How to specify/make a bean as immutable in beanfactory

[sjeelani]

Hi,

I am to newbie to spring, hence pardon for any dumb questions.

I was going thru beanfactory xml and spring has a support for specifying whether a bean is singleton or prototype. I would like to know whether there is way I can say that particular bean is immutable (ex: <bean name="yetAnotherExample" class="examples.ExampleBeanTwo" immutable="true"/>and when I declare it as immutable, spring automatically uses '==' for equality (obj1 == obj2) rather than equals().
Thanks,
Jeelani

[Colin Sampaleanu]

There's no immutable flag, but I'm confused as to when you think Spring should be comparing two instances of a bean one way vs. another (.equals vs. ==).

Please add some more details, if you want to followi this up.

Regards,

[Martin Kersten]

Are you puzzled about the term 'prototype'? By prototyping it is just said that everytime you call 'getBean' or refer to the bean a new instance is created according to the prototype specified by the <bean> tags.

It is not the same like query by example or anything like that.

Also you might refer to the Flyweight (/Lightweight) Pattern. By calling it immutable you might want to create only one instance for given property combination or something like that. This can be done by providing your own factory implementation. This is also fairly simple to do.

[sjeelani]

Hi,

Thanks for the reply. Since when we say that singleton="true" even though the framework takes care of creating only instance, we can't prevent the user apps from modifying the values. I was looking more at
'Is there a way to tell spring that make mybean in this context immutable and singleton. If we do make the bean immutable programmatically, then I can't use the same bean in other context, where mutability is required.

I was just curious to know that whether we can inject immutable property in particular contexts by allowing the framework to create some wrapper class over the bean to make it immutable.

Thanks,
Jeelani

[irbouho]

Jeelani,

You can create an Advisor that throws an exception whenever a method is invoked and apply it to your singleton bean setters methods. This will prevents "accidental" properties changees. I can see, however two issues with this scenario:
1. you can not apply your Advice to public properties.
2. you can not prevent changes to "Object" properties:

  myBean.getSubBeans().clear();

All in all, this will work only for "basic" beans. :cry:

[davison]

I was just curious to know that whether we can inject immutable property in particular contexts by allowing the framework to create some wrapper class over the bean to make it immutable.

seems an odd requirement but you can still manage it programatically quite well if I understand correctly what you're trying to do and you want to avoid AOP.

public class MyBean {
  private boolean immutable = false;
  private String propOne;

  public void setPropOne(String propOne) {
    if (immutable) throw new IllegalAccessException("no way Jose");
    this.propOne = propOne;
  }

  public void makeImmutable() { immutable = true; }
}

Now in your bean definition, make the makeImmutable method an init method..

<bean id="myBean" class="MyBean" init-method="makeImmutable">
  <property name="propOne"><value>foo</value></property>
</bean>

Your bean will be made immutable after the bean factory has set the properties on it, but your bean class can still be used elsewhere for mutable instances while ever you don't call the setImmutable() method on it.

[irbouho]

public class MyBean {
private boolean immutable = false;
private String propOne;

public void setPropOne(String propOne) {
if (immutable) throw new IllegalAccessException("no way Jose");
this.propOne = propOne;
}

public void makeImmutable() { immutable = true; }
}

this is a nice solution but it does not yet prevents users from calling

  MyBean.getSomeObject().setSomeProperty(someValue);

and of course we need here a way to prevents accidental calls to

  MyBean.makeImmutable();

[davison]

this is a nice solution but it does not yet prevents users from calling

  MyBean.getSomeObject().setSomeProperty(someValue);

correct, it was a simple answer to an incomplete set of requirements. The main point being AIUI how to prevent properties being set after the bean factory creates the bean, but without preventing other (non Spring) apps from reusing the class and setting them. In your example above, the accessor can return a clone of someObject.

[Cybernet Sauvignon]

Hi,

maybe you were inspired by J2SE Collections.unmodifiebleSet etc. methods. Please notice that for each type of Collection there is a special method/wrapper respectively. A general wrapper can be made only using AOP. And not just the simple DynaProxy solution, but the full blown code manipulation (to disable an access to the public fields).

You have one more possibility. Define your implementing class as real immutable (no direct setters, getters returning clones or immutable collections, ...)
The bean definition can then use just constructor args.

[jasonchen_nj]

The flexibility provided by Spring Framework is good. However, as I start introducing it into our project, my tech lead did ask me the following questions,

If it's a "spring singleton" defined in Spring Context, how do you prevent the other developers from doing a "new" manually? :?:

If it's not a "spring singleton”, again, how can you prevent people from just trying the "new" but not “getBean” from the framework? :?:

I think a constructor injection is a necessary. We are also considering making a "non-spring" singleton to access the Spring context in order to prevent this from happening.

Any other suggestion is all welcome.