Controllers can be created with the create-controller or generate-controller command. For example try running the following command from the root of a Grails project:

The command will create a controller at the location grails-app/controllers/myapp/BookController.groovy:

where "myapp" will be the name of your application, the default package name if one isn’t specified.

BookController by default maps to the /book URI (relative to your application root).

A controller can have multiple public action methods; each one maps to a URI:

This example maps to the /book/list URI by default thanks to the property being named list.

A controller has the concept of a default URI that maps to the root URI of the controller, for example /book for BookController. The action that is called when the default URI is requested is dictated by the following rules:

Scopes are hash-like objects where you can store variables. The following scopes are available to controllers:

Scopes can be accessed using the variable names above in combination with Groovy’s array index operator, even on classes provided by the Servlet API such as the HttpServletRequest:

You can also access values within scopes using the de-reference operator, making the syntax even more clear:

This is one of the ways that Grails unifies access to the different scopes.

Grails supports the concept of flash scope as a temporary store to make attributes available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirecting, for example:

When the delete action is requested, the message value will be in scope and can be used to display an information message. It will be removed from the flash scope after this second request.

Note that the attribute name can be anything you want, and the values are often strings used to display messages, but can be any object type.

Newly created applications have the grails.controllers.defaultScope property set to a value of "singleton" in application.yml. You may change this value to any of the supported scopes listed below. If the property is not assigned a value at all, controllers will default to "prototype" scope.

Supported controller scopes are:

To enable one of the scopes, add a static scope property to your class with one of the valid scope values listed above, for example

You can define the default strategy in application.yml with the grails.controllers.defaultScope key, for example:

A model is a Map that the view uses when rendering. The keys within that Map correspond to variable names accessible by the view. There are a couple of ways to return a model. First, you can explicitly return a Map instance:

A more advanced approach is to return an instance of the Spring ModelAndView class:

One thing to bear in mind is that certain variable names can not be used in your model:

Currently, no error will be reported if you do use them, but this will hopefully change in a future version of Grails.

In both of the previous two examples there was no code that specified which view to render. So how does Grails know which one to pick? The answer lies in the conventions. Grails will look for a view at the location grails-app/views/book/show.gsp for this show action:

To render a different view, use the render method:

In this case Grails will attempt to render a view at the location grails-app/views/book/display.gsp. Notice that Grails automatically qualifies the view location with the book directory of the grails-app/views directory. This is convenient, but to access shared views, you use an absolute path instead of a relative one:

In this case Grails will attempt to render a view at the location grails-app/views/shared/display.gsp.

Grails also supports JSPs as views, so if a GSP isn’t found in the expected location but a JSP is, it will be used instead.

If a controller defines a namespace for itself with the namespace property that will affect the root directory in which Grails will look for views which are specified with a relative path. The default root directory for views rendered by a namespaced controller is grails-app/views/<namespace name>/<controller name>/. If the view is not found in the namespaced directory then Grails will fallback to looking for the view in the non-namespaced directory.

See the example below.

Sometimes it’s easier (for example with Ajax applications) to render snippets of text or code to the response directly from the controller. For this, the highly flexible render method can be used:

The above code writes the text "Hello World!" to the response. Other examples include:

If you plan on using Groovy’s MarkupBuilder to generate HTML for use with the render method be careful of naming clashes between HTML elements and Grails tags, for example:

This will actually call the form tag (which will return some text that will be ignored by the MarkupBuilder). To correctly output a <form> element, use the following:

Actions can be redirected using the redirect controller method:

Internally the redirect method uses the HttpServletResponse object’s sendRedirect method.

The redirect method expects one of:

In the above example Grails will construct a link using the domain class id (if present).

Parameters can optionally be passed from one action to the next using the params argument of the method:

These parameters are made available through the params dynamic property that accesses request parameters. If a parameter is specified with the same name as a request parameter, the request parameter is overridden and the controller parameter is used.

Since the params object is a Map, you can use it to pass the current request parameters from one action to the next:

Finally, you can also include a fragment in the target URI:

which will (depending on the URL mappings) redirect to something like "/myapp/test/show#profile".

Actions can also be chained. Chaining allows the model to be retained from one action to the next. For example calling the first action in this action:

results in the model:

The model can be accessed in subsequent controller actions in the chain using the chainModel map. This dynamic property only exists in actions following the call to the chain method:

Like the redirect method you can also pass parameters to the chain method:

The data binder is capable of converting and assigning values in a Map to properties of an object. The binder will associate entries in the Map to properties of the object using the keys in the Map that have values which correspond to property names on the object. The following code demonstrates the basics:

To update properties of a domain object you may assign a Map to the properties property of the domain class:

The binder can populate a full graph of objects using Maps of Maps.

The data binder can populate and update Collections and Maps. The following code shows a simple example of populating a List of objects in a domain class:

That code would work in the same way if albums were an array instead of a List.

Note that when binding to a Set the structure of the Map being bound to the Set is the same as that of a Map being bound to a List but since a Set is unordered, the indexes don’t necessarily correspond to the order of elements in the Set. In the code example above, if albums were a Set instead of a List, the bindingMap could look exactly the same but 'Foxtrot' might be the first album in the Set or it might be the second. When updating existing elements in a Set the Map being assigned to the Set must have id elements in it which represent the element in the Set being updated, as in the following example:

When binding to a Map the structure of the binding Map is the same as the structure of a Map used for binding to a List or a Set and the index inside of square brackets corresponds to the key in the Map being bound to. See the following code:

When updating an existing Map, if the key specified in the binding Map does not exist in the Map being bound to then a new value will be created and added to the Map with the specified key as in the following example:

The params object that is available in a controller has special behavior that helps convert dotted request parameter names into nested Maps that the data binder can work with. For example, if a request includes request parameters named person.homeAddress.country and person.homeAddress.city with values 'USA' and 'St. Louis' respectively, params would include entries like these:

There are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' Map constructor:

The data binding happens within the code new Book(params). By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:

Then the title and author request parameters would automatically be set on the domain class. You can use the properties property to perform data binding onto an existing instance:

This has the same effect as using the implicit constructor.

When binding an empty String (a String with no characters in it, not even spaces), the data binder will convert the empty String to null. This simplifies the most common case where the intent is to treat an empty form field as having the value null since there isn’t a way to actually submit a null as a request parameter. When this behavior is not desirable the application may assign the value directly.

The mass property binding mechanism will by default automatically trim all Strings at binding time. To disable this behavior set the grails.databinding.trimStrings property to false in grails-app/conf/application.groovy.

The mass property binding mechanism will by default automatically convert all empty Strings to null at binding time. To disable this behavior set the grails.databinding.convertEmptyStringsToNull property to false in grails-app/conf/application.groovy.

The order of events is that the String trimming happens and then null conversion happens so if trimStrings is true and convertEmptyStringsToNull is true, not only will empty Strings be converted to null but also blank Strings. A blank String is any String such that the trim() method returns an empty String.

If you have a one-to-one or many-to-one association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:

Grails will automatically detect the .id suffix on the request parameter and look up the Author instance for the given id when doing data binding such as:

An association property can be set to null by passing the literal String "null". For example:

If you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type.

If you have a Set based association (the default for a hasMany) then the simplest way to populate an association is to send a list of identifiers. For example consider the usage of <g:select> below:

This produces a select box that lets you select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the books association.

However, if you have a scenario where you want to update the properties of the associated objects the this technique won’t work. Instead you use the subscript operator:

However, with Set based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a Set has no concept of order, so although we’re referring to books[0] and books[1] it is not guaranteed that the order of the association will be correct on the server side unless you apply some explicit sorting yourself.

This is not a problem if you use List based associations, since a List has a defined order and an index you can refer to. This is also true of Map based associations.

Note also that if the association you are binding to has a size of two and you refer to an element that is outside the size of association:

Then Grails will automatically create a new instance for you at the defined position.

You can bind existing instances of the associated type to a List using the same .id syntax as you would use with a single-ended association. For example:

Would allow individual entries in the books List to be selected separately.

Entries at particular indexes can be removed in the same way too. For example:

Will render a select box that will remove the association at books[0] if the empty option is chosen.

Binding to a Map property works the same way except that the list index in the parameter name is replaced by the map key:

This would bind the selected image into the Map property images under a key of "cover".

When binding to Maps, Arrays and Collections the data binder will automatically grow the size of the collections as necessary.

It is possible to bind data to multiple domain objects from the params object.

For example so you have an incoming request to:

You’ll notice the difference with the above request is that each parameter has a prefix such as author. or book. which is used to isolate which parameters belong to which type. Grails' params object is like a multi-dimensional hash and you can index into it to isolate only a subset of the parameters to bind.

Notice how we use the prefix before the first dot of the book.title parameter to isolate only parameters below this level to bind. We could do the same with an Author domain class:

Controller action arguments are subject to request parameter data binding. There are 2 categories of controller action arguments. The first category is command objects. Complex types are treated as command objects. See the Command Objects section of the user guide for details. The other category is basic object types. Supported types are the 8 primitives, their corresponding type wrappers and java.lang.String. The default behavior is to map request parameters to action arguments by name:

For primitive arguments and arguments which are instances of any of the primitive type wrapper classes a type conversion has to be carried out before the request parameter value can be bound to the action argument. The type conversion happens automatically. In a case like the example shown above, the params.accountType request parameter has to be converted to an int. If type conversion fails for any reason, the argument will have its default value per normal Java behavior (null for type wrapper references, false for booleans and zero for numbers) and a corresponding error will be added to the errors property of the defining controller.

Since "bogusValue" cannot be converted to type int, the value of accountType will be zero, the controller’s errors.hasErrors() will be true, the controller’s errors.errorCount will be equal to 1 and the controller’s errors.getFieldError('accountType') will contain the corresponding error.

If the argument name does not match the name of the request parameter then the @grails.web.RequestParameter annotation may be applied to an argument to express the name of the request parameter which should be bound to that argument:

Sometimes when performing data binding it is not possible to convert a particular String into a particular target type. This results in a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. For example:

Here we have a domain class Book that uses the java.net.URL class to represent URLs. Given an incoming request such as:

it is not possible to bind the string a-bad-url to the publisherURL property as a type mismatch error occurs. You can check for these like this:

Although we have not yet covered error codes (for more information see the section on validation), for type conversion errors you would want a message from the grails-app/i18n/messages.properties file to use for the error. You can use a generic error message handler such as:

Or a more specific one:

The BindUsing annotation may be used to define a custom binding mechanism for a particular field in a class. Any time data binding is being applied to the field the closure value of the annotation will be invoked with 2 arguments. The first argument is the object that data binding is being applied to and the second argument is DataBindingSource which is the data source for the data binding. The value returned from the closure will be bound to the property. The following example would result in the upper case version of the name value in the source being applied to the name field during data binding.

The BindUsing annotation may be used to define a custom binding mechanism for all of the fields on a particular class. When the annotation is applied to a class, the value assigned to the annotation should be a class which implements the BindingHelper interface. An instance of that class will be used any time a value is bound to a property in the class that this annotation has been applied to.

The BindInitializer annotation may be used to initialize an associated field in a class if it is undefined. Unlike the BindUsing annotation, databinding will continue binding all nested properties on this association.

The binder will do a lot of type conversion automatically. Some applications may want to define their own mechanism for converting values and a simple way to do this is to write a class which implements ValueConverter and register an instance of that class as a bean in the Spring application context.

An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented ValueConverter will be automatically plugged in to the data binding process.

A custom date format may be specified to be used when binding a String to a Date value by applying the BindingFormat annotation to a Date field.

A global setting may be configured in application.groovy to define date formats which will be used application wide when binding to Date.

The formats specified in grails.databinding.dateFormats will be attempted in the order in which they are included in the List. If a property is marked with @BindingFormat, the @BindingFormat will take precedence over the values specified in grails.databinding.dateFormats.

The formats configured by default are:

You may supply your own handler for the BindingFormat annotation by writing a class which implements the FormattedValueConverter interface and registering an instance of that class as a bean in the Spring application context. Below is an example of a trivial custom String formatter that might convert the case of a String based on the value assigned to the BindingFormat annotation.

An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented FormattedValueConverter will be automatically plugged in to the data binding process.

With that in place the BindingFormat annotation may be applied to String fields to inform the data binder to take advantage of the custom converter.

The BindingFormat annotation supports localized format strings by using the optional code attribute. If a value is assigned to the code attribute that value will be used as the message code to retrieve the binding format string from the messageSource bean in the Spring application context and that lookup will be localized.

A structured data binding editor is a helper class which can bind structured request parameters to a property. The common use case for structured binding is binding to a Date object which might be constructed from several smaller pieces of information contained in several request parameters with names like birthday_month, birthday_date and birthday_year. The structured editor would retrieve all of those individual pieces of information and use them to construct a Date.

The framework provides a structured editor for binding to Date objects. An application may register its own structured editors for whatever types are appropriate. Consider the following classes:

A Gadget has 2 Shape fields. A Shape has an area property. It may be that the application wants to accept request parameters like width and height and use those to calculate the area of a Shape at binding time. A structured binding editor is well suited for that.

The way to register a structured editor with the data binding process is to add an instance of the grails.databinding.TypedStructuredBindingEditor interface to the Spring application context. The easiest way to implement the TypedStructuredBindingEditor interface is to extend the org.grails.databinding.converters.AbstractStructuredBindingEditor abstract class and override the getPropertyValue method as shown below:

An instance of that class needs to be registered with the Spring application context:

When the data binder binds to an instance of the Gadget class it will check to see if there are request parameters with names compressedShape and expandedShape which have a value of "struct" and if they do exist, that will trigger the use of the StructuredShapeEditor. The individual components of the structure need to have parameter names of the form propertyName_structuredElementName. In the case of the Gadget class above that would mean that the compressedShape request parameter should have a value of "struct" and the compressedShape_width and compressedShape_height parameters should have values which represent the width and the height of the compressed Shape. Similarly, the expandedShape request parameter should have a value of "struct" and the expandedShape_width and expandedShape_height parameters should have values which represent the width and the height of the expanded Shape.

Typically the request parameters with "struct" as their value would be represented by hidden form fields.

The DataBindingListener interface provides a mechanism for listeners to be notified of data binding events. The interface looks like this:

Any bean in the Spring application context which implements that interface will automatically be registered with the data binder. The DataBindingListenerAdapter class implements the DataBindingListener interface and provides default implementations for all of the methods in the interface so this class is well suited for subclassing so your listener class only needs to provide implementations for the methods your listener is interested in.

There are situations where an application may want to use the data binder directly. For example, to do binding in a Service on some arbitrary object which is not a domain class. The following will not work because the properties property is read only.

An instance of the data binder is in the Spring application context with a bean name of grailsWebDataBinder. That bean implements the DataBinder interface. The following code demonstrates using the data binder directly.

See the DataBinder documentation for more information about overloaded versions of the bind method.

When batch updating properties from request parameters you need to be careful not to allow clients to bind malicious data to domain classes and be persisted in the database. You can limit what properties are bound to a given domain class using the subscript operator:

In this case only the firstName and lastName properties will be bound.

Another way to do this is is to use Command Objects as the target of data binding instead of domain classes. Alternatively there is also the flexible bindData method.

The bindData method allows the same data binding capability, but to arbitrary objects:

The bindData method also lets you exclude certain parameters that you don’t want updated:

Or include only certain properties:

The bindable constraint can be used to globally prevent data binding for certain properties.

The respond method is the preferred way to return JSON and integrates with Content Negotiation and JSON Views.

The respond method provides content negotiation strategies to intelligently produce an appropriate response for the given client.

For example given the following controller and action:

The respond method will take the followings steps:

The respond method will then look for an appriopriate Renderer for the object and the calculated media type from the RendererRegistry.

Grails includes a number of pre-configured Renderer implementations that will produce default representations of JSON responses for the argument passed to respond. For example going to the /book.json URI will produce JSON such as:

By default if you define a controller there is no priority in terms of which format is sent back to the client and Grails assumes you wish to serve HTML as a response type.

However if your application is primarily an API, then you can specify the priorty using the responseFormats property:

In the above example Grails will respond by default with json if the media type to respond with cannot be calculated from the Accept header or file extension.

If you define a view (either a GSP or a JSON View) then Grails will render the view when using the respond method by calculating a model from the argument passed to respond.

For example, in the previous listing, if you were to define grails-app/views/index.gson and grails-app/views/index.gsp views, these would be used if the client requested application/json or text/html media types respectively. Thus allowing you to define a single backend capable of serving responses to a web browser or representing your application’s API.

When rendering the view, Grails will calculate a model to pass to the view based on the type of the value passed to the respond method.

The following table summarizes this convention:

Using this convention you can reference the argument passed to respond from within your view:

You will notice that if Book.list() returns an empty list then the model variable name is translated to emptyList. This is by design and you should provide a default value in the view if no model variable is specified, such as the List in the example above:

There are cases where you may wish to be more explicit and control the name of the model variable. For example if you have a domain inheritance hierarchy where a call to list() my return different child classes relying on automatic calculation may not be reliable.

In this case you should pass the model directly using respond and a map argument:

If you simply wish to augment the calculated model then you can do so by passing a model argument:

The above example will produce a model like [bookList:books, bookCount:totalBooks], where the calculated model is combined with the model passed in the model argument.

The render method can also be used to output JSON, but should only be used for simple cases that don’t warrant the creation of a JSON view:

In this case the result would be something along the lines of:

The same dangers with naming conflicts described above for XML also apply to JSON building.

Grails supports file uploads using Spring’s MultipartHttpServletRequest interface. The first step for file uploading is to create a multipart form like this:

The uploadForm tag conveniently adds the enctype="multipart/form-data" attribute to the standard <g:form> tag.

There are then a number of ways to handle the file upload. One is to work with the Spring MultipartFile instance directly:

This is convenient for doing transfers to other destinations and manipulating the file directly as you can obtain an InputStream and so on with the MultipartFile interface.

File uploads can also be performed using data binding. Consider this Image domain class:

If you create an image using the params object in the constructor as in the example below, Grails will automatically bind the file’s contents as a byte[] to the myFile property:

It’s important that you set the size or maxSize constraints, otherwise your database may be created with a small column size that can’t handle reasonably sized files. For example, both H2 and MySQL default to a blob size of 255 bytes for byte[] properties.

It is also possible to set the contents of the file as a string by changing the type of the myFile property on the image to a String type:

Grails default size for file uploads is 128000 (~128KB). When this limit is exceeded you’ll see the following exception:

You can configure the limit in your application.yml as follows:

maxFileSize = The maximum size allowed for uploaded files.

maxRequestSize = The maximum size allowed for multipart/form-data requests.

You should keep in mind OWASP recommendations - Unrestricted File Upload

These limits exist to prevent DoS attacks and to enforce overall application performance

Command object classes are defined just like any other class.

In this example, the command object class implements the Validateable trait. The Validateable trait allows the definition of Constraints just like in domain classes. If the command object is defined in the same source file as the controller that is using it, Grails will automatically make it Validateable. It is not required that command object classes be validateable.

By default, all Validateable object properties which are not instances of java.util.Collection or java.util.Map are nullable: false. Instances of java.util.Collection and java.util.Map default to nullable: true. If you want a Validateable that has nullable: true properties by default, you can specify this by defining a defaultNullable method in the class:

In this example, both name and age will allow null values during validation.

To use command objects, controller actions may optionally specify any number of command object parameters. The parameter types must be supplied so that Grails knows what objects to create and initialize.

Before the controller action is executed Grails will automatically create an instance of the command object class and populate its properties by binding the request parameters. If the command object class is marked with Validateable then the command object will be validated. For example:

If the command object’s type is that of a domain class and there is an id request parameter then instead of invoking the domain class constructor to create a new instance a call will be made to the static get method on the domain class and the value of the id parameter will be passed as an argument.

Whatever is returned from that call to get is what will be passed into the controller action. This means that if there is an id request parameter and no corresponding record is found in the database then the value of the command object will be null. If an error occurs retrieving the instance from the database then null will be passed as an argument to the controller action and an error will be added the controller’s errors property.

If the command object’s type is a domain class and there is no id request parameter or there is an id request parameter and its value is empty then null will be passed into the controller action unless the HTTP request method is "POST", in which case a new instance of the domain class will be created by invoking the domain class constructor. For all of the cases where the domain class instance is non-null, data binding is only performed if the HTTP request method is "POST", "PUT" or "PATCH".

Normally request parameter names will be mapped directly to property names in the command object. Nested parameter names may be used to bind down the object graph in an intuitive way.

In the example below a request parameter named name will be bound to the name property of the Person instance and a request parameter named address.city will be bound to the city property of the address property in the Person.

A problem may arise if a controller action accepts multiple command objects which happen to contain the same property name. Consider the following example.

If there is a request parameter named name it isn’t clear if that should represent the name of the Product or the name of the Person. Another version of the problem can come up if a controller action accepts 2 command objects of the same type as shown below.

To help deal with this the framework imposes special rules for mapping parameter names to command object types. The command object data binding will treat all parameters that begin with the controller action parameter name as belonging to the corresponding command object.

For example, the product.name request parameter will be bound to the name property in the product argument, the buyer.name request parameter will be bound to the name property in the buyer argument the seller.address.city request parameter will be bound to the city property of the address property of the seller argument, etc…​

Command objects can participate in dependency injection. This is useful if your command object has some custom validation logic which uses a Grails service:

In this example the command object interacts with the loginService bean which is injected by name from the Spring ApplicationContext.

When a request is made to a controller action which accepts a command object and the request contains a body, Grails will attempt to parse the body of the request based on the request content type and use the body to do data binding on the command object. See the following example.

Note that the body of the request is being parsed to make that work. Any attempt to read the body of the request after that will fail since the corresponding input stream will be empty. The controller action can either use a command object or it can parse the body of the request on its own (either directly, or by referring to something like request.JSON), but cannot do both.

A common use case for command objects is a Command Object that contains a collection of another:

On this example, we want to create an Author with multiple Books.

In order to make this work from the UI layer, you can do the following in your GSP:

There is also support for JSON, so you can submit the following with correct databinding

If you prefer to avoid the overhead of data binding and simply want to convert incoming parameters (typically Strings) into another more appropriate type the params object has a number of convenience methods for each type:

The above example uses the int method, and there are also methods for boolean, long, char, short and so on. Each of these methods is null-safe and safe from any parsing errors, so you don’t have to perform any additional checks on the parameters.

Each of the conversion methods allows a default value to be passed as an optional second argument. The default value will be returned if a corresponding entry cannot be found in the map or if an error occurs during the conversion. Example:

These same type conversion methods are also available on the attrs parameter of GSP tags.

A common use case is dealing with multiple request parameters of the same name. For example you could get a query string such as ?name=Bob&name=Judy.

In this case dealing with one parameter and dealing with many has different semantics since Groovy’s iteration mechanics for String iterate over each character. To avoid this problem the params object provides a list method that always returns a list:

As of Grails 3.1, if you prefer not to rely on a resources mapping to define your mappings then you can prefix any URL mapping with the HTTP method name (in lower case) to indicate the HTTP method it applies to. The following URL mapping:

Is equivalent to:

Notice how the HTTP method name is prefixed prior to each URL mapping definition.

A single resource is a resource for which there is only one (possibly per user) in the system. You can create a single resource using the single parameter (as opposed to resources):

This results in the following URL mappings:

The main difference is that the id is not included in the URL mapping.

You can nest resource mappings to generate child resources. For example:

The above will result in the following URL mappings:

You can also nest regular URL mappings within a resource mapping:

This will result in the following URL being available:

To map a URI directly below a resource then use a collection block:

This will result in the following URL being available (without the ID):

You can link to any URL mapping created with the g:link tag provided by Grails simply by referencing the controller and action to link to:

As a convenience you can also pass a domain instance to the resource attribute of the link tag:

This will automatically produce the correct link (in this case "/books/1" for an id of "1").

The case of nested resources is a little different as they typically required two identifiers (the id of the resource and the one it is nested within). For example given the nested resources:

If you wished to link to the show action of the author controller, you would write:

However, to make this more concise there is a resource attribute to the link tag which can be used instead:

The resource attribute accepts a path to the resource separated by a slash (in this case "book/author"). The attributes of the tag can be used to specify the necessary bookId parameter.

The previous section demonstrated how to map simple URLs with concrete "tokens". In URL mapping speak tokens are the sequence of characters between each slash, '/'. A concrete token is one which is well defined such as as /product. However, in many circumstances you don’t know what the value of a particular token will be until runtime. In this case you can use variable placeholders within the URL for example:

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:

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

The above mapping would let you do things like:

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

Variables can also be used to dynamically construct the controller and action name. In fact the default Grails URL mappings use this technique:

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:

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:

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

If you wish to capture the extension of a particular path, then a special case mapping exists:

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

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

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

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:

In the above case the code within the blocks is resolved when the URL is actually matched and hence can be used in combination with all sorts of logic.

In addition you can configure handlers for individual exceptions:

With this configuration, an IllegalArgumentException will be handled by the illegalArgument action in ErrorsController, a NullPointerException will be handled by the nullPointer action, and a MyException will be handled by the customException action. Other exceptions will be handled by the catch-all rule and use the /errors/serverError view.

You can access the exception from your custom error handling view or controller action using the request’s exception attribute like so:

In versions of Grails prior to Grails 3.0, Grails supported the notion of filters. These are still supported for backwards compatibility but are considered deprecated.

The new interceptors concept in Grails 3.0 is superior in a number of ways, most significantly interceptors can use Groovy’s CompileStatic annotation to optimize performance (something which is often critical as interceptors can be executed for every request.)

Before you can start dealing with content negotiation you need to tell Grails what content types you wish to support. By default Grails comes configured with a number of different content types within grails-app/conf/application.yml using the grails.mime.types setting:

The setting can also be done in grails-app/conf/application.groovy as shown below:

The above bit of configuration allows Grails to detect to format of a request containing either the 'text/xml' or 'application/xml' media types as simply 'xml'. You can add your own types by simply adding new entries into the map. The first one is the default format.

Let’s say a controller action can return a resource in a variety of formats: HTML, XML, and JSON. What format will the client get? The easiest and most reliable way for the client to control this is through a format URL parameter.

So if you, as a browser or some other client, want a resource as XML, you can use a URL like this:

The result of this on the server side is a format property on the response object with the value xml .

You can also define this parameter in the URL Mappings definition:

You could code your controller action to return XML based on this property, but you can also make use of the controller-specific withFormat() method:

In this example, Grails will only execute the block inside withFormat() that matches the requested content type. So if the preferred format is html then Grails will execute the html() call only. Each 'block' can either be a map model for the corresponding view (as we are doing for 'html' in the above example) or a closure. The closure can contain any standard action code, for example it can return a model or render content directly.

When no format matches explicitly, a * (wildcard) block can be used to handle all other formats.

There is a special format, "all", that is handled differently from the explicit formats. If "all" is specified (normally this happens through the Accept header - see below), then the first block of withFormat() is executed when there isn’t a * (wildcard) block available.

You should not add an explicit "all" block. In this example, a format of "all" will trigger the html handler (html is the first block and there is no * block).

Every incoming HTTP request has a special Accept header that defines what media types (or mime types) a client can "accept". In older browsers this is typically:

which simply means anything. However, newer browsers send more interesting values such as this one sent by Firefox:

This particular accept header is unhelpful because it indicates that XML is the preferred response format whereas the user is really expecting HTML. That’s why Grails ignores the accept header by default for browsers. However, non-browser clients are typically more specific in their requirements and can send accept headers such as

As mentioned the default configuration in Grails is to ignore the accept header for browsers. This is done by the configuration setting grails.mime.disable.accept.header.userAgents, which is configured to detect the major rendering engines and ignore their ACCEPT headers. This allows Grails' content negotiation to continue to work for non-browser clients:

For example, if it sees the accept header above ('application/json') it will set format to json as you’d expect. And of course this works with the withFormat() method in just the same way as when the format URL parameter is set (although the URL parameter takes precedence).

An accept header of '*/\*' results in a value of all for the format property.

As of Grails 2.0, there is a separate notion of the request format and the response format. The request format is dictated by the CONTENT_TYPE header and is typically used to detect if the incoming request can be parsed into XML or JSON, whilst the response format uses the file extension, format parameter or ACCEPT header to attempt to deliver an appropriate response to the client.

The withFormat available on controllers deals specifically with the response format. If you wish to add logic that deals with the request format then you can do so using a separate withFormat method available on the request:

Grails also supports content negotiation using URI extensions. For example given the following URI:

This works as a result of the default URL Mapping definition which is:

Note the inclusion of the format variable in the path. If you do not wish to use content negotiation via the file extension then simply remove this part of the URL mapping:

To test content negotiation in a unit or integration test (see the section on Testing) you can either manipulate the incoming request headers:

Or you can set the format parameter to achieve a similar effect: