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Groovy & Grails Topics

This is work in progress

Some Groovy basics

And that compiles? But some common class doesn't have that method?

Groovy adds utility methods to a lot of common classes. You'll find the details referred to as the GDK - and its available on http://groovy.codehaus.org/groovy-jdk/

An example: 

"http://www.vg.no".toURL().text

This gives the content of the web page http://www.vg.no. Resolution is as follows.

The string is a groovy.lang.GString. This class does not have a toURL() method.

However, it has got an invokeMethod. This method does a toString, and then tries to call the method (in this case , toURL()) on the object returned from toString.

toString returns a java.lang.String. This class does not have a toURL() method. However, there are methods added by grooovy to make stuff better. These methods can be found on the GDK link above. For java.lang.String, a getURL is added which returns a java.net.URL. getText is then called on this object.

Getting started with Grails

How do I create an application I can toy around with?

Do the following: 

> cd <wherever>
> export GRAILS_HOME=<wherever>
> export PATH=${PATH}:${GRAILS_HOME}/bin
> grails create-app

See below for how to use Maven for building your Grails project.

Removing the ~/.grails directory can sometimes solve strange errors

Try it if you don't understand why you get a strange error.

How do you upgrade a Grails project to a new version of the Grails framework?

  1. download the new grails version
  2. upgrade your grails project like this: 
    > cd <prosjektdir>
> grails clean
> grails upgrade

Using Maven with Grails

How do I use Maven with Grails?

Use the Maven plugin for Grails.

Create a Maven pom for your project like this: 

mvn grails:create-pom

Note: You might need to add a log4j dependency in your pom.xml to get your project to compile.

How do I use mysql with a maven-based grails project?

Add the mysql dependency to the POM:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.6</version>
</dependency>

Then, update grails-app/conf/DataSource.groovy:

dataSource {
    pooled = true
    driverClassName = "com.mysql.jdbc.Driver"
    username = "root"
    password = ""
}
hibernate {
    cache.use_second_level_cache=true
    cache.use_query_cache=true
    cache.provider_class='com.opensymphony.oscache.hibernate.OSCacheProvider'
}
// environment specific settings
environments {
    development {
        dataSource {
            dbCreate = "create-drop" // one of 'create', 'create-drop','update'
            url = "jdbc:mysql://localhost/BuddyMap"
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:mysql://localhost/BuddyMap"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:mysql://localhost/BuddyMap"
        }
    }
}

There is an underlying assumption here: All developers and test instances will run mysql without a root password. This might not be a good idea if tests use live data.

It is also not a good idea to let the application update the schema in production use, so switch that off.

Next, start mysql and create your database 

$ mysql -uroot
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.0.51b MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> create database buddymap;
Query OK, 1 row affected (0.00 sec)

mysql>

I have not set the locale or character set for the database here. You should.

Next, package and run the application:

$ mvn package
[INFO] Scanning for projects...
(...)
$ mvn grails:run-app
(...)
[INFO] Server running. Browse to http://localhost:8080/BuddyMap

Domain classes

Which standard methods should you implement in a domain class?

It is a good idea to implement String toString() for instance like this: 

class Book {
    String author
    String title
    <snip>

    String toString() {
        "${title}, ${author}"
    }
}

This is particularly important if you are using scaffolding - otherwise the views for the scaffolded pages will use "ID: Type" in relationships (drop-down selects and cross-reference links).

How do you group domain classes in packages?

todo

How do you handle non-persistent domain classes in Grails?

todo

Services

What are grails services used for?

They are basically used for the same things that a spring service bean, or an EJB session bean are used for, i.e. providing re-usable services for the front-end part of an application, handling transactions, and handling integration with external services. The following illustration gives one example of how they can be used in a Grails + Java contextgrails_and_spring2.png (Figure from http://www.grails-exchange.com/files/DaveSyer%20-%20Grails&Spring.pdf)

How do you create a service?

Use the grails create-service method.