Java EE Pattern: The Control Layer

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1
Session: Pattern of the Control Layer
Singleton
Service Starter
Simple Workflows

2
Objectives
Learn about:
✔ How to start a service at deploy time
✔ Dealing with timers
✔ The Singleton pattern in Java EE
✔ The State Pattern in conjunction with simple workflows

3
Some orientation
Consumer
Consumer
Layer
Integration
Layer
Business Process
Layer
Services
Layer
Component
Layer
OS
Layer

4
ECB Pattern
Entity Control Boundary
✔ Based upon Robustness Diagrams
(http://www.agilemodeling.com/artifacts/robustnessDiagram.htm)
➢ Boundary: user interface
➢ Control: actual process or activity
➢ Entity: a concept from an enterprise context.
✔ Elements are generic enough to be mapped either to service-
oriented or object-oriented architectures.
Boundary Control Entity
Adam Bien

5
Services, components and patterns
Boundary Control Entity
DAO &
Domain
Store
Generic
DAO
Singleton
Service
Starter
Dual View
SOA Facade
Lightweight
asynchronous
Facade
Multichannel
Facade
TO
&
DTO
Paginator
Bean
Locator
Multichannel
Facade
Resource
Binder
Payload
Extractor
Aynchronous
Resource
Integrator
Infrastructure

6
Module
Singleton

7
Singleton
✔ Access to limited resources needs to be shared across user
requests.
✔ Amount of active SLSB is dependent on:
➢ Parallel user requests
➢ Pool settings (min/max)
➢ Both isn't covered by spec.
✔ The solution should be portable
between app.-servers.

8
Singleton
✔ Most effective in terms of caches,
application-wide configuration
parameters or cyclic actions.
✔ Often combined with Service Starter
(therefore we'll combine both labs)
@Singleton(mappedName = "ejb/facade/ServiceStarter")
@Remote(ServiceStarter.class)
public class ServiceStarterBean implements ServiceStarter
{
...

9
Module
Service Starter

10
Service Starter
✔ Some logic needs to be initialized prior to the business logic
✔ Some services might depend on each other
✔ Since EJB 3.1 these initializations are specified, in EJB 3.0 they
were not.
➢ In EJB 3.1: @Startup combined with @PostConstruct
➢ In EJB 3.0: Servlet and <load-on-startup> plus CDI

11
Example
Think of the following:
✔ Some production data needs to be retrieved for further processing
✔ You have to integrate a 3rd
party system by means of cyclic,
synchronous calls.
✔ Maybe you want to make use of
timers, be aware, you can create
timers from MDBs or Singletons
only.

12
Example
✔ ServiceStarterBean
@Singleton(mappedName = "ejb/facade/ServiceStarter")
@Startup
@Remote(ServiceStarter.class)
public class ServiceStarterBean implements ServiceStarter
{
private Logger log = Logger.getLogger(ServiceStarterBean.class);
@EJB
private PollingServiceBean pollingService;
/** the timer service */
@Resource
private javax.ejb.TimerService timerService;
@PostConstruct
public void init(){
log.info("initializing");
}
...

13
Example
✔ ServiceStarterBean (cont.)
@Schedule(second = "*/5", minute="*", hour="*", persistent = false, info="Timer:
external polling")
public void notifyPollingService(){
this.pollingService.doPoll();
}
public void stopTimer(String timerName){
Collection<Timer> timers = this.timerService.getTimers();
for (Timer timer : timers){
if(timer.getInfo().toString().equals(timerName)){
timer.cancel();
log.info("Timer: " + timerName + " has been cancelled");
}
}

14
Example
✔ External configuration: ejb-jar.xml (overwrites the annotation)
<?xml version="1.0" encoding="UTF-8"?>
<ejb-jar xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/ejb-
jar_3_1.xsd"
version="3.1">
<display-name>Pattern Training</display-name>
<enterprise-beans>
<session>
<ejb-name>ServiceStarterBean</ejb-name>
<ejb-class>de.brockhaus.userMgmt.control.ServiceStarterBean</ejb-class>
<timer>
<description>none</description>
<schedule>
<second>0/5</second>
<minute>*</minute>
<hour>*</hour>
</schedule>
<timeout-method>
<method-name>notifyPollingService</method-name>
</timeout-method>
<persistent>false</persistent>
<info>Timer: external polling</info>
</timer>
</session>
</enterprise-beans>
</ejb-jar>

15
Lab
Implement Service Activator
(not more than 15 min.)

16
Module
Simple Workflows

17
Simple Workflow
✔ Within the control layer, it might happen, you have to perform
several (identical) steps within different services / methods:
➢ Checking for existence of x, maybe in system y
➢ Calculating amounts of a, b, c (e.g. in a production plan for a certain
period)
➢ ...
✔ You might end up in the idea of decomposition into processes and
activities
➢ Has only little to do with Java EE
➢ Inspired by State Machines and
Workflow Engines
➢ No maintenance of state of a current
workflow instance

18
The abstract process
An abstract superclass providing methods to:
✔ Proceed (starting the process)
✔ Initialize the process
✔ Finalize the process

19
The abstract process
public abstract class Process{
private Logger log = Logger.getLogger(Process.class);
public static final int PROCESS_START = 1;
public static final int PROCESS_END = 999;
private int runstate = 1;
private boolean hasFinished = false;
public synchronized void proceed() throws ProcessException{
try{
this.beforeProcess();
while (runstate != Process.PROCESS_END){
runSteps();
}
this.afterProcess();
this.end();
}
catch (ProcessException e) {
...
}
catch (Exception e){
...
}
}
...

20
The activity
✔ Common interface to all process related activities:
public interface ProcessActivity
{
public <T> T proceedActivity<T>();
}

21
The activity
✔ Concrete activity (example)
➢ Make use of the constructor to pass parameters
➢ Return whatever you want from processActivity
public class StepOne implements ProcessActivity
{
private Logger log = Logger.getLogger(this.getClass());
private int value;
public StepOne(int value){
this.value = value;
}
public <T> T proceedActivity(){
log.info(this.getClass().getSimpleName() + " invoked");
if(this.value > 99){
return (T) new Boolean(true);
}
else {
return (T) new Boolean(false);
}
}
}

22
A concrete process
✔ Extend Process and overwrite runSteps() according to the activities
public class SomeProcess extends de.brockhaus.userMgmt.util.Process
{
private int amount;
public SomeProcess(int amount)
{
this.amount = amount;
}
@Override
public void runSteps() throws ProcessException
{
switch(super.getRunstate())
{
case(1): this.triggerStepOne(); break;
case(2): this.triggerStepTwo(); break;
case(3): this.triggerStepThree(); break;
default: super.setRunstate(PROCESS_END);
}
}

23
A concrete process
✔ Decide upon the activities outcome what needs to be done next
private void triggerStepOne(){
StepOne one = new StepOne(amount);
boolean ret = one.proceedActivity();
if(ret){
super.setRunstate(2);
}
else{
super.setRunstate(3);
}
}
private void triggerStepTwo() {
StepTwo two = new StepTwo();
two.proceedActivity();
super.setRunstate(PROCESS_END);
}
private void triggerStepThree() {
StepThree three = new StepThree();
three.proceedActivity();
super.setRunstate(PROCESS_END);
}

24
Relation to the control layer
✔ Obviously the process has to be connected to the SLSBs of the
control layer
@Stateless
@Remote(SomeService.class)
public class SomeServiceBean implements SomeService
{
// inject SomeProcess
@Inject
private SomeProcess p;
public void someBusinessMethod(int amount) throws ProcessException
{
// set amount at process
p.setAmount(amount);
// proceed process
p.proceed();
}
}

25
Some conclusion
Pros
✔ It's pretty simple and effective
✔ Well designed it might help a lot to stay with DRY paradigm
✔ Provides flexibility, just exchange an activity within a process and
the process will behave different.
Cons
✔ Don't think of too many activities, don't
think of fully fledged workflows spanning
several components / modules
✔ No graphical support, no modelling

26
Lab
Implement Simple Workflows
(State Machine)
(not more than 15 min.)

27
Review
Session Review:
✔ @PostConstruct, what is this good for?
✔ Can you provide some examples of using Singletons in Java EE?
✔ Why do we have processes and services in the example and what
makes the difference to workflow engines?

28
Recommeded reading
✔ http://java.sun.com/blueprints/corej2eepatterns/
✔ http://www.corej2eepatterns.com/Patterns2ndEd/
✔ Adam Bien, J2EE Patterns,
Addison Wesley 2002,
ISBN: 3-8273-1903-X
✔ Floyd Marinescu, Ed Roman:
Ejb Design Patterns: Advanced Patterns,
Processes, and Idioms; Wiley & Sons,
ISBN-10: 0471208310
✔ And other ...
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