I have been able to define multiple aspects (one is #Before and another is #Around) using Spring AOP (combined with AspectJ annotations) over a business service class.
Currently they are getting called one by one (in sequence). However I would like to know how the priority of calling the aspects can be defined and where.
Please guide me with respect to Spring AOP. Please note that I am using Spring 2.5.3 framework.
I found the answer to this problem.
One can use #Order annotation to specify the order / sequence for particular Aspect class (the class annotated with #Aspect).
Or the aspect class can implement org.springframework.core.Ordered interface to provide order value to Spring framework.
Related
I'm started a position where the backend is in Spring and I'm having trouble. In Django, Flask, Rails, Express.js, etc, there are route files where you can find the declarations of the different URLs the server has responses configured for, and find your way around from there. Spring doesn't seem to have that....there's just files everywhere with annotations that transform the various classes into endpoints.
How do you find your way around or form some sort of mental conceptualization of the codebase's structure?
Here are some general-purpose tips:
Java IDEs are great, use them extensively. They can help in many cases.
All of them support spring boot applications. So you can open, say, pom.xml file and it will load the project.
All Rest Controllers are usually annotated with #RestController annotation
If you think the #RestController is too generic and you need something much more fine-grained in your project, consider using Spring Feature called "Stereo Type annotations". In short, you can define your own annotation (like #MyOwnVeryCustomProjectController) that will be itself marked as #RestController so that Spring boot will treat the classes annotated with that custom annotation just like Rest Controllers (you can also set up any parameter with reasonable defaults with this method).
If you use Java Configurations as opposed to a purely declarative approach with annotations and component scanning, then you might check whether there are #Configuration classes that aggregate bean definitions for all controllers.
Use Spring Actuator in Spring Boot services. In short, it exposes some HTTP endpoints, to any spring-boot process. Among other (fairly useful) endpoints, there is a mappings endpoint, it displays a list of all #RequestMapping paths in Runtime.
Try to find a class marked with the annotation #RestController. This class will serve as a controller to the path in #RequestMapping("/student") with multiple URL/ endpoints marked by any of the one annotations:
#RequestMapping("delete")
#PostMapping("/create")
#GetMapping("/list")
#DeleteMapping("{id}")
#PutMapping("{id}")
Likewise, there can be multiple controllers each, in turn, having multiple endpoints.
Hope this helps.
I have just started learning Java Spring and the concept of Dependency Injection (DI) and Inversion of Control (IoC).
I learned that all objects whether it is singleton, prototype or request and sessions, are all retrieved from the container.
The container manages the dependencies between classes and the lifecycle/scope of the object.
The fundamental idea behind this is there are no "new" operators for application using Spring Framework as the backbone of the system. (Please correct me if I am wrong).
I wanted to modernize legacy applications coded without the Spring framework and manages the 3rd party libraries classes and injects them using Spring.
How should I approach this?
I learned that all objects whether it is singleton, prototype or request and sessions, are all retrieved from the container.
That is not quite right. Not all objects, but those you have the cotainer told to resolve, are retrieved from the container. In general you use the #Component annotation to mark which of your objects should the container know of. Besides #Component there are other annotations which do in principle the same, but allow a more finegrained semantics, e.g. #Repository annotation, which is at its base #Component and put #Target, #Retention, #Documented on top.
The container manages the dependencies between classes and the lifecycle/scope of the object.
Yes. The container does the wiring up for you, i.e. resolving dependencies annotated with #Ressource, #Autowired or #Inject depending on which annotation you prefer.
During the lifecycle there are possible events, which allow usage of lifecycle callbacks.
Also: You could determine the bean scope.
The fundamental idea behind this is there are no "new" operators for application using Spring Framework as the backbone of the system. (Please correct me if I am wrong).
The fundamental principle is, that you delegate the creation of objects of a certain kind to the container. Separation of creation and consumption of objects allows greater flexibility and in consequence better testability of your application.
Besides the "components" of your application, there are e.g. the typical containers like ArrayList or HashMap, upon which you use the new-operator as before.
I wanted to modernize legacy applications coded without the Spring framework and manages the 3rd party libraries classes and injects them using Spring.
How should I approach this?
From what was said above, it should be "simple":
1) Go through each class file and look for its dependencies
2) Refactor those out and put #Component on top of the class
Special case: 3rd party objects, where you do not have access to the source. Then you have to wrap its construction yourself into a factory e.g. with #Bean.
3) Add the missing dependencies via #Autowired (the spring specific annotation for marking dependencies)
4) Refactor components of the service layer with #Service annotationinstead of #Component.
5) Refactor the data access layer, instead of using #Component, you can use #Repository.
This should give you a base to work with.
I'm a little confused by the mixed use of JSF2+Spring+EJB3 or any combination of those. I know one of the Spring principal characteristics is dependency injection, but with JSF managed beans I can use #ManagedBean and #ManagedProperty anotations and I get dependency injection functionality. With EJB3 I'm even more confused about when to use it along with JSF or if there is even a reason to use it.
So, in what kind of situation would it be a good idea to use Spring+JSF2 or EJB3+JSF2?
Until now I have created just some small web applications using only JSF2 and never needed to use Spring or EJB3. However, I'm seeing in a lot of places that people are working with all this stuff together.
First of all, Spring and EJB(+JTA) are competing technologies and usually not to be used together in the same application. Choose the one or the other. Spring or EJB(+JTA). I won't tell you which to choose, I will only tell you a bit of history and the facts so that you can easier make the decision.
Main problem they're trying to solve is providing a business service layer API with automatic transaction management. Imagine that you need to fire multiple SQL queries to perform a single business task (e.g. placing an order), and one of them failed, then you would of course like that everything is rolled back, so that the DB is kept in the same state as it was before, as if completely nothing happened. If you didn't make use of transactions, then the DB would be left in an invalid state because the first bunch of the queries actually succeeded.
If you're familiar with basic JDBC, then you should know that this can be achieved by turning off autocommit on the connection, then firing those queries in sequence, then performing commit() in the very same try in whose catch (SQLException) a rollback() is performed. This is however quite tedious to implement everytime.
With Spring and EJB(+JTA), a single (stateless) business service method call counts by default transparently as a single full transaction. This way you don't need to worry about transaction management at all. You do not need to manually create EntityManagerFactory, nor explicitly call em.getTransaction().begin() and such as you would do when you're tight-coupling business service logic into a JSF backing bean class and/or are using RESOURCE_LOCAL instead of JTA in JPA. You could for example have just the following EJB class utilizing JPA:
#Stateless
public class OrderService {
#PersistenceContext
private EntityManager em;
#EJB
private ProductService productService;
public void placeOrder(Order newOrder) {
for (Product orderedproduct : newOrder.getProducts()) {
productService.updateQuantity(orderedproduct);
}
em.persist(newOrder);
}
}
If you have a #EJB private OrderService orderService; in your JSF backing bean and invoke the orderService.placeOrder(newOrder); in the action method, then a single full transaction will be performed. If for example one of the updateQuantity() calls or the persist() call failed with an exception, then it will rollback any so far executed updateQuantity() calls, and leave the DB in a clean and crisp state. Of course, you could catch that exception in your JSF backing bean and display a faces message or so.
Noted should be that "Spring" is a quite large framework which not only competes EJB, but also CDI and JPA. Previously, during the dark J2EE ages, when EJB 2.x was extremely terrible to implement (the above EJB 3.x OrderService example would in EJB 2.x require at least 5 times more code and some XML code). Spring offered a much better alternative which required less Java code (but still many XML code). J2EE/EJB2 learned the lessons from Spring and came with Java EE 5 which offers new EJB3 API which is even more slick than Spring and required no XML at all.
Spring also offers IoC/DI (inversion of control; dependency injection) out the box. This was during the J2EE era configured by XML which can go quite overboard. Nowadays Spring also uses annotations, but still some XML is required. Since Java EE 6, after having learned the lessons from Spring, CDI is offered out the box to provide the same DI functionality, but then without any need for XML. With Spring DI #Component/#Autowired and CDI #Named/#Inject you can achieve the same as JSF does with #ManagedBean/#ManagedProperty, but Spring DI and CDI offers many more advantages around it: you can for example write interceptors to pre-process or post-process managed bean creation/destroy or a managed bean method call, you can create custom scopes, producers and consumers, you can inject an instance of narrower scope in an instance of broader scope, etc.
Spring also offers MVC which essentially competes JSF. It makes no sense to mix JSF with Spring MVC. Further Spring also offers Data which is essentially an extra abstraction layer over JPA, further minimizing DAO boilerplate (but which essentially doesn't represent the business service layer as whole).
See also:
What exactly is Java EE?
JSF Controller, Service and DAO
#Stateless beans versus #Stateful beans
There's no real easy answer here as Spring is many things.
On a really high level, Spring competes with Java EE, meaning you would use either one of them as a full stack framework.
On a finer grained level, the Spring IoC container and Spring Beans compete with the combination of CDI & EJB in Java EE.
As for the web layer, Spring MVC competes with JSF. Some Spring xyzTemplate competes with the JPA interfaces (both can use eg Hibernate as the implementation of those).
It's possible to mix and match; eg use CDI & EJB beans with Spring MVC, OR use Spring Beans with JSF.
You will normally not use 2 directly competing techs together. Spring beans + CDI + EJB in the same app, or Spring MVC + JSF is silly.
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
Extending spring based interfaces is discouraged as it unnecessarily couples the code with Spring. Does the same reasoning applies to annotations as well? We need to have them imported in the source code before using them.
I'll take the opposite viewpoint--of course using a Spring-specific annotation ties the class to Spring. I claim it's self-evident that importing and using Spring annotations ties the code to Spring (although you could trivially redefine those annotations for a non-Spring environment). The key for me is how deeply it's tied to Spring.
It's easier to check for, and process, annotations, than it is to restructure a class hierarchy or duplicate the functionality of a non-marker interface. Assuming you wanted to leave the annotations in, and could duplicate the logic behind the annotations, it'll be easier to do that (IMO) than to recreate whatever class/interface hierarchy implemented similar functionality.
The other key is the word "unnecessarily". I have yet to create a Spring application and need those classes outside of a Spring environment. When I have (generally for exposed APIs) it's been at the interface level. Note, however, that I knew from the onset that this exposure would exist and I planned accordingly.
Most would argue that this does not. While you've imported those annotations needed to ensure that Spring handles the request or wraps the transactions, you are not extending or implementing a specific class or interface.
Think about it this way, when you've annotated that class you're telling Spring to do various things based upon your configuration. Take out those annotations, and what you have is a POJO which just has some methods. It's a completely valid object without those annotations, it might not do what you wish it do (i.e. handle requests), but the code is still performing the same logic as it was with the annotations -- you are just now responsible for calling it appropriately.