parameters

There are many web service providers that require nothing more than URL parameter passing to request a service. These service providers take HTTP/HTTPS request parameters as appended to a prospective consumer's URL, and process requests according to the passed parameter values. Services are delivered back to the requestor through the client's HTTP/HTTPS response message or as a separate HTTP/HTTPS request message exchange.

An example of such a real world web service is the PayPal Payments Standard payment processing service. This web service expects requests to come in on an advertised URL with request particulars appended to the URL as request parameters. Prospective consuming systems send HTTP/HTTPS request messages to the PayPal URL asking for service. Once a request for service has been accepted by PayPal, the Payments Standard web service responds and delivers service using the HTTP/HTTPS response message.

A separate web service, also involving PayPal, called the Instant Payment Notification (IPN) web service is an example of a web service in which parameters are passed on the URL from the service provider to a consumer such as OFBiz. In this case, OFBiz listens on a configured URL for an HTTP/HTTPS request message from PayPal. When one is received, OFBiz responds appropriately, taking delivery of PayPal service.

Note

Note: to implement a PayPal IPN client within an OFBiz web application, you must provide PayPal a destination URL that maps to a valid OFBiz request-map, and then process any incoming URL parameters according to the PayPal IPN directions.

Getting ready

To act as a web service client and pass parameters on the URL within an OFBiz Java Event or Service, make sure you include the following Java packages in your program:

import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import javax.Servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

How to do it...

To request a service using URL parameters, follow these steps:

  1. Within an existing OFBiz Java program (either an Event, Service, or any other) method, create a Java HashMap containing the name/value pairs that you wish to pass on the URL. For example:
    Map parameters = UtilMisc.toMap("param1", "A", "param2", "B");
  2. Use the OFBiz-provided UtilHttp.urlEncodeArgs method to properly encode the parameters directly from the HashMap into a Java String. For example:
    String encodedParameters = UtilHttp.urlEncodeArgs(parameters,
false);
  3. Build a URL and add the encoded parameter string to the URL:
    String redirectUrl = "http://www.somehost.com";
String redirectString = redirectUrl + "?" + encodedParameters;
  4. Send the request by way of the HttpServletResponse object's redirect method.
    try {
response.sendRedirect(redirectString);
}
catch (IOException e) {
// process errors here
return "error";
}

How it works...

In a very simple web service client scenario, OFBiz acts on behalf of a user or another OFBiz process and makes a web service consumer request of a remote service provider. The request is sent using a HTTP/HTTPS request message with one or more service parameters appended to the URL. The URL is the location on the web of the service provider.

Because the scenario described above is a one-way message exchange—that is, the request message is delivered to the destination URL, but OFBiz does not wait around for a response message—there must be assumptions made about how the web service provider will deliver service. Many times, service is delivered through a totally separate message exchange, initiated by first making the one-way request as described earlier.

To illustrate how this may play out, we consider the PayPal Payments Standard web service. This web service may be invoked from OFBiz in at least two different ways. For example, one approach is to include an HTML form on an OFBiz web page with a Submit button. The Submit button when clicked redirects the browser (with the request message) to the PayPal web service site passing the form contents as is.

An alternative implementation is to have an HTML form on an OFBiz web page with a Submit button that, when clicked, forwards the browser's request of an OFBiz Service (or Event). In this case, the OFBiz Event or Service will take the form attribute values (from the request message) and create a URL for the PayPal web service location. The original form attribute values and any other information as provided by the context or through database reads is appended to the URL. OFBiz then redirects the user's original request message using the sendRedirect method on the HttpServletResponse object to effectively send the user, by way of a browser and an appropriately crafted URL, to the PayPal web service.

Building URL request parameters out of plain Java strings can be tricky given the nature of the characters used to construct request parameters and delimit multiple parameter strings. For example, how do you pass a space character as part of a parameter when spaces are used as delimiters?

Enter URL encoding. URL encoding takes certain characters, deemed "special" characters, and "escapes" them so that they may be used as part of a request parameter string. OFBiz provides an easy-to-use encoder (and decoder) method(s): the UrlEncodeArgs method on the UtilHttp utility that takes as an argument a Java Map and returns an encoded string that may then be appended to a URL as shown earlier.

Note

Note: an exhaustive treatment of URL encoding is beyond the scope of this book. For more information on HTML and ASCII character codes and encoding symbols, please see http://www.ascii.cl/htmlcodes.htm

There's more...

The UrlEncodeArgs method has two modes of operation: selecting the false method-parameter value will encode using only an ampersand (&) symbol while the true parameter value tells OFBiz to use&amp as the encoding string. Based on the web service provider's instructions, you will need to determine which mode is appropriate for your application.

Many web services do not require encoded values. You will need to verify for each service whether or not it is necessary to encode and decode URL parameters. For example, the PayPal IPN web service sends request parameters without any encoding. When returning IPN messages, the client web service is, however, instructed to encode parameters.

PayPal IPN is strictly a server-to-server (there is no human intervention) messaging system where message traffic is transported across the web from one server URL to another. PayPal is the web service provider while, in this scenario, OFBiz acts as the web service client. It works something like shown in the following diagram:

There's more...
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