Spring scope using Example

This post is about to experiment about spring framework ‘singleton’ and ‘prototype’ scope. I created couple of tests to show proof of concept and clear our understanding of ‘singleton’ scope.

Those are new or fairly new to spring framework, they sometimes get confused, when they read the term ‘singleton’ in spring beans. As traditionally, we know, Singleton class pattern makes sure, we have single instance of that class per jvm. But when we talk about singleton pattern in spring framework world or lets say spring context world,  it’s singleton pattern applied per spring container. Now we can have multiple containers initiated in a java virtual machine, hence we can have multiple instance of same class possible, but single instance per container per bean definition.

Example, Lets use simple java bean, which stores it’s own creation time and also defines equals method to check that equality.

public class SpringBean {
private long createTime;

public SpringBean()
{
this.createTime = System.currentTimeMillis();
}

 

public boolean equals(final Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (this.getClass() != obj.getClass()) {
return false;
}
SpringBean other = (SpringBean) obj;
if (this.createTime != other.createTime) {
return false;
}
return true;
}
}

My spring context file defined only two beans

 

<bean id=”singletonBean”/>

<bean id=”prototypeBean” scope=”prototype”/>

 

  • <bean id=”singletonBean” class=”com.jframeworks.learn.SpringBean”/>
  • <bean id=”prototypeBean” class=”com.jframeworks.learn.SpringBean” scope=”prototype”/>

Both beans use same class but different scope. Lets define a testsuite class which will initiate these beans and run some basic tests.

 

Class: SpringScopeTestSuite, method = runTests

public SpringScopeTestSuite(ApplicationContext ctx, ApplicationContext ctx2) {
this.ctx = ctx;
this.ctx2 = ctx2;
}
...............
public void runTests(String name) throws InterruptedException
{
SpringBean singletonBean0 = this.getBeanUsingContext("singletonBean");
Thread.sleep(50);//To make sure, next bean from context is at different timestamp.
SpringBean singletonBean1 = (SpringBean) this.getCtx().getBean("singletonBean");
Thread.sleep(50);
SpringBean protoTypeBean0 = (SpringBean) this.getCtx().getBean("prototypeBean");
Thread.sleep(50);
SpringBean protoTypeBean1 = (SpringBean) this.getCtx().getBean("prototypeBean");
Thread.sleep(50);
SpringBean singletonBean2 = (SpringBean) this.getCtx2().getBean("singletonBean");
Thread.sleep(50);
SpringBean protoTypeBean2 = (SpringBean) this.getCtx2().getBean("prototypeBean");

System.out.println(name + " =====================================================");
System.out.println(name + " Equality Tests");
System.out.println(name + " =====================================================");
System.out.println(name + " Two beans references with Scope 'singleton'");
System.out.println(name + " singletonBean0.equals(singletonBean1) = " + singletonBean0.equals(singletonBean1));
System.out.println(name + " singletonBean1.equals(singletonBean2) = " + singletonBean1.equals(singletonBean2));
System.out.println(name + " singletonBean1.equals(protoTypeBean1) = " + singletonBean1.equals(protoTypeBean1));
System.out.println(name + " protoTypeBean0.equals(protoTypeBean1) = " + protoTypeBean0.equals(protoTypeBean1));
System.out.println(name + " protoTypeBean1.equals(protoTypeBean2) = " + protoTypeBean1.equals(protoTypeBean2));
System.out.println(name + " =====================================================");
}

To run this test, lets define test runner class, ‘BasicSpringScopeTest’

public class BasicSpringScopeTest
{
public static void main( final String[] args )
{
try {
ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationConfig.xml");
ApplicationContext ctx2 = new ClassPathXmlApplicationContext("applicationConfig.xml");
SpringScopeTestSuite test = new SpringScopeTestSuite(ctx, ctx2);
test.runTests("singleThread");
} catch (Exception ex) {
ex.printStackTrace();
}
}
}

Now executing this class, we see this output…

singleThread =====================================================
singleThread Equality Tests
singleThread =====================================================
singleThread Two beans references with Scope 'singleton'
singleThread singletonBean0.equals(singletonBean1) = true
singleThread singletonBean1.equals(singletonBean2) = false
singleThread singletonBean1.equals(protoTypeBean1) = false
singleThread protoTypeBean0.equals(protoTypeBean1) = false
singleThread protoTypeBean1.equals(protoTypeBean2) = false
singleThread =====================================================

Analysis.
As singletonBean0 and singletonBean1 comes from same container, and their scope is default ‘singleton’, hence they are same instance.

singletonBean1 and singletonBean2, though are same bean and ‘singleton’ scope, but they comes from different container. Hence they are different instance of same class.

singletonBean1 and prototypeBean1 comes from same container and same class, but prototypeBean1 is different bean definition and ‘prototype’ scope. Every time, this bean retrieved from container, spring container creates new instance. Hence they are not equals. They are not equal for two reasons

 

  1. They are two different bean definition (even same class type)
  2. prototypeBean is defined as ‘prototype’ scope’

 

Next equality check is for prototypeBean0 and prototypeBean1 beans.  Even they come from same container but they still not equal, because they refer to spring bean defined as ‘prototype’ scope.

Last equality check is for prototypeBean1  and prototypeBean2 beans. They are not equal for two reasons..

  1. They both comes from different spring container
  2. They refer to spring bean defined as ‘prototype’ scope.

Conclusion
Spring framework takes away boiler plate code of defining Singleton classes in code base. This aligns with their philosphy of letting developers concentrate on business logic instead of writing boiler plate code.

Just to be clear, in spring world, a bean definition is singleton not bean class. We can have multiple of bean definitions of same class, each representing different singleton class. This makes spring container light weight container. Prototype scope is not used widely, but it can specified depending upon technical requirements.

For further testing, there is another code, which runs test suite in mutli-threaded environment.

public class AdvancedSpringScopeTest
{
public static void main( final String[] args )
{
try
{
ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationConfig.xml");
ApplicationContext ctx2 = new ClassPathXmlApplicationContext("applicationConfig.xml");

SpringScopeTestSuite testSuite = new SpringScopeTestSuite(ctx, ctx2);

 

 

Thread thread1 = new Thread(testSuite, "thread1");
Thread thread2 = new Thread(testSuite, "thread2");
Thread thread3 = new Thread(testSuite, "thread3");

 

 

}
catch (Exception ex)
{
ex.printStackTrace();
}
}
}

This class output is..

thread2 =====================================================
thread2 Equality Tests
thread2 =====================================================
thread2 Two beans references with Scope 'singleton'
thread1 =====================================================
thread1 Equality Tests
thread1 =====================================================
thread1 Two beans references with Scope 'singleton'
thread1 singletonBean0.equals(singletonBean1) = true
thread3 =====================================================
thread1 singletonBean1.equals(singletonBean2) = false
thread2 singletonBean0.equals(singletonBean1) = true
thread2 singletonBean1.equals(singletonBean2) = false
thread2 singletonBean1.equals(protoTypeBean1) = false
thread1 singletonBean1.equals(protoTypeBean1) = false
thread1 protoTypeBean0.equals(protoTypeBean1) = false
thread3 Equality Tests
thread3 =====================================================
thread3 Two beans references with Scope 'singleton'
thread3 singletonBean0.equals(singletonBean1) = true
thread3 singletonBean1.equals(singletonBean2) = false
thread3 singletonBean1.equals(protoTypeBean1) = false
thread1 protoTypeBean1.equals(protoTypeBean2) = false
thread1 =====================================================
thread2 protoTypeBean0.equals(protoTypeBean1) = false
thread3 protoTypeBean0.equals(protoTypeBean1) = false
thread2 protoTypeBean1.equals(protoTypeBean2) = false
thread2 =====================================================
thread3 protoTypeBean1.equals(protoTypeBean2) = false
thread3 =====================================================

thread1.start();
thread2.start();
thread3.start();

//make sure, all threads finish.
Thread.sleep(2500);

To Interface or NOT for Value Beans in Java

What is a Java Interface

From java.sun.com

There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a “contract” that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group’s code is written. Generally speaking, interfaces are such contracts.

In this blog,  I am trying to present very common design pattern in projects, which I kind of don’t agree with.

This pattern is to define Interface for all data beans and value beans.  In most of the projects, I have worked in past (and current one too), I have observed, Architects have defined interface for any kind of bean flowing between layers of software.

For example  ‘Person’ and associated ‘Address’ bean.  So, there will be a Person interface and an Address interface. And there will be ‘PersonImpl’ and ‘AddressImpl’ object implementing respective interfaces.  Now Person interface has common getters and setters for properties like name, age, height and Address interface has getters & setters for addressLine1, addressLine2, city, zipcode, country etc.  So we can imagine implementing classes have defined class variables for corresponding each getter and setter. Now big question comes to mind, Why we have defined interface for this object? Why not to keep single class Person having getter/setter and it’s class variables.

Common Reasons (and why I think, they are wrong)

  1. It can have multiple, different implementations. As I have heard from architects, they just want to expose interface to integrating layers/client. And if later on, we change implementation, it won’t visible or require any code change on their side. Well, why we will have different implementations, in first place? These beans have getter and setter of certain property. How come you can have different implementation?
  2. We want integrating client to write their own implementation. Again, why? It’s simple data bean carrying data around. I think, this is another example of ‘abuse’ of interfaces by architects who are following more of text books version, instead of thinking out of box. Interfaces are contracts of expected behavior of operations, not data beans. Data beans doesn’t have behavior, they are mere payload objects. If they are more than payload, then I would put your design in question.
  3. To support legacy objects. Argument is, their legacy code of databeans have behavior. Now to bridge to new version of software, they need to use interfaces for databeans. Hence they provide bridged data beans implementing new data beans. And later on, when they move away from legacy, we will just use new implementation of interface. This is clear sign of ‘design’ smell. If you want to bridge to legacy code, then use bridge services objects. Bridged services will take care of handling legacy operations on legacy bean. Your new beans shouldn’t have any kind of behavior, encapsulating or hiding legacy smell. That was main reason to start new project, right?
  4. So that, we can have multiple inheritance. If your data bean’s getters/setters are not suppose to have different kind of implementation, hence why different objects will implement same implementation. Those different objects rather extend common super implementing class.
  5. (write in comments, I will add here)

Problems I have faced and I hate it

  • Whenever I need to add/remove any property, first you will have to fix interface. Hence doubling my effort in refactoring.
  • If same interface is implemented somewhere else, then changing interface (in prev step), I will have to change all other classes too. Question comes, why other implementing class, just simple extends original class.  They all are data beans,  no behavior.

Only behavior, data beans/value beans (must) contain, implementing equals() and hashCode() methods.  Well, we get those as part of Object class. Another possible ‘operations’ can from other interfaces like ‘Comparable’ or ‘Serializable’. If you get gist of it, none of  interfaces defined operations as accessors for properties.

In end, I am trying to advocate, to keep java programming simple and productive. Yes, we can boost of designing complex mutli-layered system, but at the cost of productivity and maintainability. Lets keep any kind of operability or logic in business classes.  Any logic or code in data beans, is destined for future refactoring some time.

Why I decided to host website using Amazon EC2

Summary

Cloud computing is something, we been listening and reading about from past couple of years.  From my perspective, it is virtual machines running on virtualized hardware. Hence giving us dual benefit of running any image on any kind of hardware. We can dynamically launch any image and also dynamically expand/reduce available hardware capacity. Hence the word ‘Elastic’ & ‘Cloud’. In cloud computing world, amazon is more established compare to others.  So rest of article will use Amazon EC2. Amazon elastic computing allows user to launch any image (from their growing library of public images). They are called ‘AMI’s. You pay per use of images. (hourly usage rate plans). Hence it makes it easy to choose any image w/o binding to any contract.

Why cloud computing is better than tradition web hosting?

Answer is, it is not. If you looking to run simple apache web server and few php based applications then web hosting companies give best pricing and pre-configured applications.  Decision comes, if you looking for better hardware and more RAM for your applications.

Pros, Comparing amazon EC2 smallest image & virtual private server (VPS) plans.

  1. Amazon EC2 guarantees one CPU core whereas 4-5 VPS images run per CPU core.
  2. RAM available is 1.7GB for EC2 instance and VPS gives maximum of 512MB to 756MB (for plans >$40/mo) (for simultaneous multiple J2EE web applications, you wish for more RAM.)
  3. Available capacity is elastic. You can expand it as required, compare to fixed capacity available in VPS/Dedicated plans.
  4. Once you finished configuring your images, you can make current installation as another image incase you want multiple instances running.
  5. Amazon EC2 instance are running with better reliable infrastructure comparing.
  6. If you pre-pay for EC2 account, your average cost of running website 24×7, for whole month comes to $42 something, which is comparable to good VPS plans with lots of memory.

Hence comparing to dedicated hosting, amazon ec2 seems to be better option, if we start comparing relability factor, ability to instantly increase computing capacity, price and many other options.

I decided to give it try to move my website.

Other factors to consider

  • 1.7 GB of available RAM and 160 GB of drive capacity, gives me chance to run java apps w/o worrying about available memory.
  • It was learning experience for me, (as average user of linux OS).
  • It’s easy to install applications on ubuntu image. (same goes for other OS like fedora/centOS).
  • I can run MySQL & PostGres DB server together, as required.
  • Java tomcat web server seems to be good enough, but I can try some other servers like glassfish or jboss for better experience.
  • Using it as subversion server. Ping time & bandwidth is better than other servers.
  • Will be using as build server & agent both. (Most probably open source project ‘Hudson’).