Decoupling shared code with state that needs to cleared in between uses
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- Method injection in Spring allows setting a child bean's scope to "prototype" while keeping the parent bean's scope as "singleton". This is done by injecting a factory method for the child bean into the parent bean. - An example is provided of shared validation code that needed reinstantiation. Method injection was used to set the validation bean to "prototype" while keeping the parent bean scope as "singleton". - Benefits include avoiding needless object recreation if all beans were "prototype", as well as increased testability by isolating stateful objects.
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Decoupling shared code with state that needs to cleared in between uses
- 1. Decoupling shared code with state that needs to be cleared/reinstantiated after each use Tim Fritschel Michael Fons
- 2. Why are we here today, again? • We are here to learn about method injection – specifically a kind of method injection called “lookup-method” in the Spring framework. – What it is… – How/why it is used… – To walk through an actual example.
- 3. Why are we using Spring? • Oracle 11g SOA seems to use it by default in its configuration non-BPEL services. • Our architects chose this for default configuration for new services. • Spring has many good features, of which we want to take advantage – like… – SCA integration – Dependency injection – Many other advantages
- 4. Is it more complicated to program in Spring? • Not really…mostly you just need to be aware of what the configuration file is doing too. • Mostly it is just a matter of getting used to it.
- 5. Is Dependency Injection any good? • Dependency Injection has many advantages; among them are… – Separating configuration from use/interfaces from implementation. • Increasing testability. • Changing implementations without changing the dependent class. • Promotes loose coupling and separation of responsibilities. • Promotes code readability and simplicity.
- 6. Is method injection like dependency injection? • The kind we are talking about is closely related. • This kind of method injection addresses an issue with scoping, which I will cover a bit later.
- 7. Can you give me a real example of method injection? • That’s exactly what I wanted to do next… • Our example will involve sharing common validation code in our SOA 11g framework. • As part of the evolution of our S2F SOA 11g architecture, we have debated with the architecture team to choose whether to put this shared code… – In the project – In the infrastructure library – In it’s own service
- 8. Was your shared code complex? Did it have state? • Yes; the accumulation of error messages was its “state”. • Without clearing this state, or re-instantiating the object, this state got garbled when called multiple times for that singleton.
- 9. What do you mean, Singleton? • Spring-managed beans have “scope.” • Default scope is “singleton” – for each class loader created, there is one instance created by the spring context (at Web server startup?). • “Prototype” scope can be selected – which gets a new instance each time the dependent class is instantiated.
- 10. What if we just make the shared code “prototype” and left parent code “singleton?” • In order to see any real effect with making a bean “prototype”, if it’s parent bean (i.e., the bean we are injecting the prototype bean into) is “singleton”, then we have two options within Spring – Make the parent “prototype” also. – Use method injection (“lookup-method”) and inject a factory method for the child bean, into the parent bean.
- 11. What is wrong with making everything prototype? • Well, that means that every call of the service gets a whole new set of objects allocated in memory. • That could be needlessly wasteful in some cases, and could lead to server instability. • I do not know if the architecture team has any experimental data on this possibility. They have not mentioned any yet…
- 12. OK…but “method injection” sounds weird and complex… • It’s not too bad…there is not really a difference in the lines of code required. • A group of us converted code that uses common code that had to have its state cleared each time because we had it set to singleton scope. • With method-injection we were able to set this common code bean to prototype, without changing the parent bean from its default scope of “singleton”.
- 13. Why is method injection needed in this case? • Look at http://static.springsource.org/spring/docs/2.5 .x/spring-reference.pdf • Within this document look at 3.3.7 and 3.3.7.1. • If you do you will see the way we solved this problem, was the default way to solve this using Spring. • This creates a factory method.
- 14. Are there other things you can do with method injection in Spring other than solving this scoping issue? • Yes. For example, Tim Fritschel told me that in a prior job he and his coworkers found a use of a more complex kind of Spring method injection to solve a particular kind of problem. • Please ask him, or check Google/Spring website for more information.
- 15. Couldn’t you have just used “new” to create a new instance of VaoErrors? • Yes, but VaoErrors required a datasource which was also conveniently supplied in Spring configuration file. • So with “new” the code would be messier, passing datasource down with an additional parameter. – Or further mix and match by adding a dataSource property to the Vao implementation, and injecting it there. • All this jumps around and increases code clutter and detracts from the point of the code, making it less readable.
- 16. Let’s see what you did to convert this code…
- 17. New library, ok. What else?
- 18. Spring context has some bean changes… • Yes, there were two changes to spring context. • There are two beans in use here… – Vao – this is the parent bean, that is using vaoErrors. – vaoErrors – the child bean. • Vao is Singleton scope. • VaoErrors is “prototype” scope.
- 19. Can you explain the changes further? • Vao has gone from direct property injection, to injection via the “lookup-method” element. – This creates a deploy-time implementation of the createVaoErrors method in the vao bean – When the method is called it will return an instance of the vaoErrors bean (with the scope set in spring configuration). – VaoErrors bean changes its scope to prototype now that we can use this scope without also changing the scope of the vao bean!
- 20. Great. What else do I have to do?
- 21. OK. The parent object needs the method we are injecting “declared”. What else?
- 22. What the hell? • OK, this is a bit weird, but listen… – The “lookup-method” element actually instructs spring to sub- class the parent bean, and create an implementation (or override the existing implementation) of the specified method. – The return-type of this method must be the same as the type of the bean mentioned in the “lookup-method” element’s “bean” attribute value. – The name of the method must match the “name” attribute in the lookup-method element. – It should take no arguments. – So since we will be implementing a method, it makes it easier to remember to adjust for this by making the parent class “abstract”. Alternatively you could create a method with a don’t-care implementation, but this would be misleading.
- 23. OK. Fine. Is method injection better than clearing the state? • Using this technique trades clearing the state with creating a new instance. • After comparing these two approaches on many different levels, these two seem to be equivalent. • Without the benefit of 20-20 hindsight, this equivalence was not apparent at the time we were considering our options; we thought we were improving things.
- 24. Right. What does that change look like?
- 25. OK, that makes sense. Added a factory call before the validation, and removed the “clearing” afterward. Anything else? • No, that is the last code change. • If you have JUnit tests, you will need to alter them a bit though.
- 26. Great. Is that hard? • Not too bad. You have at least two choices here – Either subclass your parent and implement the method which simply returns your new child bean (VaoErrors in this case). – Or create a test context in your SCA-INF/src in project root, and copy in your bean definitions, so that spring continues to implement this method for you.
- 27. OK. What does the first solution look like?
- 28. That’s not too bad. What does the other look like?
- 29. OK, I need a dataSource and the parent and child beans. What else?
- 30. Need a property for the parent bean and the context, and you need to instantiate the context. What else?
- 31. Well, that’s all clean-up, right? • Correct; the first change is because we no longer need to inject our datasource into the child since spring does this for us. • The second change is to get the parent bean from the context and store it on our test class property we just created. • The third change is more stuff that Spring now does for use…dependency injection, etc.
- 32. That’s great! Any other advantages?
- 33. Wow, I had not though of that! • Yes, it’s true. You can actually conduct tests to verify that your method injection is working properly!
- 34. Hey man: thanks for the info, dude! • Any time! See you at stand up!
- 35. What did you reference to get this information? • SOA 11g implementation experiences here as S2F. • http://martinfowler.com/articles/injection.ht ml • http://static.springsource.org/spring/docs/2.5 .x/spring-reference.pdf (especially chapter 3 and section 3.3.7) • http://javapapers.com/spring/dependency- injection-di-with-spring/#diadvantages
Editor's Notes
- #17: Code generation library for spring; found in lib/external on orion trunk.