Access URI template variables in view [SPR-6486]

[spring-projects-issues]

Rich Kroll opened SPR-6486 and commented

I have a URI template in the form of "/hotels/{hotel}/bookings/{booking}", and can access the path variables in the controller via @PathVariable, but when building links in the view I need a reference to the different path variables.

As an example, let's say the user requests "/hotels/Hotelier/bookings/booking-123", and as is a common use case in the view, there is a need to link back to the bookings listing "/hotels/Hotelier/bookings/". The only way currently to access the URI template variables is to manually inject them each time into the view. I suggest that a tag be created that is capable of building a link from the same URI template style that is used in the @RequestMapping, something like <spring:a link="/hotels/{hotel}">go</spring:a>.

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Affects: 3.0 M1, 3.0 M2, 3.0 M3, 3.0 M4, 3.0 RC1, 3.0 RC2, 3.0 RC3

Issue Links:

• Automatically expose @PathVariables in the Model [SPR-7543] #12200 Automatically expose @PathVariables in the Model ("is duplicated by")

7 votes, 6 watchers

[spring-projects-issues]

Craig Walls commented

I like this idea and would like to extend it a bit further...

I'd like to be able to do something like this:

<spring:a link="/hotels/{hotel}" method="delete">Delete Hotel</spring:a>

I know that the standard HTML anchor won't do here, but the proposed spring:a could automatically generate the necessary form elements required to send a POST with a hidden "_method" set to "delete" and an anchor that submits that form.

[spring-projects-issues]

James Earl Douglas commented

If we assign all of the @PathVariable variables to a Map on the model named "_pathVarMap", we can access these variables from within a new taglib.

In HandlerMethodInvoker.resolveHandlerArguments(), just take the following line:

else if (pathVarName != null) {
	args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler);
}

and replace with:

else if (pathVarName != null) {
	Object object = resolvePathVariable(pathVarName, methodParam, webRequest, handler);
	
	String modelAttributeName = pathVarName;

	if (!StringUtils.hasText(modelAttributeName)) {
		modelAttributeName = methodParam.getParameterName();
	}

	if (implicitModel.get("_pathVarMap") == null) {
		implicitModel.put("_pathVarMap", new HashMap<String, Object>());
	}
	
	((Map<String, Object>) implicitModel.get("_pathVarMap")).put(modelAttributeName, object);

	args[i] = object;
}

Of course this code should be refactored into appropriate method calls, and the name of the "_pathVarMap" model attribute should be externalized, but this is just an illustration.

The next step is to build a taglib which will look for links like:

<spring:a link="/hotels/{hotel}" method="delete">Delete Hotel</spring:a>

and replaces the {hotel} pattern with the value stored in _pathVarMap.get("hotel") from above.

[spring-projects-issues]

Scott Andrews commented

Have you tried the spring:url tag? It is modeled after the JSTL c:url tag, but will do URI template variable substitution. While it will not automatically infer the parameters or render an html tag, it should fill your requirement.

The tagdocs are currently missing from the reference manual (I'll fix that). More info about the tag is available in javadoc form.
http://static.springsource.org/spring/docs/3.0.x/javadoc-api/org/springframework/web/servlet/tags/UrlTag.html

[spring-projects-issues]

Rossen Stoyanchev commented

If path variables are exposed in the model under "pathVars" as is also suggested in #12200 then the Spring URL tag could accept a Map attribute like this:

<spring:url value="/hotels/{hotel}" params="${pathVars}">Hotel</spring:url>

Parameters could still be provided through explicit spring:param sub-elements, which would override any parameters also specified in the Map.

[spring-projects-issues]

Ted Young commented

For anyone interested in this functionality now, I wrote an example of how you can implement it as an aspect (and disable it when 3.1 is released).

http://tedyoung.me/2011/01/24/spring-expose-pathvariables-to-the-model/

[spring-projects-issues]

Ted Young commented

I am sorry, the above post was made to the wrong issue. Specifically, my article discusses how to expose @PathAttributes to the model. Please feel free to delete the comment.

[spring-projects-issues]

Stefan Haberl commented

I would like to take that even further as proposed by Richard above. It would be nice to have a more powerful, more flexible RESTful "routing" component in Spring comparable to the one PHP's Symfony framework has to offer: http://symfony.com/doc/2.0/book/routing.html

In particular it would be nice

• if you could specify additional requirements on a PathVariable for the route to match (like not-null, must be an int > 100, ...)
• if URI templates would be named, so that you can refer to them in the view by name.

If you have named URI templates you could then insert a link in your view to any another controller using the unique URI template name. This would be nice, because refactoring the request path mapping in a controller would not brake all links in the views.

[spring-projects-issues]

Rossen Stoyanchev commented

I'm marking this as complete. To summarize you have the spring:url tag for working with URI templates and also with the resolution of #12200, @PathVariables are automatically added to the model so you can just do this in the view:

<spring:url value="/hotels/{hotel}/bookings/{booking}">
   <spring:param name="hotel" value="${hotel}" />
   <spring:param name="booking" value="${booking}" />
</spring:url>

Not quite as short as the original description but pretty close and less magical while remaining explicit about the parameter values to use.