| Grails | |
| Developer: | Graeme Rocher |
| Released: | October, 2005 |
| Latest Release Version: | 6.1.2 |
| Latest Release Date: | January 31, 2024 |
| Programming Language: | Groovy |
| Operating System: | Cross-platform |
| Platform: | Cross-platform (JVM) |
| Genre: | Web application framework |
| License: | Apache License 2.0 |
Grails is an open source web application framework that uses the Apache Groovy programming language (which is in turn based on the Java platform). It is intended to be a high-productivity framework by following the "coding by convention" paradigm, providing a stand-alone development environment and hiding much of the configuration detail from the developer.
Grails was previously known as "Groovy on Rails"; in March 2006 that name was dropped in response to a request by David Heinemeier Hansson, founder of the Ruby on Rails framework.[1] Work began in July 2005, with the 0.1 release on March 29, 2006, and the 1.0 release announced on February 18, 2008.
Grails was developed to address a number of goals:
Grails has three properties that differentiate it from traditional Java web frameworks:
Creating web applications in Java traditionally involves configuring environments and frameworks at the start and during development. This configuration is very often externalized in XML files to ease configuration and avoid embedding configuration in application code.
XML was initially welcomed as it provided greater consistency to configure applications. However, in recent years, it has become apparent that although XML is great for configuration, it can be tedious to set up an environment. This may reduce productivity as developers spend time understanding and maintaining framework configuration as the application grows. Adding or changing functionality in applications that use XML configuration adds an extra step to the change process, which slows down productivity and may diminish the agility of the entire process.
Grails removes the need to add configuration in XML files. Instead, the framework uses a set of rules or conventions while inspecting the code of Grails-based applications. For example, a class name that ends with Controller (for example BookController) is considered a web controller.
When using traditional Java web toolkits, it's up to developers to assemble development units, which can be tedious. Grails provides a development environment that includes a web server to get developers started right away. All required libraries are part of the Grails distribution, and Grails prepares the Java web environment for deployment automatically.
Grails features dynamic methods on several classes through mixins. A mixin is a method that is added to a class dynamically, as if the functionality had been compiled into the program.
These dynamic methods allow developers to perform operations without having to implement interfaces or extend base classes. Grails provides dynamic methods based on the type of class. For example, domain classes have methods to automate persistence operations like save, delete and find
The Grails web framework has been designed according to the MVC paradigm.
Grails uses controllers to implement the behavior of web pages. Below is an example of a controller:list action which returns a model containing all books in the database. To create this controller the grails command is used, as shown below:
grails create-controller Book
This command creates a class in the grails-app/controller directory of the Grails project. Creating the controller class is sufficient to have it recognized by Grails. The list action maps to <nowiki>http://localhost:8080/book/list</nowiki> in development mode.
Grails supports JSP and GSP. The example below shows a view written in GSP which lists the books in the model prepared by the controller above:
This view should be saved as grails-app/views/book/list.gsp of the Grails project. This location maps to the BookController and list action. Placing the file in this location is sufficient to have it recognized by Grails.
There is also a GSP tag reference available.
Grails provides a large number of tag libraries out of the box. However you can also create and reuse your own tag libraries easily:[2] formatDate tag library above formats a object to a . This tag library should be added to the grails-app/taglib/ApplicationTagLib.groovy file or a file ending with TagLib.groovy in the grails-app/taglib directory.
Below is a snippet from a GSP file which uses the formatDate tag library:
<g:formatDate format="yyyyMMdd" date="${myDate}"/>
To use a dynamic tag library in a GSP no import tags have to be used. Dynamic tag libraries can also be used in JSP files although this requires a little more work. http://grails.org/Dynamic+Tag+Libraries
The domain model in Grails is persisted to the database using GORM (Grails Object Relational Mapping). Domain classes are saved in the grails-app/domain directory and can be created using the grails command as shown below:
grails create-domain-class Book
This command requests the domain class name and creates the appropriate file. Below the code of the Book class is shown:
When a class is defined as a domain class, that is, one managed by GORM, methods are dynamically added to aid in persisting the class's instances. http://grails.org/DomainClass+Dynamic+Methods
The save method saves an object to the database:delete method deletes an object from the database:refresh method refreshes the state of an object from the database:ident method retrieves the object's identity assigned from the database:
The count method returns the number of records in the database for a given class:exists method returns true if an object exists in the database with a given identifier:find method returns the first object from the database based on an object query statement:
The findAll method returns all objects existing in the database:findAll method can also take an object query statement for returning a list of objects:findBy* methods return the first object from the database which matches a specific pattern:findAllBy* methods return a list of objects from the database which match a specific pattern:findWhere* methods return the first object from the database which matches a set of named parameters:
Grails supports scaffolding to support CRUD operations (Create, Read, Update, Delete). Any domain class can be scaffolded by creating a scaffolding controller as shown below:<nowiki>http://localhost:8080/book</nowiki>. This works because the BookController follows the same naming convention as the Book domain class. To scaffold a specific domain class we could reference the class directly in the scaffold property:
Currently Grails does not provide scaffolding for associations.
The persistence mechanism in GORM is implemented via Hibernate. As such, legacy databases may be mapped to GORM classes using standard Hibernate mapping files.
The target audience for Grails is:
Grails is built on top of and is part of the Java platform meaning that it is very easy to integrate with Java libraries, frameworks and existing code bases. Grails offers transparent integration of classes which are mapped with the Hibernate ORM framework. This means existing applications which use Hibernate can use Grails without recompiling the code or reconfiguring the Hibernate classes while using the dynamic persistence methods discussed above. http://grails.org/Hibernate+Integration
One consequence of this is that scaffolding can be configured for Java classes mapped with Hibernate. Another consequence is that the capabilities of the Grails web framework are fully available for these classes and the applications which use them.
Grails also makes use of the Spring Inversion of Control Framework; Grails is actually a Spring MVC application under the hood.[3] The Spring framework can be used to provision additional Spring beans and introduce them into the context of the application. The SiteMesh framework is used to manage the presentation layer, simplifying the development of pages via a robust templating system.[4]
Grails applications are packaged as war artifacts that can be deployed to any servlet container or Java EE application servers.