Simple Variables

In this case by embedding a $id variable as the second token Grails will automatically map the second token into a parameter (available via the params object) called id. For example given the URL /product/MacBook, the following code will render "MacBook" to the response:

class ProductController {
     def index() { render params.id }
}

You can of course construct more complex examples of mappings. For example the traditional blog URL format could be mapped as follows:

static mappings = {
   "/$blog/$year/$month/$day/$id"(controller: "blog", action: "show")

The above mapping would let you do things like:

/graemerocher/2007/01/10/my_funky_blog_entry

The individual tokens in the URL would again be mapped into the object with values available for year, month, , id and so on.

Dynamic Controller and Action Names

Here the name of the controller, action and id are implicitly obtained from the variables controller, action and id embedded within the URL.

You can also resolve the controller name and action name to execute dynamically using a closure:

static mappings = {
    "/$controller" {
        action = { params.goHere }
    }
}

Optional Variables

Another characteristic of the default mapping is the ability to append a ? at the end of a variable to make it an optional token. In a further example this technique could be applied to the blog URL mapping to have more flexible linking:

static mappings = {
    "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")

With this mapping all of these URLs would match with only the relevant parameters being populated in the params object:

/graemerocher/2007/01/10/my_funky_blog_entry
/graemerocher/2007/01/10
/graemerocher/2007
/graemerocher

Optional File Extensions

By adding the (.$format)? mapping you can access the file extension using the response.format property in a controller:

def index() {
    render "extension is ${response.format}"
}

Arbitrary Variables

You can also pass arbitrary parameters from the URL mapping into the controller by just setting them in the block passed to the mapping:

"/holiday/win" {
     id = "Marrakech"
     year = 2007
}

This variables will be available within the object passed to the controller.

Dynamically Resolved Variables

The hard coded arbitrary variables are useful, but sometimes you need to calculate the name of the variable based on runtime factors. This is also possible by assigning a block to the variable name:

"/holiday/win" {
     id = { params.id }
}