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.
Related
I kbow Proxy object extends the target class and overrides non final methods. My question is, when the proxy object overrides target method, what extra code it adds to target method that makes advice to get called? And how proxy knows when to call an advice,before, after etc?
What you are asking is a bit too broad since the code used is quite complex.
I'm not sure why do you want to know the exact implementation details but as starting point you should check how Spring AOP works:
https://docs.spring.io/spring/docs/5.1.x/spring-framework-reference/core.html#aop
Spring implements proxying by using CGLIB or the JDK depending the situation (i.e.: if you implement your beans using interfaces, Spring will try to use the JDK).
You can check the Proxying mechanisms here: https://docs.spring.io/spring/docs/5.1.x/spring-framework-reference/core.html#aop-proxying
From there you can search for the libraries and check the code used for proxying.
I hope this serves you as a starting point for your reasearch.
I have a web application with controllers, services and simple beans.
I want to use Spring Integration as a glue to link the beans. So instead of using a reference to the next bean to be called in a bean I just want to send (return) a message (e.g. a domain object) which would be the incoming parameter in the method signature of the next bean.
Is it a good idea to use Spring Integration for this? Would SI degrade the performance?
Thanks,
V.
Please, read the Reference Manual (http://projects.spring.io/spring-integration/) and other resources before asking similar questions. Spring Integration isn't a glue.
It's an Enterprise Integration Patterns implementation Framework. Even if it can do what you are asking, its purpose is much farther.
I'd say such a requirements may be addressed just with the raw ApplicationEvent model.
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.
What is the Difference between Spring IOC and Spring AOP and their Importance ?
Have you searched the web for IoC and AOP? There are a lot of references to both.
In a nutshell, IoC allows an external force to determine what implementation will be used by code rather than the code determining the implementation. The "external force" might be a configuration file, a unit test, other different code, etc.
AOP allows cross-cutting concerns to be implemented outside of the code affected by those concerns.
The "purpose" of Spring includes IoC and AOP, but goes quite a ways beyond that in its scope.
For more details please check.
Inversion of Control Containers and the Dependency Injection pattern and
Aspect-oriented programming
Also check this
What is AOP, Dependency Injection and Inversion Of Control in Simple English
IoC, AOP and more
Spring IOC: In simple answer normally you create object with new operator and set yourself for getter and setter. So, yes we use new operator in Java to create object. There is no any bad in doing this. But, when your project size grows and lots of developers are working, and you want to achieve POJO-based programming, you can use DI. So then maybe your question arises - why I can not code it myself? Of course you can use the power of reflection, annotation, and XML. But, some other had already coded this then why not reuse the third party one? There are lots of options for you to choose; Spring can be the best one. It manages your object life cycle from object creation to its destruction. You use the objects created and set by Spring DI container but you do not create them yourself.
Spring AOP: It is related to cross cutting concern. What it mean is in large system the common functionality is scattered throughout different modules. So AOP provides an easiest way to take out a common implementation in the form of 'aspect'. You can also in this case write own implementation using proxy concept but you can reuse the code of proxy based that is implementation of APO alliance using Spring.
Objective of Spring IOC is to reduce explicit dependencies between components, while purpose of Spring AOP is to wire components together possibly by enforcing certain common behavior (read: NOT Interface)
Since purpose of Spring AOP is to enforce certain behavior across components.So, Spring IOC comes in handy to achieve this purpose
All websites state that the Spring core container is the basis for complete Spring framework i.e., it is used across
the all modules like AOP, JDBC module, Web module, etc. As per my understanding, the Spring core container's main purpose is
to inject dependencies, so avoiding the need of factory classes and methods. Is that correct?
Second question: When it is said, Spring core container is the basis for complete Spring framework (e.g., for Spring AOP). As per my understanding, in Spring AOP also, getting the object of classes like
ProxyFactoryBean is achieved by core container. Right?
Thirdly, it is stated that Spring core container avoids the need for programming the use of singletons. How come singleton
classes are avoided by core container?
yep
yep
All beans declared in Spring config files are singleton by default. They are instantiated when your application starts.
First off, your understanding of what you get from Spring is about right. So let's get on to your third question, the interesting one.
The key is it's not that you don't have singletons, it's that they're singletons by configuration. This is a vital difference, as it means you can avoid all the complicated singleton enforcement code (the source of frequent problems) and instead just write exceptionally simple programs that focus on the business end of things. This is particularly important when you are writing a program with non-trivial object lifetimes: for example, in a webapp it makes it very easy to manage the lifespan of objects that hold state associated with a user's session, since if the objects have session scope, they'll be “singleton per user session”. That's enormously easier to work with than many of the alternatives.
The fact that Spring can also help out with transactions is just perfect as transaction handling is distinctly non-trivial, and AOP is the best solution to them in Java that I've seen (other languages have other options open) with Spring supporting a pretty straight-forward way of doing it. Try to do it properly without if you don't believe me. Spring's pretty much wonderful.