We are trying to utilize pf4j and pf4j-spring to provide a flexible application based on Spring.
Ideally, we would like to define Spring beans (#Service, #Repository and others) in plugins and inject them in the main application.
From what I can see, it seems to fail due to timing issues. Or in other words, Springs expects the beans to be available before the PluginManager gets instantiated.
There is an example repository that illustrates the issue on GitHub.
The question would be: Can I change something, so that Spring instantiates the PluginManager first? Is there another approach to make this work?
Note: Yes, we are aware of sbp. Unfortunately, it seems to be dead, and we didn't get it working properly either.
I'm providing a Spring RestController as a part of a library. Currently I have reusable jaxrs services, but I need to make Spring Boot alternatives. One RestController for example has 2 dependencies: one is a service that I could see being a bean and the other a String property.
I'm wondering what is the idiomatic way to expect to get those dependencies from users consuming the library. I had a few ideas about how it might happen, but wasn't sure what was the right or at least best practice way to do it.
Should users construct the RestController manually using the constructor (not using dependency injection)? I actually couldn't even figure out how to do this such that the Spring Boot Application knew about it and didn't see it in guides, so I was assuming this isn't the normal way to provide RestControllers. I only wondered if this was the right way to go as dependency injection being used for a third party library class's dependencies seems like it could be hard to manage.
Should both be beans, with the String property being a named bean? I have this one working, but I'm wondering if consumers of the library having to provide beans that the library's RestController expects is tricky or a bad practice.
Should the simple service be a bean and the String injected via #Value?
Is there some alternative or better way?
I have read that dependency injection is good for testing, in that a class can be tested without its dependencies, but the question comes to my mind if Class A depends on Class B or C or any class, testing Class A independent of some class is yielding a test result of zero, not a failed or past test.
Class A was created to do something and if it is not fed anything whether using new key word or setting up the extra files in Spring, Class A won't do any work.
About the idea of making code modular, readable and maintainable: so business classes became cleaner, but all we did was shift confusion from dirty Java business classes to convoluted XML files and having to delete interfaces used to inject to our loosened objects.
In short, it seems we have to make edits and changes to a file somewhere,right?
Please feel free to put me in my place if my understanding is lacking, just a little irritated with learning Spring because I see the same amount of work just rearranged.
Dependency injection is good for unit testing because you can individually test each method without that method depending on anything else. That way each unit test can test exactly one method.
I would say that if the xml is what’s annoying you check out Spring boot. It’s based on a java configuration so no xml and it simplifies a lot of configuration for you based on your class path. When I first started spring I found the xml very daunting coming from a java background but the annotation based configuration and the auto configuring done by spring boot is extremely helpful for quickly getting applications working.
IMO biggest advantage of using the spring is dependency injection which makes your life easy. For example if you would like to create a new service with three dependencies, then you can create a class very easily using Spring. But without spring, you will end up writing different factory methods which will return you the instances you are looking for. This makes your code very verbose with static method calls. You may want to take a look at the code repositories before spring era.
Again if you would like to use Spring or not is your personal call based on project complexity. But it's other features/advantages cant be overlooked.
And XML files or Java configs are the ways of achieving spring configuration - where you would like to add your business logic is personal flavour. Only thing is you should be consistent all across your project.
I would suggest that you read Martin Fowler's great article on Inversion of Control and Dependency Injection to gain a better understanding of why frameworks like Spring can be really useful to solve a well known set of common dependency injection problems when writing software.
As others have mentioned, there is no obligation to use Spring; and whatever you can do with Spring, you can probably do it by other means like abstract factories, factory methods, or service locators.
If your project is small enough, then you probably wouldn't mind solving the dependency injection issues on your own using some design patterns like those mentioned above. However, depending on the size of your project, many would prefer to use a framework or a library that already packs a bunch of solutions to these recurrent head scratchers.
In regards to the advantages of dependency injection frameworks when doing unit testing is the idea that you don't need to test the dependencies of your class, but only your class.
For example, most likely your application has a layered design. It is very common to have a data access class or a repository that you use to retrieve data from a datasource. Logically, you also have a class where you use that DAO.
Evidently, you already wrote unit testing for your DAO, and therefore, when you're testing your business class (where the DAO is being used) you don't care about testing your DAO again.
Fortunately, since Spring requires some form of dependency injection for your DAO, this means your class must provide a constructor or a setter method through which we can inject that DAO into our business class, right?
Well, then during unit testing of your business class, you can conveniently use those injection points to inject your own fake DAO (i.e. a mock object). That way, you can focus on the testing of your business class and forget about retesting the DAO again.
Now compare this idea with other solutions you may have done on your own:
You inject the dependency directly by instantiating the DAO within your business class.
You use a static factory method within your code to gain access to the DAO.
You use a static method from a service locator within your code to gain access to the DAO.
None of these solutions would make your code easy to test because there is no simple manner to get in the way of choosing exactly what dependency I want injected into my business class while testing it (e.g. how do you change the static factory method to use a fake DAO for testing purposes?).
So, in Spring, using XML configuration or annotations, you can easily have different dependencies being injected into your service object based on a number of conditions. For example, you may have some configurations for testing that evidently would be different than those used in production. And if you have a staging environment, you may even have different XML configurations of dependencies for your application depending on whether it is running in production or integration environments.
This pluggability of dependencies is the key winning factor here in my opinion.
So, as I was saying, my suggestion to you is that you first expand your understanding of what problems Spring core (and in general all dependency injection frameworks) is trying to solve and why it matters, and that will give you a broader perspective and understanding of these problems in a way that you could to determine when it is a good idea to use Spring and when it is not.
Diving deeper into Spring AOP I already understood that Spring Framework chooses a proxy-based strategy for weaving in aspects. I read that these Proxies are created at runtime and just in time, i.e. "lazy".
Now the following question came up to me: Which scope does such a proxy object have, considering a web appliaction? Is there a way to find out?
I'm looking forward to your answers!
Proxies are usually created by a BeanPostprocessor (in AbstractAutoProxyCreator hirearchy) so them have the same scope as the target bean.
If you create proxies by other ways, like using a ProxyFactoryBean you can change the scope, but in general isn't a good idea.
I know that Spring is most useful for dependency injection. But what I don't understand is how do I have one common "ApplicationContext ac=....." for the whole project, lets say I have a WebApplication and it has multiple number of packages but all of them still in one project , so how do I make the ApplicationContext instantiate only once. I had read somewhere that in Spring objects are initialized only once something as singleton beans but what I dont understand is that how is it different than Singleton design pattern, there is one question on this in SO but I couldn't quite clearly understand the answer as I am a newbie to Spring trying to learn by myself. Any help would really be appreciated. Sorry if the Q is too long. Hope I was able to explain my doubt clearly.
Spring veans are by default Singleton, although it is possible to configure them as prototype which means that a new bean will be created upon each request. In practice, singleton means one instance per context and are their lifecycle is managed by Spring nwhich also provides hooks into the various stages. Spring does not manage prototype beans once they have been created.
It is common in a SpringMVC application to have more than one context (one for the business services, the other for the web controllers). You would only need to create an ApplicationContext when building a standalone application. SpringMVC applications use the ContextLoaderListener to create the necessary contexts.