Could someone provide a sample code snippet that stitches two java interfaces using spring-aop introduction (mixin)?
I'm looking for AspectJ annotation style configuration. Also, the specific use case I have is to stitch a few java beans each implementing their own interfaces together. So, rather than having a delegate coded, if I could just get away by using Spring XML, it'd be awesome.
You can use #DeclareParents or <aop:declare-parents> to get the mixin behavior. For example,
#DeclareParents(value="service.*", defaultImpl=AuditRecorderDefaultImpl.class)
private AuditRecorder mixin;
will mixin all classes in the service package with the AuditRecorder interface automatically forwarding each method to AuditRecorderDefaultImpl.
You can see working examples of this from AspectJ in Action's downloadable sources. You can also see detailed explanation in Spring documentation.
A demo based on Spring in Action book 4th edition is here, the configuration is JavaConfig style with #ComponentScan
Related
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.
Over the course of writing Spring Boot apps, our team adds in a lot of #Value annotations to help make things configurable. At some point we start to lose track of exactly what we added and what can be configured. We get a lot of questions from the QA and DevOps teams about what exactly can be configured and what can't.
Currently we just do a grep through the code base and apply some crude regular expressions to try and parse out the meaningful pieces. But this doesn't catch 100% of cases and inevitably we end up digging through the code to find out what fields can be configured.
I know we could use JavaDoc to somewhat achieve our goal, but the documentation would be buried with other JavaDoc (methods, fields, classes, etc) and it's still reliant on developers to remember to add the JavaDoc to each field.
Has anyone found a more automated way to document their #Value fields? I'm thinking something like Swagger, but specifically for Spring and the various ways it can externalize configuration.
Javadoc is indeed a way to document for developers, not the QA or the operators.
Your question is really interesting but answering to that canonically is hard because #Value are implementation details of components. Swagger that you quote documents REST contracts, that is an important difference.
Here some ideas :
Writing a BDD test for them that could be used too as documentation makes really no sense functionally but technically it makes.
Indeed, you could write a BDD integration test (with Cucumber or any other library) where you document and test the presence of each expected property.
Not a perfect solution, but you could at least retrieve exposed properties and a little more with these Spring Boot actuators :
configprops : Displays a collated list of all #ConfigurationProperties.
env : Exposes properties from Spring’s ConfigurableEnvironment.
Whenever you can, favor #ConfigurationProperties injection to group properties that work together rather than #Value. Isolating them in #ConfigurationProperties classes and adding javadoc for them is not bad at all to document their presence and usage.
as suggested by caco3 you can also generate your own metadata by using the Annotation Processor :
You can easily generate your own configuration metadata file from
items annotated with #ConfigurationProperties...
The processor picks up both classes and methods that are annotated
with #ConfigurationProperties. The Javadoc for field values within
configuration classes is used to populate the description attribute.
It joins with the previous point : favoring #ConfigurationProperties whenever it is possible.
I started learning spring today and i have a question regarding what happens to the annotations when java files with annotations is compiled ?.
The reason i am asking this is because of the fundamental difference i see when we choose to use the xml approach vs the annotations approach , and what i think is the philosophy of spring. The way i understand is spring says that all your java classes can be simple pojo's and all the spring related config should be kept independent (Like xml file.)
In case of developing spring application using xml *.java files have no idea about spring container and are compiled in to .class without any spring related dependencies.
But now when we annotate the .java file and the file is compiled the compiled file now has all spring related dependencies hard baked in to it and no longer are your classes simple pojo's.
Is this correct ? I am not sure if i am missing some thing here.
Annotations can be considered as metadata of a class or its element (method, field, local variable...). When you put annotation, you don't implement any behaviour. You just give additional info on an element.
That way, Spring, which is in charge of instanciating its bean can collect the info with reflection (see also this site) and process it.
To conclude, your Spring beans still remain POJO and there is no difference with the XML way (...from that point of view) since Spring gets from annotations the information it would have got from XML .
I think you are right and your question is justifiable, that's the way how I think about it too.
Not only compiled code but also dependency on spring jars bother me. Once you use this annotations your resulting jar depends on spring library.
It's reasonable to store beans in model according to DDD but spring is some kind of infrastructure layer so I didn't like the dependency.
Even if you would use XML, it's useful for few placed to use attributes. E.g. #Required attribute which is useful to verify that linked bean was injected. So, I've decide to use constructor dependency injection to omit this attribute, see my article. I completely leave out the dependency on spring in the code.
You can probably find such mind hook for many annotation you want/force to use.
You can use annotations only for your configuration classes, without marking them actual bean classes. In such scenario if you not use spring you just not load configuration classes.
I want to create a new application in Spring MVC, before start it, I want to learn how to use Spring MVC with annotations and JDBC template. I search many blogs and tutorials about that, but they are pretty much confusing as well.
Hopefully somebody could give me a good link where I could learn step by step for the annotation driven spring mvc application.
Have a look at following series:
Barebones Spring MVC
Articles about writing sample Spring Finance Manager
Both have a git/svn repositories with working and complete code, so if you can't find something in article you may read the code :-)
Also I can't see any problems with using JDBC templates: it's just a way how you are implementing your DAO and nothing more. All other code interacts with DAO by interface and know nothing about implementation.
Very helpfull for me is to separate code to different layers which of them interacts between each other, like in this picture:
(image stolen from http://www.captaindebug.com)
Spring provides even special annotations to allow grouping classes to layers. They are: #Controller for controllers, #Service for classes with business logic and #Repository for marking your DAOs.
I am starting to learn the Spring Framework. I came across this link but I can't understand in which order to learn from these?
Can anybody help me out?
The order of the entries on that page isn't organized so that you can gradually learn the concepts.
I'd rather advise you to try and go through the official Spring documentation first and take a look at the samples that come together with Spring. It'll give you an idea of the possibilities. Also, don't forget to make sure that you understand what the Inversion of Control (IoC) pattern is and why it's useful.
Here's what I'd recommend to someone starting out with Spring and IoC:
You should first try to use Spring in a very simple command-line application (hello world style).
Create an application context in xml and load it from your main method
Define a bean and retrieve it from your freshly loaded application context
Try to add a second bean definition in the application context and play with the bean definitions
Learn how to inject beans in properties, in constructors, ...
Play with those for a while in order to get a good feeling of what Spring core actually does for you (the IoC container) and how it can help you to decouple components in your code
Once you have a clear understanding of that, you can move on and read about Spring annotations and how you can either use xml or annotations (or even combine both approaches) to wire up your beans
You should only start using Spring in a Web application after having played around enough with the above. Once you have all that under control, then it'll be time to discover more advanced stuff and other Spring portfolio projects such as Spring Security, Spring MVC, Spring AOP, ...
The following are nice to have on the desk:
- Spring Configuration Refcard
- Spring Annotations Refcard
In any case, have fun! :)
I suggest you to learn from a books
I use Spring Recipes Second Edition to learn spring, the books is very technical and explain a good concept about spring