Services in Grails

1. Grails Services

2. Agenda
● Separation of concern
● Controller’s Responsibility
● Introduction of Service
● Create a Service
● Declaratives Transaction
● Transactions Rollback and Session
● Scoped Services
● Dependency Injection and Services
● Using Services from java
● Injecting a Service

3. hello!
I am Vijai Kumar Shukl
You can find me at vijay@nextho

4. 1.
Separation of Concern

5. “Consider this analogy: Imagine a company where employees are assigned tasks on different nature of
work.
Consider Employee has following responsibility
● Handle accounting and releasing of check payments
● Take calls from customers for product support
● Handle administrative tasks such as booking flights plus accommodation of executives
● Manage the schedule of the CEO
Problem with this
1. Complicated to John
2. He have to change his frame of mind
3. It’s not easy to replace that employee

6. 2.
Controller’s Responsibility

7. Consider a controller in Grails. Inside a controller, we can do the following:
● Handle routing logic
● Invoke GORM operations to manipulate data in the database
● Render text and show it to the user.
The real purpose of a controller is to deal with routing logic- which means
✘ Receive requests from users
✘ Invoke the most appropriate business logic
✘ Invoke the view to display the result

8. 3.
Service Introduction

9. ✘ All Business logic should be implemented in service layer.
✘ Grails has default support and handling for the service Layer.
✘ Services in Grails are the place to put the majority of the logic in your application, leaving
controllers responsible for handling request flow with redirects and so on.

10. 4.
Creating A Service

11. grails create-service [package_name] [service_name]
A service's name ends with the convention Service,
other than that a service is a plain Groovy class
package com
class PublicService{
}
It will create two files:-
PublicService (Inside service folder)
PublicServiceSpec (Inside test folder)
grails create-service com Public
def publicService

12. 5.
Declarative Transactions

13. Services are typically involved with coordinating logic between domain classes, and hence often involved with
persistence that spans large operations.
The nature of services, they frequently require transactional behaviour.
All services are transactional by default. To disable this set the transactional property to false
Note*:- Dependency is the only way that declarative transactions work. You will not get transactional behaviour if you
use new operator such as new PublicService()
The result is that all methods are wrapped in a transaction and automatic rollback occurs if a method throws a runtime
exception (i.e. one that extends RuntimeException) or an Error.
class PublicService{
static transactional = false
}

14. Custom Transaction Configuration
Grails also provides @Transactional and @NotTransactional annotations for cases:-
x Where you need more fine-grained control over transactions at a per-method level
x Need to specify an alternative propagation level.
x For example, the @NotTransactional annotation can be used to mark a particular method to be
skipped when a class is annotated with @Transactional.
class BookService{
@Transactional(readOnly=true)
def listBook(){
Book.list()
}
}

15. 6.
Transactions Management

16. ✘ Transaction is a very important concept. Hope you are well aware of ACID(Atomicity, Consistency,
Isolation, Durability) properties .
✘ Usually we wish certain sequence of database changes to be successful. If not possible we want no
operation to happen at all.
✘ Imagine a code that deducts money from one account (accountFrom.balance =
accountFrom.balance - amount), and adds money to another account (accountTo.balance =
accountTo.balance + amount). Imagine if something happened (an Exception) after deducting from
the source account and the destination account was not updated. Money will be lost and not
accounted for. For this type of codes, we want an "all or nothing" behavior. This concept is also called
atomicity.
✘ Since Grails supports transactions, it automatically do these things to us when we declare a service
to be transactional:
○ If all db operations are successful, reflect the changes to the database (this is also called commit) •
○ If one db operation result in exception, return to the original state and forget/undo all the previous operations
(this is also called rollback) •
✘ By default all services are transactional, if you don’t want transactional behavior of the service you
can get rid of it by class TestService(){ static transactional=false }

17. ✘ When a transaction is rolled back the Hibernate Session used by GORM is cleared.
✘ Means an object within the session become detached and accessing uninitialized lazy-loaded
collections will lead to LazyInitiallizationExceptions.

18. 7.
Scoped Services

19. By default, access to service methods is not synchronized, so nothing prevents concurrent execution of
those methods. In fact, because the service is a singleton and may be used concurrently, you should be
very careful about storing state in a service.
You can change this behaviour by placing a service in a particular scope. The supported scopes are:
● prototype - A new service is created every time it is injected into another class
● request - A new service will be created per request
● flash - A new service will be created for the current and next request only
● flow - In web flows the service will exist for the scope of the flow
● conversation - In web flows the service will exist for the scope of the conversation. ie a root flow
and its sub flows
● session - A service is created for the scope of a user session
● singleton (default) - Only one instance of the service ever exists

20. 8.
Dependency Injection

21. A key aspect of Grails services is the ability to use Spring Framework's dependency injection features.
Grails supports "dependency injection by convention".
you can use the property name representation of the class name of a service to automatically inject them
into controllers, tag libraries, and so on.
class BookController{
def bookService
{
class BookController{
BookService bookService
{
class AuthorService{
def bookService
}

22. Our process is easy
create inject use

23. def appContext =
ServletContextHolder.servletContext.getAttribute(GrailsApplicationAttributes.APPLICATION_CONTEXT)
def scrapeScoreService = appContext.scrapeScoreService

24. References
http://grails.asia/grails-tutorial-for-beginners-grails-service-layer/
http://docs.grails.org/2.4.4/guide/services.html

25. thanks!
Any questions?
You can find me at
vijay@nexthoughts.com